{{Short description|Extinct order of basal arthropods}} {{Use dmy dates|date=August 2024}} {{Automatic taxobox | fossil_range = {{fossilrange|Cambrian Series 2|Early Devonian}} | image = 20191201 Radiodonta Amplectobelua Anomalocaris Aegirocassis Lyrarapax Peytoia Laggania Hurdia.png | image_caption = Left to right, top to bottom: ''Amplectobelua symbrachiata'', ''Anomalocaris canadensis'', ''Aegirocassis benmoulai'', ''Peytoia nathorsti'', ''Lyrarapax unguispinus'', ''Cambroraster falcatus'', and ''Hurdia victoria'' | taxon = Radiodonta | authority = Collins, 1996 | synonyms = | subdivision_ranks = Families | subdivision = * {{extinct}}Anomalocarididae * {{extinct}}Amplectobeluidae * {{extinct}}Tamisiocarididae * {{extinct}}Hurdiidae * Genera with uncertain classification ** {{extinct}}''Caryosyntrips''? ** {{extinct}}''Cucumericrus''? ** {{extinct}}''Houcaris'' ** {{extinct}}''Innovatiocaris'' ** {{extinct}}''Laminacaris'' ** {{extinct}}''Paranomalocaris'' ** {{extinct}}''Shucaris'' **{{extinct}}''Zhenghecaris''? }}
'''Radiodonta''' is an extinct order of stem-group arthropods that was successful worldwide during the Cambrian period. Radiodonts are distinguished by their distinctive frontal appendages, which are morphologically diverse and were used for a variety of functions. Radiodonts were among the earliest large predators, and they also included sediment sifters and filter feeders.<ref name=":5">{{Cite journal|last1=De Vivo|first1=Giacinto|last2=Lautenschlager|first2=Stephan|last3=Vinther|first3=Jakob|date=28 July 2021|title=Three-dimensional modelling, disparity and ecology of the first Cambrian apex predators|journal=Proceedings of the Royal Society B: Biological Sciences|volume=288|issue=1955|article-number=20211176|doi=10.1098/rspb.2021.1176|pmc=8292756|pmid=34284622}}</ref> Some of the most famous species of radiodonts are the Cambrian taxa ''Anomalocaris canadensis'', ''Hurdia victoria'', ''Peytoia nathorsti'', ''Titanokorys gainesi, Cambroraster falcatus'' and ''Amplectobelua symbrachiata''. The later surviving members include the subfamily Aegirocassisinae from the Early Ordovician of Morocco and the Early Devonian member ''Schinderhannes bartelsi'' from Germany.
==Etymology== The name Radiodonta (Latin for ''radius'' "spoke of a wheel" and Greek for ''odoús'' "tooth") refers to the radial arrangement of tooth plates (oral cone) surrounding the mouth,<ref name="collins1996"/> although this feature is suggested to be absent in some radiodont species.<ref name=":2" /><ref name=":0">{{Cite journal |last1=Cong |first1=Pei-Yun |last2=Edgecombe |first2=Gregory D. |last3=Daley |first3=Allison C. |last4=Guo |first4=Jin |last5=Pates |first5=Stephen |last6=Hou |first6=Xian-Guang |date=2018 |title=New radiodonts with gnathobase-like structures from the Cambrian Chengjiang biota and implications for the systematics of Radiodonta |url=https://serval.unil.ch/notice/serval:BIB_A3714F0A56F0 |journal=Papers in Palaeontology |language=en |volume=4 |issue=4 |pages=605–621 |doi=10.1002/spp2.1219 |issn=2056-2802 |s2cid=90258934 |doi-access=free}}</ref>
==Definition== The original diagnosis of order Radiodonta in 1996 is as follows:<ref name="collins1996">{{cite journal |first=Desmond |last=Collins |year=1996 |title=The "evolution" of ''Anomalocaris'' and its classification in the arthropod class Dinocarida (nov.) and order Radiodonta (nov.) |journal=Journal of Paleontology |volume=70 |issue=2 |pages=280–293 |url=http://www.citeulike.org/user/vburton82/article/3498460|archive-url=https://web.archive.org/web/20110927153321/http://www.citeulike.org/user/vburton82/article/3498460|url-status=usurped|archive-date=27 September 2011|doi=10.1017/S0022336000023362 |s2cid=131622496 |url-access=subscription }}</ref>
{{Blockquote|text=Radiodontids are bilaterally symmetrical, elongate arthropods with a nonmineralized cuticle typically most robust in the jaws and claws. The body is subdivided into two tagmata, much like the prosoma and opisthosoma of chelicerate arthropods. Typically, the front part shows no external segmentation, bears one pair of preoral claws, a pair of prominent eyes, and ventral jaws with radiating teeth. Some forms have additional rows of teeth and three or four postoral gnathobasic limb pairs. The trunk is metameric, typically with about 13 segments laterally developing imbricating lobes for swimming and gills for respiration, and may end in a prominent three-part tail. Some forms have gnathobasic trunk limbs.}}
In 2014, the clade Radiodonta was defined phylogenetically as a clade including any taxa closer to ''Anomalocaris canadensis'' than ''Paralithodes camtschaticus''.<ref name="vinther2014">{{cite journal |first1=Jakob |last1=Vinther |first2=Martin |last2=Stein |first3=Nicholas R. |last3=Longrich |first4=David A. T. |last4=Harper |author-link4=David Harper (palaeontologist) |year=2014 |title=A suspension-feeding anomalocarid from the Early Cambrian |journal=Nature |volume=507 |issue=7493 |pages=496–499 |doi=10.1038/nature13010 |pmid=24670770|bibcode=2014Natur.507..496V |s2cid=205237459 |url=http://dro.dur.ac.uk/21270/1/21270.pdf }}</ref> In 2019, it was redefined morphologically as animal bearing head carapace complex with central (H-) and lateral (P-) elements; outgrowths (endites) from frontal appendages bearing auxiliary spines; and reduced anterior flaps or bands of lamellae (setal blades) and strong tapering of body from anterior to posterior.<ref name=":1">{{Cite journal |last1=Moysiuk |first1=J. |last2=Caron |first2=J.-B. |date=14 August 2019 |title=A new hurdiid radiodont from the Burgess Shale evinces the exploitation of Cambrian infaunal food sources |journal=Proceedings of the Royal Society B: Biological Sciences |volume=286 |issue=1908 |article-number=20191079 |doi=10.1098/rspb.2019.1079 |pmc=6710600 |pmid=31362637}}</ref>
Members of Radiodonta are known as radiodonts,<ref name=":1" /><ref name=":0" /><ref name=":6" /> radiodontans,<ref name=":2">{{Cite journal |last1=Cong |first1=Peiyun |last2=Daley |first2=Allison C. |last3=Edgecombe |first3=Gregory D. |last4=Hou |first4=Xianguang |date=30 August 2017 |title=The functional head of the Cambrian radiodontan (stem-group Euarthropoda) ''Amplectobelua symbrachiata'' |journal=BMC Evolutionary Biology |volume=17 |issue=1 |page=208 |doi=10.1186/s12862-017-1049-1 |issn=1471-2148 |pmc=5577670 |pmid=28854872 |doi-access=free}}</ref><ref name=":7">{{Cite journal |last1=Cong |first1=Peiyun |last2=Daley |first2=Allison C. |last3=Edgecombe |first3=Gregory D. |last4=Hou |first4=Xianguang |last5=Chen |first5=Ailin |date=September 2016 |title=Morphology of the radiodontan ''Lyrarapax'' from the early Cambrian Chengjiang biota |url=https://www.researchgate.net/publication/305769020 |journal=Journal of Paleontology |language=en |volume=90 |issue=4 |pages=663–671 |doi=10.1017/jpa.2016.67 |issn=0022-3360 |s2cid=88742430}}</ref> radiodontids,<ref name="collins1996" /> anomalocarids,<ref name="vinther2014" /> or anomalocaridids,<ref name="daley2009" /><ref name="cong2014" /><ref name="vanroy2015">{{cite journal |last1=Van Roy |first1=Peter |last2=Daley |first2=Allison C. |last3=Briggs |first3=Derek E. G. |year=2015 |title=Anomalocaridid trunk limb homology revealed by a giant filter-feeder with paired flaps |journal=Nature |volume=522 |issue=7554 |pages=77–80 |bibcode=2015Natur.522...77V |doi=10.1038/nature14256 |pmid=25762145 |s2cid=205242881}}</ref> although the last two originally refer to the family Anomalocarididae, which previously included all species of this order but is now restricted to only a few species.<ref name="vinther2014" />
==Description== thumb|left|300px|Size estimation and comparison of radiodont species known by nearly complete specimens
Most radiodonts were significantly larger than the other Cambrian fauna, with typical body lengths of large taxa varying from {{cvt|30|to|50|cm|in}}.<ref>{{cite book|author=Foster, J.|year=2014|chapter=Magical Mystery Tour: The Biological Psychedelia of the Burgess Shale|title=Cambrian Ocean World: Ancient Sea Life of North America|publisher=Indiana University Press|pages=195–252|isbn=978-0-253-01182-4}}</ref> The largest described radiodont is the Early Ordovician species ''Aegirocassis benmoulai'', which may have grown up to {{cvt|2|m|ft}} long.<ref name="vanroy2015" /><ref name=":6" /> A nearly complete specimen of a juvenile ''Lyrarapax unguispinus'' measured only {{cvt|18|mm|in}}, making it among the smallest radiodont specimens known, though adults reached a length of {{cvt|8.3|cm|in}}<ref name=":6"/><ref name=":13">{{Cite journal|last1=Liu|first1=Jianni|last2=Lerosey-Aubril|first2=Rudy|last3=Steiner|first3=Michael|last4=Dunlop|first4=Jason A.|last5=Shu|first5=Degan|last6=Paterson|first6=John R.|date=1 November 2018|title=Origin of raptorial feeding in juvenile euarthropods revealed by a Cambrian radiodontan|journal=National Science Review|language=en|volume=5|issue=6|pages=863–869|doi=10.1093/nsr/nwy057|issn=2095-5138|doi-access=free}}</ref> An isolated frontal appendage of a hurdiid from the Ordovician with a length less than half that of the juvenile ''Lyrarapax'' is known, but it is not known whether this specimen pertains to an adult.<ref name="Patesetal2020"/> The largest known Cambrian radiodont was ''Amplectobelua'', reaching lengths of up to {{cvt|90|cm|in}} based on an incomplete specimen.<ref name=":84">{{Cite journal |last1=Wu |first1=Yu |last2=Pates |first2=Stephen |last3=Pauly |first3=Daniel |last4=Zhang |first4=Xingliang |last5=Fu |first5=Dongjing |date=3 November 2023 |title=Rapid growth in a large Cambrian apex predator |journal=National Science Review |volume=11 |issue=3 |article-number=nwad284 |doi=10.1093/nsr/nwad284 |issn=2095-5138|doi-access=free |pmid=38312385 |pmc=10833464 }}</ref> ''Anomalocaris canadensis'' was also relatively large, estimated up to {{cvt|34.2|-|37.8|cm|in}} long,<ref name=":6" /> and the Cambrian hurdiid ''Titanokorys'' approached around {{cvt|50|cm|in}} long.<ref name="CaronMoysiuk2021"/>
The body of a radiodont could be divided into two regions: head and trunk. The head is composed of only one body segment<ref name=":8" /> known as the ocular somite, covered by sclerites (head carapace complex), bore arthropodized frontal appendages, ventral mouthparts (oral cone), and stalked compound eyes. The tapering trunk is composed of multiple body segments, each associated with pairs of flaps and gill-like structures (setal blades).<ref name=":1" />
=== Frontal appendage === {{multiple image | align = center | width = 350 | footer = Frontal appendages morphology of the radiodont families Anomalocarididae/Amplectobeluidae and Hurdiidae | image1 = 20191213_Radiodonta_frontal_appendage_Anomalocarididae_Amplectobeluidae.png | image2 = 20191213_Radiodonta_frontal_appendage_Hurdiidae.png }}
The anterior structures on the head are a pair of frontal appendages which have been referred to as 'claws', 'grasping appendages', 'feeding appendages', or 'great appendages' in previous studies (the last term is discouraged since the homology between frontal appendages and the original, morphologically distinct great appendages of megacheirans is questionable.<ref name=":8" /><ref name=":18">{{Cite journal|last1=Zeng|first1=Han|last2=Zhao|first2=Fangchen|last3=Niu|first3=Kecheng|last4=Zhu|first4=Maoyan|last5=Huang|first5=Diying|date=2020|title=An early Cambrian euarthropod with radiodont-like raptorial appendages|url=https://www.nature.com/articles/s41586-020-2883-7|journal=Nature|language=en|volume=588|issue=7836|pages=101–105|doi=10.1038/s41586-020-2883-7|pmid=33149303|bibcode=2020Natur.588..101Z|s2cid=226248177|issn=1476-4687|url-access=subscription}}</ref>). They are sclerotized (hardened) and arthropodized (segmented), bearing ventral endites (spines) on most of their podomeres (segmental units), and the endites may bear additional rows of auxiliary spines on their anterior and posterior margins.<ref name=":3">{{Cite journal|last1=Pates|first1=Stephen|last2=Daley|first2=Allison C.|last3=Butterfield|first3=Nicholas J.|date=11 June 2019|title=First report of paired ventral endites in a hurdiid radiodont |journal=Zoological Letters|volume=5|issue=1|page=18|doi=10.1186/s40851-019-0132-4|issn=2056-306X|pmc=6560863|pmid=31210962 |doi-access=free }}</ref><ref name=":1" /> The frontal appendage consists of two regions: the shaft ('peduncle',<ref name=":6" /> 'base'<ref name=":19">{{Cite journal|last1=Wu|first1=Yu|last2=Ma|first2=Jiaxin|last3=Lin|first3=Weiliang|last4=Sun|first4=Ao|last5=Zhang|first5=Xingliang|last6=Fu|first6=Dongjing|date=1 May 2021|title=New anomalocaridids (Panarthropoda: Radiodonta) from the lower Cambrian Chengjiang Lagerstätte: Biostratigraphic and paleobiogeographic implications|url=https://www.sciencedirect.com/science/article/pii/S0031018221001188|journal=Palaeogeography, Palaeoclimatology, Palaeoecology|language=en|volume=569|article-number=110333|doi=10.1016/j.palaeo.2021.110333|bibcode=2021PPP...56910333W|s2cid=233565727|issn=0031-0182|url-access=subscription}}</ref> or 'proximal region'<ref name=":6" /> in some studies) and the distal articulated region<ref name=":3" /> (also referred to as 'claw'<ref name=":19" />). A triangular region covered by soft cuticle (arthrodial membrane) may occur on the ventral side between podomeres and provide flexibility.<ref name=":16" /><ref name=":5" /> Their purported pre-ocular and protocerebral origin suggest they are homologous to the primary antennae of Onychophora and the labrum of Euarthropoda (all arose from ocular somite),<ref name=":8">{{Cite journal|last1=Ortega-Hernández|first1=Javier|last2=Janssen|first2=Ralf|last3=Budd|first3=Graham E.|date=1 May 2017|title=Origin and evolution of the panarthropod head – A palaeobiological and developmental perspective|journal=Arthropod Structure & Development|volume=46|issue=3, ''Evolution of Segmentation''|pages=354–379|doi=10.1016/j.asd.2016.10.011|issn=1467-8039|pmid=27989966|doi-access=free}}</ref><ref name="cong2014">{{cite journal |first1=Peiyun |last1=Cong |first2=Xiaoya |last2=Ma |first3=Xianguang |last3=Hou |first4=Gregory D. |last4=Edgecombe |first5=Nicholas J. |last5=Strausfeld |year=2014 |title=Brain structure resolves the segmental affinity of anomalocaridid appendages |journal=Nature |doi=10.1038/nature13486 |pmid=25043032 |volume=513 |issue=7519 |pages=538–42|bibcode=2014Natur.513..538C |s2cid=4451239 }}</ref> while subsequent studies also suggest a deutocerebral origin and homologous with the chelicerae of Chelicerata and the antennae or 'great appendages' of other arthropods (all arose from post-ocular somite 1).<ref name=":41" /> Since the morphology of the frontal appendages, especially those of the spines, always differs between species, it is one of the most important means of species identification.<ref name=":3" /> In fact, many radiodonts are only known from a handful of fossilized frontal appendages.<ref name=":16" /><ref name=":3" />
<gallery mode="packed" heights="200"> File:20191221 Radiodonta frontal appendage Anomalocarididae Amplectobeluidae.png|Frontal appendages of Anomalocarididae, Amplectobeluidae, and possibly related species File:20191228 Radiodonta frontal appendage Tamisiocarididae Cetiocaridae.png|Frontal appendages of Tamisiocarididae File:20191229 Radiodonta frontal appendage Hurdiidae.png|Frontal appendages of Hurdiidae </gallery>
=== Oral cone === thumb|Oral cones of various radiodonts
The mouth is on the ventral side of the head, behind the attachment point of frontal appendages and is surrounded by a ring of tooth plates, forming the mouthpart known as oral cone ('jaws' in previous studies<ref name="collins1996" />). Three or four tooth plates might be enlarged, giving the oral cone a triradial (e.g. ''Anomalocaris'', ''Echidnacaris'') or tetraradial (e.g. Hurdiidae, ''Lyrarapax'') appearance.<ref name=":14">{{Cite journal|last1=Daley|first1=Allison C.|last2=Bergström|first2=Jan|date=April 2012|title=The oral cone of ''Anomalocaris'' is not a classic ''peytoia''|url=https://www.researchgate.net/publication/223958266|journal=Naturwissenschaften|language=en|volume=99|issue=6|pages=501–504|doi=10.1007/s00114-012-0910-8|issn=0028-1042|pmid=22476406|bibcode=2012NW.....99..501D|s2cid=2042726}}</ref><ref name=":13" /><ref name=":42" /> The inner margin of tooth plates have spikes facing towards the mouth opening. Additional rows of internal tooth plates may occur in some hurdiid genera.<ref name="daley2009" /><ref name=":1" /> Detail reconstruction of some amplectobeluid oral cones are speculative, but they possibly did not present a typical radial arrangement.<ref name=":2" /><ref name=":0" />
=== Head sclerites, eyes and trunk === {{multiple image | align = center | width = 350 | footer = | image1 = 20210516 Radiodonta head sclerites.png | image2 = Anomalocaris and Echidnacaris eyes.jpg | direction = | total_width = | alt1 = | caption1 = Head sclerite complexes of various radiodonts | caption2 = Preserved compound eyes of ''Anomalocaris'' (top, F-H) and ''Echidnacaris'' (bottom, I-K), showing preserved individual ommatidia lenses. The density of the lenses in ''Anomalocaris'' is comparable to that of living dragonflies. Scale bar = 5 mm in F, G, I, J 0.3 mm in H and 1 mm in K }}
Three head sclerite (carapace) complex formed by a central H-element (anterior sclerite or head shield) and a pair of P-elements (lateral sclerites) cover the dorsal and laterovental surface of the animal's head.<ref name=":1" /> The P-elements may connect to each other as well as the H-element by a narrow anterior extension (P-element neck or 'beak').<ref name="daley2009" /><ref name=":1" /> The head sclerites are small and ovoid in Anomalocarididae and Amplectobeluidae,<ref name=":2" /><ref name=":1" /> but often enlarged in Hurdiidae, corresponded to their distinct body shapes (streamlined in Anomalocarididae/Amplectobeluidae but often compact in Hurdiidae).<ref name=":1" /> The head bore two stalked compound eyes, which may have had mobility,<ref name=":9">{{Cite journal|last1=Strausfeld|first1=Nicholas J.|last2=Ma|first2=Xiaoya|last3=Edgecombe|first3=Gregory D.|last4=Fortey|first4=Richard A.|last5=Land|first5=Michael F.|last6=Liu|first6=Yu|last7=Cong|first7=Peiyun|last8=Hou|first8=Xianguang|date=August 2015|title=Arthropod eyes: The early Cambrian fossil record and divergent evolution of visual systems|journal=Arthropod Structure & Development|language=en|volume=45|issue=2|pages=152–172|doi=10.1016/j.asd.2015.07.005|pmid=26276096|doi-access=free}}</ref> and are located between the gaps formed by the posterior regions of the H-element and P-elements.<ref name="daley2009" /><ref name=":1" /> In ''Anomalocaris'', the stalked eyes had a over 24,000 lenses (ommatidia) in each eye, with the resolution being comparable to a dragonfly. The resolution of the eyes of ''Echidnacaris'' was somewhat lower though still considerable, with over 13,000 lenses per eye.<ref name="Paterson2020" /> The compound eyes of ''Echidnacaris,'' exceptionally among known radiodonts, are unstalked.<ref name="Patesetal2020" /> Some species of Hurdiid possess an additional median eye behind the H-element.<ref name=":41" />
{{multiple image | align = center | width = 350 | footer = Anterior region of two generalized ''Anomalocaris'' and Hurdiid radiodont, showing distinct morphology A: Dorsal view, B: Ventral view, Fa: Frontal appendage, He: H-element, Pe: P-element, Ey: Eye, Oc: Oral cone, Af: Anterior (neck) flap, Bf/Vf:, Ventral flap, Sb: Setal blade | image1 = 20190908_Radiodonta_Anomalocaris_Anterior.png | image2 = 20190908_Radiodonta_Hurdiidae_Anterior.png | direction = | total_width = | alt1 = | caption1 = | caption2 = }}
Contrary to the original diagnosis, the division of body segments (segmental boundaries) can be visible externally<ref name="vanroy2015" /><ref name=":7" /><ref name=":1" /> and no known member of Radiodonta (except the putative radiodont ''Cucumericrus''<ref name="vanroy2015" /><ref name=":4">{{Cite journal |last1=Xian-Guang |first1=Hou |last2=Bergström |first2=Jan |last3=Ahlberg |first3=Per |date=September 1995 |title=''Anomalocaris'' and other large animals in the lower Cambrian Chengjiang fauna of southwest China |journal=GFF |language=en |volume=117 |issue=3 |pages=163–183 |doi=10.1080/11035899509546213 |issn=1103-5897}}</ref>) is known to have pediform trunk appendages (legs).<ref name="daley2014">{{cite journal |first1=Allison C. |last1=Daley |first2=Gregory D. |last2=Edgecombe |year=2014 |title=Morphology of ''Anomalocaris canadensis'' from the Burgess Shale |journal=Journal of Paleontology |volume=88 |issue=1 |pages=68–91 |doi=10.1666/13-067|s2cid=86683798 }}</ref> The trunk has numerous body segments (somites), tapering from anterior to posterior, with the anterior three or four segments significantly constricted into a neck region.<ref name=":1" />
<gallery mode="packed" heights="200"> File:20210914 Radiodonta body flaps lobes structures variations.png|Variations of radiodont body flaps File:Radiodonta body flaps movement.gif|The movement of radiodont flapping appendages File:Video animation of Cambroraster.webm|Video animation of ''Cambroraster falcatus'', showcasing the movement of the trunk flaps File:20200103 Radiodonta gnathobase‐like structures.png|Ventral view of a generalized GLS-bearing radiodont, showing gnathobase‐like structures (GLSs) associated with reduced anterior flaps </gallery>
The trunk appendages were fin-like body flaps ('lateral flaps' or 'lobes' in some studies), usually one pair of ventral flaps per body segment, each slightly overlapping the one more anterior to it, but additional, non-overlapping sets of small dorsal flaps may occur in some Hurdiid species.<ref name="vanroy2015" /> The flaps may have numerous vein-like structures (referred to as 'strengthening rays',<ref name=":7" /> 'flap rays',<ref name=":1" /> 'tranverse rods',<ref name="vanroy2015" /> 'transverse lines'<ref>{{Cite journal|last1=Daley|first1=Allison C.|last2=Paterson|first2=John R.|last3=Edgecombe|first3=Gregory D.|last4=García-Bellido|first4=Diego C.|last5=Jago|first5=James B.|date=2013|title=New anatomical information on Anomalocaris from the Cambrian Emu Bay Shale of South Australia and a reassessment of its inferred predatory habits|journal=Palaeontology|language=en|volume=56|issue=5|pages=971–990|doi=10.1111/pala.12029|s2cid=128955512 |issn=1475-4983|doi-access=free}}</ref> or 'veins'<ref name=":20">{{Cite journal|last1=Chen|first1=Jun-yuan|last2=Ramsköld|first2=Lars|last3=Zhou|first3=Gui-qing|date=27 May 1994|title=Evidence for Monophyly and Arthropod Affinity of Cambrian Giant Predators|url=https://www.science.org/doi/abs/10.1126/science.264.5163.1304 |journal=Science|volume=264|issue=5163|pages=1304–1308|language=EN|doi=10.1126/science.264.5163.1304|pmid=17780848|bibcode=1994Sci...264.1304C|s2cid=1913482|url-access=subscription}}</ref>). The flaps on the neck region (referred to as 'reduced flaps',<ref name=":2" /> 'neck flaps',<ref name=":7" /> 'head flaps',<ref name="daley2014" /> 'anterior flaps'<ref name=":26">{{Cite journal|last1=Aria|first1=Cédric|last2=Zhao|first2=Fangchen|last3=Zeng|first3=Han|last4=Guo|first4=Jin|last5=Zhu|first5=Maoyan|date=8 January 2020|title=Fossils from South China redefine the ancestral euarthropod body plan|journal=BMC Evolutionary Biology|volume=20|issue=1|page=4|doi=10.1186/s12862-019-1560-7|issn=1471-2148|pmc=6950928|pmid=31914921 |doi-access=free }}</ref> or 'differentiated flaps'<ref name=":18" />) are significantly reduced. In some species, jaw-like feeding appendages called gnathobase-like structures (GLSs) arose from each of the bases of their reduced neck flaps.<ref name=":2" /><ref name=":0" /> Numerous elongated blade-like extensions (referred to as lanceolate blades or lamellae<ref name=":1" />) arranged in a row, forming bands of gill-like structures known as setal blades, covered the dorsal surface of each body segment.<ref name="vanroy2015" /> At least in ''Aegirocassis'', each of the lanceolate blades are covered in wrinkles.<ref name="vanroy2015" /> The ventral flaps may be homologous to the endopod of the biramous limbs of euarthropods and lobopodous limbs (lobopods) of gilled lobopodians, and the dorsal flaps and setal blades may be homologous to the exite and gill-bearing dorsal flaps of the former taxa.<ref name="vanroy2013">{{cite conference |first1=Peter |last1=Van Roy |first2=Allison C. |last2=Daley |first3=Derek E. G. |last3=Briggs |year=2013 |title=Anomalocaridids had two sets of lateral flaps |conference=57th Annual Meeting of The Paleontological Association |location=Zurich, Switzerland }}</ref><ref name="vanroy2015" /> The trunk may end either with a tail fan compose of 1 to 3 pairs of blades,<ref name=":20" /><ref name="daley2014" /><ref name=":1" /> a pair of long furcae,<ref name=":20" /><ref name=":13"/><ref name=":1" /> an elongated terminal structure,<ref name="daley2014" /> or a featureless blunt tip.<ref name="vanroy2015" />
<gallery mode="packed" heights="120"> File:20220724 Hurdiidae.png|''Stanleycaris'', ''Hurdia'', ''Aegirocassis'', ''Peytoia'' and ''Cambroraster'' are all examples of hurdiid radiodonts. These were the most diverse and long lasting of the radiodont families, surviving from the Cambrian up until the Devonian. File:20210626 Anomalocaris.png|''Anomalocaris'' is a member of the anomalocarididae family, which at one point included all radiodonts, but now only includes a few genera such as ''Lenisicaris''. File:20210912 Amplectobeluidae.png|''Amplectobelua'' and ''Lyrarapax'' are representatives of the amplectobeluidae which is a very inclusive family of mainly Chinese radiodonts. File:20191228 Radiodonta frontal appendage Tamisiocarididae Cetiocaridae.png|''Echidnacaris'' and ''Tamisiocaris'' are examples of the family tamisiocarididae which were exclusively suspension feeding radiodonts from the Cambrian. </gallery>
=== Internal structures === left|thumb|Digestive system of a radiodont. thumb|Various interpretations of radiodont brain. A: after Cong et al. 2014,<ref name="cong2014" /> B: after Moysiuk & Caron 2022<ref name=":41" /> Traces of muscles, digestive system and nervous system were described from some radiodont fossils. Pairs of well-developed muscles were connected to the ventral flaps located at the lateral cavities of each body segment.<ref name="daley2014" /><ref name="cong2014" /> Between the lateral muscles is a sophisticated digestive system, formed by a widening of the foregut and hindgut, both connected by a narrow midgut associated with six pairs of gut diverticula (digestive glands).<ref name="daley2014" /><ref name=":7" /><ref name=":10">{{Cite journal|last1=Vannier|first1=Jean|last2=Liu|first2=Jianni|last3=Lerosey-Aubril|first3=Rudy|last4=Vinther|first4=Jakob|last5=Daley|first5=Allison C.|date=2 May 2014|title=Sophisticated digestive systems in early arthropods|journal=Nature Communications|language=en|volume=5|issue=1|page=3641|doi=10.1038/ncomms4641|issn=2041-1723|pmid=24785191|bibcode=2014NatCo...5.3641V|doi-access=free}}</ref>
The brain of radiodonts was simpler than the three-segmented (compose of pro-, deuto- and tritocerebrum) brains of euarthropods, but further interpretations differ between studies. Based on Cong et al. 2014, the brain composed of only one brain segment originating from the ocular somite, the protocerebrum. The nerves of the frontal appendages and compound eyes arose from the anterior and lateral regions of the brain.<ref name="cong2014" /><ref name=":8" /> Based on Moysiuk & Caron 2022, the frontal appendage nerves arose from the ventral deutocerebrum, the second brain segment. The previous "frontal appendage nerves" actually represent median eye nerve.<ref name=":41" /> In both interpretations, posterior to the brain was a pair of apparently unfused ventral nerve cords which ran through the animal's neck region.<ref name="cong2014" /><ref name=":41" />
== Paleoecology == === Physiology === {{multiple image | align = right | direction = horizontal | width = 250 | header = | image1 = Shucaris and Erratus.jpg | alt1 = | caption1 = Reconstruction of ''Shucaris'', an anomalocaridid or amplectobeluiid radiodont, using its frontal appendages to capture a fleeing ''Erratus'' | image2 = Fezouata_Biota.jpg | alt2 = | caption2 = Reconstruction of the Fezouata Biota, featuring ''Aegirocassis'', a filter feeding radiodont, and two species of ''Pseudoangustidontus'' }}
Radiodonts were interpreted as nektonic or nektobenthic animals, with their morphology suggesting an active swimming lifestyle. The muscular, overlapping ventral flaps may have propelled the animal through the water, possibly by moving in a wave-like formation resembling modern rays and cuttlefish.<ref name=":31">{{Cite journal|last1=Whittington|first1=Harry Blackmore|last2=Briggs|first2=Derek Ernest Gilmor|date=14 May 1985|title=The largest Cambrian animal, ''Anomalocaris'', Burgess Shale, British-Columbia|url=https://royalsocietypublishing.org/doi/10.1098/rstb.1985.0096|journal=Philosophical Transactions of the Royal Society of London. B, Biological Sciences|volume=309|issue=1141|pages=569–609|doi=10.1098/rstb.1985.0096|bibcode=1985RSPTB.309..569W|url-access=subscription}}</ref><ref>{{Cite journal|last=Usami|first=Yoshiyuki|date=7 January 2006|title=Theoretical study on the body form and swimming pattern of Anomalocaris based on hydrodynamic simulation|journal=Journal of Theoretical Biology|volume=238|issue=1|pages=11–17|doi=10.1016/j.jtbi.2005.05.008|issn=0022-5193|pmid=16002096|bibcode=2006JThBi.238...11U}}</ref> Pairs of dorsal flaps, which make up a tail fan in some species, may have helped steering and/or stabilizing the animal during locomotion.<ref name="vanroy2015" /><ref name=":12" /> In ''Anomalocaris'', morphology of the tail fan even suggests it could rapidly change its swimming direction efficiently.<ref>{{Cite journal|last1=Sheppard|first1=K. A.|last2=Rival|first2=D. E.|last3=Caron|first3=J.-B.|date=1 October 2018|title=On the Hydrodynamics of Anomalocaris Tail Fins|journal=Integrative and Comparative Biology|language=en|volume=58|issue=4|pages=703–711|doi=10.1093/icb/icy014|issn=1540-7063|pmid=29697774|doi-access=free|hdl=1974/22737|hdl-access=free}}</ref> On the other hand, some hurdiids have features significantly specialized for a nektobenthic lifestyle, such as ''Cambroraster'' with its dome-like H-element similar to the carapace of a horseshoe crab.<ref name=":1" /> Bands of setal blades with wrinkling lanceolate blades may have increased the surface area, suggesting they were gills, providing the animal's respiratory function.<ref name="daley2014" /><ref name="vanroy2015" /> Abundance of the remains of scleritzed structures such as disarticulated frontal appendages and head sclerite complexes, suggest that mass moulting events may have occurred among radiodonts,<ref name="vanroy2015" /><ref name=":1" /> a behavior which also has been reported in some other Cambrian arthropods such as trilobites.<ref>{{Cite journal|last1=Daley|first1=Allison|last2=Drage|first2=Harriet|date=1 September 2015|title=The fossil record of ecdysis, and trends in the moulting behaviour of trilobites|url=https://www.researchgate.net/publication/282594514|journal=Arthropod Structure & Development|volume=45|issue=2|doi=10.1016/j.asd.2015.09.004|pmid=26431634|pages=71–96|doi-access=free}}</ref>
=== Diet === {{multiple image | align = right | total_width = 300 | perrow = 2 | caption_align = center | footer = Suggested frontal appendage mobility and movement of various radiodonts<ref name=":16" /><ref name=":5" /> | image1 = 20210629 Anomalocaris canadensis frontal appendage mobility.gif | caption1 = ''Anomalocaris canadensis'' | image2 = 20210812 Amplectobelua stephenensis frontal appendage mobility.gif | caption2 = ''Amplectobelua stephenensis'' | image3 = 20210812 Hurdia frontal appendage mobility.gif | caption3 = ''Hurdia victoria'' | image4 = 20210630 Tamisiocaris borealis frontal appendage mobility.gif | caption4 = ''Tamisiocaris borealis'' | image5 = 20210812 Peytoia nathorsti Laggania cambria frontal appendage mobility.gif | caption5 = ''Peytoia nathorsti'' | image6 = 20210813 Cambroraster falcatus frontal appendage mobility.gif | caption6 = ''Cambroraster falcatus'' }}
Radiodonts had diverse feeding strategies, which could be categorized as raptorial predators, sediment sifters, or suspension, filter feeders.<ref name=":6" /><ref name=":15">{{Cite web|last1=De Vivo |first1=Giacinto |last2=Lautenschlager |first2=Stephan |last3=Vinther |first3=Jakob |date=16 December 2016 |title=Reconstructing anomalocaridid feeding appendage dexterity sheds light on radiodontan ecology|url=https://www.researchgate.net/publication/325343898}}</ref><ref name=":5" /><ref name=":27" /><ref name=":28">{{Cite journal|last1=Caron|first1=J.-B.|last2=Moysiuk|first2=J.|date=2021|title=A giant nektobenthic radiodont from the Burgess Shale and the significance of hurdiid carapace diversity|journal=Royal Society Open Science|volume=8|issue=9|article-number=210664|doi=10.1098/rsos.210664|doi-access=free|pmc=8424305|pmid=34527273|bibcode=2021RSOS....810664C}}</ref> For example, raptorial predators like ''Anomalocaris'' and Amplectobeluids might have been able to catch agile prey by using their raptorial frontal appendages; the latter even bore a robust endite for holding prey like a pincer.<ref name=":4" /><ref name=":16">{{Cite journal|last1=Daley|first1=Allison C.|last2=Budd|first2=Graham E.|date=2010|title=New anomalocaridid appendages from the Burgess Shale, Canada |journal=Palaeontology |language=en |volume=53 |issue=4 |pages=721–738 |doi=10.1111/j.1475-4983.2010.00955.x |bibcode=2010Palgy..53..721D |issn=1475-4983|doi-access=free}}</ref><ref name=":2" /><ref name=":5" /> With the smaller head carapace complex and large surface of arthrodial membranes, frontal appendages of these taxa had greater flexibility.<ref name=":13"/> Stout frontal appendages of sediment sifters like ''Hurdia'' and ''Peytoia'' have serrated endites with mesial curvature, which could form a basket-like trap for raking through sediment and passing food items towards the well-developed oral cone.<ref name=":1" /><ref name=":5" /> Endites of frontal appendages from suspension/filter feeders like ''Tamisiocaris'' and ''Aegirocassis'' have flexible, densely packed auxiliary spines, which could filter out organic components such as mesozooplankton and phytoplankton down to 0.5mm.<ref name="vinther2014" /><ref name="vanroy2015" /> Frontal appendages of ''Caryosyntrips'', which are unusual for radiodonts in having the direction of endite-bearing surfaces opposing one another and may have been able to manipulate and crush prey in a scissor-like slicing or grasping motion.<ref name=":16" /><ref name=":35">{{Cite journal |last1=Pates |first1=S. |last2=Daley |first2=A. C. |date=2017 |title=''Caryosyntrips'': a radiodontan from the Cambrian of Spain, USA and Canada |url=https://ora.ox.ac.uk/objects/uuid:2e8264d8-6b2b-4851-81f6-a0d3e57f3db0 |journal=Papers in Palaeontology |language=en |volume=3 |issue=3 |pages=461–470 |doi=10.1002/spp2.1084 |s2cid=135026011 |issn=2056-2802}}</ref>
Oral cones of radiodonts may have been used for suction and/or biting.<ref name=":14" /><ref name=":15" /><ref name=":1" /> Together with the great variety of frontal appendages in different species of radiodonts, differentiation of oral cones between species suggests preferences of different diets as well.<ref name=":15" /><ref name=":5" /> For example, the triradial oral cone of ''Anomalocaris'' with irregular, tuberculated toothplates and a small opening may have been adapted to small and nektonic prey,<ref name=":14" /><ref name=":5" /> while the rigid tetraradial oral cones of ''Peytoia'', ''Titanokorys'', ''Hurdia'', and one isolated oral cone attributed to ''Cambroraster'' with a larger opening and sometimes additional tooth plates may have been capable to consume larger food items relative to their body size and probably benthic or endobenthic prey.<ref name=":14" /><ref name=":15" /><ref name=":1" />
==Classification== === Taxonomic affinities === {{cladogram |title= |align= right |caption= Summarized phylogeny between Radiodonta and other Ecdysozoan taxa<ref name=":11" /> |cladogram= {{clade| style=width:30em;font-size:100%;line-height:100% |label1=<small>Ecdysozoa</small>|1={{clade |label1=<small>Cycloneuralia</small>|1=Priapulida 60px and relatives |label2=<small>Panarthropoda</small>|2={{clade |1=Onychophora 80px |2=Tardigrada 80px |label3=†|3=Lobopodian grade<br />(paraphyletic) 80px |4={{clade |label1=†|1=Siberiid lobopodians 80px |2={{clade |label1=†|1=''Pambdelurion'' 80px |label2=†|2=''Kerygmachela'' 80px |3={{clade |label1=†|1=Opabiniidae 80px |label2=†|2='''Radiodonta''' 80px |3=Deuteropoda 60px }} }} }} }} }} }} }}
<gallery mode="packed" heights="100"> File:20220213 Opabiniidae Opabiniids.png|The opabiniids ''Opabinia'' (top) and ''Utaurora'' (bottom), were close relatives of the radiodonts File:20210310 Kylinxia zhangi.png|The presumed radiodont/opabiniid-euarthropod intermediate ''Kylinxia'', shares many of the characteristics found in both dinocaridids and euarthropods — later studies have considered this relationship far less direct<ref>{{cite journal |last1=Izquierdo-López |first1=Alejandro |last2=Caron |first2=Jean-Bernard |title=The problematic Cambrian arthropod Tuzoia and the origin of mandibulates revisited |journal=Royal Society Open Science |date=December 2022 |volume=9 |issue=12 |doi=10.1098/rsos.220933 |doi-access=free|url=https://royalsocietypublishing.org/doi/full/10.1098/rsos.220933 |language=en |issn=2054-5703}}</ref> File:Erratus.png|A life reconstruction of the basal deuteropod ''Erratus'', which helped reveal the evolution of arthropod trunk appendages File:20221119 Mieridduryn bonniae diagrammatic reconstruction.png|''Mieridduryn'' is a dinocaridid panarthropod from the Middle Ordovician that shares traits with both opabiniids and radiodonts </gallery>
Most phylogenetic analyses suggest that radiodonts, alongside opabiniids (''Opabinia'' and ''Utaurora''<ref name=":22" />), are stem-group arthropods just basal to deuteropoda,<ref name=":11">{{Cite journal|last=Ortega-Hernández|first=Javier|date=Dec 2014|title=Making sense of 'lower' and 'upper' stem-group Euarthropoda, with comments on the strict use of the name Arthropoda von Siebold, 1848|journal=Biological Reviews of the Cambridge Philosophical Society|volume=91|issue=1|pages=255–273|doi=10.1111/brv.12168|issn=1469-185X|pmid=25528950|s2cid=7751936}}</ref> a clade including upper stem (e.g. fuxianhuiids and bivalved arthropods) and crown Euarthropoda (e.g. Artiopoda, Chelicerata and Mandibulata).<ref name="daley2009" /><ref>{{Cite journal|last=Edgecombe|first=Gregory D.|date=1 March 2010|title=Arthropod phylogeny: An overview from the perspectives of morphology, molecular data and the fossil record|url=https://www.sciencedirect.com/science/article/pii/S1467803909000541|journal=Arthropod Structure & Development|series=Fossil Record and Phylogeny of the Arthropoda|language=en|volume=39|issue=2|pages=74–87|doi=10.1016/j.asd.2009.10.002|pmid=19854297|issn=1467-8039|url-access=subscription}}</ref><ref>{{Cite journal|last1=Legg|first1=David|last2=Sutton|first2=Mark|last3=Edgecombe|first3=Gregory|last4=Caron|first4=Jean-Bernard|date=10 October 2012|title=Cambrian bivalved arthropod reveals origin of arthrodization|url=https://www.researchgate.net/publication/232231696|journal=Proceedings. Biological Sciences |volume=279|issue=1748|pages=4699–4704|doi=10.1098/rspb.2012.1958|pmid=23055069|pmc=3497099}}</ref><ref name="Legg 2013 493–501">{{Cite journal|last=Legg|first=David|date=2013|title=Multi-Segmented Arthropods from the Middle Cambrian of British Columbia (Canada)|url=https://www.cambridge.org/core/journals/journal-of-paleontology/article/abs/multisegmented-arthropods-from-the-middle-cambrian-of-british-columbia-canada/9CA5AE5D58832452EDA388ED438774CB|journal=Journal of Paleontology|language=en|volume=87|issue=3|pages=493–501|doi=10.1666/12-112.1|s2cid=86725173|issn=0022-3360|url-access=subscription}}</ref><ref>{{Cite journal|last1=Legg|first1=David|last2=Sutton|first2=Mark|last3=Edgecombe|first3=Gregory|date=30 September 2013|title=Arthropod fossil data increase congruence of morphological and molecular phylogenies|url=https://www.researchgate.net/publication/257205419|journal=Nature Communications|volume=4|page=2485|doi=10.1038/ncomms3485|pmid=24077329|bibcode=2013NatCo...4.2485L|doi-access=free}}</ref><ref>{{Cite journal|last1=Legg|first1=David A.|last2=Vannier|first2=Jean|date=2013|title=The affinities of the cosmopolitan arthropod Isoxys and its implications for the origin of arthropods|url=https://onlinelibrary.wiley.com/doi/abs/10.1111/let.12032|journal=Lethaia|language=en|volume=46|issue=4|pages=540–550|doi=10.1111/let.12032|issn=1502-3931|url-access=subscription}}</ref><ref name="vinther2014" /><ref name="cong2014" /><ref name="vanroy2015" /><ref name=":6" /><ref name=":1" /><ref name=":26" /><ref name=":18" /><ref name=":27" /><ref name=":28" /><ref>{{Cite journal|last1=O'Flynn|first1=Robert J.|last2=Williams|first2=Mark|last3=Yu|first3=Mengxiao|last4=Harvey|first4=Thomas H. P.|last5=Liu|first5=Yu|date=2022|title=A new euarthropod with large frontal appendages from the early Cambrian Chengjiang biota|url=https://palaeo-electronica.org/content/2022/3551-a-new-chengjiang-euarthropod|journal=Palaeontologia Electronica|language=English|volume=25|issue=1|pages=1–21|doi=10.26879/1167|s2cid=246779634|issn=1094-8074|doi-access=free}}</ref><ref name=":30" /><ref name=":22">{{Cite journal|last1=Pates|first1=Stephen|last2=Wolfe|first2=Joanna M.|last3=Lerosey-Aubril|first3=Rudy|last4=Daley|first4=Allison C.|last5=Ortega-Hernández|first5=Javier|date=9 February 2022|title=New opabiniid diversifies the weirdest wonders of the euarthropod stem group|journal=Proceedings of the Royal Society B: Biological Sciences|volume=289|issue=1968|article-number=20212093|doi=10.1098/rspb.2021.2093|pmc=8826304|pmid=35135344}}</ref> This interpretation is supported by numerous arthropod groundplan found on radiodonts and opabiniids, such as stalked compound eyes,<ref name=":9" /> digestive glands,<ref name=":10" /> trunk appendages forming by dorsal and ventral elements (precursor of arthropod biramous appendages).<ref name="vanroy2015" /><ref name=":30" /> Compared to opabiniids, which possess posterior mouth opening and fused frontalmost appendages (comparable to euarthropod posterior-facing labrum/hypostome complex),<ref name=":8" /><ref name=":22" /> radiodonts on the other hand featured euarthropod-like dorsal sclerite (H-element) and arthropodization (frontal appendages) on their head regions,<ref>{{Cite journal|last=Ortega-Hernández|first=Javier|date=15 June 2015|title=Homology of Head Sclerites in Burgess Shale Euarthropods|journal=Current Biology|volume=25|issue=12|pages=1625–1631|doi=10.1016/j.cub.2015.04.034|issn=0960-9822|pmid=25959966|doi-access=free}}</ref><ref name=":8" /><ref name=":22" /> alongside cuticularized gut termini.<ref name="daley2014" /> The fact that both radiodonts and opabiniids lack exoskeleton on their trunk region suggests that the origin of compound eyes and arthropodization (segmented appendages) precede arthrodization (full set of trunk exoskeleton) in the arthropod stem lineage.<ref name=":11" /><ref name=":32">{{Cite journal|last1=Giribet|first1=Gonzalo|last2=Edgecombe|first2=Gregory D.|date=17 June 2019|title=The Phylogeny and Evolutionary History of Arthropods|journal=Current Biology|language=English|volume=29|issue=12|pages=R592–R602|doi=10.1016/j.cub.2019.04.057|issn=0960-9822|pmid=31211983|s2cid=189926344|doi-access=free}}</ref><ref name=":33">{{Cite journal|last=Edgecombe|first=Gregory D.|date=2 November 2020|title=Arthropod Origins: Integrating Paleontological and Molecular Evidence|url=https://www.annualreviews.org/doi/10.1146/annurev-ecolsys-011720-124437|journal=Annual Review of Ecology, Evolution, and Systematics|volume=51|issue=1|pages=1–25|doi=10.1146/annurev-ecolsys-011720-124437|s2cid=225478171|issn=1543-592X|url-access=subscription}}</ref> The constricted neck region with feeding appendicular structures of some radiodont may also shed light on the origin of the sophisticated arthropod head, which was formed by the fusion of multiple anterior body segments.<ref name=":2" /><ref name=":8" /> Basal deuteropods that possess a mixture of radiodont/opabiniid characters like ''Kylinxia'' and ''Erratus'', may represent intermediate forms between radiodonts, opabiniids and other euarthropods.<ref name=":18" /><ref name=":30">{{Cite journal|last1=Fu|first1=Dongjing|last2=Legg|first2=David A.|last3=Daley|first3=Allison C.|last4=Budd|first4=Graham E.|last5=Wu|first5=Yu|last6=Zhang|first6=Xingliang|date=28 March 2022|title=The evolution of biramous appendages revealed by a carapace-bearing Cambrian arthropod|journal=Philosophical Transactions of the Royal Society B: Biological Sciences|volume=377|issue=1847|article-number=20210034|doi=10.1098/rstb.2021.0034|pmid=35125000|pmc=8819368 |s2cid=246608509}}</ref>
<gallery mode="packed" heights="120"> File:20191217 Siberiida Siberion Megadictyon Jianshanopodia.png|The siberiid lobopodians ''Siberion'' (upper left), ''Megadictyon'' (bottom center) and ''Jianshanopodia'' (upper right) File:20210730 Gilled lobopodians Pambdelurion Kerygmachela.png|The 'gilled lobopodians' ''Pambdelurion'' (left) and ''Kerygmachela'' (right) File:20191028 Megacheirans Leanchoilia Haikoucaris Yohoia Fortiforceps.png|Megacheira or 'great appendage arthropods', a class of possible stem-chelicerate previously thought to be radiodont's close relative </gallery>
Taxa just basal to the radiodont, opabiniid and euarthropod branch are 'gilled lobopodians' like ''Pambdelurion'' and ''Kerygmachela'', which are occasionally united under the class Dinocaridida with opabibiids and radiodonts.<ref name=":21">{{Citation|last1=Xianguang|first1=Hou|title=Dinocaridids – anomalous arthropods or arthropod-like worms?|date=2006|last2=Jan|first2=Jan Bergström|last3=Jiayu|first3=In Rong|last4=Zongjie|first4=Fang|last5=Zhanghe|first5=Zhou|last6=Renbin|first6=Zhan|last7=Xiangdong|first7=Wang|last8=Xunlai|first8=Yuan|last9=Xianguang|first9=Hou|citeseerx=10.1.1.693.5869}}</ref><ref name="Legg 2013 493–501"/> They have body flaps, digestive glands, large (presumely compound) eyes and specialized frontal appendages like the former taxa, but their frontal appendages are not arthropodized nor fused, eyes sessile, gill-like structures less prominent, and certainly bore lobopod underneath each of their flaps.<ref>{{Cite journal|last=Budd|first=Graham E.|date=1998|title=The morphology and phylogenetic significance of Kerygmachela kierkegaardi Budd (Buen Formation, Lower Cambrian, N Greenland)|url=https://www.cambridge.org/core/journals/earth-and-environmental-science-transactions-of-royal-society-of-edinburgh/article/abs/morphology-and-phylogenetic-significance-of-kerygmachela-kierkegaardi-budd-buen-formation-lower-cambrian-n-greenland/AF165229724342F0BD90933A037CB05F|journal=Earth and Environmental Science Transactions of the Royal Society of Edinburgh|language=en|volume=89|issue=4|pages=249–290|doi=10.1017/S0263593300002418|s2cid=85645934 |issn=1473-7116|url-access=subscription}}</ref><ref name="vanroy2015" /><ref>{{Cite journal|last1=Young|first1=Fletcher J.|last2=Vinther|first2=Jakob|date=2017|title=Onychophoran-like myoanatomy of the Cambrian gilled lobopodian Pambdelurion whittingtoni|url=https://onlinelibrary.wiley.com/doi/abs/10.1111/pala.12269|journal=Palaeontology|language=en|volume=60|issue=1|pages=27–54|doi=10.1111/pala.12269|bibcode=2017Palgy..60...27Y |hdl=1983/92180ef0-2205-4c65-9a70-90d59cfea2f4 |s2cid=55477207 |issn=1475-4983|hdl-access=free|url-access=subscription}}</ref><ref name=":22" /> Taxa even basal to 'gilled lobopodians' are siberiids like ''Megadictyon'' and ''Jianshanopodia'',<ref name=":11" /> a group of lobopodians that bore robust frontal appendages and digestive glands, but no body flaps. Such intermediate forms between lobopodian and radiodont/euarthropod suggest that the total-group Arthropoda arose from a paraphyletic lobopodian grade, alongside the other two extant panarthropod phyla Tardigrada and Onychophora.<ref>{{Cite journal|last=Edgecombe|first=Gregory D.|date=2010|title=Arthropod phylogeny: An overview from the perspectives of morphology, molecular data and the fossil record|url=https://www.academia.edu/15185231|journal=Arthropod Structure & Development|volume=39|issue=2–3|pages=74–87|doi=10.1016/j.asd.2009.10.002|pmid=19854297|issn=1467-8039}}</ref><ref name=":11" /><ref name=":8" /><ref>{{Cite journal|last1=Daley|first1=Allison C.|last2=Antcliffe|first2=Jonathan B.|last3=Drage|first3=Harriet B.|last4=Pates|first4=Stephen|date=22 May 2018|title=Early fossil record of Euarthropoda and the Cambrian Explosion|journal=Proceedings of the National Academy of Sciences|language=en|volume=115|issue=21|pages=5323–5331|doi=10.1073/pnas.1719962115|issn=0027-8424|pmc=6003487|pmid=29784780|bibcode=2018PNAS..115.5323D|doi-access=free}}</ref><ref name=":32" /><ref name=":33" />
Previous studies may suggest radiodonts as a group other than stem-arthropods, such as a hitherto unknown phylum;<ref name=":31" /> cycloneuralian worms undergone convergent with arthropods (based on the cycloneuralian-like radial mouthparts);<ref>{{Cite journal|last1=Xian-Guang|first1=Hou|last2=Bergström|first2=Jan|last3=Ahlberg|first3=Per|date=September 1995|title=Anomalocaris and other large animals in the lower Cambrian Chengjiang fauna of southwest China|url=https://www.researchgate.net/publication/233050167|journal=GFF|language=en|volume=117|issue=3|pages=163–183|doi=10.1080/11035899509546213|issn=1103-5897}}</ref><ref name=":21" /> stem chelicerate euarthropods alongside megacheirans also known as great appendage arthropods (based on the similarity between radiodont frontal appendages, megacheiran great appendages and chelicerae);<ref>{{Cite journal|last1=Haug|first1=Joachim T.|last2=Waloszek|first2=Dieter|last3=Maas|first3=Andreas|last4=Liu|first4=Yu|last5=Haug|first5=Carolin|date=March 2012|title=Functional morphology, ontogeny and evolution of mantis shrimp-like predators in the Cambrian: MANTIS SHRIMP-LIKE CAMBRIAN PREDATORS|url=https://www.researchgate.net/publication/241247067|journal=Palaeontology|language=en|volume=55|issue=2|pages=369–399|doi=10.1111/j.1475-4983.2011.01124.x|s2cid=82841481 |doi-access=free}}</ref> or ''Schinderhannes bartelsi,'' which resolved as a hurdiid radiodont in recent analyses,<ref name=":11" /><ref name="vinther2014" /><ref name="vanroy2015" /><ref name=":6" /><ref name=":1" /><ref name=":27" /><ref name=":28" /> as a species more closely related to euarthropods than other radiodonts (based on some putative euarthropod-like features found on ''Schinderhannes'').<ref name=":12">{{Cite journal|last1=Kühl|first1=Gabriele|last2=Briggs|first2=Derek E. G.|last3=Rust|first3=Jes|date=6 February 2009|title=A Great-Appendage Arthropod with a Radial Mouth from the Lower Devonian Hunsrück Slate, Germany|journal=Science|language=en|volume=323|issue=5915|pages=771–773|doi=10.1126/science.1166586|issn=0036-8075|pmid=19197061|bibcode=2009Sci...323..771K|s2cid=47555807}}</ref> However, neither each of them were supported by later investigations. The radial mouthparts are not cycloneuralian-exclusive and more likely present result of convergent evolution or ecdysozoan plesimorphy, since they also have been found in panarthropods such as tardigrade and some lobopodians;<ref>{{Cite journal|last1=Smith|first1=Martin R.|last2=Caron|first2=Jean-Bernard|date=June 2015|title=Hallucigenia 's head and the pharyngeal armature of early ecdysozoans|journal=Nature|language=en|volume=523|issue=7558|pages=75–78|doi=10.1038/nature14573|issn=1476-4687|pmid=26106857|bibcode=2015Natur.523...75S|s2cid=205244325|url=http://dro.dur.ac.uk/20476/1/20476.pdf}}</ref> radiodonts lacking definitive euarthropod features such as trunk tergites and multiple head appendages,<ref name=":11" /> and the megacheiran great appendages were considered to be deutocerebral,<ref>{{Cite journal|last1=Tanaka|first1=Gengo|last2=Hou|first2=Xianguang|last3=Ma|first3=Xiaoya|last4=Edgecombe|first4=Gregory D.|last5=Strausfeld|first5=Nicholas J.|date=October 2013|title=Chelicerate neural ground pattern in a Cambrian great appendage arthropod|journal=Nature|language=en|volume=502|issue=7471|pages=364–367|doi=10.1038/nature12520|issn=1476-4687|pmid=24132294|bibcode=2013Natur.502..364T|s2cid=4456458}}</ref><ref>{{Cite journal|last1=Ortega-Hernández|first1=Javier|last2=Lerosey-Aubril|first2=Rudy|last3=Pates|first3=Stephen|date=18 December 2019|title=Proclivity of nervous system preservation in Cambrian Burgess Shale-type deposits|journal=Proceedings of the Royal Society B: Biological Sciences|volume=286|issue=1917|article-number=20192370|doi=10.1098/rspb.2019.2370|pmid=31822253|pmc=6939931}}</ref> which could be non-homologous to the radiodont protocerebral frontal appendages;<ref name="cong2014" /><ref name=":8" /> putative euarthropod characters found on the single ''Schinderhannes'' fossil is questionable and may present other radiodont-like structures.<ref name=":11" />
=== Interrelationships === {{cladogram |title= |align= right |caption=Phylogeny of Radiodonta after Moysiuk & Caron 2021<ref name=":27" /> |cladogram= {{clade| style=width:36em;font-size:90%;line-height:100% |1={{clade |3=''Caryosyntrips'' 80px |2=Euarthropoda |1={{clade |label1=Tamisiocarididae|1={{clade |1=''Houcaris saron'' 80px |2={{clade |1=''Echidnacaris briggsi'' 80px |2=''Tamisiocaris'' 80px }} }} |label2=Anomalocarididae+<br/>Amplectobeluidae|2={{clade |1=''Laminacaris'' 80px |2=''Houcaris magnabasis'' 80px |3=''Anomalocaris'' 80px |4={{clade |1=''Lyrarapax'' 70px |2=''Amplectobelua'' 90px }} |5={{clade |1=''"Anomalocaris" kunmingensis'' 80px |2=''Ramskoeldia consimilis'' 80px |3=''Ramskoeldia platyacantha'' 80px |4=''Paranomalocaris'' 80px }} }} |label3=Hurdiidae|3={{clade |1=''Peytoia'' 80px |2=cf. ''Peytoia'' 70px |3=''Stanleycaris'' 80px |4='' Schinderhannes'' 70px |5={{clade |1=''Aegirocassis'' 100px |2={{clade |1=''Hurdia'' 80px |2={{clade |1=''Pahvantia'' 70px |2={{clade |1=''Cambroraster'' 80px |2={{clade |1=''Titanokorys'' 70px |2=''Cordaticaris'' 70px }} }} }} }} }} }} }} }} }} }}
Traditionally, all radiodont species have been placed within one family, Anomalocarididae,<ref name="collins1996" /> hence the previous common name 'anomalocaridid'<ref name=":4" /><ref name="daley2009" /> and it was still occasionally used to refer the whole order even after reclassification.<ref name="cong2014" /><ref name="vanroy2015" /> Since the reassignment done by Vinther ''et al.'' 2014, most of the radiodont species were reclassified within three new families: Amplectobeluidae, Tamisiocarididae<ref name=":6" /><ref name=":1" /> (formerly Cetiocaridae<ref name="vinther2014" />), and Hurdiidae.<ref name="vinther2014" /><ref name="vanroy2015" /><ref name=":6" /><ref name=":1" /> Including Anomalocarididae, the four recent radiodont families may form the clade Anomalocarida.<ref name="vinther2014" />
The original description of the order Radiodonta included ''Anomalocaris'', ''Laggania'' (later known as ''Peytoia''), ''Hurdia'', ''Proboscicaris'', ''Amplectobelua'', ''Cucumericrus'', and ''Parapeytoia''.<ref name="collins1996" /> However, ''Proboscicaris'' is now regarded as a junior synonym of ''Hurdia'', and ''Parapeytoia'' is considered to be a Megacheiran.<ref name="daley2009">{{cite journal |first1=Allison C. |last1=Daley |first2=Graham E. |last2=Budd |first3=Jean-Bernard |last3=Caron |first4=Gregory D. |last4=Edgecombe |first5=Desmond |last5=Collins |year=2009 |title=The Burgess Shale anomalocaridid ''Hurdia'' and its significance for early euarthropod evolution |journal=Science | volume=323 |pages=1597–1600 |doi=10.1126/science.1169514 |pmid=19299617 |issue=5921|bibcode=2009Sci...323.1597D |s2cid=206517995 }}</ref><ref name="daley2014" /><ref name="vanroy2015" /> Due to the limited discovery, The position of ''Cucumericrus'' within Radiodonta is unclear, as it was either unselected by phylogenetic analysis<ref name="vinther2014" /><ref name=":1" /><ref name=":6" /><ref name=":27">{{Cite journal|last1=Moysiuk|first1=Joseph|last2=Caron|first2=Jean-Bernard|date=2021|title=Exceptional multifunctionality in the feeding apparatus of a mid-Cambrian radiodont|journal=Paleobiology|language=en|volume=47|issue=4|pages=704–724|doi=10.1017/pab.2021.19|bibcode=2021Pbio...47..704M |s2cid=236552819|issn=0094-8373|doi-access=free}}</ref><ref name=":28" /> or resolved in a polytomy with Radiodonta and Euarthropoda.<ref name="vanroy2015" /><ref name=":13" />
<gallery mode="packed" heights="120"> File:20210207 Cucumericrus decoratus trunk appendage.png|One of the poorly-known body parts (trunk appendage) of ''Cucumericrus decoratus''; this species may not represent a true radiodont. File:20191228 Radiodonta frontal appendage Anomalocaris briggsi.png|Frontal appendage of ''Echidnacaris briggsi'', a tamisiocarid radiodont that was once suggested to belong to the ''Anomalocaris'' genus until its description in 2023. </gallery>
The first in-depth phylogenetic analysis of Radiodonta was conducted by Vinther ''et al''. in 2014,<ref name="vinther2014" /> followed by a handful of subsequest studies with more or less modified results.<ref name="cong2014" /><ref name="vanroy2015" /><ref name=":6" /><ref name=":13" /><ref name=":1" /><ref name=":27" /><ref name=":28" /><ref name=":22" /> In most analysis, ''Caryosyntrips'' is the basal-most genus, but either resolved in a polytomy with other radiodonts and Euarthropoda (alongside ''Cucumericrus'' if included<ref name="vanroy2015" /><ref name=":13" />) or outside of Radiodonta, casting doubt on its radiodont affinity.<ref name=":43">{{Cite journal |last=McCall |first=Christian R. A. |date=13 December 2023 |title=A large pelagic lobopodian from the Cambrian Pioche Shale of Nevada |url=https://www.cambridge.org/core/journals/journal-of-paleontology/article/abs/large-pelagic-lobopodian-from-the-cambrian-pioche-shale-of-nevada/11B0704C49A7730AA3E8F46EB2CA1C95 |journal=Journal of Paleontology |volume=97 |issue=5 |language=en |pages=1009–1024 |doi=10.1017/jpa.2023.63 |issn=0022-3360|url-access=subscription }}</ref> With the exclusion of questionable ''Caryosyntrips'' and ''Cucumericrus'', the monophyly of Radiodonta is widely supported,<ref name="vinther2014" /><ref name="cong2014" /><ref name="vanroy2015" /><ref name=":6" /><ref name=":13" /><ref name=":1" /><ref name=":27" /><ref name=":28" /> with a few results suggest possible paraphyly (either the Anomalocarididae+Amplectobeluidae clade or Hurdiidae sister to Euarthropoda).<ref name=":26" /><ref name=":22" /> Putative synapomorphies of monophyletic Radiodonta include tripartite head sclerite complex and differentiated neck region.<ref name=":1" /> The genus ''Anomalocaris'' in a broader sense always found to be polyphyletic, usually with ''"Anomalocaris" kunmingensis'' and ''"Anomalocaris" briggsi'' resolved as a member of Amplectobeluidae and Tamisiocarididae respectively.<ref name="vinther2014" /><ref name="cong2014" /><ref name="vanroy2015" /><ref name=":6" /><ref name=":1" /><ref name=":27" /><ref name=":28" /> Interrelationship of Amplectobeluidae is uncertain, as the amplectobeluid affinities of ''Lyrarapax'' and ''Ramskoeldia'' were occasionally questioned.<ref name=":0" /><ref name=":1" /><ref name=":28" /> Monophyly of the speciose family Hurdiidae was recovered by most analysis and well-supported by several synapomorphies (e.g. distal articulated region of frontal appendage with proximal 5 podomeres bearing subequal endites<ref name=":3" /><ref name=":1" />). Tamisiocarididae was often suggested to be sister group of Hurdiidae in the 2010s,<ref name="vinther2014" /><ref name="vanroy2015" /><ref name=":6">{{Cite journal|last1=Lerosey-Aubril|first1=Rudy|last2=Pates|first2=Stephen|date=14 September 2018|title=New suspension-feeding radiodont suggests evolution of microplanktivory in Cambrian macronekton|journal=Nature Communications|language=en|volume=9|issue=1|page=3774|doi=10.1038/s41467-018-06229-7|issn=2041-1723|pmid=30218075|pmc=6138677|bibcode=2018NatCo...9.3774L}} [https://datadryad.org/stash/dataset/doi:10.5061/dryad.1cf2fb0 Dryad Data]</ref><ref name=":1" /> but this position became questionable in subsequent studies.<ref name=":41" /><ref name=":42" />
* '''Radiodonta''' ** ?''Cucumericrus''<ref name=":4" /> (radiodont affinity questionable<ref name=":23">{{Cite journal |last1=Zeng |first1=Han |last2=Zhao |first2=Fangchen |last3=Yin |first3=Zongjun |last4=Zhu |first4=Maoyan |date=2 January 2018 |title=Morphology of diverse radiodontan head sclerites from the early Cambrian Chengjiang Lagerstätte, south-west China |journal=Journal of Systematic Palaeontology |volume=16 |issue=1 |pages=1–37 |doi=10.1080/14772019.2016.1263685 |issn=1477-2019 |s2cid=133549817}}</ref><ref name=":43" />) ** ?''Caryosyntrips''<ref name=":16" /> (radiodont affinity questionable<ref name=":43" />) ** '''Anomalocarida''' *** ''Paranomalocaris''<ref name=":36">{{Cite journal|last1=Yuanyuan|first1=Wang|last2=Huang|first2=Diying|last3=Hu|first3=Shixue|date=1 November 2013|title=New anomalocardid frontal appendages from the Guanshan biota, eastern Yunnan|url=https://www.researchgate.net/publication/257689210|journal=Chinese Science Bulletin|volume=58|issue=32|pages=3937–3942|doi=10.1007/s11434-013-5908-x|bibcode=2013ChSBu..58.3937W|s2cid=83910779|doi-access=free}}</ref> (placed within Anomalocarididae by some studies.<ref name=":13" /><ref name=":29" />) *** ''Laminacaris''<ref name=":37">{{Cite journal|last1=Guo|first1=J.|last2=Pates|first2=S.|last3=Cong|first3=P.|last4=Daley|first4=A. C.|last5=Edgecombe|first5=G. D.|last6=Chen|first6=T.|last7=Hou|first7=X.|date=2018|title=A new radiodont (stem Euarthropoda) frontal appendage with a mosaic of characters from the Cambrian (Series 2 Stage 3) Chengjiang biota|url=https://ora.ox.ac.uk/objects/uuid:a2d090f6-c0cb-4603-8d89-a4894fe55f02|journal=Papers in Palaeontology|language=en|volume=5|issue=1|issn=2056-2799}}</ref> (placed within Amplectobeluidae by some studies.<ref name=":6" />) *** ''Houcaris'' (either placed within Anomalocarididae,<ref name="vanroy2015" /><ref name=":13" /><ref name=":22" /> Amplectobeluidae<ref name="vinther2014" /><ref name=":6" /><ref name=":43" /> or Tamisiocarididae.<ref name=":17" /><ref name=":27" />) *** ''Innovatiocaris''<ref name="Innovatiocaris">{{Cite journal |last1=Zeng |first1=Han |last2=Zhao |first2=Fangchen |last3=Zhu |first3=Maoyan |date=7 September 2022 |title=''Innovatiocaris'', a complete radiodont from the early Cambrian Chengjiang Lagerstätte and its implications for the phylogeny of Radiodonta |journal=Journal of the Geological Society |volume=180 |doi=10.1144/jgs2021-164 |s2cid=252147346 |issn=0016-7649}}</ref> *** '''Anomalocarididae''' **** ''Anomalocaris'' (in a broader sense, some species may placed within the other families.<ref name="vinther2014" /><ref name="vanroy2015" />) **** ''Lenisicaris''<ref name=":19" /> **** Shucaris (either placed within Anomalocarididae or Amplectobeluidae<ref name=":44" /><ref name=":43" />) **** ''Verrocaris''<ref name=":47" /> *** '''Amplectobeluidae''' **** ''Lyrarapax''<ref name="cong2014" /> (position questioned by some studies.<ref name=":0" />) **** ''Amplectobelua''<ref name=":4" /> **** ''Ramskoeldia''<ref name=":0" /> (position questioned by some studies.<ref name=":1" />) **** ''Guanshancaris''<ref name=":29">{{Cite journal|last1=Jiao|first1=De-guang|last2=Pates|first2=Stephen|last3=Lerosey-Aubril|first3=Rudy|last4=Ortega-Hernandez|first4=Javier|last5=Yang|first5=Jie|last6=Lan|first6=Tian|last7=Zhang|first7=Xi-guang|date=2021|title=The endemic radiodonts of the Cambrian Stage 4 Guanshan biota of South China|journal=Acta Palaeontologica Polonica|language=en|volume=66|doi=10.4202/app.00870.2020|s2cid=236683029|issn=0567-7920|doi-access=free}}</ref><ref name=":Guanshancaris">{{Cite journal |last1=Zhang |first1=Mingjing |last2=Wu |first2=Yu |last3=Lin |first3=Weiliang |last4=Ma |first4=Jiaxin |last5=Wu |first5=Yuheng |last6=Fu |first6=Dongjing |date=11 April 2023 |title=Amplectobeluid Radiodont Guanshancaris gen. nov. from the Lower Cambrian (Stage 4) Guanshan Lagerstätte of South China: Biostratigraphic and Paleobiogeographic Implications |journal=Biology |language=en |volume=12 |issue=4 |page=583 |doi=10.3390/biology12040583 |pmid=37106783 |issn=2079-7737 |pmc=10136193 |doi-access=free }}</ref> **** Shucaris (either placed within Anomalocarididae or Amplectobeluidae<ref name=":44" /><ref name=":43" />) *** '''Tamisiocarididae''' **** ''Tamisiocaris'' **** ''Echidnacaris'' **** ''Houcaris'' *** '''Hurdiidae'''/'''Peytoiidae'''<ref name=":43" /> ****Aegirocassisinae<ref name=":40">{{Cite journal |last1=Potin |first1=Gaëtan J.-M. |last2=Gueriau |first2=Pierre |last3=Daley |first3=Allison C. |date=2023 |title=Radiodont frontal appendages from the Fezouata Biota (Morocco) reveal high diversity and ecological adaptations to suspension-feeding during the Early Ordovician |journal=Frontiers in Ecology and Evolution |volume=11 |doi=10.3389/fevo.2023.1214109 |issn=2296-701X |doi-access=free }}</ref> ***** ''Aegirocassis''<ref name="vanroy2015" /> ***** ''Pseudoangustidontus'' ****''Peytoia'' **** ''Schinderhannes'' (position questioned by some studies.<ref name=":112">{{Cite journal |last=Ortega-Hernández |first=Javier |date=Dec 2014 |title=Making sense of 'lower' and 'upper' stem-group Euarthropoda, with comments on the strict use of the name Arthropoda von Siebold, 1848 |journal=Biological Reviews of the Cambridge Philosophical Society |volume=91 |issue=1 |pages=255–273 |doi=10.1111/brv.12168 |issn=1469-185X |pmid=25528950 |s2cid=7751936}}</ref><ref>{{cite journal |author1=Zhu, X. |author2=Lerosey-Aubril, R. |author3=Ortega-Hernández, J. |year=2021 |title=Furongian (Jiangshanian) occurrences of radiodonts in Poland and South China and the fossil record of the Hurdiidae |url=https://www.researchgate.net/publication/352983445 |journal=PeerJ |volume=9 |at=e11800 |doi=10.7717/peerj.11800 |pmc=8312493 |pmid=34386302 |doi-access=free |language=en}}</ref><ref>{{cite journal |last1=Potin |first1=Gaëtan J.-M. |last2=Daley |first2=Allison C. |date=2023 |title=The significance of ''Anomalocaris'' and other Radiodonta for understanding paleoecology and evolution during the Cambrian explosion |journal=Frontiers in Earth Science |volume=11 |bibcode=2023FrEaS..1160285P |doi=10.3389/feart.2023.1160285 |issn=2296-6463 |doi-access=free}}</ref>) **** ''Stanleycaris''<ref name=":38">{{Cite book|last1=Pates|first1=Stephen|url=https://www.researchgate.net/publication/326142899|title=Reply to Comment on "Aysheaia prolata from the Utah Wheeler Formation (Drumian, Cambrian) is a frontal appendage of the radiodontan Stanleycaris" with the formal description of Stanleycaris|last2=Daley|first2=Allison|last3=Ortega-Hernández|first3=Javier|date=3 March 2018}}</ref> **** ''Mosura'' **** Falciscaris<ref name=":46">{{Cite journal |last=Gaëtan J.-M. |first=Potin |last2=Claisse |first2=Pénélope |date=2025-12-02 |title=A new giant nektobenthic radiodont benthivore from the Early Ordovician Fezouata Biota in Morocco |url=https://www.app.pan.pl/article/item/app012782025.html |journal=Acta Palaeontologica Polonica |volume=70 |issue=4 |pages=709–722 |doi=10.4202/app.01278.2025|doi-access=free }}</ref> ****Hurdiinae ***** ''Hurdia'' ***** ''Pahvantia''<ref name=":6" /> ***** ''Ursulinacaris''<ref name=":3" /> ***** ''Cambroraster''<ref name=":1" /> ***** ?''Zhenghecaris'' (putative hurdiid radiodont<ref name=":23" />) ***** ''Cordaticaris''<ref name=":24">{{Cite journal|last1=Sun|first1=Zhixin|last2=Zeng|first2=Han|last3=Zhao|first3=Fangchen|date=1 August 2020|title=A new middle Cambrian radiodont from North China: Implications for morphological disparity and spatial distribution of hurdiids|url=http://www.sciencedirect.com/science/article/pii/S0031018220303928|journal=Palaeogeography, Palaeoclimatology, Palaeoecology|volume=558|language=en|article-number=109947|doi=10.1016/j.palaeo.2020.109947|bibcode=2020PPP...55809947S|s2cid=224868404|issn=0031-0182|url-access=subscription}}</ref> ***** ''Buccaspinea''<ref name=":25">{{Cite journal|last1=Pates|first1=Stephen|last2=Lerosey-Aubril|first2=Rudy|last3=Daley|first3=Allison C.|last4=Kier|first4=Carlo|last5=Bonino|first5=Enrico|last6=Ortega-Hernández|first6=Javier|date=19 January 2021|title=The diverse radiodont fauna from the Marjum Formation of Utah, USA (Cambrian: Drumian)|journal=PeerJ|language=en|volume=9|article-number=e10509|doi=10.7717/peerj.10509|pmid=33552709| pmc=7821760 |issn=2167-8359|doi-access=free}}</ref> ***** ''Titanokorys''<ref name="CaronMoysiuk2021"/> ***** Huangshandongia (may be a synonym of Hurdia)<ref name=":45">{{Cite journal |last=Daley |first=Allison C. |last2=Budd |first2=Graham E. |last3=Caron |first3=Jean-Bernard |date=October 2013 |title=Morphology and systematics of the anomalocaridid arthropod Hurdia from the Middle Cambrian of British Columbia and Utah |url=https://www.tandfonline.com/doi/full/10.1080/14772019.2012.732723 |journal=Journal of Systematic Palaeontology |volume=11 |issue=7 |pages=743–787 |doi=10.1080/14772019.2012.732723 |issn=1477-2019|url-access=subscription }}</ref> ***** Liantuoia (may be a synonym of Hurdia)<ref name=":45" /> {| class="wikitable sortable mw-collapsible" |+ {{nowrap|Described species of Radiodonta}} |- ! scope="col" | Species ! scope="col" | Original description ! scope="col" | Year named ! scope="col" | Family ! scope="col" | Age ! scope="col" | Location ! scope="col" class="unsortable" | Frontal appendage !Head sclerite complex |- | ''Cucumericrus decoratus'' || Hou, Bergström, & Ahlberg || 1995<ref name=":4" /> || (unassigned) ||Cambrian Stage 3 ||{{Flag|China}} || Unknown |Unknown |- | '' Caryosyntrips serratus'' || Daley & Budd || 2010<ref name=":16" /> ||(unassigned) | data-sort-value="509" |Wuliuan–Drumian || {{flag|Canada}} {{flag|United States}} || 200px |Unknown |- | '' Caryosyntrips camurus'' || Pates & Daley || 2017<ref name=":35" /> ||(unassigned) | data-sort-value="509" |Wuliuan || {{flag|Canada}} {{flag|United States}} || 200px |Incomplete<ref name=":25" /> |- | '' Caryosyntrips durus'' || Pates & Daley || 2017<ref name=":35" /> ||(unassigned) | data-sort-value="504.5" |Drumian || {{flag|United States}} || 200px |Unknown |- | ''Paranomalocaris multisegmentalis'' || Wang, Huang, & Hu || 2013<ref name=":36" /> || Anomalocarididae? ||Cambrian Stage 4 || {{flag|China}} || 200px |Unknown |- | ''Paranomalocaris simplex'' || Jiao, Pates, Lerosey-Aubril, Ortega-Hernandez, Yang, Lan, Zhang || 2021<ref name=":29" /> || Anomalocarididae? ||Cambrian Stage 4 || {{flag|China}} || 200px |Unknown |- | ''Laminacaris chimera'' || Guo, Pates, Cong, Daley, Edgecombe, Chen, & Hou || 2018<ref name=":37" /> || (controversial) ||Cambrian Stage 3 || {{flag|China}} || 200px |Unknown |- | ''Innovatiocaris maotianshanensis'' || Zeng, Zhao, Zhu || 2022<ref name="Innovatiocaris" /> || (unassigned) | data-sort-value="514" |Cambrian Stage 3 || {{flag|China}} || 200px |P-element unknown<ref name="Innovatiocaris" /> |- | ''Innovatiocaris''? ''multispiniformis'' || Zeng, Zhao, Zhu || 2022<ref name="Innovatiocaris" /> || (unassigned) | data-sort-value="514" |Cambrian Stage 3 || {{flag|China}} || 200px |Unknown |- | ''Anomalocaris canadensis'' || Whiteaves || 1892<ref name="whiteaves1892" /> || Anomalocarididae ||Wuliuan ||{{flag|Canada}} || 200px |200x200px |- | ''Lenisicaris pennsylvanica'' (formerly ''Anomalocaris pennsylvanica'')<ref name=":19" /> || Resser || 1929 || Anomalocarididae | data-sort-value="529" |Cambrian Stage 3 || {{flag|United States}} || 200px |Unknown |- | ''Lenisicaris lupata'' || Wu, Ma, Lin, Sun, Zhang, & Fu || 2021<ref name=":19" /> || Anomalocarididae | data-sort-value="514" |Cambrian Stage 3 || {{flag|China}} || 200px |Unknown |- | ''Anomalocaris daleyae'' || Paterson, García-Bellidob & Edgecombe || 2023 || Anomalocarididae ||Cambrian Stage 4 ||{{flag|Australia}} || 200px |Unknown |- | ''Houcaris magnabasis'' (formerly ''Anomalocaris magnabasis'')<ref name=":17" /> || Pates, Daley, Edgecombe, Cong & Lieberman || 2019 || (controversial) | data-sort-value="514" |Cambrian Stage 4 || {{flag|United States}} || 200px |Unknown |- | ''Houcaris saron'' (formerly ''Anomalocaris saron'')<ref name=":17">{{Cite journal|last1=Wu|first1=Yu|last2=Fu|first2=Dongjing|last3=Ma|first3=Jiaxin|last4=Lin|first4=Weiliang|last5=Sun|first5=Ao|last6=Zhang|first6=Xingliang|date=1 June 2021|title=''Houcaris'' gen. nov. from the early Cambrian (Stage 3) Chengjiang Lagerstätte expanded the palaeogeographical distribution of tamisiocaridids (Panarthropoda: Radiodonta)|journal=PalZ|language=en|volume=95|issue=2|pages=209–221|doi=10.1007/s12542-020-00545-4|s2cid=235221043|issn=1867-6812}}</ref> || Hou, Bergström, & Ahlberg || 1995 || (controversial) | data-sort-value="529" |Cambrian Stage 3 || {{flag|China}} || 200px |Unknown |- | ''Echidnacaris briggsi''<ref name=":42">{{Cite journal |last1=Paterson |first1=John R. |last2=García-Bellido |first2=Diego C. |last3=Edgecombe |first3=Gregory D. |date=1 January 2023 |title=The early Cambrian Emu Bay Shale radiodonts revisited: morphology and systematics |journal=Journal of Systematic Palaeontology |language=en |volume=21 |issue=1 |doi=10.1080/14772019.2023.2225066 |s2cid=259719252 |issn=1477-2019|doi-access=free }}</ref> || Nedin || 1995 || Tamisiocarididae ||Cambrian Stage 4 || {{flag|Australia}} || 200px |Possible H-element and unique lateral sclerites associated with compound eyes<ref name="Patesetal2020" /><ref name=":42" /> |- | ''Ramskoeldia platyacantha'' || Cong, Edgecombe, Daley, Guo, Pates, & Hou || 2018<ref name=":0" /> || Amplectobeluidae || Cambrian Stage 3 ||{{flag|China}} || 200px |Incomplete<ref name=":0" /> |- | ''Houcaris? consimilis'' || Cong, Edgecombe, Daley, Guo, Pates, & Hou || 2018<ref name=":0" /> || (controversial) ||Cambrian Stage 3 ||{{flag|China}} || 200px |Incomplete<ref name=":0" /> |- | ''Lyrarapax unguispinus'' || Cong, Ma, Hou, Edgecombe, & Strausfield || 2014<ref name="cong2014" /> || Amplectobeluidae | data-sort-value="514" |Cambrian Stage 3 || {{flag|China}} || 200px |P-element neck unknown |- | ''Lyrarapax trilobus'' || Cong, Daley, Edgecombe, Hou, & Chen || 2016<ref name=":7" /> || Amplectobeluidae | data-sort-value="514" |Cambrian Stage 3 || {{flag|China}} || 200px |P-element unknown |- | ''Amplectobelua symbrachiata'' || Hou, Bergström, & Ahlberg || 1995<ref name=":4" /> || Amplectobeluidae ||Cambrian Stage 3 ||{{flag|China}} || 200px |200x200px |- | ''Amplectobelua stephenensis'' || Daley & Budd || 2010<ref name=":16" /> || Amplectobeluidae ||Wuliuan ||{{flag|United States}} || 200px |Unknown |- | ''Guanshancaris kunmingensis'' || Zhang ''et al.'' || 2023<ref name=":Guanshancaris" /> || Amplectobeluidae ||Cambrian Stage 4 ||{{Flag|China}} || 200px |Unknown |- | ''Tamisiocaris borealis'' || Daley & Peel || 2010 || Tamisiocarididae ||Cambrian Stage 3 || {{flag|Greenland}} || 200px |Incomplete<ref name="vinther2014" /> |- | ''Ursulinacaris grallae'' || Pates, Daley & Butterfield || 2019 || Hurdiidae | data-sort-value="509" |Wuliuan || {{flag|Canada}} {{flag|China}} || 200px |Unknown |- | ''Schinderhannes bartelsi'' || Kühl, Briggs, & Rust || 2009<ref name=":12" /> || Hurdiidae | data-sort-value="407.6" |Emsian || {{flag|Germany}} || Incomplete<ref name=":1" /> |Incomplete<ref name=":1" /> |- | ''Stanleycaris hirpex'' || Pates, Daley, & Ortega-Hernández || 2018<ref name=":38" /> || Hurdiidae ||Wuliuan || {{flag|Canada}} || 200px |P-element is unknown, possibly absent<ref name=":41">{{Cite journal |last1=Moysiuk |first1=Joseph |last2=Caron |first2=Jean-Bernard |date=8 August 2022 |title=A three-eyed radiodont with fossilized neuroanatomy informs the origin of the arthropod head and segmentation |journal=Current Biology |language=en |volume=32 |issue=15 |pages=3302–3316.e2 |doi=10.1016/j.cub.2022.06.027 |pmid=35809569 |s2cid=250361698 |issn=0960-9822|doi-access=free }}</ref> |- | ''Peytoia nathorsti'' || Walcott || 1911<ref name=":34" /> || Hurdiidae ||Wuliuan–Drumian ||{{flag|Canada}} {{flag|United States}} || 200px |Incomplete<ref name=":1" /> |- | ''Peytoia infercambriensis'' (formerly ''Cassubia infercambriensis'')<ref>{{Cite journal|last1=Daley|first1=Allison|last2=Legg|first2=David|date=2 July 2015|title=A morphological and taxonomic appraisal of the oldest anomalocaridid from the Lower Cambrian of Poland|url=https://www.researchgate.net/publication/279805320|journal=Geological Magazine|volume=-1|issue=5|pages=949–955|doi=10.1017/S0016756815000412|bibcode=2015GeoM..152..949D|s2cid=130745134}}</ref> || Lendzion || 1975 || Hurdiidae | data-sort-value="514" |Cambrian Stage 3 || {{flag|Poland}} || 200px |Unknown |- | ''Aegirocassis benmoulai'' || Van Roy, Daley, & Briggs || 2015<ref name="vanroy2015" /> || Hurdiidae (Aegirocassisinae) | data-sort-value="485.4" |Tremadocian || {{flag|Morocco}} || 200px |257x257px |- | ''Hurdia victoria'' || Walcott || 1912<ref name=":39" /> || Hurdiidae | data-sort-value="509" |Wuliuan–Drumian || {{flag|Canada}} {{flag|Czechia}} || 200px |200x200px |- | ''Hurdia triangulata'' || Walcott || 1912<ref name=":39" /> || Hurdiidae | data-sort-value="509" |Wuliuan || {{flag|Canada}} || 200px |200x200px |- | ''Cambroraster falcatus'' || Moysiuk & Caron || 2019<ref name=":1" /> || Hurdiidae | data-sort-value="509" |Wuliuan || {{flag|Canada}} || 200px |229x229px |- | ''Pahvantia hastata'' || Robison & Richards || 1981 || Hurdiidae | data-sort-value="504.5" |Drumian || {{flag|United States}} || 200px |200x200px |- | ''Cordaticaris striatus'' || Sun, Zeng, & Zhao || 2020<ref name=":24" /> || Hurdiidae | data-sort-value="504.5" |Drumian || {{flag|China}} || Incomplete<ref name=":24" /> |200x200px |- | ''Zhenghecaris shankouensis'' || Vanner, Chen, Huang, Charbonnier, & Wang || 2006 || Hurdiidae | data-sort-value="514" |Cambrian Stage 3 || {{flag|China}} || Unknown <ref name=":23" /> |left|200x200px |- | ''Buccaspinea cooperi'' || Pates, Lerosey-Aubril, Daley, Kier, Bonino & Ortega-Hernández || 2021<ref name=":25" /> || Hurdiidae | data-sort-value="504.5" |Drumian || {{flag|United States}} || 200px |Unknown |- | ''Titanokorys gainesi'' || Caron & Moysiuk || 2021<ref name=":28" /> || Hurdiidae | data-sort-value="509" |Wuliuan || {{flag|Canada}} || 200px |200x200px |- | ''Pseudoangustidontus duplospineus'' || Van Roy & Tetlie || 2006 || Hurdiidae (Aegirocassisinae) | data-sort-value="485.4" |Tremadocian || {{flag|Morocco}} || 200px |Unknown |- | ''Pseudoangustidontus izdigua'' || Potin, Gueriau & Daley || 2023 || Hurdiidae (Aegirocassisinae) | data-sort-value="485.4" |Tremadocian || {{flag|Morocco}} || 200px |Incomplete<ref name=":40"/> |- |''Shucaris ankylosskelos'' |Wu ''et al.'' |2024 |(controversial) |Cambrian Stage 2–Cambrian Stage 3 |{{flag|China}} |200px |Incomplete<ref name=":44">{{Cite journal |last1=Wu |first1=Yu |last2=Pates |first2=Stephen |last3=Liu |first3=Cong |last4=Zhang |first4=Mingjing |last5=Lin |first5=Weiliang |last6=Ma |first6=Jiaxin |last7=Wu |first7=Yuheng |last8=Chai |first8=Shu |last9=Zhang |first9=Xingliang |last10=Fu |first10=Dongjing |date=31 December 2024 |title=A new radiodont from the lower Cambrian (Series 2 Stage 3) Chengjiang Lagerstätte, South China informs the evolution of feeding structures in radiodonts |journal=Journal of Systematic Palaeontology |language=en |volume=22 |issue=1 |doi=10.1080/14772019.2024.2364887 |issn=1477-2019|doi-access=free |hdl=10871/136118 |hdl-access=free }}</ref> |- |''Stanleycaris qingjiangensis'' |Wu ''et al.'' |2024 |Hurdiidae |Cambrian stage 3 |{{flag|China}} |200px |Incomplete<ref>{{Cite journal |last1=Wu |first1=Yu |last2=Pates |first2=Stephen |last3=Zhang |first3=Mingjing |last4=Lin |first4=Weiliang |last5=Ma |first5=Jiaxin |last6=Liu |first6=Cong |last7=Wu |first7=Yuheng |last8=Zhang |first8=Xingliang |last9=Fu |first9=Dongjing |date=July 2024 |title=Exceptionally preserved radiodont arthropods from the lower Cambrian (Stage 3) Qingjiang Lagerstätte of Hubei, South China and the biogeographic and diversification patterns of radiodonts |url=https://onlinelibrary.wiley.com/doi/10.1002/spp2.1583 |journal=Papers in Palaeontology |language=en |volume=10 |issue=4 |doi=10.1002/spp2.1583 |issn=2056-2799|url-access=subscription }}</ref> |- |''Mosura fentoni'' |Moysiuk & Caron |2025 |Hurdiidae |Wuliuan |{{flag|Canada}} |200px | P-element unknown, possibly absent<ref>{{cite journal |last1=Moysiuk |first1=Joseph |last2=Caron |first2=Jean-Bernard |date=14 May 2025 |title=Early evolvability in arthropod tagmosis exemplified by a new radiodont from the Burgess Shale |journal=Royal Society Open Science |volume=12 |issue=5 |doi=10.1098/rsos.242122 |pmc=12076883 |doi-access=free|pmid=40370603 }}</ref> |- |Falciscaris mumakiana<ref name=":46" /> |Potin ''et al.'' |2025 |Hurdiidae |Tremadocian |{{flag|Morocco}} |200px |Unknown |- |''Verrocaris kerrymatti''<ref name=":47">{{Cite journal |last=Oxman |first=Katherine L. |last2=Minkowitz |first2=Conner |last3=Thomas |first3=Roger D. K. |date=2025-12-29 |title=Verrocaris kerrymatti n. gen. n. sp., a new “misfit” anomalocaridid radiodont (Euarthropoda) from the Kinzers Formation (Cambrian, Series 2, Stage 4) of Pennsylvania and its implications |url=https://www.cambridge.org/core/journals/journal-of-paleontology/article/verrocaris-kerrymatti-n-gen-n-sp-a-new-misfit-anomalocaridid-radiodont-euarthropoda-from-the-kinzers-formation-cambrian-series-2-stage-4-of-pennsylvania-and-its-implications/56265B098699F16609EBE20E79BDA325 |journal=Journal of Paleontology |language=en |pages=1–14 |doi=10.1017/jpa.2025.10194 |issn=0022-3360|doi-access=free }}</ref> |Oxman ''et al.'' |2025 |Anomalocarididae |Cambrian Stage 4 |{{flag|United States}} |200px |Unknown |}
==History== [[File:Laggania cambria Peytoia nathorsti USNM 57555.jpg|thumb|180px|Body specimen of ''Peytoia nathorsti'', the original "''Laggania cambria''"]] <gallery mode="packed" heights="150"> File:Anomalocaris canadensis grasping claw, Burgess Shale.jpg|Frontal appendage of ''Anomalocaris canadensis'' File:Peytoia nathorsti Laggania cambria oral cone, Burgess Shale.jpg|Oral cone of ''Peytoia nathorsti'' File:Hurdia victoria USNM PAL 57718.jpg|H-element of ''Hurdia victoria'' File:USNM PAL 57490.jpg|Paired frontal appendages from an unnamed hurdiid radiodont<ref name=":27" /> </gallery>
The history of radiodonts is complex. Incomplete specimens pertaining to different body parts of the same species had historically been interpreted as belonging to different species and even different phyla.<ref name=collins1996/><ref name=daley2009/> Prior to their recognition as a group, radiodont specimens had been assigned to five different phyla: Porifera, Cnidaria, Echinodermata, Annelida, and Arthropoda.<ref name=collins1996/>
The first known radiodont specimens were collected from the trilobite beds of Mount Stephen by Richard G. McConnell of the Geological Survey of Canada in 1886<ref name=collins1996/> or 1888.<ref name=whiteaves1892/> These specimens were named ''Anomalocaris canadensis'' in 1892 by GSC paleontologist Joseph Whiteaves.<ref name=whiteaves1892/> Whiteaves interpreted the specimens, now known to be isolated frontal appendages, as the abdomen of a phyllocarid crustacean.<ref name="whiteaves1892" /> Additional radiodont specimens were described in 1911 by Charles Walcott.<ref name=":34">Walcott, C. D. 1911a. [https://repository.si.edu/handle/10088/23427 Middle Cambrian holothurians and medusae]. Cambrian geoogy and paleontology II. Smithsonian Miscellaneous Collections, 57: 41–68.</ref> He interpreted an isolated oral cone, which he named ''Peytoia nathorsti'', as a jellyfish, and a poorly preserved but relatively complete specimen, which he named ''Laggania cambria'', as a holothurian.<ref name=":34" /> In 1912 Walcott named ''Hurdia victoria'' and ''H. triangulata'' based on isolated H-elements, which he interpreted as the carapaces of crustaceans.<ref name=":39">WALCOTT, C. D. 1912. [https://repository.si.edu/bitstream/handle/10088/23430/SMC_57_Walcott_1910_6_145-245.pdf?sequence=1&isAllowed=y Middle Cambrian Branchiopoda, Malacostraca, Trilobita and Merostomata]. Smithsonian Miscellaneous Collections, 57: 145–228.</ref> Isolated frontal appendages of ''Peytoia'' and ''Hurdia'', collectively known as "Appendage F" in Briggs 1979, were all identified as those of ''Sidneyia'' at that time.<ref name=":34" /> A ''Hurdia'' P-element was named ''Proboscicaris'' in 1962, and interpreted as the carapace of a bivalved arthropod.<ref>ROLFE, W. D. I. 1962. Two new arthropod carapaces from the Burgess Shale (Middle Cambrian) of Canada. Breviora Museum of Comparative Zoology, 60: 1–9.</ref>
The Geological Survey of Canada initiated a revision of Burgess Shale fossils in 1966, overseen by Cambridge University paleontologist Harry B. Whittington.<ref name=collins1996/> This revision would ultimately lead to the discovery of the complete radiodont body plan. In 1978, Simon Conway Morris recognized that the mouthparts of ''Laggania'' were ''Peytoia''-like, but he interpreted this as evidence that it was a composite fossil made up of a ''Peytoia'' jellyfish and a sponge.<ref name=conwaymorris1978/> In 1979, Derek Briggs recognized that the fossils of ''Anomalocaris'' were appendages, not abdomens, but interpreted them as walking legs alongside "Appendage F".<ref name=briggs1979/> It was not until 1985 that the true nature of the fossils of ''Anomalocaris'', ''Laggania'', and ''Peytoia'' was recognized, and they were all assigned to a single genus, ''Anomalocaris''.<ref name=":31" /> Subsequently, it was recognized that ''Anomalocaris'' was a distinct form from the other two, resulting in a split into two genera, the latter of which was variously named ''Laggania'' and ''Peytoia'' until it was determined that both represent the same species and ''Peytoia'' had priority.<ref name=":14" /> It was later recognized that some of the fossils assigned to these taxa belonged to another form, which was recognized as bearing a carapace made up of ''Hurdia'' and ''Proboscicaris'' elements. Finally, in 2009, these specimens were redescribed as ''Hurdia''.<ref name="daley2009" /> Even after these recognitions, partial misidentifications (e.g. oral cone and frontal appendages of ''Peytoia'' had been assigned to ''Anomalocaris''<ref name="collins1996" /> and ''Hurdia'',<ref name="daley2009" /> respectively) had been revealed by subsequent studies as well.<ref name=":14" /><ref>{{Cite journal|last1=Daley|first1=Allison|last2=Budd|first2=Graham|last3=Caron|first3=Jean-Bernard|date=1 October 2013|title=The morphology and systematics of the anomalocarid Hurdia from the Middle Cambrian of British Columbia and Utah|url=https://www.researchgate.net/publication/263406650|journal=Journal of Systematic Palaeontology|volume=11|doi=10.1080/14772019.2012.732723|s2cid=86465719}}</ref>
The taxon Radiodonta itself was coined in 1996 by Desmond Collins, after it was established that ''Anomalocaris'' and its kin represented a distinctive lineage with arthropod affinities rather than a hitherto unknown phylum.<ref name=collins1996/> Collins also established the class Dinocarida to contain the order Radiodonta as well as the Opabiniidae, which he recognized as distinct due to its lacking the distinctive oral cone structure of radiodonts.<ref name="collins1996" /> Radiodonta was first given a phylogenetic definition in 2014.<ref name="vinther2014"/> Radiodonta was originally viewed as containing a single family, Anomalocarididae, but it was divided into four families in 2014: Amplectobeluidae, Anomalocarididae, Cetiocaridae, and Hurdiidae.<ref name="vinther2014" /> The name Cetiocaridae did not conform to the International Code of Zoological Nomenclature and so was renamed Tamisiocarididae in 2019.<ref name=Pates2019/>
Until the 2010s, radiodonts were typically considered to be uniformly large apex predators, but discoveries of new species over the course of that decade led to a considerable increase in the known ecological and morphological diversity of the group.<ref name="vinther2014" /><ref name="vanroy2015" /><ref name=":6" /><ref name=":1" /><ref name=Paterson2020/><ref name=":25" /><ref name=":27" /><ref name=":28" />
==References== {{Reflist|refs= <ref name=briggs1979>{{Cite journal| volume = 22| issue = 3| pages = 631–664| last = Briggs| first = D. E. G.| title = Anomalocaris, the largest known Cambrian arthropod| journal = Palaeontology| date = 1979}}</ref> <ref name="CaronMoysiuk2021">{{cite journal |last1=Caron |first1=J.-B. |last2=Moysiuk |first2=J. |title=A giant nektobenthic radiodont from the Burgess Shale and the significance of hurdiid carapace diversity |journal=Royal Society Open Science |date=September 2021 |volume=8 |issue=9 |article-number=210664 |doi=10.1098/rsos.210664|pmid=34527273 |pmc=8424305 |bibcode=2021RSOS....810664C |doi-access=free }}</ref> <ref name=conwaymorris1978>{{Cite journal| volume = 52| issue = 1| pages = 126–131| last = Conway Morris| first = S.| title = Laggania cambria Walcott: A Composite Fossil| journal = Journal of Paleontology| date = 1978 | jstor = 1303799}}</ref> <ref name=Paterson2020>{{cite journal | first1 = John R. | last1 = Paterson | first2 = Gregory D. | last2 = Edgecombe | first3 = Diego C. | last3 = García-Bellido | title = Disparate compound eyes of Cambrian radiodonts reveal their developmental growth mode and diverse visual ecology | journal = Science Advances | year = 2020 | volume = 6 | issue = 49 | doi = 10.1126/sciadv.abc6721 | pmid = 33268353 | pmc = 7821881 | bibcode = 2020SciA....6.6721P | hdl = 10141/622906 | hdl-access = free }}</ref> <ref name=Pates2019>{{Cite journal| doi = 10.1017/S0016756818000547| volume = 156| issue = 7| pages = 1233–1246| last1 = Pates| first1 = Stephen| last2 = Daley| first2 = Allison C.| title = The Kinzers Formation (Pennsylvania, USA): the most diverse assemblage of Cambrian Stage 4 radiodonts| journal = Geological Magazine| date = 2019 | bibcode = 2019GeoM..156.1233P| s2cid = 134299859| url = https://www.cambridge.org/core/product/identifier/S0016756818000547/type/journal_article}}</ref> <ref name="Patesetal2020">{{Cite journal| doi = 10.1098/rsos.200459| doi-access = free| issn = 2054-5703| volume = 7| issue = 6| article-number = 200459| last1 = Pates| first1 = Stephen| last2 = Botting| first2 = Joseph P.| last3 = McCobb| first3 = Lucy M. E.| last4 = Muir| first4 = Lucy A.| title = A miniature Ordovician hurdiid from Wales demonstrates the adaptability of Radiodonta| journal = Royal Society Open Science| date = 2020| pmid = 32742697| pmc = 7353989| bibcode = 2020RSOS....700459P}}</ref> <ref name=whiteaves1892>{{Cite journal| volume = 5| issue = 4| last = Whiteaves| first = J. F.| title = Description of a new genus and species of phyllocarid Crustacea from the Middle Cambrian of Mount Stephen, B. C.| journal = The Canadian Record of Science| date = 1892}}</ref> }}
{{Radiodonta}} {{Taxonbar|from=Q3291852}}
* Category:Cambrian Series 2 first appearances Category:Dinocaridida Category:Early Devonian extinctions Category:Prehistoric arthropod orders