Progress and future directions in archosaur phylogenetics

Journal of Paleontology, Nov 2001 by Brochu, Christopher A

The number of times this type of snout evolved within the crown group depends on how one resolves the placement of Gavialis (see below), but it minimally arose five times independently. Similar snouts are found in various non-crocodylian crocodyliforms, such as the dyrosaurids, pholidosaurs, and thalattosuchians. Thalattosuchians and pholidosaurs are restricted to the Jurassic and Cretaceous, but dyrosaurids survived until the Eocene. These have historically been viewed as representing independent derivations of slender-snouted morphology separate from those within the crown group (Kahn, 1955; Buffetaut, 1982).

The earliest cladistic analyses of Crocodyliformes seemed to confirm this view-thalattosuchians were one of the basalmost members of Mesoeucrocodylia, and dyrosaurids were more closely related to Crocodylia (Benton and Clark, 1988; Norell and Clark, 1990). But more recent studies found support for a clade including the three non-eusuchian slender snouted lineages (Clark, 1994; Wu et al., 1997). This was regarded as very surprising, and the first analysis to suggest this result-that of Clark (1994)--expressed at least some suspicion that these groups were being drawn together by a suite of nonindependent characters linked to elongation of the snout. But when characters presumed to be correlated with longirostry were eliminated from the analysis, the same result was obtained (Clark, 1994). In effect, the pholidosaurids and dyrosaurids are drawing thalattosuchians crownward; when pholidosaurids and dyrosaurids are excluded from the matrix, thalattosuchians return to their expected position at the root of Mesoeucrocodylia (Buckley and Brochu, 1999).

In my view, two scenarios are involved. First, the analyses supporting this conclusion were conducted at a very high taxonomic level, including taxa from the Triassic through the Cenozoic. A great deal of anatomical detail could not be included in those studies. It is very possible that inclusion of more morphological information from throughout the skeleton will support independent derivations of slender snout morphology in crocodyliforms. Second, these data sets may be accurately reflecting phylogeny, and our previous suppositions of snout evolution in crocodyliforms may have been mistaken.

Because we cannot actually know the true phylogeny, deciding between these scenarios will be difficult. Resolution of this issue will require further examination of fossil crocodyliforms and inclusion of a larger number of morphological characters, especially from regions of the skeleton not directly involved in snout shape.

Gavialis.-Because two of the key players in this story are slender-snouted crocodylians (Gavialis and Tomistoma), one is tempted to regard this as part of the "longsnout problem" in crocodyliform systematics. In fact, it represents two distinct points of disagreement between morphological and molecular data (Fig. 5). One is topological-morphological data have historically regarded Gavialis as the sister taxon to all other living crocodylians (Kahn, 1955; Tarsitano et al., 1989; Norell, 1989; Brochu, 1997), but most molecular data sets support a sister group relationship between Gavialis and Tomistoma (Densmore, 1983; Densmore and Owen, 1989; Densmore and White, 1991; Gatesy and Amato, 1992; Hass et al., 1993; White and Densmore, 2001). The other is temporal--did Gavialis diverge from its closest living relatives (whatever they are) in the Mesozoic, as paleontologists have long argued (Kahn, 1955), or did it last share a common ancestor with another living crocodylian in the Late Tertiary (Densmore and Dessauer, 1984; Hass et al., 1993)?