Due to the current fossil whale craze, I’ve decided to dedicate this week’s Monday Organism to another famous fossil related to whales. Well, I say “related to whales”, and this much is true for any ancient organism, but the truth is, Indohyus is really, if anything, only a sister taxon to ancient cetaceans – it’s unlikely that whales descended from Indohyus directly.
Indohyus was a fox-sized artiodactyl (even-toed ungulate) that lived around 48 million years ago, exactly when you’d expect to find land mammals, and it was found exactly in the right location: in Pakistani sediments (since Ambulocetus natans was found there, too) – but the show’s not over yet.
Indohyus almost certainly lived on water. Isotope measurements confirmed that Indohyus spent a considerable amount of time underwater, and the structure of his dense bones further reaffrims it. In addition, Indohyus has a particular ear anatomy that, prior to Indohyus’ discovery, was thought to be unique to whales and their fossil ancestors – a structure called “an auditory bulla” (since I know jack diddly about animal anatomy, I’ll leave it unelaborated). The important thing to know here is that this auditory bones in the Indohyus fossil confirm at least a partially marine habitat – and rather hits the nail on the head as far as Indohyus’ marine history is concerned.
Indohyus is a curious find, and is definitely a good clue of how whale ancestors might have evolved, but it’s unlikely that Indohyus is, in fact, the ancestor of whales. For starters, Indohyus is herbivorous, and as such, puts another important node between it and whale ancestors, who are, including modern whales, all carnivorous.
That in mind, it’s important to note that even though fossils provide wonderful evidence for the evolution of taxa, every fossil we use to depict as “ancestral” is almost certainly not, strictly speaking, ancestral per se. Evolution predicts that you’d find fossils of ancestors and their nearest relatives, and that their nearest relatives would have similar traits to ancestors and descendants. Since it’s much more likely to find ancestral relatives (sister taxa) – it’s more parsimonious to assume that this is what we find in most fossils.
For me – Indohyus being herbivorous is a clear sign that this isn’t an ancient whale, or possibly not even a sister taxa to an ancient whale – although it’s definitely a “cousin” or “second-degree cousin” taxon for ancient whales, as clearly shown by its aquatic and whale-like attributes.