Convergent evolution in large theropods

Looking within Tetanurae, we see a striking quality of theropod evolution. Superficially, big thero-pods all resemble each other (they were once all united as "carnosaurs"). Clearly, as theropods evolved to large sizes, lineages independently developed some of the same features. Such similar, although independent, evolution is called convergent. In the case of large theropods, features such as proportionally large heads and a tendency toward shorter arms occurred convergently. The same features occur independently in neocera-tosaurs (for example, Carnotaurus), in primitive tetanurans (Spinosaurus, Szechuanosaurus, Megalosaurus, and Afrovenator), and in avetheropods (Giganotosaurus, Allosaurus, and all the tyrannosauroids).

As we have seen, however, despite their superficially convergent morphology, these animals behaved very differently. Why, then, did they independently develop similar morphologies? The answer likely resides in the logistics of growing BIG. Recent work has suggested that to increase size yet maintain reasonable agility as bipedal theropods, compromises must be made in the sizes and power of various body parts. The superficially similar morphology of all large theropods is likely the result of those compromises, as large size was attained.

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Figure 9.29. Semi-lunate carpal (colored) in the left hand of Ingenia, an oviraptorosaurian maniraptoriform.

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Figure 9.29. Semi-lunate carpal (colored) in the left hand of Ingenia, an oviraptorosaurian maniraptoriform.

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