Basilosaurus: An Eocene Whale

Basilosaurus was a member of the cetacean family that now includes whales, dolphins, and porpoises. It lived during the Eocene Epoch in the Cenozoic Period and represents great strides that mammals made in the oceans after adapting to life in the water in the early Eocene.

Fossils of Basilosaurus were first discovered in a belt of Eocene mineral deposits in Louisiana, Mississippi, and Alabama. Fossils of this ancient whale were so common in this area of the United States that many residents used the bones in construction of cabins. Other basilosaurus fossils have been found around the world in Australia and Egypt.

"Basilosaurus" is actually a bit of a misnomer due to the fact that "saurus" means lizard and the animal in question was an endothermic mammal and not an ectothermic reptile. Although large reptiles lived in the oceans during the Mesozoic Era, they had all died off following the Cretaceous-Tertiary extinction event. It took about twenty million years for mammals to enter the water and begin to fill the marine niches left by these giant reptilian predators which included the plesiosaurs, nothosaurs, and mososaurs. Basilosaurus evolved from earlier amphibious cetaceans like the ambulocetus that could walk on land as well as swim in water. Whereas ambulocetus had legs instead of flippers, basilosaurus represents the point in cetacean evolution when whales left the land behind completely in favor of the bounty of the water.

Structurally, basilosaurus possessed a more streamlined, eel-like body as opposed to the bulkier, front-heavy bodies of modern baleen and toothed whales. Also, unlike modern whales, basilosaurus did not have a blow-hole at the top of the head for respiration and thus would need to raise its mouth out of the water to take a breath. It is unlikely that this large marine predator could have stayed underwater for very long, so it inhabited shallow seas instead of deep water. It was a very large cetacean, with lengths averaging 45 to 70 feet in preserved specimens. This would have made it on par with some of the largest baleen whales alive today, although basilosaurus was not a direct ancestor of the whales we now know in modern oceans. From the fossil record, it is clear that basilosaurus had terrestrial ancestry due to their presence of preserved canine teeth which are characteristic of land-based mammalian predators. Furthermore, basilosaurus had tiny remnants of hind legs as its back fins and its forelimbs contain similar, albeit altered bone structure, to terrestrial mammals.

It is clear from its size that basilosaurus was at the top of marine food chains during the Eocene Epoch. Although its head was small compared to modern whales, it sported forty teeth of various sizes in a mouth that was up to eight feet long. This would have made it a very effective predator of not only fish, but also other predators in the oceans at the times such as sharks. In order to maintain its enormous size, this whale ancestor could not have been a very picky eater, and was probably an opportunistic hunter. Any animal it could possibly fit in its mouth that it came into contact with was probably fair game.

Basilosaurus did not survive past the Eocene period, and it is likely that the change in global climate affected its success. Depending on large prey which in turn depended on smaller organisms would have made it vulnerable to disruptions at the bottom of the food chain. Indeed, plankton and other small creatures were dying off in large numbers due to increased glaciation at the poles and a global drop in climate.