The hind legs of whales

February 10, 2011 • 11:00 am

by Greg Mayer

Snakes are not the only tetrapods (or even lizards) to have lost their legs. Whales have lost their hind legs (the front ones are now their flippers), and we have a pretty good fossil record of how they did so, thanks in large part to the work of Phil Gingerich of the University of Michigan Museum of Paleontology (see his great whale evolution site here) and his collaborators.

Although whales lack external hind legs (except as rare teratologies), they do have internal rudiments of the hind limbs and pelvic girdle, as I was reminded during a recent visit to my and Jerry’s alma mater, the Museum of Comparative Zoology, where one of the Museum’s killer whale skeletons now hangs in the building across a new courtyard: note the remnant hind limb girdle at lower left.

Killer whale, Orcinus orca; note remnant hind limb girdle at lower left (the MCZ is the building in the background).

Here’s a closer view.

Killer whale hind limb girdle remnant.

In addition to the killer whale skeleton, there’s also a bottlenose whale skeleton; here’s its remnant hind limb girdle.

Bottlenose whale, Hyperoodon ampullatus, hind limb girdle.

Both whales were hung in this new building (which is where the MCZ parking lot was in my and Jerry’s days) at the initiative of the MCZ’s director, Jim Hanken. The figure below, from Phil Gingerich, shows some of the fossil whales through which limb loss has been traced.

Whale evolution.

Here’s his caption for the figure (go to his website for full citations to the papers mentioned):

Figure 1. Skeletons of the archaeocetes Dorudon atrox and Rodhocetus balochistanensis compared to that of Elomeryx armatus, which is here taken as a model for the extinct group of artiodactyls (Anthracotheriidae, s.l.) that we now think may have given rise to archaic whales. Pakicetus has a distinctive skull and lower jaw, but is not demonstrably different from early protocetids postcranially. Note changes in body proportions and elongation of feet for foot-powered swimming in Rodhocetus, then later reduction of the hind limbs and feet as the tail-powered swimming of modern cetaceans evolved in Dorudon.

A. Elomeryx drawing from W. B. Scott, first published in 1894. B. Pakicetus skull from Gingerich et al. (1983). Terrestrial interpretation is pure speculation: what little is known of the skeleton resembles Rodhocetus. C. Rodhocetus skeletal reconstruction from Gingerich et al. (2001). D. Dorudon skeletal reconstruction from Gingerich and Uhen (1996). Figure may be reproduced for non-profit educational use.

I showed photos of the hind limb remnants of Maiacetus, Basilosaurus, and Dorudon skeletons at the USNM in an earlier post.

(Thanks to Jon Losos for checking whale ID’s for me.)