Family level classification within Thylacocephala, with comments on their evolution and possible relationships

in Crustaceana
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Thylacocephala are among the most problematic of arthropod fossils. Various authors have allied them with all manner of crustacean groups, including branchiopods, cirripedes, remipedes, and malacostracans. They have a very apomorphic body plan often marked by hypertrophy of the compound eyes, three pairs of large raptorial subchelate limbs, eight sets of well-developed phyllobranch gills, and from 8 to at least 16 posterior trunk somites bearing paddle-like limbs. They have been thought of as a distinct class composed of two orders, Concavicarida and Conchyliocarida, but membership within the orders varies according to authors, and no familial divisions have been proposed within the orders until now. This lack of taxonomic structure inhibits organization of available information concerning the paleoecology, paleogeography, and phylogenetic relationships of thylacocephalans. A working hypothesis for the higher taxonomy within the class is proposed here. This entails a redefinition of the two orders, and recognition of seven families, five of them new: Austriocarididae Glaessner, 1931, Clausocarididae Arduini, 1992 (new status), Concavicarididae n. fam., Dollocarididae, n. fam., Microcarididae n. fam., Ostenocarididae n. fam. and Protozoeidae n. fam.

Family level classification within Thylacocephala, with comments on their evolution and possible relationships

in Crustaceana



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    The as-yet-undescribed thylacocephalan from the early Silurian of Wisconsin, U.S.A., see text for details (modified from Mikulic et al., 1985).

  • View in gallery

    Order Concavicarida. A, Austriocaris carinata (modified from Rolfe, 1969); B, Concavicaris milesi (modified from Briggs & Rolfe, 1983); C, Harrycaris whittingtoni (modified from Briggs & Rolfe, 1983); D, Clausocaris pinnai (modified from Arduini, 1992); E, Atropicaris rostrata (from Arduini & Brasca, 1984); F, Thylacocephalus cymolopos (modified from Schram et al., 2003); G, Protozoea hilgendorfi (modified from Schram et al., 1999).

  • View in gallery

    Order Conchyliocarida. A, Dollocaris ingens (from Secretan, 1985); B, Mayrocaris bucculata, left valve of carapace in outline only to reveal soft anatomy visible beneath the carapace shield (from Polz, 1994); C, Ostenocaris cypriformis (modified from Arduini & Pinna, 1989); D, Paraostenia voultensis (modified from Rolfe, 1985).

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    Major types of arrangements of the compound eyes in Thylacocephala. A, small and stalked; B, sessile and bilobed; C, sessile and unilobed (modified from Vannier et al., 2006).

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    A phylogram of Thylacocephala based on current morphological information analyzed manually employing the method of Hennig. Numbered clades represent: 1, order Conchyliocarida; 2, Dollocarididae; 3, Ostenocarididae; 4, order Concavicarida; 5, concavicarids + austriocarids; 6, Austriocarididae; 7, Concavicarididae; 8, Microcarididae; 9, protzoeids + clausocarids; 10, Calusocarididae; 11, Protozeidae. For discussion of the characters of the numbered clades, see the text.

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