Thylacocephala

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. These were typically assigned to the phyllocarids despite an apparent lack of abdomen and appendages. 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, 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. The group took the name Thylacocephala by priority, with Concavicarida and Conchyliocarida subjugated to orders, erected by Rolfe, and modified by Schram. ==Taxonomy==

'' 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: • Concavicarida Briggs & Rolfe, 1983 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 Secrétan, 1983: • 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, 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). == Genera ==

'' |220x220px''Class: Thylacocephala • Ainiktozoon • Ankitokazocaris • Eodollocaris • Falcatacaris • Ligulacaris • Paraostenia • Polzia • Rugocaris • Silesicaris • Stoppanicaris • Thylacares • Victoriacaris • Zazrivacaris • Pseudoprotozoea • Bohemiacaris • Family: Microcarididae • Atropicaris • Ferrecaris • Keelicaris • Thylacocaris • Family: Ostenocarididae • Kilianocaris • Ostenocaris == Anatomy ==

'' Based on Vannier, Alternatively these could originate from three anterior trunk segments. The posterior trunk has a series of eight to twenty 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 ==

'', 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. 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. 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: • antennules, antennae and maxillipeds }}}} }} }}}}}} }}}} }}}}}}}}}}}}}}}}}}}}}}}}}} ==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. • 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). 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, 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, while Schram suggests that the stomach region extending into the cephalic sac could result from an inflated foregut or anteriorly directed caecum. 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==

.''|328x328px Numerous modes of life have been suggested for the Thylacocephala. Secrétan suggested Dollocaris ingens was too large to swim, ==References==