Jessica Theodor and I just recently had a paper published in Nature magazine. Here’s the press release:

Summary of “Hippopotamus and whale phylogeny” by Jonathan H. Geisler and Jessica M. Theodor. March 19, 2009 issue of Nature, Volume 458, Number 7236. Published online. doi:10.1038/nature07776

Geisler and Theodor (2009) questions some of the conclusions of J. G. M. Thewissen and colleagues published in the December 27, 2007 issue of the journal Nature. In that paper, Thewissen and colleagues developed an evolutionary tree for Cetacea (the group that includes whales and dolphins) and artiodactyls (the group of even-hoofed mammals that includes deer, cow, and hippos). They found cetaceans to be a primitive branch of the artiodactyl group, and not closely related to the hippopotamus, a result that conflicts with previously published data that supports a close relationship between hippos and cetaceans.

Geisler and Theodor reanalyzed the data collected by Thewissen and his colleagues and included previously published anatomical and DNA data from living species. Contrary to Thewissen and colleagues, they found the hippopotamus to be the closest living group to cetaceans. Among extinct taxa, both studies found the hoofed mammal Indohyus to be closest to cetaceans. Indohyus is found in northern Pakistan, the same region where the oldest cetaceans are found, suggesting that 55 to 50 million years ago the ancestors of whales first entered rivers and lakes in that region. In a 2005 paper, Jonathan Geisler and Mark Uhen (Alabama Museum of Natural History) suggested that Indohyus was close to whales, a result confirmed by Thewissen and his colleagues in 2007. Combining information from living hippos and the extinct Indohyus, new hypotheses for how the ancestor of whales invaded aquatic environments can be developed. The ancestor of whales went through a hippo-like stage where they were amphibious, walked on river bottoms instead of swam, and ventured onto land to forage. Then they went through an Indohyus-like stage where they spent nearly all of their time in the water, possibly feeding on aquatic vegetation. Later, their teeth evolved for fish eating and the earliest whales lost their ability to move on land. This scenario can be tested and further refined by studies that incorporate data from living and extinct species.

Robert Boessenecker excavating a new skull of the baleen whale Herpetocetus from the late Miocene Purisima Formation of Central California, July 2007.