Origins

How to find the ancestors of dinosaurs? As we've seen (in Chapters 1 and 3), we don't! Yet, the hierarchy of characters in the cladogram specifies for us what features ought to be present in an ancestor. It is then simply a question of finding an organism that most closely matches the expected combinations of characters and character states. As we have seen, the likelihood that the actual progenitor of a lineage will be fossilized is nil; however, we can commonly find representatives of closely related lineages that embody most of the features of the hypothetical ancestor. Using that criterion, though, there are two current hypotheses about dinosaur ancestry (Figure 4.17).

Two hypotheses of dinosaur origins

Hypothesis no. 1. According to American dinosaur specialists J. A. Gauthier and K. Padian, pterosaurs - otherwise highly modified for flight (see Figure 4.13) - may be the closest archosaurian relatives to dinosaurs, together sharing derived features as ornithodirans (Figure 4.17a). The clade of "pterosaurs +

Figure 4.16. The fully erect posture in dinosaurs. Unlike in, for example, a human, the bones of the leg restrict movement to only one plane: forward and backward.

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