The big news in this week’s issue of Nature was the discovery of a small ornithischian dinosaur covered in bristles, but there was another, shorter paper that caught my eye. In December 2007 Nature printed a short communication on Indohyus, a small artiodactyl that seemed like a good candidate for the type of creature that whales evolved from. Paleontologists Hans Thewissen and Lisa Noelle Cooper explain the significance of Indohyus to whale evolution in this video;
There was something that bothered me about the systematic analysis of Indohyus, however. In the paper’s phylogenetic tree raoellids (the group to which Indohyus belongs) and cetaceans were grouped together as sister groups, themselves forming a clade distinct from all other artiodactyls. While molecular analysis showed that hippos were the closest living relatives to whales they were far removed from this position in the 2007 paper.
Others apparently noticed this same issue, for in this week’s issue of Nature Jonathan Geisler and Jessica Theodor published a revised phylogenetic tree that incorporated molecular evidence. What they found was that Indohyus and its relatives among the Raoellidae are the closest relatives of cetaceans. Since they are extinct, however, hippos are the closest living relatives of whales, making them the sister group to the raoellids and cetaceans together. (It is curious, however, that perissodactyls are placed as closer to artiodactyls as a group than mesonychids are. I have to wonder if this is primarily a result of including the molecular data, as there is none for the mesonychids to include.)
As Geisler and Theodor explain this reinforces the idea that adaptations to life in the water shared by hippos and whales were inherited from a common ancestor. One such trait is osteosclerosis (which Geisler and Theodor call pachyostosis), or a thickening of the bones. This would naturally cause bones to be heavier and function as a kind of “bone ballast” in aquatic and semi-aquatic vertebrates.
If this adaptation was inherited from a common ancestor, Geisler and Theodor argue, then it may have appeared before the thickening of the inner ear bones that are characteristic of raoellids and whales. The thickening of the ear bones that mark these two groups, but not hippos, would have simply been an extension of osteosclerosis in the rest of the skeleton. If this was so then the thickened inner ear bones could then have been co-opted for underwater hearing.
So what do Thewissen and colleagues think of this? In a reply they state that the new analysis backs up their primary conclusion that the raoellids are the closest relatives of cetaceans. Their original phylogenetic tree was based on fossil evidence and that is why the placement of hippos came into dispute.
They also state that the thickening of the ear bones in raoellids and cetaceans appears to have been affected by a different kind of osteological process than osteosclerosis. If this is correct then the thickening of the inner ear bones would not simply be an extension of osteosclerosis into the skull as Geisler and Theodor suggest. (Indeed, if the processes are different it might explain why hippos did not convergently evolve thickened ear bones.) As of this moment, however, the hypothesis has not been confirmed or refuted. It is an interesting question that will require study of both paleontology and development to unravel.
So there you have it. It is not the sort of thing that is going to make the news but it is good to see hypotheses tested and discussed further. (Phylogenetic trees are, after all, hypotheses that are constantly being revised.) With the confirmation that whales are closest to raoellids and that both form a sister group to hippos there are plenty of new questions to delve into, and I am glad to see how detailed our understanding of early whale evolution is becoming.