Choristodera, COPE, 1876

Gao, Ke-Qin & Fox, Richard C., 2005, A new choristodere (Reptilia: Diapsida) from the Lower Cretaceous of western Liaoning Province, China, and phylogenetic relationships of Monjurosuchidae, Zoological Journal of the Linnean Society 145 (3), pp. 427-444 : 439

Order Choristodera

For more than a century since Cope’s (1876) naming of the group, the order Choristodera has been known mostly from the Champsosauridae and Simoedosauridae . The contents of the Choristodera , however, have been significantly expanded recently by recognition of the membership of Cteniogenys and Monjurosuchus in this group ( Evans, 1989, 1990; Gao et al., 2000), and by the discovery of new morphotypes including the long-necked Hyphalosaurus from China ( Gao et al., 1999) and Shokawa from Japan ( Evans & Manabe, 1999). By current understanding, the order Choristodera includes five families: Cteniogenidae , Monjurosuchidae , Hyphalosauridae , Simoedosauridae and Champsosauridae . The relationships of these families are depicted in cladograms resulting from this study ( Fig. 6 ).

The taxonomically problematic taxon Lazarussuchus is excluded from the order Choristodera in this paper (see Gao & Fox, 1998, for discussion). Independent phylogenetic analyses consistently place Lazarussuchus outside the choristoderan clade ( Gao & Fox, 1998; Evans & Manabe, 1999; Ksepka et al., 2005; this paper; but see also Evans & Klembara, 2005, for different interpretations). Lazarussuchus shares with choristoderes several character states including: parietal foramen absent (4-1; also absent in many other aquatic reptiles); caudal zygapophyses lie nearly or completely vertical (58-1); and fibula narrow at proximal end and flared at distal end (70-1). None of these are individually diagnosable for the Choristodera , and possible homologies of these characters in Lazarussuchus with choristoderes need to be further investigated. According to Hecht’s (1992) description, Lazarussuchus displays several features that are not otherwise known in choristoderes: paired narial openings located far back from the tip of the snout; prefrontals entering narial opening; vertebral centrum amphicoelous and notochordal; and presence of four sacral vertebrae with fused ribs. In addition, uniquely diagnostic character states, such as presence/absence of a neomorph bone in the braincase and presence/ absence of a pterygoquadrate foramen, are unknown for Lazarussuchus . Until these anatomical ambiguities can be clarified, Lazarussuchus is best treated as a taxon outside the Choristodera .

With the exclusion of Lazarussuchus , the order Choristodera can be unambiguously diagnosed by the following derived character states: prefrontals having median sutural contact for their entire length (2-2); parietal foramen absent (4-1; possibly homoplastic in Lazarussuchus and in many other aquatic reptiles); choana retracted close to midpoint of marginal tooth row (20-1); pterygoid flange consisting of pterygoid and ectopterygoid with a horizontal overlap (23-1); basipterygoid/pterygoid joint a sutural contact (24-1); parasphenoid and pterygoid having a clear sutural contact (25-1); postorbital process of jugal much shorter than anteroventral process (28-1); interpterygoid vacuity enclosed anteriorly by pterygoids and posteriorly by parasphenoid (33-1; further modification in Champsosauridae ); neomorph in braincase present as part of external wall of braincase and medial wall of temporal fossa (40-1; unknown for Lazarussuchus ); free odontoid process unfused to axis (50-1); vertebral centra amphiplatyan with notochordal canal closed (52-1); sacral vertebrae three in number (56-1); sacral and caudal ribs remain free from vertebrae (57-1).

Figures

Figure 6. Comparison of the two equally parsimonious trees (TL = 123 steps; CI = 0.85; RI = 0.87) derived from analysis of data presented in Appendices 1 and 2.