{{Short description|Extinct order of arthropods}} {{Automatic taxobox | fossil_range = {{Fossil range|Guzhangian|Katian}} | image = 20240212 Aglaspis spinifer.png | image_caption = ''Aglaspis spinifer'' | image2 = Beckwithia collection Paul Hille.jpg | image2_caption = Fossil of ''Beckwithia typa'' | display_parents = 2 | taxon = Aglaspidida | authority = Raasch, 1939 | subdivision_ranks = Subtaxa | subdivision_ref = | subdivision = *{{extinct}}''Australaglaspis'' *{{extinct}}''Beckwithia'' *{{extinct}}'''Tremaglaspididae''' **{{extinct}}''Brachyaglaspis'' **{{extinct}}''Chlupacaris'' **{{extinct}}''Cyclopites'' **{{extinct}}''Flobertia'' **{{extinct}}''Quasimodaspis'' **{{extinct}}''Tremaglaspis'' *{{extinct}}'''Aglaspididae''' **{{extinct}}''Aglaspella'' **{{extinct}}''Aglaspis'' **{{extinct}}''Aglaspoides'' **{{extinct}}''Chraspedops'' **{{extinct}}''Glypharthrus'' **{{extinct}}''Gogglops'' **{{extinct}}''Hesselbonia'' **{{extinct}}''Setaspis'' **{{extinct}}''Tuboculops'' **{{extinct}}''Uarthrus'' }}
'''Aglaspidida''' is an extinct order of marine arthropods known from fossils spanning the Middle Cambrian to the Upper Ordovician. Initially considered chelicerates, modern anatomical comparisons demonstrate that the aglaspidids cannot be accommodated within this group,<ref name=Ortega2010>{{Cite journal| volume = 53| issue = 5| last1 = Ortega-Hernández | first1 = J.| last2 = Braddy | first2 = S. J.| last3 = Jago | first3 = J. B.| last4 = Baillie | first4 = P. W.| title = A new aglaspidid arthropod from the Upper Cambrian of Tasmania| pages = 1065–1076| journal=Palaeontology| year = 2010| doi = 10.1111/j.1475-4983.2010.00974.x| doi-access = | bibcode = 2010Palgy..53.1065O}}</ref> and that they lie instead within the Artiopoda, thus placing them closer to the trilobites,<ref name="Ortega2013">{{Cite journal| volume = 29| last1 = Ortega-Hernández | first1 = J.| last2 = Legg | first2 = D. A.| last3 = Braddy | first3 = S. J.| title = The phylogeny of aglaspidid arthropods and the internal relationships within Artiopoda| pages = 15–45| journal=Cladistics| year = 2013| issue = 1 | doi = 10.1111/j.1096-0031.2012.00413.x| pmid = 34814371 }}</ref> being placed in the artiopod subgroup Vicissicaudata.<ref name=":0">{{Cite journal |last1=Lerosey-Aubril |first1=Rudy |last2=Zhu |first2=Xuejian |last3=Ortega-Hernández |first3=Javier |date=2017-09-11 |title=The Vicissicaudata revisited – insights from a new aglaspidid arthropod with caudal appendages from the Furongian of China |journal=Scientific Reports |language=en |volume=7 |issue=1 |page=11117 |doi=10.1038/s41598-017-11610-5 |issn=2045-2322 |pmc=5593897 |pmid=28894246|bibcode=2017NatSR...711117L }}</ref>
With 38 known valid species as of 2017, they represent one of the most diverse groups of early Paleozoic arthropods, after trilobites.<ref name=":3">Lerosey-Aubril, R., Ortega-Hernández, J., Van Roy, P., Zhu, X., (2017). [https://www.researchgate.net/publication/319535415_The_Aglaspidida_a_poorly-known_yet_important_order_of_early_Palaeozoic_arthropods The Aglaspidida: a poorly-known, yet important order of early Palaeozoic arthropods]. ''International Workshop on Evolution of Cambrian Arthropods - Taxonomy, Ontogeny and Phylogeny''. 1-6 Sep. 2017, Xian, China.</ref> Aglaspidid fossils are found in North America (United States and Canada), Europe, Australia, and China.
== Description == The exoskeletons of aglaspidids have frequently been suggested to have been phosphatic,<ref name=":0" /> though this may actually represent post-mortem taphonomic mineral replacement, and at least some members of the group definitively had calcified exoskeletons.<ref name=":4" /> The headshields ancestrally have a pair of unstalked eyes attached to the upper surface, which are attached to a raised region of the head that merges with the rest of the headshield frontwards (anteriorly) and towards the midline (medially),<ref name=":3" /> though some species appear to have lost their eyes entirely. The underside of the headshield (cephalon) had either four or possibly five pairs of attached limbs (including a pair of antennae<ref name="Ortega2013" />). The trunk segments (tergites) were freely articulating (with the exception of the posteriormost one<ref name=":1">{{Cite journal |last1=Ortega-HernáNdez |first1=Javier |last2=Van Roy |first2=Peter |last3=Lerosey-Aubril |first3=Rudy |date=May 2016 |title=A new aglaspidid euarthropod with a six-segmented trunk from the Lower Ordovician Fezouata Konservat-Lagerstätte, Morocco |url=https://www.cambridge.org/core/product/identifier/S0016756815000710/type/journal_article |journal=Geological Magazine |language=en |volume=153 |issue=3 |pages=524–536 |doi=10.1017/S0016756815000710 |bibcode=2016GeoM..153..524O |issn=0016-7568|url-access=subscription |hdl=1854/LU-8714213 |hdl-access=free }}</ref>), and bore outwardly projecting pleurae.<ref name=":1" /> The body ends with a tailspine.<ref name=":0" /> A distinctive feature of aglaspididans are "postventral plates", a pair of two flat sclerotized plates located on the underside of the posterior final few segments of the body, covering the base of the tailspine.<ref name="Ortega2013" /> Most members of the group are around {{Convert|2|cm|in}} in length, though their length varies from {{Convert|1|cm|in}} up to a maximum of {{Convert|13.5|cm|in}} including the tailspine.<ref name=":3" />
Members of the family Agaspididae (which constitute the majority of the group) are additionally defined as having a flat, wide body, with the posterior outer corners of the headshield having an acute to spinose shape, with the trunk ancestrally composed of 12 tergites, with the tailspine being elongate and fused to the posteriormost 12th trunk segment/tergite.<ref name=":0" />
Members of Tremaglaspididae are characterised by having a short tailspine (less than half the length of the trunk), reduction or complete loss of eyes, the headshield having rounded angles, the trunk having a vaulted shape and being composed of 11 or less (minimum 6) tergites.<ref name=":0" />
{{gallery|20240202 Aglaspidid ventral.png|Diagram of aglaspididan underside anatomy|Lerosey-Aubril et al. 2017b Aglaspidida.png|Diagram of ''Beckwithia typa'', a basal aglaspididan (left), ''Aglaspis spinifer'' (Agaspididae) (middle) and ''Tremaglaspis vanroyi'' (Tremaglaspididae) (right)|41598 2017 11610 Fig5 HTML.webp|View of ''Glypharthrus trispinicaudatus'' (Agaspididae) from above|20240205 Brachyaglaspis singularis.png|Life restoration of ''Brachyaglaspis singularis'' (Tremaglaspididae)|20240209 Tremaglaspis unite.png|Life restoration of ''Tremaglaspis unite'' (Tremaglaspididae)|||width=185|height=|lines=|align=center|title=Gallery of aglaspididans}}
== Ecology == [[File:Gogglops ensifer.jpg|thumb|Restoration of ''Gogglops'' (Aglaspididae) on the seafloor (note:body is hypothetical)]] Aglaspidids are thought to have primarily inhabited shallow marine environments.<ref name=":2" /> Aglaspidids are suggested to have been predatory or scavengers, using their legs to pass food towards the (probably posteriorly directed) mouth. They are thought to have mostly been benthic animals that inhabited the seafloor (with trace fossils probably made by aglaspidids on the seafloor having been reported from the Upper Cambrian of Wisconsin), though ''Cyclopites'' may have been a swimming, nektonic animal.<ref>{{Cite journal |last=Hesselbo |first=Stephen P. |date=November 1992 |title=Aglaspidida (Arthropoda) from the Upper Cambrian of Wisconsin |url=https://doi.org/10.1017/s0022336000021016 |journal=Journal of Paleontology |volume=66 |issue=6 |pages=885–923 |doi=10.1017/s0022336000021016 |issn=0022-3360|url-access=subscription }}</ref>
== Evolution == Aglaspidida first appeared in Laurentia (what is now North America) during the Guzhangian stage of the upper Miaolingian (Cambrian Series 3), before undergoing a major radiation during the Furongian (Upper Cambrian), reaching a worldwide distribution during this period. During the Ordovician, remains are known from Avalonia (modern Wales), Gondwana (Morocco) and South China, with the youngest representative being known from the Katian stage of the Upper Ordovician in Morocco.<ref name=":0" />
== Taxonomy == When aglaspidids were first described in the 1860s, they were initially considered to be crustaceans. Following the pioneering work on the group by Gilbert O. Raasch in his 1939 publication ''Cambrian Merostomata'', who named the group and many genera of aglaspidids, they were considered to be chelicerates. However, research in the late 20th century challenged this hypothesis, and during the 21st century it became recognised that they belonged to a group called Vicissicaudata with taxa like ''Sidneyia'', ''Emeraldella'' (both formerly classified as "xenopods") Cheloniellida, and ''Kodymirus'', based on shared traits of the organisation of the final segments of the trunk.<ref name="Ortega2013" /> Vicissicaudata is generally placed within the Artiopoda, the broader group which contains trilobites and their close relatives.<ref name=":0" />
=== List of genera === * {{extinct}}''Australaglaspis'' (Late Cambrian, Australia) * {{extinct}}''Beckwithia'' (Middle-Late Cambrian, United States) * {{extinct}}'''Tremaglaspididae''' ** {{extinct}}''Brachyaglaspis'' (Early Ordovician, Morocco) ** {{extinct}}''Chlupacaris'' (Late Ordovician, Morocco) ** {{extinct}}''Cyclopites'' (Late Cambrian, United States) ** {{extinct}}''Flobertia'' (Late Cambrian, United States) ** {{extinct}}''Quasimodaspis'' (Middle-Late Cambrian, United States) ** {{extinct}}''Tremaglaspis'' (Middle Cambrian, United States, Early Ordovician, Wales, unnamed possible species also known from the Early Ordovician of Morocco<ref name=":2">{{Cite journal |last=Lerosey-Aubril |first=Rudy |last2=Ortega-HernáNdez |first2=Javier |last3=Kier |first3=Carlo |last4=Bonino |first4=Enrico |date=September 2013 |title=Occurrence of the Ordovician-type aglaspidid Tremaglaspis in the Cambrian Weeks Formation (Utah, USA) |url=https://www.cambridge.org/core/product/identifier/S001675681300037X/type/journal_article |journal=Geological Magazine |language=en |volume=150 |issue=5 |pages=945–951 |doi=10.1017/S001675681300037X |issn=0016-7568|url-access=subscription }}</ref>) * {{extinct}}'''Aglaspididae''' ** {{extinct}}''Aglaspella'' (Late Cambrian, United States, South China) ** {{extinct}}''Aglaspis'' (Late Cambrian, United States) ** {{extinct}}''Aglaspoides'' (Late Cambrian, United States, possibly a synonym of ''Glyptarthus''<ref name="Ortega2013" />) ** {{extinct}}''Chraspedops'' (Late Cambrian, United States) ** {{extinct}}''Glypharthrus'' (Late Cambrian, United States, Canada, South China) ** {{extinct}}''Gogglops'' (Upper Ordovician, China) ** {{extinct}}''Hesselbonia'' (Late Cambrian, United States) ** {{extinct}}''Setaspis'' (Late Cambrian, United States) ** {{extinct}}''Tuboculops'' (Late Cambrian, United States) ** {{extinct}}''Uarthrus'' (Late Cambrian, United States) Additionally, ''Obrutschewia, Angarocaris'' and ''Intejocaris'', known from the Ordovician of the Siberian Platform, may also be members of the group.<ref name=":4">{{Cite journal |last=Siveter |first=Derek J. |last2=Fortey |first2=Richard A. |last3=Zhu |first3=Xuejian |last4=Zhou |first4=Zhiyi |date=October 2018 |title=A three-dimensionally preserved aglaspidid euarthropod with a calcitic cuticle from the Ordovician of China |url=https://www.cambridge.org/core/product/identifier/S0016756817000309/type/journal_article |journal=Geological Magazine |language=en |volume=155 |issue=7 |pages=1427–1441 |doi=10.1017/S0016756817000309 |issn=0016-7568|url-access=subscription }}</ref> Certain other similar extinct arthropods are suggested to possibly be closely related to the aglaspidids, including members of the poorly known order Strabopida.<ref name="Ortega2013" />
Cladogram of the position of Aglaspdida within Vicissicaudata, after McCoy et al. 2025:<ref name="tardisia2">{{cite journal |last1=McCoy |first1=Victoria E. |last2=Herrera |first2=Fabiany |last3=Wittry |first3=Jack |last4=Mayer |first4=Paul |last5=Lamsdell |first5=James C. |date=2025 |title=A possible vicissicaudatan arthropod from the Late Carboniferous Mazon Creek Lagerstätte |journal=Geological Magazine |volume=162 |doi=10.1017/S001675682400044X |doi-access=free}}</ref>{{Clade|{{clade |label1=Vicissicaudata |1={{Clade sequential |1=''Molaria spinifera'' 70px |2=''Emeraldella brocki'' |3=''Emeraldella brutoni'' 100px |4={{Clade sequential |1=''Sidneyia inexpectans'' 100px |2=''Tardisia broedeae'' |3=Cheloniellida 150px }} |5=''Carimersa neptuni'' |6=''Kodymirus vagans'' 70px |7=''Eozetetes gemelli'' |8=Aglaspidida 100px }} }}|label1=}}
Cladogram of Aglaspidida after Lerosey-Aubril, Zhu & Ortega-Hernández, 2017:<ref name=":0" />{{clade|{{clade |1=''Australaglaspis'' |2={{clade |1=''Beckwithia'' |2={{clade |label1=Tremaglaspididae |1={{clade |1=''Flobertia'' |2={{clade |1=''Cyclopites'' |2={{clade |1=''Chlupacaris'' |2={{clade |1=''Quasimodaspis'' |2={{clade |1=''Tremaglaspis unite'' |2={{clade |1=''Tremaglaspis vanroyi'' |2=''Brachyaglaspis'' }}}}}}}}}}}} |label2=Aglaspididae |2={{clade |1=''Uarthrus'' |2={{clade |1=''Aglaspella'' |2={{clade |1=''Gogglops'' |2={{clade |1=''Glypharthrus simplex'' |2=''Aglaspis'' }}}} |3={{clade |1=''Chraspedops'' |2={{clade |1=''Glypharthrus thomasi'' |2={{clade |1=''Glypharthrus trispinicaudatus'' |2=''Glypharthrus magnoculus'' }}}}}}}}}}}}}}}}|label1='''Aglaspidida'''}}
== References == {{Reflist}} *Hesselbo, SP. 1992. Aglaspidida (Arthropoda) from the Upper Cambrian of Wisconsin. Journal of Paleontology 66(6)885-923. *Raasch, GO. 1939. Cambrian Merostomata. Geological Society of America Special Paper 19, 146p.
==External links== {{Commons category}} *[http://www.fossilmuseum.net/fossils/aglaspida.htm Virtual fossli museum on Aglaspida] *[http://www.fossilmall.com/EDCOPE_Enterprises/invertebrates/invert14/invfossil-14.htm Fossilmall Australian Aglaspida] *[https://web.archive.org/web/20070802130258/http://strata.ummp.lsa.umich.edu/jack/showgenera.php?taxon=280&rank=class Merostomata genera] *[http://www.backtothepast.com.mx/ebonino/html/aglaspidi.html Enrico Bonino Aglaspida description page (in Italian)]
{{Artiopoda}} {{Taxonbar|from=Q394207}}
Category:Aglaspidida Category:Prehistoric arthropod orders Category:Miaolingian first appearances Category:Late Ordovician extinctions