Carl Zimmer in his excellent blog, The Loom:
Last week I wrote about a new study that identified a fossil mammal as the closest relative to whales, helping to shed light on how whales moved from land to sea. The mammal, Indohyus, was a small four-legged creature that probably spent a fair amount of time in water and ate vegetation. The authors of the new study proposed that the ancestors of whales originally lived this way. Gradually, the whale lineage became more adapted to life in water and shifted to eating meat, as exemplified by early whales like Ambulocetus, which was something like a furry alligator.
In the comment thread, Noumenon asked this question:
I don’t understand how Indohyus and Ambulocetus, both dated to around 47 mya, can both be the ancestors of today’s whales. You say carnivory was an important transition for whales. Then Indohyus would have had to split off before Pakicetus, before whales became carnivorous.
Via email, I got a similar question from a biologist I know who is working on a book about evolution. He had read about the discovery in this article by Ian Sample in the Guardian, who declared:
Fossil hunters have discovered the remains of the earliest ancestor of the modern whale: a small deer-like animal that waded in lagoons and munched on vegetation.
So how can an ancestor be younger than its descendants?