An interesting paper by Hans Thewissen and four co-authors was published in this week’s Nature (read the abstract–the full article is by subscription; and the Pharyngula story) The article suggests a possible artiodactyl ancestor to the whales, and the evidence centers around ear bones.
Artiodactyls include modern animals such as cows, deer, and pigs, and while it has long been known that whales were related to them, it has not been clear exactly which artiodactyl group was closest to the whales.
The artiodactyl group in question is an obscure family called the Raoellidae, which are known from the Eocene of Asia. Specifically, the article discusses the racoon-sized raoellid Indohyus from Pakistan. Indohyus is a typical small Eocene artiodactyl, with fairly long, slender legs and a hoof on each toe. There is nothing in the overall appearance of Indohyus to suggest that it was aquatic (see Nature’s reconstruction.)
To review, the middle ear of mammals is enclosed by the bowl-shaped tympanic (or auditory) bulla. In land mammals the tympanic bulla includes a structure to support the eardrum, and attaches to the petrosal on each side. Below is the tympanic bulla from a modern artiodactyl (a white-tailed deer from my wife’s personal skull collection–you had to guess that my wife would have a personal skull collection!):
Deer tympanic bulla, with the front at the upper right. A is the lateral attachment point with the petrosal, and B is medial attachment point. C is the medial edge of the bulla; notice that it is very thin, only about 0.5 mm thick.
In all whales, the inner (medial) attachment between the tympanic bulla and the petrosal is lost; there is a gap between the two bones at that point. Moreover, the medial edge of the bulla is greatly thickened into a structure called the involucrum:
Tympanic bullae of various whales (not to the same scale). Medial is at the bottom, and the red arrows indicate the involucrum. A, Eobalaenoptera (primitive baleen whale); B, Diorocetus (primitive baleen whale); C, Balaenoptera (fin whale); D, Eubalaena (right whale); E, Squalodon (primitive toothed whale); F, Orycterocetus (extinct sperm whale). A and B are VMNH specimens; C-F are from the National Museum of Natural History.
This is thought to be related to whales’ ability to hear underwater. The eardrum structure is good at detecting vibrations in the air, but when immersed in water the movement of the eardrum is dampened. When we put our heads underwater the sounds we hear are mostly transmitted by vibrations of the bones in our skull.
Whales use these skull vibrations to maximum advantage. The separation of the medial side of the tympanic bulla from the petrosal allows the tympanic bulla to vibrate more. The thick, heavy involucrum amplifies these vibrations. (This is largely based on a paper by Nummela et al.; here’s the abstract.)
So the involucrum is associated with using skull vibrations to detect sound underwater. This mechanism isn’t the only way to hear in the water, as seals and sea cows don’t have it, but it is the method found in all whales. To use the lingo, the presence of the involucrum is a synapomorphy of the whales; a unique feature that whales inherited from their ancestor.
Indohyus, like all the whales and in contrast to all other artiodactyls, has an involucrum. This indicates a close relationship between the the raoellids and the whales, and suggests that the raoellids may be the direct ancestors to the whales. There are actually a few other features they share with early whales, including the arrangement of the incisors and the shape and wear patterns on the premolars.
Upon closer examination, it turns out that Indohyus has other subtle features that suggest an aquatic lifestyle. The limb bones of Indohyus are osteosclerotic. This means that the outer dense layer of the bones is unusually thick, making the bone heavier. This is found in a whole range of aquatic animals, including sea cows, beavers, hippos, and early whales (later whales reversed this trend.)
Another indicator of aquatic habits is the ratio of Oxygen 18 to Oxygen 16 in Indohyus’ tooth enamel (see my article in the Richmond Times-Dispatch on oxygen isotope studies). The ratio in Indohyus is comparable to that seen in aquatic mammals such as Hippopotamus, and very different from land mammals.
Another interesting point is that the Carbon 12/Carbon 13 ratios in Indohyus are different than in early whales such as Pakicetus. This, combined with the crushing molars in Indohyus, indicates a different diet than the early whales. The authors suggest that Indohyus may have been an herbivore (all later whales are carnivores).
Determining evolutionary relationships and interpreting the lifestyles of ancient organisms often hinges on careful observation of small details.