Dorudon atrox (Mammalia, Cetacea): Form, function, and phylogenetic relationships of an archaeocete from the late middle Eocene of Egypt.

Dorudon atrox is a medium-sized archaeocete that was in life about five meters long. It is known from the Gehannam and Birket Qarun Formations of Fayum, Egypt, which are latest Bartonian and earliest Priabonian in age (middle-late Eocene, ca. 41 to 40 million years before present). The skull of D. atrox shows that the bones are arranged in a normal mammalian pattern; it lacks cranial telescoping seen in later mysticetes or odontocetes. The vertebral column of D. atrox shows many characteristics shared with modern cetaceans including compressed cervical vertebrae, uniform size of posterior tharacics through anterior caudals, and darsoventrally flattened posterior caudals. Forelimbs of Dorudon atrox are modified into flippers. Movement at the elbow joint was restricted to the anteroposterior plane, with no pronation or supination possible. The carpals lack saddle-shaped articular surfaces and were capable of limited movement between adjacent bones. Hind limbs of D. atrox were present but very reduced. Much of the soft anatomy of Dorudon atrox has been reconstructed from muscle scars on bones, grooves and foramina, and cranial endocasts that show origins and insertions of muscles, paths of blood vessels and cranial nerves, and the shape of the brain. D. arrox had muscles more like primitive terrestrial mammals than modern cetaceans and D. atrox had a much smaller brain relative to its body size than modern cetaceans. The skeleton and inferred anatomy are used to reconstruct the life habits and physical abilities of Dorudon atrox. Limbs of D. atrox show that it was incapable of supporting its weight on land, and D. atrox is one of the earliest cetaceans to have had a fully aquatic existence. Lastly, this study includes a phylogenetic analysis of Dorudon atrox, other archaeocetes, and modern cetaceans. Subfamily Dorudontinae (including D. atrox), has long been suggested to include the ancestor of the modern cetaceans. The present phylogenetic analysis indicates that dorudontines gave rise to modern cetaceans, although it is unclear which of known dorudontines is most closely related to modern cetaceans.PhDEarth SciencesPaleontologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/129905/2/9635629.pd