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Progress and future directions in archosaur phylogenetics
Journal of Paleontology, Nov 2001 by Brochu, Christopher A
The ancestral dinosaur was an enhancement of the morphology seen in nondinosaurian dinosauromorphs such as Marasuchus. It was probably a bipedal animal, probably predatory, and probably the size of a large dog. Early ornithischians and sauropodomorphs already show modifications for herbivory, but basal members of these Glades remained small and bipedal (Sereno, 1991b; Benton et al., 2000). The ancestral archosaur was very likely able to keep the belly off the ground by rotating the hindlimb into a vertical orientation during locomotion, a mode of walking seen frequently in living crocodylians (Gatesy, 1991). The ancestral dinosaur took this a step further by forcing its hindlimbs under the body with an interned femoral head, constraining hindlimb movement to the parasagittal plane. These modifications are thought to reflect a decoupling of breathing and walking and to permit breathing while running, something the ancestral diapsid could not do (Carrier, 1991; Carrier and Farmer, 2000).
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Various aspects of dinosaurian relationships might appear contentious to the casual observer, especially when they are discussed in public. In fact, the different hypotheses are highly congruent (Holtz, 1999). Figures 3 and 4 summarize much of the work over the past decade (Gauthier, 1986; Holtz, 1994, 2000; Sues, 1997; Forster et al., 1998; Makovicky and Sues, 1998; Sereno, 1999, 2001; Currie and Carpenter, 2000; Norell et al., 2001; Chiappe, 2001), with particular attention to the theropods. Additional work discussing relationships within the terminal taxa shown in Figure 3 can be found in Sereno (1986, 1991b), Weishampel et al. (1990), Forster (1990, 1997), Upchurch (1995), Currie and Padian (1997), Wilson and Sereno (1998), Chinnery and Weishampel (1998), Head (1998), and Makovicky (2001).
One of the more interesting resolutions in recent years is that therezinosauroids, or segnosaurs, are highly modified theropods. The segnosaurian skull preserves a confusing mixture of ornithischian, sauropodomorph, and theropod features, and it was difficult to figure out what these animals were (Perle, 1979, 1982; Gauthier, 1986). New material described in the 1990s (Russell and Dong, 1993; Clark et al., 1994; Xu et al., 1999) firmly establishes them as theropods. The only remaining controversy is whether therezinosauroids are closer to ornithomimosaurs (Sereno, 1999) or to oviraptorosaurs (Holtz, 2000; Norell et al., 2001).
That birds are derived theropod dinosaurs is no longer the subject of scholarly dispute. Every phylogenetic analysis conducted on archosaur relationships supports this conclusion (Gauthier, 1986; Benton and Clark, 1988; Holtz, 1994, 2000; Novas and Puerta, 1997; Padian and Chiappe, 1998; Forster et al., 1998; Chiappe et al., 1998; Chatterjee, 1999; Sereno, 1999; Norell et al., 2001), and arguments to the contrary are rhetorical more than evidential (Padian, 2001). The recent discovery of nonavian theropods with apparent feathers or featherlike structures in northeastern China (Chen et al., 1998; Xu et al., 1999, 2000, 2001; Ji et al., 1998, 2001) confirms this as tightly as the idea that humans are derived primates or that fruit flies are derived arthropods. The only real controversy over basal bird relationships involves the bizarre alvarezsaurids, which were first thought to be highly modified flightless birds (Perle et al., 1994; Novas, 1996; Chiappe et al., 1996, 1998), but which subsequent analyses suggest may be outside Avialae, though closely related to it (Holtz, 2000; Norell et al., 2001; Chiappe, 2001), or close relatives of ornithomimosaurs (Sereno, 1999, 2001).
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