Evolution of the brain in Theropoda (Dinosauria)

Franzosa, Jonathan William
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The description of natural and artificial endocasts has become routine since the work of Tilly Edinger. Most workers write little more than simple descriptions of isolated specimens because endocasts are rare, and scattered throughout the world. Harry Jerison took the next step and broadly compared endocasts in detail, using them for evolutionary studies of brain size and intelligence. In those studies, Jerison concluded that dinosaurs (excluding birds) had the brain size expected for "reptiles" of their size. However insightful, many equations and assumptions Jerison used are now questioned. The goals of this study are to continue the comparative work of Edinger and Jerison, and to employ more modern techniques and information than was available when Jerison's studies were conducted, to either verify or overturn his hypotheses. Most of this study was completed using the Department's High Resolution X-ray Computed Tomographic (CT) Scanner, which allowed digital endocasts to be made from intact braincases. This increased the number of taxa available for analyses to 18. These endocasts are described in detail, allowing 14 characters to be identified and scored. Several analyses are done, one using just the examined taxa and characters, and three in which the characters are added to existing data matrices. These analyses showed the 14 characters contain useful phylogenetic information, and one of the phylogenies had a polytomy resolved owing to the addition of the characters. The endocast characters also show similarities in brain architecture between pterosaurs and birds, and indicate that an avian-style brain evolved gradually through the theropod lineage. The character transitions that show the gradual evolution in the cerebral hemispheres, optic lobes, floccular lobes, and other brain features in the sequence of studied endocasts form the basis for testing inferences about theropod biology, behavior, and intelligence. Trends concerning theropod sensory and prey catching ability are tested and supported, indicating that theropods became more dependent on sight, and less dependent on smell, over time, and developed better hand-eye coordination, agility, and balance for capturing smaller prey, whereas using Encephalization Quotients for intelligence determination is disputed. This hypothesis testing is one of the strengths of digital endocasts.