River Dolphins and Evolution

A while back I was looking through my 1970s Funk & Wagnals’ Wildlife Encyclopedia and read the entry on River Dolphins. In it a particular passage caught my eye because it was not very parsimonious.

We usually take it for granted that because whales, porpoises and dolphins are so obviously descended from land animals, their ancestors must have returned directly to the sea. As a result, it is something of a surprise to learn there are such animals as river dolphins, and we suppose they have come up rivers from the sea. In fact, the ancestors of dolphins could just as easily have left the land fro the rivers, and later invaded the sea. This is supported by the primitive characters exhibited in the skeletons of the freshwater dolphins. In most whales, porpoises and dolphins the seven neck vertebrae are squashed together and fused, and the animals have no visible neck. In freshwater dolphins the vertebrae are separate and there is still some sign of a neck. Also, the skull of a freshwater dolphin has not undergone the same fundamental changes as the skulls of its marine relatives, and in several ways is more like the skull of the extinct dolphin Squalodon of 15 millions years ago.

The problem of this hypothesis is that it requires the river dolphins of the Amazon, Ganges, Yangtze, etc. to all be autochthonous and essentially unrelated, since those river systems are separated by good span of geography. So I went digging to see how science has progressed in 30 years, and found this gem.

Hamilton H et al. (2001) Evolution of River Dolphins. The Royal Society Proceedings: Biological Sciences, 268: 549-556.