Fossil Porpoises Muddy the Waters

Harbour porpoise

Dolphins and porpoises belong to two separate families, and there are clear differences between them. However, this isn't immediately obvious at first glance: they're both small cetaceans, with flippers, tail flukes, and so on. A comparison between, say, a bottlenose dolphin (Tursiops truncatus) and a harbour porpoise (Phocoena phocoena) does show a number of differences that are broadly true of their respective relatives - but not entirely so.

Porpoises, for instance, are smaller than dolphins, on average. But only 'on average', because, in fact, the smallest dolphin is smaller than the smallest porpoise. Similarly, dolphins usually have a 'beak' - a narrow snout projecting forward from the front of the head - that is missing in the more rounded heads of porpoises. But, once again, there are dolphins that don't have the beak, such as Risso's dolphin (Grampus griseus).