Hagerman Fossil Beds National Monument - Muskrat (U.S. National Park Service)

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A modern muskrat

Pliopotamys - Not just a minor member of the Hagerman Fauna

The diverse variety of animals found as fossils at Hagerman Fossil Beds National Monument reflects the variety of habitats that existed in the area 3.5 million years ago. One of these habitats, the wetlands, is especially well preserved within the monument and species of frogs, birds, mammals and turtles associated with this environment are quite abundant as fossils. Among these species that are so common is the ancestor to the living muskrat that still thrives in the Hagerman Valley today.

This ancestral muskrat, also referred to as the pygmy muskrat, since it is smaller than its living descendent is known by the scientific name Pliopotamys minor which is a rather appropriate and descriptive name since Plio refers to the Pliocene- the time it lived, pota means river (Ever hear of the Potomac River? which translates to the word river) and mys means mouse. The species name minor is in reference to its small size. So literally our animal is the small Pliocene river mouse but pygmy muskrat works just as well.

Pliopotamys was first described in 1933 based on jaws collected by Smithsonian at Hagerman. It's relationship to the living muskrat was immediately recognized because it was first given the scientific name of Ondatra idahoensis minor but later study determined that there were enough differences that the Hagerman form should be recognized as being distinct from the living muskrat Ondatra and was given the name Pliopotamys. Changes in scientific names (such as the recent change of the well known Brontosaurus to the previously unknown name Apatosaurus) may make some people uncomfortable or just plain irritated. After all, once you give something a name shouldn't it stay that way? But despite the inconvenience of having to substitute a new name for a well known animal the changing of names actually reflects an improvement in our understanding of these extinct animals and is really a type of progress, sometimes major, sometimes minor.

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Extinct Muskrat

It is important to remember that when one is working with fossil animals, unlike modern species, we don't always have a complete individual with which to work. Once an animal dies it is easy for the bones of the skeleton to become separated and scattered. Many of the fossil species have been described based on a single bone or sometimes only part of a single bone. While this is not always ideal, since a single bone is not likely to provide all the information you need to understand the animal, it is often all you have preserved in the fossil record. Frequently it is only after years of collecting additional material that paleontologists have enough to recognize that a previously described species is the same as another one. What if one species is described based on a humerus (upper arm bone) and another based on a jaw? You wouldn't know they were from the same type of animal until you found a skeleton with both the jaw and humerus preserved. By the same token you may not be able to tell how really different two animals are until you have enough pieces to make a real comparison. Larger samples permit better comparisons and often let us make more accurate determinations as to how similar or different two described species are.

The first specimen of Pliopotamys described was a lower jaw and it turns out that most of the specimens known of this animal, especially at Hagerman, are jaws and isolated teeth. A few limb bones that resemble smaller versions of the living muskrat have been found but most of our knowledge of the animal is based on the well preserved teeth and jaws. There is not a single skeleton known for Pliopotamys minor and such a find at Hagerman would greatly enhance our knowledge of the animal. In fact, despite Pliopotamys being common as a fossil, it wasn't until 1969 that a complete skull was found at Hagerman. Prior to that discovery all that was known of the skull was a few partial palates and other bits and pieces.

So what would a complete skeleton of Pliopotamys tell us? For one thing it would provide good information as to whether this ancestral muskrat was a good swimmer like its living descendent or was at the early stages of moving into the water. There is frustration in that we don't have all the pieces. Although we see a pattern and the story is emerging, it would be nice to be able to fill in some gaps, such as having a complete skeleton of Pliopotamys. But then, that's what Hagerman Fossil Beds is all about - to protect an area that preserves an important part of earth history so that those missing pieces may come to light someday and fill in the gaps.

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This article originally appeared in The Fossil Record, December 1995

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