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Progress and future directions in archosaur phylogenetics
Journal of Paleontology, Nov 2001 by Brochu, Christopher A
The role of taxon sampling in this disparity is debated (van Tuinen et al., 2000; Johnson, 2001). But strikingly, most nonmitochondrial analyses suggest that Galloanserae is the basalmost lineage within Neognathae. Because of the close relationship between Paleognathae and Galloanserae in mitochondrial analyses, one is tempted to suspect some sort of rooting issue. Groth and Barrowclough (1999) found that application of a more slowly-- evolving gene supports a more conventional set of relationships, with Paleognathae and Galloanserae forming consecutive sister taxa to all other extant birds.
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Divergence timing of extant bird Glades.-At present, there are two general models for the diversification of avian lineages. One of these has been most explicitly developed by Feduccia (1995, 1996) and is based on an interpretation of the fossil record. Most Cretaceous neognath fossils are referrable to predominantly aquatic or shore-dwelling groups (e.g., Olson, 1985, 1992; Olson and Parris, 1987; Noriega and Tambussi, 1995; Feduccia, 1996; Kurochkin, 2000). Some authors argue that all such fossils are either charadriiforms or "transitionals" between charadriiforms and other bird groups (Feduccia, 1995). The vast majority of neognath lineages first appear in the Eocene or later. A literal reading of this pattern leads to the conclusion that the ancestral neognath was shorebird-like, and that Neognathae experienced an explosive radiation following the Cretaceous-Tertiary extinction event, which claimed the lives of the widespread enantiornithine birds.
The second model is based primarily on molecular analyses of neognath relationships. Nucleotide sequence data uniformly indicates divergences among extant neognath "orders" within rather than after the Cretaceous (Hedges et al., 1996; Cooper and Penny, 1997; Waddell et al., 1999; van Tuinen and Hedges, 2001). This implies a large number of ghost lineages crossing the Cretaceous-Tertiary boundary and is partly congruent with earlier vicariance models of avian historical biogeography (Cracraft, 1974). A similar conflict is encountered with the divergence of mammalian "orders"-most first appear in the Tertiary, but molecular data suggest divergences in the Mesozoic (Bromham et al., 1999). If the primary divergences predated the Cretaceous-Tertiary boundary, one could conclude that the explosive radiation posited by the first model is an artifact of an incomplete fossil record and not a real phylogenetic pattern.
This conflict has been couched in "fossils versus molecules" rhetoric, and this is not altogether inappropriate-first appearances for crown-group birds are younger than 100 m.y. But in some ways, arguments against the fossil-based model are not arguing against a phylogenetic hypothesis as much as an ecological scenario. This is fundamentally different from the phylogenetic pattern used to derive molecular estimates. In a way, this is an apples-and-oranges comparison.
The conclusion that neognath groups sprang from a pool of shoredwelling ancestors is based on a literal reading of the fossil record and is not expressed as a hierarchical pattern. Regardless of the neognath phylogeny one chooses to accept, the presence of anseriforms and charadriiforms in the Late Cretaceous implies the presence of many other lineages at the same time (Cracraft, 2001). Presbyornis, a ducklike bird from the Lower Tertiary and, possibly, the Late Cretaceous (Olson, 1985; Noriega and Tambussi, 1995; Hope, 1998; Stidham, 1998b), was thought to represent a transitional form between basal shorebirds and anseriforms (Feduccia and Olson, 1980; Feduccia, 1994). But phylogenetic analyses clearly place Presbyornis within Anseriformes and distant from charadriiforms (Livezey, 1997; Ericson, 1997; Stidham, 1998b). If anseriforms and galliforms are sister taxa, then discovery of a Cretaceous anseriform implies the presence of Galliformes in the Cretaceous, even if no such fossils have been found. Other interesting fossils, when considered in a phylogenetic framework, imply even more ghost lineages in the Cretaceous (Hope, 1998; Stidham, 1998a), though the identifications are not always accepted (Dyke and Mayr, 1999).
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