Meet <i>Pakicetus</i>, the Terrestrial Mammal BioLogos Calls a "Whale" - Evolution News & Views

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Meet Pakicetus, the Terrestrial Mammal BioLogos Calls a "Whale"

In a previous post, we noted some fish-related problems with BioLogos's page discussing the fossil record. But these aren't the only marine mistakes on the page. BioLogos says regarding the evolution of whales:

Recently, a 52-million-year-old whale fossil, Pakicetus, was found in Pakistan. It was clearly a small, wolf-sized whale, but it did not have the characteristic fat-pad, a structure that allows the whale's jaw vibrations to be used for hearing. Also, its teeth were much like those of the terrestrial animals already thought to be related to whales.
Aside from the fact that Pakicetus was discovered in 1983 (not exactly "recently"), there's quite a bit more that should be said about this fossil. The claim that Pakicetus is a whale is a bit misleading, and depends on how you define "whale"; the claim requires a very non-whale-like definition of "whale".

Most of us think of whales as aquatic marine mammals, and in fact that's usually part of the definition of "whale". Pakicetus is often claimed to be an ancestor of whales based on its ear-bones and other skull-bones, but it was a terrestrial land mammal. And its lack of a fat-pad is by no means its only decidedly non-whale characteristic. Pakicetus discoverer J.G.M. Thewissen reviewed many aquatic features of true whales and notes that "Pakicetids display none of these features."

Moreover, Pakicetus had a multitude of other terrestrial features which somehow didn't receive any mention on BioLogos's page. After reviewing many features of Pakicetus which "are commonly interpreted as adaptations for running," Thewissen concluded in Nature:

Taken together, the features of the skull indicate that pakicetids were terrestrial, and the locomotor skeleton displays running adaptations. Some features of the sense organs of pakicetids are also found in aquatic mammals, but they do not necessarily imply life in water. Pakicetids were terrestrial mammals, no more amphibious than a tapir.

(J. G. M. Thewissen, E. M. Williams, L. J. Roe, & S. T. Hussain, "Skeletons of terrestrial cetaceans and the relationship ofwhales to artiodactyls," Nature, Vol. 413:277-281 (September 20, 2001).)

Evolutionary thinking may force-fit this terrestrial mammal into being labeled a "cetacean," but by any standard definition of the term, it was certainly no "whale."

Postscript: I have received some extremely positive feedback on these posts responding to BioLogos (see Part 1, Part 2, and Part 3). I want to make it clear, however, that nothing in these posts has been intended to suggest or imply that anybody has acted dishonestly. These posts are simply a response and correction to what I view as inaccuracies and deficiencies in the BioLogos fossil record page.