“P” is for Ptilodus

“P” is for Ptilodus.

P was a particularly difficult letter for which to select an appropriate genus. There are many very important “P” genera, including: Plesiadapis and Phenacodus. So I covered a bit about Plesiadapis here, and will cover a bit about Phenacodus when I get to Tetraclaenodon.

Ptilodus is an equally important genus in the Paleocene. It is a member of the multituberculates, a now-extinct group of mammals that most likely filled an ecological niche similar to that of modern rodents. In fact, it could be argued that the extinction of the multituberculates was caused by the advent of rodents, though they did manage to co-exist for several million years.

Skull of Ptilodus, a paleocene multituberculate, after Vaughan, 1986, pencil drawing. Nobu Tamura, from Wikipedia.

There are at least five different species of Ptilodus in The Breaks. They share in common a large, smoothly arched, blade-like lower fourth premolar (p4) with many distinct serrations along the edge. From each serration comes a long ridge going down each side of the tooth. Compare this with Fractinus for a different potential shape of a multituberculate lower fourth premolar.

The distinctions between species are made on the basis of tooth measurements, the curvature of the blade edge of the tooth, and the number of serrations. Ptilodus looks strikingly like Baiotomeus, which was described earlier.

Sketches of the comparison between Baiotomeus lamberti and Ptilodus gnomus

Sketches of the comparison between Baiotomeus lamberti and Ptilodus gnomus. In both of these sketches the front of the tooth is to the left.

Even though I describe these teeth as ‘large,’ I should hasten to qualify that statement. Ptilodus titanus (actually called Ptilodus sp. T in the literature because it hasn’t been formally published), the biggest member of the genus, has a total length of the p4 of about one centimeter. That’s less than half and inch. That’s huge for the Paleocene, but pretty tiny otherwise.

Here’s some SEM images of a smaller member of the genus, called Ptilodus gnomus.

Two SEM images of the same specimen of Ptilodus gnomus (UW 23093 from locality V-90045. This is a left lower fourth premolar. The images differ by the image collection method. The left is a Backscattered electon image; the right is a secondary electron image.

Two SEM images of the same specimen of Ptilodus gnomus (UW 23093 from locality V-90045. This is a left lower fourth premolar. The images differ by the image collection method. The left is a backscattered elecrton image; the right is a secondary electron image. This is a buccal view, looking at the side of the tooth that would be against the cheek. The front of the jaw was to the left.

The blade-like lower fourth premolar occludes against the upper fourth premolar, which is of an entirely different morphology.

The right upper fourth premolar of Ptilodus gnomus. Notice the rows of pointed cusps, characteristic of multituberculates.

The right upper fourth premolar of Ptilodus gnomus. Notice the rows of pointed cusps, characteristic of multituberculates. This is an occlusal view, looking down on the chewing surface of the tooth.

In The Breaks, there were five certain species of Ptilodus, and a bunch of other specimens that were definitely Ptilodus, but I wasn’t sure which species. All together, of about 800 specimens total that were identified as distinct genera,  at least 231 represent definitive species of Ptilodus and come from 24 different localities within The Breaks.

I spent a long time looking at teeth of Ptilodus.

Part of the Blogging from A to Z challenge

For 4-18-13

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