The study of the origin of whales has undergone so much change during my own lifetime that it’s sometimes hard to keep up. When I was very young, Basilosaurus was the fossil whale representative, but being that it was already a whale it didn’t solve the problem of whale origins. Newer discoveries of older cetaceans in Asia like Pakicetus, Ambulocetus, Kutchicetus, and Rodhocetus provided a group of transitional types that stunned vertebrate paleontologists, but a major debate loomed over these fossils. While it first appeared that these”walking whales” were closely related to the carnivorous ungulates called mesonychids, genetic analysis, the presence of a “double-pulley” ankle bone, and the distribution of weight across the toes suggested that whales evolved from artiodactyl ancestors, their closest living relatives being hippos. There are still major problems, however, and the debate is as vigorous as ever. Do cetaceans belong within the Artiodactyla, supporting the construction of the Cetartiodactyla to contain both groups in one superorder? Or do cetaceans and artiodactyls (including hippos) belong to sister superorders, the Cetartiodactyla representing the grouping of the two together without nesting cetaceans within the artiodactyla? The fact that the fossil trail of hippos and cetaceans don’t mesh well has further muddied the waters of the issue, but in a new paper presented today in the journal Nature, a fossil creature called Indohyus is proposed as a representative of what the earliest whale ancestors might have been like.
As it stands now, the earliest known cetaceans are about 50 million years old, the best known (or at least most widely-cited) being Pakicetus, an animal that has undergone something of a makeover since it was discovered in 1983. Early representations based upon partial material made Pakicetus look like a chubby seal with a tail, but more complete finds revealed that Pakicetus was still more terrestrially-adapted despite its aquatic habits, illuminating a key point in the transition into the water. This presents a bit of a problem when we compare the earliest known cetaceans to the earliest known hippopotimids; the earliest known members of the hippo family are about 15 million years old with the earliest known Asian representatives being about 6 million years old. Genetic analysis may continue to support a close relationship but the fossil record in this case has yet to provide sufficient clues to show us how cetaceans are related to hippos, and Indohyus may be a representative of a better candidate for a sister group to cetaceans, the Raoellidae.
The primary character that, according to the new research, links Indohyus to cetaceans is the presence of an involucrum, a thickening of the tympanic bone in the ear that has been thought to be characteristic of cetaceans. Given this, the study argues, the Raoellidae should be grouped within the Cetacea or be the sister group to it, the position of sister group being preferred as grouping within the Cetacea seemed to cause instability in the phylogenetic trees. According to the cladogram presented, then, the Raoellidae and Cetacea would comprise sister groups that would together form a sister group to all other artiodactyl groups. (Indohyus could not be an ancestor to the Cetacea as it is slightly younger than Pakicetus and Raoellids temporally overlapped with the evolving cetaceans, an inferred common ancestor being absent at this point.) It’s unlikely that such a distinctive trait evolved more than once so Indohyus may very well then be part of a sister group to cetaceans (although other Raoellids need to be tested), but we are still left with the question of whether both groups should be nested within the existing Artiodactyla (to make a monophyletic Cetartiodactyla) or should remain outside, in which case it might not be possible to group the two superorders under the name Cetartiodactyla. This is the area that requires more resolution, and I think Indohyus alone isn’t sufficient to shake the evolutionary bush as much as might be supposed.
In order to bolster their case, however, the authors present some additional evidence that Indohyus had aquatic habits. Many aquatic animals, extinct and extant, have had their bones adapted to act as a sort of ballast, the condition being known as osteosclerosis. When the limb bones of Indohyus were studied it too exhibited this weight-increasing adaptation, which the researchers infer as meaning that it may have walked along the bottom of nearby watery habitats in much the same way that hippos do today.
Isotope data from the teeth were a little more difficult to interpret, however, as the values arrived at for Indohyus did not fall within groupings detected for terrestrial mammals or Archaeocetes like Pakicetus. Whatever it was eating remains ambiguous, although it could have been eating on land while spending considerable time in the water. The authors briefly ponder some aquatic food sources for this animal, but they note that further study of tooth wear and other factors will be needed to figure out what this animal was consuming. When contrasted to early cetaceans like Pakicetus, though, Indohyus looks more like a herbivore than a carnivore, which does have important implications for whale evolution. If Raoellids really are the sister group to Cetaceans, then perhaps the common ancestor was more of an omnivore than either a carnivore or an herbivore. Either that or the ancestor was a carnivore or herbivore and one group went in the opposite direction while another retained most of the ancestral habits, but until more fossils are found it seems difficult to tell how the two groups that both lived during the same time could have arrived at different body types and food preferences. (As a result of my recent work on primate carnivory for my term paper, as well, there are important physiological costs to switching from becoming an herbivore to a carnivore, as well as problems with disease and parasites. The switch can be made and it obviously has been in the past, but for now I wouldn’t say Indohyus can be taken as being the archetype of cetaceans earlier than Pakicetus.) The hypothesis presented in the paper is that Indohyus is representative of an cetacean ancestor, but while it might provide a model for some aspects of early cetacean evolution it does present a large number of problems as well. If Raoellids really are the sister group to cetaceans, we’ll have to go back further in the fossil record of Asia as cetaceans and Raoellids were clearly on different evolutionary paths during the time period being discussed.
Overall, the position of Raoellids as a sister group to cetaceans based upon Indohyus seems pretty flimsy, especially when presented in a few scant pages in Nature. Much more research is required to help resolve this issue, especially a greater number of well-preserved Indohyus specimens, hopefully one that is articulated. Indeed, the skeletal reconstruction is a composite and it appears that the comparison of the inner ear, essential to the argument being made in the paper, was based only on one specimen, so at the moment all I can really say about the hypothesis is “It’s an interesting idea.” The osteosclerosis in the bones of Indohyus suggests that it was spending a good amount of time in the water (and not just fleeing into the water as suggested in the paper and some popular reports), but the data from the teeth don’t seem to fit in with a wholly aquatic lifestyle, so what Indohyus was doing on the land and in the water remains inconclusive. If nothing else, this is an area that merits some more research and the hypothesis could be tested by going back to Asia to try and find some older fossils closer to when Raoellids and Cetaceans would have split from each other (especially on the cetacean line), but I guess we’ll just have to wait.
Media-coverage surrounding this new paper isn’t helping to resolve misunderstandings about evolution, however. In the AP piece reproduced via Yahoo!News, writer Seth Borenstein can’t seem to figure out just what Indohyus is. His confusion is apparent from the first line of the article;
It sounds like a stretch, but a new study suggests that the missing evolutionary link between whales and land animals is an odd raccoon-sized animal that looks like a long-tailed deer without antlers. Or an overgrown long-legged rat.
Borenstein scrabbles to lump Indohyus in with some modern animals in a feeble attempt to get people to understand the fossil find, but I can’t help but wonder if such a comparison does more harm than good. I’m not defending Indohyus as the archetype for whale ancestors, but the reporting style and comparison of unique extinct animals with modern ones (we’ve got raccoons, rats, deer, pigs, and dogs all in the same article) obscures rather than illuminates evolution. It’s this sort of description that makes it so easy for creationists to conjure up cartoons of cows with dolphin torsos in order to foster incredulity about evolution. Sure, Raoellid doesn’t really roll off the tongue, but I think we need to stop assuming that the general public just won’t understand what these animals were and take a paragraph or two to explain where they fit into the scheme of things rather than saying “It’s a raccoon-deer-rat-thing.” If we are to educate people about these discoveries we have to respect that they can understand what we’re talking about if we choose our words carefully and take the time to do so, although I wouldn’t expect some writers in the mass media to change tactics anytime soon.
[As Neil notes, though, Borenstein seems to have taken a cue from lead author Thewissen in using somewhat awkward animal analogies, and at least the errors aren't as egregious as that whole Leaky-gate thing from a few months ago. Given some of the comments I've seen about Indohyus in comment threads and in message boards, though, some people thing that scientists are saying that whales evolved from deer or rats, so my aggravation remains justified.]
Thewissen et al. (2007) “Whales originated from aquatic artiodactyls in the Eocene epoch of India,” Nature, Vol. 450, pp. 1190-1195