Content provided in partnership with

Progress and future directions in archosaur phylogenetics

Journal of Paleontology, Nov 2001 by Brochu, Christopher A

<< Page 1 Continued from page 10. Previous | Next

Just as with the more general crocodyliform longsnout problem, we are left to wonder if the snout-related characters diagnosing Gavialoidea and Tomistominae are independent. Not all support for these groups is concentrated in the rostrum, but much of it is (Brochu, 1997; Trueman, 1999). Furthermore, some fossil slender-- snouted crocodylians have yet to be incorporated into the analysis, and there is a rather long stratigraphic void in the eusuchian record between Hylaeochampsa in the Barremian and the earliest crown-group crocodylians in the Campanian (Brochu, 2001). Sampling for crocodylians is similarly poor (though not entirely barren) in the Paleocene and Oligocene, at least relative to the latest Cretaceous and Eocene. We cannot assume that the sample currently available will reflect the signal supported by a larger sample should new discoveries be made in the future.

Most Popular Articles in Reference: The importance of understanding organizational culture; Credit card attitudes and behaviors of college students; What factors attract foreign direct investment?; Libraries Need Relationship Marketing - mutual interest marketing concept, ...; How to set performance goals: employee reviews are more than annual critiques; More »

Crown-group bird relationships.-For extant birds, the Linnean "order" seems to have served the same functional purpose served by the "phylum" at the metazoan level-it expresses the boundary between (mostly) uncontested monophyly and unresolved relationships. No one questions the monophyly of Piciformes (woodpeckers and allies) or Apodiformes (swifts and hummingbirds), but determining how piciforms and apodiforms are related to other bird "orders" is another matter. This problem persists to this day.

A similar problem faces mammal systematists, and similar explanations have been given for both problems-most commonly, that birds and mammals underwent rapid bursts of diversification early in their histories, leaving few transitional fossils that would tie lineages together and leaving long branches connected by short internodes. Whether these bursts occurred before or after the Cretaceous-Tertiary boundary is a complex issue itself (see below), but is independent of our inability to recover a robust, universally-- accepted hypothesis of relationships within crown-group Aves.

There is broad (though not universal) agreement that the paleognaths-living ratites and tinamous-form a clade. This has long been a controversial idea; some authorities thought the characters shared in common by ratites were convergences related to flightlessness, and the palate morphology shared by ratites and tinamous was argued to be plesiomorphic (Olson, 1985; Feduccia, 1996). The Gondwanan distribution of these animals was also problematic if one assumed a Tertiary divergence, as the ranges of different extant ratite species are separated by marine barriers that would favor aerial dispersal, thus implying a flying ancestor and multiple derivations of flightlessness (Feduccia, 1996). Parsimony analyses of morphology (Cracraft, 1974, 2001; Lee et al., 1997) and multiple molecular data sets (Sibley and Ahlquist, 1990; Lee et al., 1997; Harlid et al., 1998; van Tuinen et al., 2000) support ratite and paleognath monophyly, although the interrelationships within the group remain unclear.

Whether Paleognathae is the sister group to a monophyletic Neognathae is less certain. Morphology and some molecular data sets support this conclusion (Cracraft, 1988; Groth and Barrowclough, 1999; van Tuinen et al., 2000), but others do not (e.g., Hdrlid et al., 1998; Harlid and Arnason, 1999; Mindell et al., 1997, 1999). These latter studies are based on mitochondrial genes and ally the paleognaths with a clade including galliforms and anseriforms (Galloanserae), with Passeriformes (songbirds) forming the basalmost avian lineage (Fig. 4).

Progress and future directions in archosaur phylogenetics

Journal of Paleontology, Nov 2001 by Brochu, Christopher A

Find Research Guides for:

Find Featured Titles for: Business

Most Popular Articles in Reference

Most Popular Publications in Reference