In 1887, the English paleontologist Harry Seeley first recognized a fundamental division among dinosaurs. Ornithischia (ornis - bird; ischia - hip) were all those dinosaurs that had a bird-like pelvis, in which at least a part of the pubis runs posteriorly, along the lower rim of the ischium (Figure 4.19). Saurischia (sauros - lizard) were those that had a pelvis more like a lizard, in which the pubis is directed anteriorly, and slightly downward (Figure 4.20). This pelvic distinction has held sway ever since.
That dinosaurs had one or the other kind of pelvis was of great importance to understanding the evolution of these animals, and, in Seeley's hands, it went considerably further. For it implied to him that the ancestry of Ornithischia and Saurischia was to be found separately and more deeply embedded in a heterogeneous group of primitive archosaurs once called "Thecodontia".6 To Seeley, therefore, Dinosauria was not monophyletic.
All that changed in 1986, when cladistic analysis, in the skilled hands of J. A. Gauthier, provided powerful evidence for a monophyletic Dinosauria. And, since Gauthier's studies, numerous other analyses by other paleontologists, using both newly discovered and familiar taxa, have confirmed that dinosaurs are monophyletic.
6. The group "Thecodontia," is based on the same characters that diagnose all archosaurs. If we're discussing archosaurs, we can't cherry pick a few basal ones; we need to include all members of the group (including dinosaurs and pterosaurs) that bear the diagnostic characters. In short, the term "Thecodontia," though venerable, has not withstood cladistic scrutiny (see Chapters 10 and 14).
Is Saurischia more primitive than Ornithischia?
The distinction between Ornithischia and Saurischia is valid and, as we'll see later in this book, both groups are diagnosed by suites of well-established characters. Indeed, the split between ornithischians and saurischians is the fundamental division within Dinosauria. But which is more primitive? Saurischians, with their claws and teeth, appear to be a lot like their archosaurian forebearers - claws, teeth, etc. (see Figure 4.18) - and in many books, particularly the older ones, they are treated first, reflecting the intuitive notion that they are more primitive, that somehow ornithischians must have evolved from a saurischian ancestor. But is this true?
The cladistic answer to this question is, in so far as we yet know, clearly "No": Figure 4.14 shows that Saurischia is no less derived than Ornithischia. Thus we really don't know which came first. To underscore this important evolutionary point, we'll begin by studying Ornithischia first.
In this chapter, then, we've used clado grams to locate dinosaurs within the vertebrate world. Cladograms have brought us to an understanding rather different from that generally held concerning the place of dinosaurs and the significance of many familiar groups of tetrapods (see Box 4.2). Now, understanding who dinosaurs are - and how they evolved -we'll begin to look at them a bit more closely, studying not just their relationships, but also what is known of their behavior and habitats.
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