BIOGEOGRAPHIC DISTRIBUTION OF ARCHAEOCETES IN NORTH AMERICA

Archaeocetes are known from the latest early Eocene to late Oligocene. The earliest archaeocetes are restricted to Indo-Pakistan, but by the late middle Eocene, they are known from Egypt, west Africa, and the southeastern United States. The earliest cetaceans in North America are semi-aquatic animals in the family Protocetidae that had well-developed hind limbs. All archaeocetes are thought to be coastal animals that prefer shallow, warm water environments. Despite this, protocetids may have crossed the Atlantic directly rather than following the northern coastal route based on paleocurrent reconstructions and the dispersal of other organisms.

Basilosaurids are fully-aquatic archaeocetes with greatly reduced hind limbs and significantly more vertebrae than protocetids, and are thought to be ancestral to modern whales. In North America, basilosaurids are known from the Gulf Coast and southeastern Atlantic Coastal Plain sediments of middle to late Eocene age. Interestingly, the most advanced protocetids (Georgiacetus, Eocetus) are both known from North America, suggesting perhaps that basilosaurids evolved in North America, and then dispersed back to the Old World and around the world from here.

North American basilosaurids are known from Arkansas, Louisiana, Mississippi, Alabama, Georgia, Florida, South Carolina, and North Carolina. While Basilosaurus and Cynthiacetus are known across this region, Zygorhiza and Dorudon appear to have disjunct, non-overlapping distributions with Zygorhiza restricted to the Gulf Coast, and Dorudon restricted to the southeastern Atlantic Coast. This distribution correlates with a significant change in lithology from the Gulf Coast to the Atlantic Coast (as well as with onshore to offshore), located in central Georgia. On the Gulf Coast side, there is considerably more clay in the deposits where dorudontines are collected. On the Atlantic Coast side, there is much less clay, and dorudontines are usually found in carbonates or phosphate-rich deposits. This disjunct distribution may represent a habitat preference by these closely related whales or it may represent separate dispersal events to North America from other parts of the world. It is interesting to note that Zygorhiza has only been reliably identified outside of North America in New Zealand, and Dorudon in Africa.