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Progress and future directions in archosaur phylogenetics
Journal of Paleontology, Nov 2001 by Brochu, Christopher A
There is similar consensus on the close relationship between Scleromochlus, Marasuchus, Pseudolagosuchus, Lagerpeton, and Dinosauria (Gauthier, 1986; Sereno, 1991a; Sereno and Arcucci, 1994a, 1994b; Novas, 1996; Benton, 1999a; Fig. 3). Pterosaurs are usually regarded as members of this assemblage, but some recent reviews argue against their archosaurian affinities (see below).
The ancestral archosaur was a predator that could probably locomote on two or four legs. It also probably had several softtissue and behavioral features known in extant archosaurs, such as a four-chambered heart and at least some degree of nest building and parental care (Gauthier et al., 1988). The basalmost pseudosuchians remained quadrupedal and robustly built, but basal dinosauromorphs show a tendency to lengthen the hindlimb relative to the body and to have more gracile limb proportions.
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The only pseudosuchian lineage to survive the Triassic is Crocodylomorpha, which includes an assemblage of gracile "sphenosuchians" (the monophyly of which is debated, e.g., Benton and Clark, 1988; Walker, 1990; Sereno and Wild, 1992; Wu and Chatterjee, 1993; Clark et al., 2000) and crocodyliforms. Crocodyliformes includes those animals conventionally called "crocodilians" in the prephylogenetic literature, though the term "crocodylian" is now restricted to the crown group. Crocodyliformes was traditionally grouped into a series of three grades-Protosuchia, Mesosuchia, and Eusuchia-on the basis of palate and vertebral morphology (e.g., Langston, 1973; Buffetaut, 1982), but of these, only Eusuchia is demonstrably monophyletic (Benton and Clark, 1988; Brochu, 1997; Buscalioni et al., 2001). The earliest eusuchian is Hylaeochampsa vectiana from the Early Cretaceous of England (Clark and Norell, 1992); there are scattered putative eusuchian occurrences throughout the Cretaceous (Stromer, 1925, 1933; Efimov, 1982), and the first crown-group crocodylians appear in the Campanian (Erickson, 1972; Wu et al., 1996; Williamson, 1996).
Crocodylian phylogenetics in Figure 2 are summarized from Brochu (1997), Salisbury and Willis (1996), and Buscalioni et al. (2001) and reflect the morphological perspective on gavialoid relationships. For the most part, molecular and morphological data agree rather strongly on the details of crocodylian phylogeny-- Alligatoridae is monophyletic and Crocodylus, Osteolaemus, and Tomistoma are more closely related to each other than any are to Alligatoridae (Poe, 1996; Brochu and Densmore, 2001). There is even broad agreement on divergence timing estimates, with notable exceptions (see below).
Dinosaurian monophyly is supported by virtually all phylogenetic analyses of Archosauria. Herrerasaurids and Eoraptor may be outside Dinosauria or basal theropods (Gauthier, 1986; Brinkman and Sues, 1987; Novas, 1992, 1994; Padian and May, 1993; Sereno, 1993, 1999; Sereno and Novas, 1993; Holtz and Padian, 1995; Galton, 1999; Holtz, 2000), but the earliest dinosaurian fossils include primitive sauropodomorphs and ornithischians (Sereno, 1991b; Flynn et al., 1999). The primary branches of Dinosauria were thus distinct by the Carnian.
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