Thylacocephala

{{Short description|Extinct group of arthropods}} {{Automatic taxobox | fossil_range = Late Ordovician (Sandbian) to Late Cretaceous (Santonian) {{fossilrange|458|84|earliest=515|}} (possible early Cambrian record) | image = Clausocaris lithographica reconstruction.png | image_caption = Restoration of ''Clausocaris'', a Concavicarida | taxon = Thylacocephala | authority = Pinna ''et al.'', 1982 | subdivision_ranks = Orders | subdivision = *Concavicarida *Conchyliocarida | image2 = Thylacares_brandonensis_S12862-014-0159-2-10.jpg | image2_caption = Reconstruction of ''Thylacares'', once considered to be the earliest known thylacocephalan }}

'''Thylacocephala''' (from the Greek {{lang|el|θύλακος}} or ''{{lang|el|thylakos}}'', meaning "pouch", and {{lang|el|κεφαλή}} or ''{{lang|el|cephalon}}'' meaning "head") is an extinct group of mandibulate arthropods,<ref name=":02">{{Cite journal |last1=Pulsipher |first1=M. A. |last2=Anderson |first2=E. P. |last3=Wright |first3=L. S. |last4=Kluessendorf |first4=J. |last5=Mikulic |first5=D. G. |last6=Schiffbauer |first6=J. D. |year=2022 |title=Description of ''Acheronauta'' gen. nov., a possible mandibulate from the Silurian Waukesha Lagerstätte, Wisconsin, USA |journal=Journal of Systematic Palaeontology |volume=20 |issue=1 |at=2109216 |doi=10.1080/14772019.2022.2109216 |s2cid=252839113}}</ref> that are generally regarded as a kind of crustacean, though their exact position within this group is uncertain.<ref name=":0">{{Cite journal |last=Laville |first=Thomas |last2=Haug |first2=Carolin |last3=Haug |first3=Joachim T. |last4=Forel |first4=Marie-Béatrice |last5=Charbonnier |first5=Sylvain |date=2021-02-16 |title=Morphology and anatomy of the Late Jurassic Mayrocaris bucculata (Eucrustacea?, Thylacocephala) with comments on the tagmosis of Thylacocephala |url=https://www.tandfonline.com/doi/full/10.1080/14772019.2021.1910584 |journal=Journal of Systematic Palaeontology |language=en |volume=19 |issue=4 |pages=289–320 |doi=10.1080/14772019.2021.1910584 |issn=1477-2019|url-access=subscription }}</ref> As a class they have a short research history, having been erected in the early 1980s.<ref name="Pinna1982">{{cite journal |author=G. Pinna, P. Arduini, C. Pesarini & G. Teruzzi |year=1982 |title=Thylacocephala: una nuova classe di crostacei fossili |journal=Atti della Società Italiana di Scienze Naturali e del Museocivico di Storia Naturale di Milano |volume=123 |pages=469–482}}</ref><ref name="SecRiou">{{cite journal |author1=S. Secrétan |author2=B. Riou |name-list-style=amp |year=1983 |title=Un groupe énigmatique de crustacés, ses représentants du Callovien de la Voulte−Sur−Rhône |journal=Annales de Paléontologie |volume=69 |pages=59–97}}</ref><ref name="BriggsRolfe">{{cite journal |author=D. E. G. Briggs, & W. D. I. Rolfe |year=1983 |title=New Concavicarida (new order: ?Crustacea) from the Upper Devonian of Gogo, Western Australia, and the palaeoecology and affinities of the group |journal=Special Papers in Palaeontology |volume=30 |pages=249–276}}</ref>

They typically possess a large, laterally flattened carapace that encompasses the entire body. The compound eyes tend to be large and bulbous, and occupy a frontal notch on the carapace. They possess three pairs of large raptorial limbs, and the abdomen bears a battery of small swimming limbs. Their size ranges from ~15 mm to potentially up to 250 mm.<ref name="Vannier"/>

Inconclusive claims of thylacocephalans have been reported from the lower lower Cambrian (''Zhenghecaris''),<ref name="Vannier">{{cite journal |author=Jean Vannier, Jun–Yuan Chen, Di–Ying Huang, Sylvain Charbonnier & Xiu–Qiang Wang |year=2006 |title=The Early Cambrian origin of thylacocephalan arthropods |journal=Acta Palaeontologica Polonica |volume=51 |issue=2 |pages=201–214 |url=http://www.app.pan.pl/archive/published/app51/app51-201.pdf }}</ref> but later study considered that genus as radiodont or arthropod with uncertain systematic position.<ref>{{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=2021-01-19 |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> The oldest unequivocal fossils are Upper Ordovician (Sandbian) in age, around 460-450 million years old.<ref name="Mikulic">{{cite journal |author=D. G. Mikulic, D. E. G. Briggs & J. Kluessendorf |year=1985 |title=A new exceptionally preserved biota from the Lower Silurian of Wisconsin, U.S.A. |journal=Philosophical Transactions of the Royal Society |volume=311 |issue=1148 |pages=75–85 |doi=10.1098/rstb.1985.0140 |jstor=2396972 |bibcode=1985RSPTB.311...75M|doi-access= }}</ref><ref>{{Cite journal|last1=Van Roy|first1=Peter|last2=Rak|first2=Štěpán|last3=Budil|first3=Petr|last4=Fatka|first4=Oldřich|date=2021-04-26|editor-last=Jagt|editor-first=John|title=Upper Ordovician Thylacocephala (Euarthropoda, Eucrustacea) from Bohemia indicate early ecological differentiation|url=https://onlinelibrary.wiley.com/doi/10.1002/spp2.1363|journal=Papers in Palaeontology|volume=7 |issue=3 |language=en|pages=1727–1751|doi=10.1002/spp2.1363|bibcode=2021PPal....7.1727V |s2cid=235557325 |issn=2056-2799|url-access=subscription|hdl=1854/LU-8714218|hdl-access=free}}</ref> Thylacocephala survived at least until the Santonian stage of the Upper Cretaceous, around 84 million years ago.<ref name="Schram1999">{{cite journal |author=F. R. Schram, C. H. J. Hof & F. A. Steeman |year=1999 |title=Thylacocephala (Arthropoda: Crustacea?) from the Cretaceous of Lebanon and implications for Thylacocephalan systematics |journal=Palaeontology |volume=42 |pages=769–797 |doi=10.1111/1475-4983.00097 |issue=5|bibcode=1999Palgy..42..769S |doi-access=free }}</ref><ref>{{cite journal | last1=Haug | first1=Carolin | last2=Briggs | first2=Derek E G. | last3=Mikulic | first3=Donald G. | last4=Kluessendorf | first4=Joanne | last5=Haug | first5=Joachim T. | title=The implications of a Silurian and other thylacocephalan crustaceans for the functional morphology and systematic affinities of the group | journal=BMC Evolutionary Biology | date=2014 | volume=14 | issue=1 | article-number=159 | doi=10.1186/s12862-014-0159-2 | pmid=25927449 | pmc=4448278 | bibcode=2014BMCEE..14..159H | doi-access=free }}</ref>

Beyond this, there remains much uncertainty concerning fundamental aspects of the thylacocephalan anatomy, mode of life, and relationship to the Crustacea, with whom they have always been cautiously aligned.

==Research history== The Thylacocephala is only recently described as a class, yet species now included within the group were first described at the turn of the century.<ref name="Meek">{{cite journal |author=F. Meek |year=1872 |title=Descriptions of new western Palaeozoic fossils mainly from the Cincinnati Group of the Lower Silurian series of Ohio |journal=Proceedings of the Academy of Natural Sciences of Philadelphia |volume=23 |pages=308–337}}</ref><ref name="VanStraelen">{{cite journal |author=V. Van Straelen |year=1923 |title=Les mysidacés du Callovien de la Voulte−Sur−Rhône (Ardèche) |journal=Bulletin de la Société Géologique de France|series= Notes et Mémoires |volume=23 |pages=431–439}}</ref> These were typically assigned to the phyllocarids despite an apparent lack of abdomen and appendages.<ref name="Schram1990">{{cite journal |author=F. R. Schram |year=1990 |title=On Mazon Creek Thylacocephala |journal=Proceedings of the San Diego Society of Natural History |volume=3 |pages=1–16 |oclc=24814481}}</ref> In 1982/83, three research groups independently created higher taxa to accommodate new species. Based on a specimen from northern Italy, Pinna ''et al.'' designated a new class,<ref name="Pinna1982"/> Thylacocephala, while Secrétan – studying ''Dollocaris ingens'', a species from the La Voulte-sur-Rhône konservat-lagerstätte in France – erected the class '''Conchyliocarida'''.<ref name="Secretan1983">{{cite journal |author=S. Secrétan |year=1983 |title=Une nouvelle classe fossile dans la super−classe des Crustacés: Conchyliocarida |journal=Les Comptes rendus de l'Académie des sciences |volume=296 |pages=437–439}}</ref> Briggs & Rolfe, working on fossils from Australia's Devonian deposits were unable to attribute certain specimens to a known group, and created an order of uncertain affinities, the '''Concavicarida''', to accommodate them.<ref name="BriggsRolfe"/> It was apparent the three groups were in fact working on a single major taxon (Rolfe noted disagreements over interpretation and taxonomic placement largely resulted from a disparity of sizes and differences in preservation.)<ref name="Rolfe">{{cite journal |author=W. D. I. Rolfe |year=1985 |title=Form and function in Thylacocephala, Conchyliocarida and Concavicarida (?Crustacea): a problem of interpretation |journal=Transactions of the Royal Society of Edinburgh |volume=76 |issue=2–3 |pages=391–399 |doi=10.1017/s0263593300010609|bibcode=1985EESTR..76..391R |s2cid=84613876 }}</ref> The group took the name Thylacocephala by priority, with '''Concavicarida''' and '''Conchyliocarida''' subjugated to orders, erected by Rolfe,<ref name="Rolfe"/> and modified by Schram.<ref name="Schram1990"/>

==Taxonomy== [[File:Ankitokazocaris.JPG|thumb|''Ankitokazocaris'' fossil (Triassic)]]Researchers agree the Thylacocephala represent a class. Some efforts have been made at further classification: Schram split currently known taxa into two orders:<ref name="Schram1990"/> *'''Concavicarida''' <small>Briggs & Rolfe, 1983</small><ref name="BriggsRolfe"/> which possesses: **A large, well developed optic notch **A discrete compound eye ** A fused rostrum ** 8 to 16 homologous well-demarcated trunk segments diminishing in height anteriorly and posteriorly ** Order includes ''Ainiktozoon'' (Silurian), ''Harrycaris'' (Devonian), ''Concavicaris'' (Devonian to Carboniferous), ''Dollocaris'' (Jurassic). *'''Conchyliocarida''' <small>Secrétan, 1983</small>:<ref name="Secretan1983"/> ** Lacks an optic notch ** Eyes on a protruding sac-like cephalon ** No rostrum. ** Order includes ''Convexicaris'' (Carboniferous), ''Yangzicaris'' (Triassic), and ''Atropicaris'', ''Austriocaris'', ''Clausocaris'', ''Kilianocaris'', ''Ostenocaris'', and ''Paraostenia'' from the Jurassic.

The accuracy of this scheme has been questioned in recent papers,<ref name="Vannier"/> as it stresses differences in the eyes and exoskeletal structure, which – in modern arthropods – tend to be a response to environmental conditions. Thus it has been suggested these features are too strongly controlled by external factors to be used alone to distinguish higher taxa. The problem is exacerbated by the limited number of thylacocephalan species known. More reliable anatomical indicators would include segmentation and appendage attachments (requiring the internal anatomy, currently elusive as a result of the carapace).

* ''Ainiktozoon'' * ''Ankitokazocaris'' * ''Eodollocaris'' * ''Falcatacaris''<ref name= ":3">{{Cite journal |last1=Braig|first1=Florian|last2=Haug|first2=Joachim|last3=Schädel|first3=Mario|last4=Huag|first4=Carolin|date=2019-06-28 |title=A new thylacocephalan crustacean from the Upper Jurassic lithographic limestones of southern Germany and the diversity of Thylacocephala|journal=Palaeodiversity|language=en |volume=12|issue=69|page=69 |doi= 10.18476/pale.v12.a6 |doi-access=free}}</ref> * ''Ligulacaris'' * ''Paraostenia'' * ''Polzia'' * ''Rugocaris'' * ''Silesicaris'' *''Stoppanicaris''<ref name= ":26">{{Cite journal |last1=Ji|first1=Cheng|last2=Tintori|first2=Andrea|date=2024-11-04 |title=New Material of Thylacocephala from the Early Ladinian (Middle Triassic) of Northern Grigna (Lecco, Lombardy, Northern Italy)|journal=Diversity|language=en |volume=16|issue=11|page=677 |doi= 10.3390/d16110677|bibcode=2024Diver..16..677J |doi-access=free}}</ref> * ''Thylacares'' * ''Victoriacaris'' * ''Zazrivacaris'' <ref name= ":Gerbe2025">{{Cite journal |last1=Gerbe|first1=Alexis|last2=Hyžný|first2=Matúš|last3=Schlögl|first3=Ján|date=2025-09-10 |title=New Early Jurassic thylacocephalan assemblage from the Western Carpathians in Slovakia|journal=Acta Palaeontologica Polonica|language=en |volume=70|issue=3|page=557-571 |doi=10.4202/app.01184.2024|doi-broken-date=10 September 2025 |doi-access=free}}</ref> * ''Pseudoprotozoea''<ref name=":1">{{Cite journal |last=Van Roy |first=Peter |last2=Rak |first2=Štěpán |last3=Budil |first3=Petr |last4=Fatka |first4=Oldřich |date=2021 |title=Upper Ordovician Thylacocephala (Euarthropoda, Eucrustacea) from Bohemia indicate early ecological differentiation |url=https://onlinelibrary.wiley.com/doi/abs/10.1002/spp2.1363 |journal=Papers in Palaeontology |language=en |volume=7 |issue=3 |pages=1727–1751 |doi=10.1002/spp2.1363 |issn=2056-2802|hdl=1854/LU-8714218 |hdl-access=free }}</ref> * ''Bohemiacaris''<ref name=":1" /> * '''Order Concavicarida''' ** '''Family Austriocarididae''' *** ''Austriocaris'' *** ''Yangzicaris'' ** '''Family Clausocarididae''' *** ''Clausocaris'' *** ''Convexicaris'' ** '''Family Concavicarididae''' *** ''Concavicaris'' *** ''Harrycaris'' *** ''Paraconcavicaris''<ref name=":4">{{Cite journal |last1=Broda|first1=Krzysztof|last2=Rak|first2=Štěpán|last3=Hegna|first3=Thomas|date=2019-06-24 |title=Do the clothes make the thylacocephalan? A detailed study of Concavicarididae and Protozoeidae (?Crustacea, Thylacocephala) carapace micro-ornamentation |journal=Journal of Systematic Palaeontology|language=en |volume=18|issue=11|pages=911–930 |doi= 10.1080/14772019.2019.1695683 |doi-access=free}}</ref> ** '''Family: Microcarididae''' *** ''Atropicaris'' *** ''Ferrecaris'' *** ''Keelicaris'' <ref name=":5"/> *** ''Microcaris'' *** ''Thylacocephalus'' ** '''Family: Protozoeidae''' *** ''Globulocaris''<ref name=":5"/> *** ''Hamaticaris'' <ref name=":5"/> *** ''Protozoea'' *** ''Pseuderichthus'' * '''Order Conchyliocarida''' ** '''Family: Dollocarididae''' *** ''Dollocaris'' *** ''Mayrocaris'' *** ''Paradollocaris'' <ref name=":5">{{Cite journal |last1=Charbonnier|first1=Sylvain|last2=Teruzzi|first2=Giorgio|last3=Audo |first3=Denis|last4=Lasseron|first4=Maxime|last5=Haug |first5=Carolin|last6=Haug|first6=Joachim|date=2018-04-16 |title=New thylacocephalans from the Cretaceous Laggerstätten of Lebanon |journal=Bulletin de la Société Géologique de France |language=en |volume=188 |issue=19 |page=19 |doi=10.1051/bsgf/2017176|doi-access=free}}</ref> *** ''Thylacocaris''<ref name= ":5"/> ** '''Family: Ostenocarididae''' *** ''Kilianocaris'' *** ''Ostenocaris''

== Anatomy == [[File:Ostenocaris ribeti reconstruction.png|thumb|''Ostenocaris ribeti'']] Based on Vannier,<ref name="Vannier" /> modified after Schram:<ref name="Schram1990" /> The Thylacocephala are bivalved arthropods with morphology exemplified by three pairs of long raptorial (predatory) appendages and hypertrophied eyes. They have a worldwide distribution. A laterally compressed, shield−like carapace encloses the entire body, and often has an anterior rostrum−notch complex and posterior rostrum. Its lateral surface can be externally ornamented, and evenly convex or with longitudinal ridges. Spherical or drop-shaped eyes are situated in the optic notches, and are often hypertrophied, filling the notches or forming a paired, frontal globular structure. No prominent abdominal features emerge from the carapace, and the cephalon is obscured. Even so, some authors have suggested the presence of five cephalic appendages, three of which could be the very long genticulate and chelate raptorials protruding beyond the ventral margin.<ref name="Schram1990" /><ref name="Pinna">{{cite journal |author=G. Pinna, P. Arduini, C. Pesarini & G. Teruzzi |year=1985 |title=Some controversial aspects of the morphology and anatomy of ''Ostenocaris cypriformis'' (Crustacea, Thylacocephala) |journal=Transactions of the Royal Society of Edinburgh |volume=76 |issue=2–3 |pages=373–379 |doi=10.1017/s0263593300010580|bibcode=1985EESTR..76..373P |s2cid=85401617 }}</ref> Alternatively these could originate from three anterior trunk segments.<ref name="Secretan1985" /> The posterior trunk has a series of eight to twenty<ref name="BriggsRolfe" /><ref name="Mikulic" /> styliform, filamentous pleopod-like appendages, decreasing in size posteriorly. Most Thylacocephala have eight pairs of well developed gills, found in the trunk region.

Beyond this there is a lack of knowledge about even basic thylacocephalan anatomy, including the number of posterior segments, origin of the raptorials, number of cephalic appendages, shape and attachment of gills, character of mouth, stomach and gut. This results from the class's all–encompassing carapace, which prevents the study of their internal anatomy in fossils. == Affinities == [[File:Dollocaris ingens reconstruction.jpg|thumb|''Dollocaris ingens'', from the Jurassic aged Voulte-sur-Rhône lagerstätte in France.]] It is universally accepted that the Thylacocephala are arthropods, yet the position within this phylum is debated. It had formerly been cautiously assumed that the class was a member of the Crustacea, but no conclusive proof exists. The strongest apomorphy aligning the class with other crustaceans is the carapace. As this feature has evolved independently numerous times within the Crustacea and other arthropods, it is not a very reliable pointer, and such evidence alone remains insufficient to align the class with the crustaceans.<ref name="Schram1990"/>

Of the features which could prove crustacean affinities, the arrangement of mouthparts would be the easiest to find in the Thylacocephala. The literature features some mention of such a head arrangement, but none definitive. Schram reports the discovery of mandibles in the Mazon Creek thylacocephalan ''Concavicaris georgeorum''.<ref name="Schram1990"/> Secrétan also mentions – with caution – possible mandibles in serial sections of ''Dollocaris ingens'', and traces of small limbs in the cephalic region (not well preserved enough to assess their identity).<ref name="Secretan1985">{{cite journal |author=S. Secrétan |year=1985 |title=Conchyliocarida, a class of fossil crustaceans: relationships to Malacostraca and postulated behaviour |journal=Transactions of the Royal Society of Edinburgh |volume=76 |issue=2–3 |pages=381–389 |doi=10.1017/s0263593300010592|bibcode=1985EESTR..76..381S |s2cid=131035558 }}</ref> Lange ''et al.'' report a new genus and species, ''Thylacocephalus cymolopos'', from the Upper Cretaceous of Lebanon, which has two possible pairs of antennae, but note the possession of two pairs of antennae alone does not prove the class occupies a position in the crown-group Crustacea.<ref name="Lange2001">{{cite journal |author=S. Lange, C. H. J. Hof, F. R. Schram & F. Steeman |year=2001 |title=New genus and species from the Cretaceous of Lebanon links the Thylacocephala to the Crustacea |journal=Palaeontology |volume=44 |pages=905–912 |doi=10.1111/1475-4983.00207 |issue=5|bibcode=2001Palgy..44..905L |s2cid=84659286 |doi-access=free }}</ref>

Despite a lack of evidence for a crustacean body plan, several authors have aligned the class with different groups of crustaceans. Schram provides an overview of possible affinities:<ref name="Schram1990"/> *Nothing in either Uniramia or Cheliceriformes seems likely. *Conchostraca is possible, but there is no strong supporting evidence. *A maxillopodan connection is possible. Largely considered due to the Italian researchers' insistence (see disagreements). *Stomatopods show many parallels but have no comparison to cephalon or body regions. *Remipedes show some parallels. *Decapod-like gills suggest malacostracan affinities.<ref name="Rolfe"/> In these various interpretations, numerous different limb arrangements for the three raptorials have been proposed: *antennules, antennae and mandibles<ref name="Arduini">{{cite journal |author1=P. Arduini, G. Pinna |author2=G. Teruzzi |name-list-style=amp |year=1980 |title=A new and unusual lower cirripede from Osteno in Lombardy: ''Ostenia cypriformis'' n. g. n. sp. (preliminary note) |journal=Atti della Società Italiana di Scienze Naturali e del Museocivico di Storia Naturale di Milano |volume=121 |pages=360–370}}</ref> *antennules, antennae and maxillipeds<ref name="Pinna"/> *thoracic (in keeping with stromatopod analogies)<ref name="Secretan1985"/> *maxillules, maxillae, maxillipedes<ref name="Schram1990"/><ref name="Rolfe"/> Further work is necessary to provide any solid conclusions.

A study in 2022 describing a new arthropod from Wisconsin, ''Acheronauta'', suggested thylacocephalans occupied a position more primitive than the crustaceans and myriapods as basal stem-group mandibulates.<ref name=":02"/> {{clade|{{clade |1=†''Parioscorpio venator'' 60px |2={{clade |1=†''Cascolus ravitis'' 60px |2={{clade |1=†''Tanazios dokeron'' |2={{clade |1={{clade |1=†''Captopodus poschmanni'' |2={{clade |1=†''Acheronauta stimulapis'' 60px |2=†Thylacocephala 60px }} }} |2={{clade |1=†''Occacaris oviformis'' 60px |2={{clade |1={{clade |1=Myriapoda 60px |2={{clade |1=†''Ercaicunia multinodosa'' 60px |2={{clade |1={{clade |1=†''Clypecaris pteroidea'' 60px |2={{clade |1={{clade |1={{clade |1=†''Waptia fieldensis'' 60px |2=†''Perspicaris dictynna'' 60px }} |2={{clade |1=†''Canadaspis perfecta'' 60px |2={{clade |1=†''Tokummia katalepsis'' 60px |2=†''Branchiocaris pretiosa'' 60px }} }} }} |2={{clade |1={{clade |1=†''Nereocaris exilis'' 60px |2=†''Odaraia alata'' 60px }} |2={{clade |1=†Euthycarcinoidea 60px |2=†Fuxianhuiida 60px }} }} }} }} }} }} }} |label2= |2=Pancrustacea (including Hexapoda) 60px 60px }} }} }} }} }} }}|style=|grouplabel1=|label1=Total group Mandibulata}}This would place them outside of the crustaceans as a more basal branch of the arthropod family tree.<ref name=":02" /> However, other authors have supported a crustacean placement, with a 2021 study of the anatomy of ''Mayrocaris'' supporting a remipede affinity for thylacocephalans within Crustacea.<ref name=":0" />

A 2025 study by Laville and colleagues used synchrotron X-ray tomography on fossil thylacocephalan specimens in order to gain a detailed understanding of their internal anatomy in order to determine the position of Thylacocephala within Crustacea. They recovered thylacocephalans as true crustaceans most closely related Malacostraca. A version of their phylogeny is presented below:<ref>{{Cite journal |last=Laville |first=Thomas |last2=Forel |first2=Marie-Béatrice |last3=King |first3=Andrew |last4=Charbonnier |first4=Sylvain |date=2025-11-01 |title=Synchrotron X-ray tomography sheds light on the phylogenetic affinities of the enigmatic thylacocephalans within Pancrustacea |url=https://royalsocietypublishing.org/doi/10.1098/rspb.2025.1612 |journal=Proceedings of the Royal Society B: Biological Sciences |language=en |volume=292}}</ref>

{{clade |label1=Mandibulata |1={{clade |1=†''Aquilonifer'' 100px |2={{clade |1=†''Tanazios'' |2={{clade |1={{clade |1=†''Acheronauta'' 100px |2=†''Captopodus'' }} |2={{clade |1={{clade |1=†''Nereocaris'' 100px |2={{clade |1=†Fuxianhuiida 100px |2=†Hymenocarina 100px }}}} |2={{clade |1={{clade |1=†Euthycarcinida 100px |2=Myriapoda (including centipedes and millipedes) 100px }} |label2=Pancrustacea |2={{clade |label1=Allotriocarida |1={{clade |1=Branchiopoda (tadpole shrimp/''Triops'', fairy shrimp, clam shrimp, water fleas) 100px |2={{clade |1=Hexapoda (including springtails and insects) 100px |label2=Xenocarida |2={{clade |1=Cephalocarida (horseshoe shrimp) 100px |2=Remipedia 100px }}}}}} |2={{clade |label1=Oligostraca |1={{clade |1=Branchiura (fish lice) 100px |2={{clade |1=Ostracoda 100px |2=Mystacocaridida 100px }}}} |label2=Multicrustacea |2={{clade |1=Copepoda 100px |2={{clade |1=Cirripedia (barnacles) 100px |2={{clade |1=†Cyclida (Americlidae) 100px |2={{clade |1=†Thylacocephala 100px |2={{clade |1=Malacostraca (shrimp, prawn, lobsters, mantis shrimp, crab, woodlice, etc) 100px }}}}}}}}}}}}}}}}}}}}}}}}}}

==Disagreements== Numerous conflicts of opinion surround the Thylacocephala, of which the split between the “Italian school” and rest of the world is the most notable. Based on poorly preserved ''Ostenocaris cypriformis'' fossils from the Osteno deposits of Lombardy, Pinna ''et al.'' erected the class Thylacocephala.<ref name="Pinna"/> Based on inferred cirripede affinities the authors concluded the frontal lobed structure was not an eye, but a 'cephalic sac'. This opinion arose from the misinterpretation of the stomach as a reproductive organ (its contents included vertebral elements of fish, thought to be ovarian eggs).<ref name="Vannier"/> Such an arrangement is reminiscent of cirripede crustaceans, leading the authors to suggest a sessile, filter feeding mode of life, the 'cephalic sac' used to anchor the organism to the seabed. The researchers have since conceded it is highly improbable the ovaries are situated in the head, but maintain that the frontal structure is not an eye. Instead they suggest the 'cephalic sac' is covered with microsclerites, their arguments most recently presented in Alessandrello ''et al.''<ref name="Alessandrello">{{cite book |author=A. Alessandrello, P. Arduini, P. Pinna & G. Teruzzi |year=1991 |chapter=New observations on the Thylacocephala (Arthropoda, Crustacea) |editor=A. M. Simonetta & S. Conway Morris |title=The Early Evolution of Metazoa and the Significance of Problematic Taxa |pages=245–251 |publisher=Cambridge University Press |location=Cambridge |isbn=0-521-40242-5}}</ref> *The structure is complex and "presumably multipurpose" *“Apart from a few features” it shows little affinity with a compound eye *There is a close connection with stomach residues, sac muscular system and outer hexagonal layer *Having a stomach between the eyes is unusual *Sclerites that should correspond to rhabdoms in 'eye theory' are interstitial to the hexagons, not at centre as would be expected for individual ommatidium. *Structural analogy with cirriped peduncle

Instead the authors suggest the sac is used to break down coarse chunks of food and reject indigestible portions.

All other parties interpret this as a large compound eye, the hexagons being preserved ommatidia (all researchers agree these are the same structure).<ref name="Rolfe"/><ref name="Secretan1985"/> This is supported by fossils of ''Dollocaris ingens'' which are so well preserved that individual retinula cells can be discerned. The preservation is so exceptional that studies have shown the species' numerous small ommatidia, distributed over the large eyes, could reduce the angle between ommatidia, thus improve their ability to detect small objects.<ref name="Frohlich">{{cite journal |author=F. Fröhlich, A. Mayrat, B. Riou & S. Secrétan |year=1992 |title=Structures rétiniennes phosphatisées dans l'oeil géant de ''Dollocaris'', un crustacé fossile |journal=Annales de Paléontologie |volume=78 |pages=193–204}}</ref> Of the arguments above, it is posited by opponents that eyes are complex structures, and those in the Thylacocephala display clear and numerous affinities with compound eyes in other arthropod fossils, down to a cellular level of detail. The 'cephalic sac' structure itself is poorly preserved in Osteno specimens, a possible reason for interstitial 'sclerites'. The structural analogy with a cirripede peduncle lost supporting evidence when the 'ovaries' were shown to be alimentary residues,<ref name="Vannier"/> and the sac muscular system could be used to support the eyes. The unusual position of the stomach is thus the strongest inconsistency, but the Thylacocephala are defined by their unusual features, so this is not inconceivable. Further, Rolfe suggests the eyes' position can be explained if they have a large posterior area of attachment,<ref name="Rolfe"/> while Schram suggests that the stomach region extending into the cephalic sac could result from an inflated foregut or anteriorly directed caecum.<ref name="Schram1990"/>

Discussion of the matter has ceased in the last decade, and most researchers accept the anterior structure is an eye. Confusion is most likely the result of differing preservation in Osteno.

==Mode of life== [[File:Reconstruction of Concavicaris submarinus.jpg|thumb|Reconstruction of ''Concavicaris submarinus'' preying on a conodont]] [[File:Ainiktozoon loganese.jpg|thumb|The strange Silurian thylacocephalan ''Ainiktozoon loganense.''|328x328px]] Numerous modes of life have been suggested for the Thylacocephala.

Secrétan suggested ''Dollocaris ingens'' was too large to swim,<ref name="Secretan1985"/> so inferred a predatory 'lurking' mode of life, lying in wait on the sea bed and then springing out to capture prey. The author also suggested it could be necrophagous, supported by Alessandrello ''et al.'',<ref name="Alessandrello"/> who suggest they would have been incapable of directly killing the shark remains found in the Osteno specimens' alimentary residues. Instead they surmise the Thylacocephala could have ingested shark vomit which included such remains.

Vannier ''et al.'' note the Thylacocephala possess features which would suggest adaptations for swimming in dim-light environments – a thin, non-mineralized carapace, well-developed rostral spines for possible buoyancy control in some species, a battery of pleopods for swimming, and large prominent eyes.<ref name="Vannier"/> This is supported by the Cretaceous species from Lebanon, which show adaptations for swimming,<ref name="Schram1999"/> and possibly schooling.

Rolfe provides many possibilities, but concludes a realistic mode of life is mesopelagic, by analogy with hyperiid amphipods.<ref name="Rolfe"/> Further suggests floor-dwelling is also possible, and that the organism could rise to catch prey during the day and return to the sea floor at night. Another notable proposal is that, like hyperiids, the class could gain oil from their food source for buoyancy, an idea supported by their diet (known from stomach residues containing shark and coleoid remains, and other Thylacocephala).

Alessandrello ''et al.'' suggest a head-down, semi-sessile life on a soft bottom,<ref name="Alessandrello"/> in agreement with that of Pinna ''et al.'', based on cirripede affinities. A necrophagous diet is suggested.<ref name="Pinna"/>

Briggs & Rolfe report that all the Gogo formation Thylacocephala are found in a reef formation, suggesting a shallow water environment.<ref name="BriggsRolfe"/> The authors speculate that due to the terracing of the carapace an infaunal mode of life is possible, or the ridges could provide more friction for hiding in crevices of rock.

Schram suggests a dichotomy in size of the class results from different environments;<ref name="Schram1990"/> larger Thylacocephala could have lived in a fluid characterized by turbulent flow, and relied on single power stroke of trunk limbs to position themselves. He suggests that smaller forms may have resided in a viscous medium, characterized by laminar flow, and used a lever to generate the speed necessary to capture prey.

Fossil evidence from the Holy Cross Mountains of Poland shows that thylacocephalans were regularly preyed on by predatory jawed fish during the Late Devonian epoch.<ref>{{Cite journal |last1=Broda |first1=Krzysztof |last2=Wolny |first2=Mateusz |last3=Zatoń |first3=Michał |date=October 2015 |title=Palaeobiological significance of damaged and fragmented thylacocephalan carapaces from the Upper Devonian of Poland |url=https://www.sciencedirect.com/science/article/pii/S0016787815000644 |journal=Proceedings of the Geologists' Association |language=en |volume=126 |issue=4–5 |pages=589–598 |doi=10.1016/j.pgeola.2015.05.005 |bibcode=2015PrGA..126..589B |access-date=20 August 2025 |via=Elsevier Science Direct|url-access=subscription }}</ref>

==References== {{reflist|25em}}

Category:Thylacocephala Category:Prehistoric crustaceans Category:Arthropod classes Category:Prehistoric protostome classes Category:Cambrian first appearances Category:Late Cretaceous extinctions