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Progress and future directions in archosaur phylogenetics

Journal of Paleontology, Nov 2001 by Brochu, Christopher A

ABSTRACT-The basic structure of archosaurian phylogeny is understood to include two primary crown-group lineages-one leading to living crocodiles and including a broad diversity of Triassic animals (e.g., phytosaurs, rauisuchians, aetosaurs), and another leading to dinosaurs (living and extinct). These lineages were established by the middle Triassic. A few extinct groups remain controversial, such as the pterosaurs, and debate persists over the phylogenetic relationships among extant bird lineages, which have proved difficult to resolve, and divergence timing estimates within Aves and Crocodylia remain the source of contention. A few analyses support a close relationship between archosaurs and turtles, or even a nesting of turtles within Archosauria. All sources of information used to resolve these issues have weaknesses, and these problems all involve highly derived lineages when they first appear in the fossil record.

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ARCHOSAURS, REPRESENTED today by birds and crocodylians, are only a minor fragment of the total scope of living and extinct biodiversity. They are dwarfed in diversity and abundance by the arthropods and flowering plants that truly own the planet, and even among vertebrates the bony fish are much more diverse (Wilson, 1988). But archosaurs adopt a disproportionately large role in lay considerations of biology and paleontology. We see birds every day and find them attractive. And nonavian dinosaurs bring instant recognition and capture the imagination.

Archosaurs have arguably played a pivotal role in the history of paleontology and phylogenetics. Early dinosaur discoveries created a sensation in the 19th century, and the fevered rush to collect giant dinosaurs for museums, led to the discovery of several important fossil localities around the world, many of which are still productive (Colbert, 1968; Desmond, 1982; Cadbury, 2000; McGowan, 2001). The scientific study of archosaurs has seen a resurgence over the past three or four decades, driven by new discoveries, by new analytical techniques, and by a more sophisticated scientific philosophy centered as much on important questions as on the intrinsic allure of the animals themselves. Because most people find dinosaurs (living and extinct) appealing, new discoveries and surrounding controversies often find their way to the broadcast media. Dinosaurs and their relatives can be seen as ambassadors of science to the general public (e.g., Padian, 1992).

Modern phylogenetic systematics was first brought to vertebrate paleontology by students of fossil fish, but its application to archosaurs showed the power of a phylogenetic approach to the broader community. Pioneering phylogenetic surveys in the 1980s laid out the general framework of archosaurian relationships and confirmed growing suspicions that birds are derived theropod dinosaurs (Gauthier, 1984, 1986; Benton, 1984; Gauthier and Padian, 1985; Benton and Clark, 1988; Sereno and Arcucci, 1990). When added to a broader data set for amniotes (Gauthier et al., 1988), these helped demonstrate the central role fossils could play in phylogeny reconstruction. Attention to phylogeny has characterized studies of archosaur historical biogeography (e.g., Wilson and Sereno, 1998; Sereno, 2000; Cracraft, 2001), functional evolution (e.g., Gatesy and Middleton, 1997; Carrano, 1998; Carrier and Farmer, 2000; Hutchinson, 2001a, 2001b), developmental biology (Larsson, 1998), and paleobiology (e.g., Varricchio et al., 1999; Homer et al., 1999; Erickson and Brochu, 2000).

The purpose of this paper is to review current understanding of archosaur phylogenetics and to indicate the questions that, in this author's view, present the next set of challenges archosaur systematists face. Archosaurs have a rich fossil record, but also include living members. Despite the sampling of diverse sources of data, from skeletal anatomy to nucleic acid sequences, and despite the expanded methodological toolbox available to the systematist, some portions of the archosaur tree resist resolution. Phylogenetic relationships among crown-group bird "orders" are perhaps the most significant of these. We also encounter interesting conflicts between fossil and molecular data sets regarding lineage divergence timing within both birds and crocodylians, and at least some sequence-based analyses argue that turtles belong within Archosauria. These challenges lie at the interface between paleontology and neontology.

ARCHOSAUR RELATIONSHIPS-WHERE WE AGREE (USUALLY)

Archosauria was one of the first groups for which phylogeny-- based taxon names were applied (Gauthier, 1984, 1986; Gauthier and Padian, 1985; Benton and Clark, 1988). In phylogenetic nomenclature, taxon name definitions are based on ancestry and descent rather than the possession of subjective "key" characters (de Quieroz and Gauthier, 1990, 1992; Cantino and de Quieroz, 2000). Phylogenetic name definitions will be used throughout this paper.

Archosauria, as defined by Gauthier and Padian (1985) and based on Gauthier's (1984, 1986) pioneering work, is defined in reference to the last common ancestor of birds and crocodylians and all of its descendants. Crown-group Archosauria excludes a few animals previously classified as archosaurs that possessed what were thought to be the hallmark features of the group-- socketed teeth and an antorbital fenestra. The name Archosauriformes was erected for Archosauria sensu lato-Archosauria, Proterosuchidae, Proterochampsidae, Euparkeria, and Erythrosuchidae (Fig. 1). The term "Thecodontia" refers to the paraphyletic assemblage of archosauriforms not belonging to Crocodyliformes, Dinosauria, or Pterosauria (Gauthier and Padian, 1985) and is not considered a taxon name. Some authors continue to use Archosauria in its prephylogenetic sense (e.g., Benton, 1999; Gower, 2000), and in this sense taxonomy is not one of the issues on which archosaur systematists agree. However, nomenclatural issues are neither as important nor as interesting as conflicts over the data itself.

Progress and future directions in archosaur phylogenetics

Journal of Paleontology, Nov 2001 by Brochu, Christopher A

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