Drivers and mechanisms of convergent forelimb reduction in non-avian theropod dinosaurs

Why were the arms of Tyrannosaurus rex, Carnotaurus, and other theropods so short as to be useless? This fine paper explains why.

Results indicate that forelimb reduction is strongly correlated with cranial robusticity and gigantism. Reduced/vestigial forelimbs evolved in at least five theropod lineages in concert with increased cranial robusticity and gigantism. Abelisaurids, carcharodontosaurids and tyrannosaurids show the greatest forelimb reduction relative to the skull. Repeated forelimb reduction across theropods was facilitated by increased cranial robusticity and greater body size that was potentially influenced by an upward trend in prey body size. These events resulted in a shift from subduing prey using grasping forelimbs to using powerful bites and robust skulls.

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