A Whale of a Missing Link : Indohyus

Yet another missing link has been found! This new find links whales to quadrupedal land mammals.

Thewissen et al. report in Nature new fossil material from the Middle Eocene of Kashmir, India. This species (in the genus Indohyus is represented by a remarkable set of remains, including cranial and post cranial material. Previous studies using DNA had linked whales to the artiodactyls (even-toed ungulates such as deer, antelope, and bison). However, there is a great deal of uncertainty, and some contradictory evidence, as to where exactly in this group the whales arose.

Thewissen et al. are suggesting that a middle Eocene artiodactyl family known as Raoellidae are closely related to early whales. Until recently, the Raoellids were only known from dental material. This study, reports newly studied cranial and postcranial remains of the genus Indohyus dating to just under 50 million years ago, or just this side (more recent) of the likely split between whales and other artiodactyls. The fossils show morphological affinities with contemporary whales. In other words, Indohyus is a sister group to the whales.

Assuming this is true, it is possible to make more detailed claims regarding the origin and evolution of whales. Knowing something about the morphology and biogeography, and thus the ecology and adaptations of, a closely related form can be very suggestive of the nature of an evolutionary divergence.

One of the problems with whale evolution was that DNA and morphological considerations had linked whales to hippos. This is problematic because hippos evolved very recently (compared to whales) and on a different continent (Africa for hippos, Asia for whales). The present analysis knocks hippos out of the picture and links whales to a more suitable nearest (yet extinct) group which existed more closely in both space and time to the earliest whales.

The similarities that link whales to Indohyus include the presence of an "involucrum." This is a thickening of part of the auditory bulla (a roundish thing at the base of the skull that contains ear-parts). In addition, the incisors are arranged in the jaw in similar ways in the two groups, and there are similarities in the teeth.

This graph shows the relationshiop between whales (the open squares) and artiodactyls that are not whales (the triangles along the bottom). Indohyus is clearly not in the artiodactyl group. Nor is it in the whale group. Clearly the thickening of the tympanic walls is shifted in the whale direction, but the width is not. (Figure 3 from the paper)

What is important here is that these traits are not found in species that could be ancestral to the two groups, but are shared by the two groups. Had these traits been found in ancestral forms or a lot of other related forms, they (the traits) could be shared by whales and Indohys because everybody has them. But since the traits are derived with respect to other artiodactyls, and shared by these two forms, an evolutionary link is strongly suggested.

When an analysis of traits including only living forms and some ancient whales is done, hippos still link with whales. This paper goes much farther in the analysis by including several fossil remains. This causes the phyogenetic tree that includes whales to be drawn very differently, pushing aside the enigmatic relationship with hippos and indicating a close relationship between whales and the Raoellid Indohys

Assuming that this new phylogenetic analysis is correct, what does it say about the nature of whale evolution? Well, there is evidence that Indohyus was aquatic. Very thick cortical bone in the limbs indicate this (this occurs in virtually all semi-aquatic or aquatic quadrupeds). Stable isotope studies support this as well. Oxygen and carbon isotope values of aquatic mammals tend to have a characteristic pattern, which may be seen in Indohyus.

This leads the authors to suggest an evolutionary hypothesis for whale origins:

Indohyus was a small, stocky artiodactyl, roughly the size of the raccoon ... It was not an adept swimmer; instead it waded in shallow water, with its heavy bones providing ballast to keep its feet anchored. Indohyus may have fed on land, although a specialized aquatic diet is also possible.

...Our working hypothesis for the origin of whales is that raoellid ancestors, although herbivores or omnivores on land, took to fresh water in times of danger. Aquatic habits were increased in Indohyus (as suggested by osteosclerosis and oxygen isotopes), although it did not necessarily have an aquatic diet (as suggested by carbon isotopes). Cetaceans originated from an Indohyus-like ancestor and switched to a diet of aquatic prey. Significant changes in the morphology of the teeth, the oral skeleton and the sense organs made cetaceans different from their ancestors and unique among mammals.

Do I believe this? No, not yet. The link between Indohyus and whales via the traits that were indicated are interesting, and even suggestive. However, the same problem could be operating here as occured with the hippos, but with the authors lulled into seeing the link as more likely because of the geographical closeness of Indohyus and the presumed area of origin of whales. A careful examination of the morphology shows that while Indohyus resembles ancient whales more than it does other forms, it does not fall neatly into the whale measurement for the features of the auditory bulla.

The isotopic study is also a close call but not necessarily close enough. The isotopic signature associated with Indohyus is not a dead ringer for extinct aquatic organisms. The argument can be made that the animal is aquatic on the basis of the morphology (the thickened limb bone cortex) but the morphology supports the isotopic study more than the other way around.

I see nothing in this analysis that excludes the possibility that Indohyus is indeed a close relative of whales. Even if it does not represent a common ancestor, it can be thought of as a hypothetical model for early whale evolution. So yes, this is a good story ... a strong hypothesis regarding the relationship between Indohyus and early whales, and a plausible evolutionary scenario. But there are still some uncertainties.

What is needed to firm up this analysis? Simple: Go find more fossils! Never mind that the best collecting localities are in the middle of a perennial war zone.

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J. G. M. Thewissen, Lisa Noelle Cooper, Mark T. Clementz, Sunil Bajpai & B. N. Tiwari (2007) Whales originated from aquatic artiodactyls in the Eocene epoch of India. Nature 450, 1190-1194 (20 December 2007) doi:10.1038/nature06343; Received 26 June 2007; Accepted 3 October 2007

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