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Progress and future directions in archosaur phylogenetics

Journal of Paleontology, Nov 2001 by Brochu, Christopher A

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Archosauriformes is part of a much broader clade named Archosauromorpha, which is defined on the basis of crocodylians, birds, and all taxa more closely related to them than to lepidosaurs. This group includes a bizarre assemblage of predominantly Triassic forms, such as trilophosaurids, rhynchosaurs, and protorosaurians (Gauthier et al., 1988; Renesto, 1994; de Braga and Rieppel, 1997; Merck, 1997; Dilkes, 1998; Peters, 2000). The non-archosauriform relationships in Figure 1 are derived primarily from Merck (1997).

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The placement of champsosaurs (Choristodera) within Archosauromorpha was controversial at first, because the group was until recently known only from the Late Cretaceous through Tertiary (Evans and Hecht, 1993). A rootward position within Archosauromorpha implies an unsampled history minimally into the Triassic, where choristodere sister taxa appear. Putative choristoderes were subsequently identified from Jurassic and Late Triassic deposits (Evans, 1989, 1991; Storrs and Gower, 1993).

Sister groups to Archosauria are first known from relatively complete material in the Early Triassic, although one probable proterosuchid is known from the Late Permian of Russia (Tatarinov, 1960; Gower and Sennikov, 2000). With the exception of the Permian occurrence, the proterosuchids and erythrosuchids are restricted to the Early and Middle Triassic (Gower and Sennikov, 1997, 2000). Proterochampsids are found in Late Triassic (Camian) units of South America (Reig, 1959; Sill, 1967; Romer, 1972). These were quadrupedal carnivores with a powerful bite, as evidenced by the socketed, serrated teeth and enlarged attachment space for the jaw adductors and trigeminal musculature. They also bear an antorbital fenestra, the function of which remains unclear (Witmer, 1997). The largest non-archosaur archosauriforms had half-meterlong skulls. Detailed anatomical and taxonomic reviews of these taxa can be found in Walker (1964), Parrish (1992), Gower and Sennikov (1997, 2000), Gower and Weber (1998), and Welman (1998).

The oldest-known crown-group archosaurs are Anisian "rauisuchians." The interrelationships and monophyly of Rauisuchia remain controversial (Gower, 2000), but there is general agreement that "rauisuchians," broadly conceived, are more closely related to crocodylians than to birds (Benton and Clark, 1988; Sereno and Arcucci, 1990; Sereno, 1991a; Parrish, 1993; Juul, 1994; Gower and Wilkinson, 1996). Other crown-group archosaurian forms are first known from fossils in the Ladinian (Benton, 1995).

Most analyses agree that Parasuchia (phytosaurs), Aetosauria, and the various "rauisuchian" lineages are close relatives of Crocodylomorpha (Gauthier, 1986; Benton and Clark, 1989; Sereno and Arcucci, 1990; Sereno, 1991a; Parrish, 1993; Juul, 1994; Gower and Wilkinson, 1996; Fig. 2). All of these animals (pseudosuchians) have a so-called "crocodile-normal" ankle, with a rotary ankle joint in which the proximal tarsals (astragalus and calcaneum) move against each other with a peg-and-socket articulation. In these taxa, the peg is on the astragalus and the socket on the calcaneum. Whether the "crocodile-reversed" archosaurs (ornithosuchids, where the peg is on the calcaneum and socket on the astragalus) are related to crocodylians (Sereno, 199 la; Parrish, 1993) or closer to dinosauromorphs with plesiomorphic mesotarsal articulations (Gauthier, 1986; Juul, 1994) is a matter of controversy, but the difference is one or two nodes. Cruickshank (1979), Sereno (1991a), Parrish (1986, 1993), Gower (1996), and Dyke (1998) provide valuable reviews of archosaur ankle morphology and phylogenetic relevance.

Progress and future directions in archosaur phylogenetics

Journal of Paleontology, Nov 2001 by Brochu, Christopher A

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