Whales, Hominins and Semicircular Canals: Part One

George Gaylord Simpson is probably responsible. I say that because in his influential The principles of classification and a classification of mammals he has this to say about whales:

Because of their perfected adaptation to a completely aquatic life, with all its attendant conditions of respiration, circulation, dentition, locomotion, etc., the cetaceans are on the whole the most peculiar and aberrant of mammals. Their place in the sequence of cohorts and orders is open to question and is indeed quite impossible to determine in any purely objective way.

Within five years or so biochemical techniques were being applied to the question of cetacean relationships. Results indicated that cetaceans were ultimately derived from artiodactyls. In the meantime, paleontologists were linking to condylarth mesonychids (based largely on the work of Van Valen). Mesonychids are:

…probably the most dedicated terrestial predators. They can be compared to dogs, hyaenas and bears in general appearance. Their main weapons for attacking prey were the large, pointed canines, a typical trait of carnivorous mammals. Unlike other predators, however, mesonychids did not dispose of sharp claws to tear their prey. Instead, their feet retained a telltale sign of their ungulate ancestors, which is quite unusual for a carnivore: Hoofs. The cheek teeth of mesonychids were modified for cutting meat, but the shearing mechanism was very different from – and obviously less effective than – the carnassial structure that carnivores, creodonts and carnivorous marsupials acquired. In fact, mesonychids may have used their teeth mainly for grasping chunks of meat, which were torn off with movements of the head while the feet anchored the prey.

This would put the artiodactyla as a more distant sister taxon of both cetacea and the Mesonychidae, something clearly at odds with the biochemical evidence. Then, in 2000, two studies were published that pretty much clinched the argument in favor of the artiodactlya. Gingerich, in Science, and Thewissen, in Nature, both reported finding primitive cetaceans with a double pulleyed astragalus – a trait only artiodactyls have. Additionally, Thewissen argued that his specimens displayed a number of other traits indicative of terrestrial cursoriality (although this is somewhat controversial). At this point research is focussed on identifying which artiodactyl the cetacea are most closely related too. The hippopotamus is the leading candidate.
What does all this have to do with hominins and semicircular canals? This is actually a multipart series and you will have to wait for the final post to find the answer to that question.


2 Responses

  1. The last ARO (Assoc for Research in Otolaryn.) there were a few talks on this topic, as the semicircular canals are part of the vestibular system of the inner ear. I think I know where you’re heading with this, but don’t wanna spoil your punchline. However, don’t forget about the cochlea in this story too. 🙂

  2. Did they mention Fred Spoor?

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