Thylacocephala (from the Greek θύλακος or thylakos, meaning "pouch", and κεφαλή or cephalon meaning "head") is an extinct group of mandibulate arthropods, that are generally regarded as a kind of crustacean, though their exact position within this group is uncertain. 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 (Sandbian) in age, around 460-450 million years old. Thylacocephala survived at least until the Santonian stage of the Upper Cretaceous, around 84 million years ago. 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, 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
Class: Thylacocephala Ainiktozoon Ankitokazocaris Eodollocaris Falcatacaris Ligulacaris Paraostenia Polzia Rotundaparma Rugocaris Silesicaris Stoppanicaris Thylacares Triangulusinus Victoriacaris Zazrivacaris Pseudoprotozoea Bohemiacaris Order Concavicarida Quadrangulucaris Family Austriocarididae Austriocaris Yangzicaris Family Clausocarididae Clausocaris Convexicaris Family Concavicarididae Concavicaris Harrycaris Paraconcavicaris Atropicaris Family: Microcarididae Ferrecaris Keelicaris Microcaris Thylacocephalus Family: Protozoeidae Globulocaris Hamaticaris Protozoea Pseuderichthus Order Conchyliocarida Family: Dollocarididae Dollocaris Mayrocaris Paradollocaris Thylacocaris Family: Ostenocarididae Kilianocaris Ostenocaris
Anatomy
Based on Vannier, modified after Schram: 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. 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
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