| The four living species of tapirs are the remnants of a lineage whose evolutionary origin traces back at least fifty million years, from their divergence from the rhinoceroses. Although rare, fossil tapirs document many patterns of morphological change leading to the derived morphologies of extant forms. These patterns, and associated patterns of individual and ontogenetic variation are here documented for both Recent and fossil tapirs. In particular, the following issues are explored: the nature of morphological variation and variability in a new species of Eocene tapir, the phylogenetic relationships within Ceratomorpha, the determination of ontogenetic polymorphism in Recent Tapirus, and the use of computed tomography (CT) to document cryptic cranial morphologies in Tapirus.; The first chapter presents Hesperaletes borineyi gen. et sp. nov., from the middle Eocene of southern California. Hesperaletes has a deeply retracted narial incision, indicating prehensile proboscis development. This feature also characterizes the early Oligocene North American Protapirus, and has been used to diagnose Tapiridae. While late Uintan samples are referred to H. borineyi anomalous variability and morphological variation leads to referral of much smaller early Uintan samples to Hesperaletes, sp. indet. Analysis of tapiroid phylogeny suggests that Colodon is more closely related to Tapirus than is North American Protapirus. Cranial similarities resolve North American Protapirus and Hesperaletes as members of an unnamed clade, implying that Hesperaletes is a tapirid. Because of difficulties with character coding assumptions, however, this result is considered suspect.; The second chapter presents a new method that uses parsimony algorithms to determine ontogenetic sequences; formalizing reticulating sequence network maps, and accounting for sequence polymorphism. The method is used to analyze human and tapiroid postnatal skeletal ossification patterns, and illustrate both its validity, and its potential for evolutionary and phylogenetic analyses.; The final chapter develops computed tomographic (CT) data for T. terrestris, T. bairdii, and T. indicus. These data comprise series of 1 mm thick coronal slices, that were subsequently digitally resliced along orthogonal planes. The original and resliced CT images were labeled for anatomical structures, rendered in three dimensions, and animated. Descriptions of cranial cavities are provided. |
