Bone-crunching serpent whales - Basilosaurids

A selection of the early cetaceans known as basilosaurids, to scale.

Palaeontology is unfortunately littered with mistakes. Edward Cope (he of the late 19th Century 'Bone Wars' fame) famously reconstructed the long-necked plesiosaur, Elasmosaurus, with the head on the short tail end. It was a regrettable decision when his rival, Othniel Marsh, was the one to point out his mistake. When giant metre long claws were discovered in Mongolia during 1948, the beast was thought to be some giant turtle-like reptile. It took until 1970 for Therizinosaurus to have it's dinosaur identity established. More recently earlier this year a sensational fossil was discovered encased in amber. Named Oculudentavis, it was described as a minute primitive bird, only for it to be quickly pointed out that it had features more in common with lepidosaur reptiles. Finally another example was when the first bones of one of the largest proto-whales was discovered in 1834, only for them to be interpreted as a sea-going reptile and thus given the name of King lizard. We know it as Basilosaurus, however the mistake is perpetuated by the name given to the entire family, Basilosauridae.

Prior to the first basilosaurids, early cetaceans were relatively small and still tied to the shore. Propulsion in water was mostly by hindfoot such as seen in phocid seals. By the mid Eocene basilosaurids had finally ditched returning to land for a fully marine existence. Although resembling modern whales, basilosaurids lacked the melon head characteristic of toothed whales. Whales also have very homogenous dentition, while basilosaurids retained the well defined range of teeth seen in other mammal groups used for chewing their prey. This consisted of mostly fish, squid and other marine mammals. Young Dorudon fossils have been found with bite marks consistant with predation from the larger Basilosaurus. Basilosaurids are characteristically long serpentine animals due to an elongation of the lumbar vertebrae. This facilitated a movement via tail propulsion similar to that seen in modern whales. Like the early proto-whales basilosaurids still possessed hind limbs but these were tiny and not used for locomotion. It is thought they may have been used claspers for mating.

The relationship between the basilosaurids and modern whales is confusing with some species possibly being ancestral to modern whales. The end of the Eocene was also the end of the last proto-whales.

* Note - updated image; wasn't happy with some of the images.