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Upper Colorado Region
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Released On: March 20, 2004

Stegomastodon Mandible Found on Bureau of Reclamation Land Near Elephant Butte Reservoir
The mandible (lower jaw) of a Stegomastodon (also called Stegomastodont) was found by a New Mexico State Parks ranger on Bureau of Reclamation land near Elephant Butte Reservoir.

Reclamation issued a permit to New Mexico Museum of Natural History, a repository for federal property, to remove the lower jaw from the side of an arroyo, clean and stabilize it, and display it at the museum if appropriate or otherwise properly store it as federal property.

Although it is not yet prepared, Gary Morgan, paleontologist with New Mexico Museum of Natural History, is fairly certain that the jaw belongs to an extinct mastodon named Stegomastodon rexroadensis.

This is the most complete lower jaw of Stegomastodon ever found in New Mexico.

There are several extinct species in this extinct genus.

The original fossil specimens of this species were found in Kansas and Arizona.

The Elephant Butte Stegomastodon lived during the Pliocene geologic epoch and is about three million years old.

Stegomastodon was shorter and stockier than the living Asian elephant; it was probably about eight feet tall at the shoulder and weighed several tons.

Stegomastodon had a pair of long rather straight upper tusks about six feet long, but lacked lower tusks. The lack of lower tusks, although typical of the two species of living elephants, is actually unusual for a Pliocene mastodon, most of which possessed a small rounded tusk in the lower jaw.

The lower molars or grinding teeth were elongated and tall compared to other Pliocene mastodons, suggesting that they ate rather coarse vegetation such as grasses as well as softer leaves and twigs.

Stegomastodon lived throughout New Mexico during the Pliocene epoch, but went extinct early in the Ice Age (Pleistocene epoch) about one million years ago.

Based on the sediment in which it was preserved (sands and gravels of the Palomas Formation), the Elephant Butte Stegomastodon's bones were buried alongside the ancestral Rio Grande.

If a visitor finds a resource such as a fossil or any cultural artifacts on State or Federal land, do not touch or remove it. Note its location and contact the authorities so that the resource can be properly handled.

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