The evolution of endocranial space in mammals and non-mammalian cynodonts
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I examined cranial endocasts of extinct and extant mammals and non-mammalian cynodonts in order to study the evolution of endocranial space in Mammalia. A cranial endocast represents the three-dimensional space within the braincase (= endocranial space). During life, the endocranial space of mammals is largely filled by the brain but also contains associated nerves, blood vessels, and meninges. Therefore, cranial endocasts provide a proxy for studying the brains of extinct mammals. To study the evolution of endocranial space in mammals, I described new digital endocast data from four fossil mammals and a non-mammalian mammaliaform. The non-destructive nature of computed tomography allowed me to examine the cranial endocasts of these rare and unique specimens. I produced anatomical descriptions of the new cranial endocast material, and determined encephalization quotients for all of the taxa examined in this dissertation. Encephalization quotients (or EQs) are measures of the size of endocranial volume relative to body size for a particular taxon. EQs are determined from allometric comparisons of a number of closely related taxa. Based on new endocast data combined with data obtained from the literature, I scored 39 endocast characters by incorporating data from 23 taxa. I examined these characters across a hypothesis of phylogenetic relationships for Mammalia and nonmammalian cynodonts. Examination of the distribution of these 39 endocast characters across mammal phylogeny revealed synapomorphies for the following clades: Mammalia, Monotremata, Theria, Marsupialia, and Placentalia. The 39 endocast characters examined across mammal phylogeny were also examined in a sample of endocasts from a single mammalian species, Monodelphis domestica. The purpose of this analysis was to begin exploring the extent to which ontogenetic and individual variation of these 39 characters affects how the characters might be scored for a phylogenetic analysis. Results indicate that although there is detectable ontogenetic and individual variation among these characters when the entire M. domestica endocast sample is examined, if only qualitative characters are examined on adult specimens of Monodelphis, none of the characters are variable. Additional studies are required to determine the taxonomic extent of individual and ontogenetic variation of these characters.