Home: Scrying: For the love of species IV: Jurassic Mammals

For the love of species IV: Jurassic Mammals

February 24, 2006

A handful of the millions of species alive todayIt may be difficult for human researchers to pinpoint just what makes a species a species, but nature doesn’t seem to be bothered. Rather, nature has defined millions of species for us; while we may quibble over shat is what, the organisms have no problems filling their distinct niches.

We recognize differences between species when it is convenient to do so. As John Wilkins suggests, while we struggle for classification, the members of the group have no trouble making the distinction:

For a start, biologists and nonbiologists alike recognise much the same sorts of base level taxa in the natural world. This might be an artefact of human cognitive propensities (Hey 2001a), but as in the famous case of the Rana pipiens complex, the organisms themselves recognise these differences even if humans do not (Littlejohn 1969; Littlejohn and Oldham 1968), in this case based on differing mating calls, and the taxonomy followed suit once these differences were recognised. (Wilkins 625*)

It makes me wonder how many species have slipped definition. Of course, new species are discovered all the time, even in our own humble class of mammals. Recently, we’ve discovered new species of mammals who currently live in the remote forests of Papua New Guinea as well as some who lived in China 164 million years ago.A cross between a beaver, an otter, and a platypus, this mammal swam the rivers of China 164 million years ago

164 million years ago, you might have guessed, dinosaurs were rather prolific. Mammals were thought to be mere tiny rodents at the time, certainly the underdog in the ecosystem. The latest fossil discovery in the Liaoning Province (of feathered fossil fame) shows that our furry little ancestors were more prolific than once thought.

According to the New York Times, the fossil finds revealed a beaver-like mammal that swam and fished in the Jurassic period:

Thomas Martin, an authority on early mammals at Senckenberg Research Institute in Frankfurt, said the find pushed back “the mammalian conquest of the waters by more than 100 million years” and “impressively contradicts” the conventional view.

The creature, dubbed Castorocauda lutrasimilis was well suited for fishing rivers and lakes, and was probably rather cute as well:
The extinct species appears to have been an amalgam of animals. It had a broad, scaly tail, flat like a beaver’s. Its sharp teeth seemed ideal for eating fish, like an otter’s. Its likely lifestyle — burrowing in tunnels on shore and dog-paddling in water — reminds scientists of the modern platypus.

Its skeleton suggests that it was about 20 inches long, from snout to the tip of its tail, about the length of a small house cat….

The skeleton is accompanied by fur and scale imprints and the suggestion of soft-tissue webbing in the hind limbs. Dr. Luo said the fur was to keep water from the animal’s skin. It is the most primitive known mammal to be preserved with hair, evidence for its evolution before the appearance of more complex mammals.

The scientists said the tail and limbs of the newfound specimen were well developed for aquatic life. They surmised that like the platypus, Castorocauda swam in rivers and lakes, ate aquatic animals and insects and built nests in burrows along the shore. The animal had molars specialized for feeding on small fish and small aquatic invertebrates.

“So far, it is the only semiaquatic mammal from the Jurassic,” Dr. Luo said.

Of course, even though we are exploring the diversity of mammals in the fossil records, our furry relatives are only the icing on the cake. In the next few days, I’ll share a variety of fossils, some from my own personal collection. Also, I’ll look at the role of extinction, and more on how distinct species come to be.


Castorocauda Image by Mark A. Klinger of the Carnegie Museum of Natural History via the Times article, linked above.

*-John S. Wilkins “How to be a chaste species pluralist-realist: the origins of species modes and the synapomorphic species concept” in Biology and Philosophy vol 18: 621-638, 2003