Concavicarida

The Thylacocephala (from the Greek or ', meaning " pouch", and or ' meaning "head") are group of extinct probable mandibulate arthropods, that have been considered by some researchers as having possible crustacean affinities. As a class they have a short research history, having been erected in the early 1980s.

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.

Inconclusive claims of thylacocephalans have been reported from the lower lower Cambrian ('' Zhenghecaris''), but later study considered that genus as radiodont or arthropod with uncertain systematic position. The oldest unequivocal fossils are Upper Ordovician and Lower Silurian in age. As a group, the Thylacocephala survived to the Santonian stage of the Upper Cretaceous, around 84 million years ago.The implications of a Silurian and other thylacocephalan crustaceans for the functional morphology and systematic affinities of the group
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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. 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, Thylacocephala, while Secrétan – studying ''Dollocaris ingens'', a species from the La Voulte-sur-Rhône konservat-lagerstätte in France – erected the class Conchyliocarida. 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. 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.) The group took the name Thylacocephala by priority, with Concavicarida and Conchyliocarida subjugated to orders, erected by Rolfe, and modified by Schram.

Taxonomy

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