SUMMER 2003 No. 17

Page 5

Walking with Otters

Stephen Carroll takes us on a journey in search of the elusive ancestors of the elusive otter…

With TV series like Walking with Beasts, Extinct and Ice Age prehistoric environments and creatures have been bought to vivid, be-whiskered life right in our living rooms. Amongst all the sabre-teeth and woolly fur though, to my otter-attuned eyes there has been one beast conspicuous by its absence....

This led me to undertake an expedition millions of years back in time to find out about the dawn of prehistoric otters and their extinct ancestors, via the internet and some hours in the library. The story begins just after computer-generated dinosaurs ruled the earth, 65Ma (Ma = million years ago). Up until then, and for some while afterwards, mammals really were small insectivorous creatures scurrying around the primeval forest floor.

evolutionary timeline

It was not until the Palaeocene, some 5 million years later, that 11 new mammal orders became prominent. Two groups, the Carnivora (to which the otters will belong), and the now extinct Creodonta showed distinct adaptations for eating meat. The secret of the new designs was all in the teeth: in both Creodonts and Carnivores the rear molars became modified to form specialised meat-shearing surfaces.

Palaeocene (65-54Ma) and Eocene (54-38Ma)

The Creodonts were a diverse group of some 50 genera (including Hyaenodon from Walking with Beasts), and the dominant placental mammal predators from 60-30Ma, eventually disappearing in the Late Miocene 5Ma (hence they were at different times co-existent with the dinosaurs and with our otter ancestors). Meanwhile the early Carnivora remained diminutive and insignificant, possibly tree-dwelling, weasel or cat-sized animals. These diversified at around 55Ma (the beginning of the Eocene) into two recognisable genera: the Viverravidae and the Miacidae. It is not clear how closely related the Miacids might be to modern Carnivores, but one, called Miacis, from Germany seems to be similar to a Pine Marten, except about 20cm long.

Oligocene (38-26Ma)

By the end of the Eocene and beginning of the Oligocene (38-35Ma), as Creodont pre-eminence was waning, modern Carnivore groups had begun to evolve from the two ancestral genera. There is little fossil evidence to demonstrate this though until the early Oligocene; however, when fossil remains do reappear, many of the modern taxonomic families seen today are well formed and identifiable. Furthermore, by this time the Carnivore families were apparently well established in the Oligocene world. There may have been an environmental trigger for this. A high turnover in species and the decline of the Creodonts coincide with rapid climatic changes as the polar ice caps formed and global conditions cooled and became drier. At the same time developments are seen in Carnivore skulls: a more complex ossification of the inner ear bones and a larger brain pan indicate that carnivores were becoming more perceptive and intelligent.