The First Whales to Use Sonar?

Xenorophus

Other than their obvious physical adaptations, one of the most familiar features of dolphins and whales is their ability to use ultrasound to echolocate. All dolphins, porpoises, and toothed whales (collectively called odontocetes) can echolocate and, while some of the fine details do vary between, say, sperm whales and some of the smaller species, the basic mechanism is much the same. Ultrasound would not have been useful to the large, ground-dwelling ancestors of whales in the same way that it is for bats, so it must have evolved after they entered the water. But how soon after?

We can put some limits on this. At the younger end, since all odontocetes echolocate, it's unlikely to have evolved any later than their last common ancestor, which is estimated to have lived around 34 million years ago. On the other hand, it's notable that the toothless, baleen, whales do not use ultrasound; in fact, they are specialised in the exact opposite direction to produce sounds well below, not above, the range of human hearing. This suggests that they evolved along different lines, and that the origins of ultrasonic echolocation lie somewhere after the two split. 

Age of Mammals: The Oligocene (Pt 1)

It's been over a decade since I started including bimonthly looks at specific slices of Earth's past in this blog. In that time, I have covered three epochs: the Pleistocene, Pliocene, and Miocene. Together with the current Holocene epoch, these comprise what we currently consider to be two "periods": the Quaternary and Neogene. Both of these are dominated, more or less, by mammals of the sort we'd generally recognise today, even if the details are different. All of the earlier chunks of time since the extinction of the non-avian dinosaurs, however, constitute a different period, the Paleogene, where this was much less true.

When Charles Lyell devised the current system of dividing the "Age of Mammals" into epochs in 1833, he originally defined four. A few years later, he revised this to five, but even then, the entirety of what we'd now call the Paleogene was placed into a single epoch, the Eocene. In 1854, however, German palaeontologist Heinrich Beyrich, split off the later part of the Eocene into a new epoch, which he saw as a distinct period of transition in the development of fossil seashells. He called this the Oligocene, and it proved useful beyond his original mollusc-based definition, and so has remained in use to this day. (Beyrich's wife, incidentally, was a children's author, and made the unusual step of favourably commenting on the work of Charles Darwin in a novel for young girls at a time when it was still controversial).