The Walking Whales by Thewissen J. G. M. "Hans"; Thewissen J G M;

Author:Thewissen, J. G. M. "Hans"; Thewissen, J G M;
Language: eng
Format: epub
Publisher: University of California Press

HOW MANY BONES MAKE A SKELETON?

The critical question that we hoped to answer with fossils from H-GSP locality 62 was this: What are whales related to? More pakicetid fossils would answer that question, and this skull is an important part of the answer. For more than two decades, there was no controversy among paleontologists: whales are related to a long-extinct group called mesonychians (Mesonychia in Latin). It was an idea proposed by the brilliant and eccentric paleontologist Leigh Van Valen.1 He observed that mesonychian teeth looked just like those of early whales. In both, a lower molar has a high trigonid with a single cusp and a low talonid with one cusp, very unusual for a mammal (figure 34). A lot of fossils are known for mesonychians—dentitions, skulls, and skeletons from North America, Europe, and Asia—and they lived at the right time to have given rise to whales.2 Their dentition suggests a meat diet, and their body is vaguely wolf-like, but they are hoofed mammals: five toes per foot, with a tiny hoof at the end of each. However, the paleontological romance with the mesonychian-whale hypothesis encountered trouble from molecular biologists who found that, in terms of proteins and DNA, modern whales are very similar to modern artiodactyls. So similar, actually, that it appears that cetaceans should be included in even-toed ungulates: their closest relatives are hippos, and hippos are more closely related to cetaceans than to any other artiodactyl. That is called a sister-group relationship. Of course, mesonychians are extinct, and their proteins and DNA cannot be studied, and that leaves the possibility open that cetaceans and mesonychians were sister groups but that the two of them combined form a group that is the sister group of hippos. However, that did not sit well with the paleontologists either: the double-trochleated astragalus occurs in all artiodactyls, including hippos, but not in mesonychians, which seemed to exclude mesonychians from the artiodactyl group. In cetaceans, it is impossible to tell what the astragalus looked like since all modern and nearly all fossil cetaceans have lost their hind limbs. In basilosaurids, the ankle bones are fused into an unrecognizable lump, and in remingtonocetids, no anklebones are known. Ambulocetus was disappointing in this regard too: we found half of an astragalus, but not the part that would have solved the problem.

This is why a pakicetid skeleton is needed. It would provide a skeleton of a cetacean sufficiently primitive to allow us to make direct comparisons to artiodactyls and mesonychians. Ankles would be of particular importance to solve the artiodactyl-mesonychian riddle.

So, back in the United States, that is what we are going for. Ellen is extracting the bones from the blocks from locality 62 in the hope of finding enough of them to build a skeleton of a pakicetid. The trouble with the locality is that there are no single skeletons: the bones of lots of individuals and species are jumbled together here. Ellen has prepared drawers full of locality 62 bones, and there are

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