Evolution of Tertiary Gulf Coast Marine Vertebrate Faunas

Gulf Coast marine vertebrate faunas underwent a dramatic transition at the Cretaceous/Paleogene boundary with the loss of top tetrapod predators including mosasaurs and plesiosaurs. A new secondarily aquatic tetrapod fauna did not get established in the Gulf Coast until the late middle Eocene. This new fauna included semi-aquatic, predatory protocetid whales; as well as herbivorous semi-aquatic sirenians. It appears that these animals dispersed from the Old World tropics across the Atlantic Ocean to establish themselves on the Atlantic and Gulf Coasts of North America. There is also evidence from other organisms (palms, crustaceans, mollusks etc.) that these too followed a strong current originating in the area of the modern Mediterranean Sea, and flowing westward towards the southeastern margin of North America.

Although data is somewhat sparse, it appears that by the late Eocene at least, the Gulf Coast had a somewhat different marine tetrapod fauna when compared to the Atlantic Coast. This is probably due to differences in the depositional environments in each region, resulting in somewhat different organisms inhabiting each.

The record of post-Eocene marine tetrapods in the Gulf Coast becomes poor for various reasons. The Oligocene is well-represented in the stratigraphic record, but despite abundant fossils of other sorts, marine tetrapods are almost non-existent. Much of the Miocene and Pliocene is poorly represented in the stratigraphic record, except in Florida which has abundant rocks, as well as marine tetrapod fossils. It is also interesting to note that Miocene and Pliocene marine tetrapods are well-represented along the Atlantic Coast, but that Florida still has a fauna that is distinct from these other regions.

Future collecting efforts will hopefully clarify the reasons for missing marine tetrapods from the fossil record, as well as discover some fossils to close the gaps that currently remain.