General Information
During the Eocene, there lived Dorudon, which are the fossil ancestors of modern whales. Dorudon were similar to whales in appearance, but their anatomical structure showed a sharper, more unsoftened skeleton shape. Their primary diet consisted of small fish and mollusks, and Dorudon lived only in warm marine waters. In particular, Dorudon remains have been found near Egyptian territories, as well as in North America. Although Dorudon was a predator, it could also prey on Basilosaurus, which were more dangerous and larger fossil forms of cetaceans. The lack of echolocation made Dorudon non-social animals, which means they mostly lived alone or in small flocks. Dorudon is considered an obligate aquatic animal, which means that going on land was excluded for it.
Skull structure
The critical difference between Dorudon and modern whales lies in the brain structure: the brain of fossil forms did not have an additional fat layer, which was necessary for effective echolocation. Thus, Dorudon’s communication and “sense of smell” system were much more primitive. The absence of a fat layer in the brain made their brains small, which also changed the structure of the skull: there was no rounded protrusion characteristic of modern whales. The eyeballs were directed to the sides, with the nostril openings shifted higher. Interestingly, Dorudon had some semblance of an external ear, which was lost during evolution in the descendants.
Structure of the postcranial skeleton
The peculiar shape of the skeleton structure indicates a more dynamic movement mechanism of Dorudon compared to modern whales. The small body size (up to 5 meters) and flexible tail section of the spine were reminiscent of modern dolphins. It is believed that Dorudon is the final formation of the bony tail, with the cetacean still retaining rudimentary hind limbs that served as hind fins. The forelimbs also survived and were modified into anterior fins. The cross-section of the spine underwent the most significant changes: the pelvic bones were no longer connected to it, and the vertebral structure was similar to any other vertebrae of the entire spine.