New Discoveries from Old Bones at Hagerman Fossil Beds National Monument

Article by Kari Prassack, Paleontologist, Hagerman Fossil Beds National Monument
for Park Paleontology Newsletter, Spring 2018

Dr. Kari Prassack is the park paleontologist at Hagerman Fossil Beds National Monument. When not rummaging through collections or tumbling down steep embankments she can be found surrounded by stacks of papers and more than a few stuffed animal toys.

NPS Photo

Hagerman Fossil Beds National Monument is a relatively small and unassuming fossil site located along the sinuous Snake River in southern Idaho. Its fossils date from the early half of the Pliocene and range in age from approximately 4.2 to 3 million years in age. The Monument is best known for its fossil horses, representing the earliest species of the modern horse genus Equus. This single-toed horse has been recovered by the hundreds since the Smithsonian first excavated here in the 1930s and provide paleontologists with important information on Pliocene horse behavior. This horse, however, is just one of nearly two hundred species of animals that have been recorded at Hagerman from their fossilized remains. This includes water and wetland birds like swans, cormorants, and rails; rodents ranging from the diminutive deer mouse to the large and muscular beaver; larger mammals like peccaries, pronghorn, and ground sloths; and the many carnivorans (e.g., bears, cats, dogs, otters) that would have hunted them. This great diversity and the number of holotypes (new species first described from these deposits) are what makes Hagerman so special.

Imagine going back 3.5 million years. While southern Idaho today is a flood-scoured plain of montane-steppe vegetation, it was a very different place during the Pliocene. The chorus of frogs is interrupted by a splash as a small otter grapples with one of the many salmonid fish calling the river home. Wetlands and stands of woodlands dot the surrounding landscape as a mastodon makes its way through a stand of tall reeds. This diverse landscape and its animal community provide us with important information about ancient migrations, evolutionary changes, a shifting climate, and even animal behavior. Each fossil contains a packet of information, which, however small, further contributes to our understanding of both the ancient past and how past events shape the world we see today. That small fishing otter, in particular, has provided us with a large packet of data crucial to understanding the story of otter evolution; a story that was almost never told.

Fig. 1. Greater Grison. Grisons, which are members of the Mustelidae family, evolved in North America but are restricted today to South and Central America. Fossils of three different species of grison are known from the Hagerman Fossil Beds.

Image from WikiCommons.

This new species of river otter, Lontra weiri, is represented by a jaw and humerus (arm bone) that were collected in the 1980s before the Monument became a unit of the National Park Service. These and other fossils from Hagerman were brought to the Idaho Museum of Natural History to join an impressive collection of fossils dating back hundreds of millions of years in age. It was there where these two fossils were assigned to the small, extinct grison, Trigonictis cooki. Grisons (Fig. 1) are members of the weasel family Mustelidae which also includes things like otters, badgers, and martins. Grison fossils are well known from the Hagerman Fossil Beds; however, these fossils did not belong to a grison. It was not until 2014, decades after their initial discovery, that these fossils were properly identified as the oldest example of the New World river otter genus, Lontra.

Otters are members of the family Mustelidae, which includes weasels, badgers, and martins. Otters, which first appeared in the Miocene, have a strong fossil record throughout the Old World but a poor record in North and South America. There are thirteen species of otter worldwide today with species present on every continent except for Antarctica and Australia. In the Americas, the sea otter (Enhydra lutris), giant Brazilian otter (Pteronura brasiliensis), and four species of the river otter genus (Lontra) are found. Fossils of Lontra are especially uncommon in the fossil record, with the previous oldest fossils of Lontra dating back only 1.8 million years ago, during the Pleistocene epoch, and belonging to the extant (still living) North American river otter, Lontra canadensis. Paleontologists thought Lontra evolved from an otter in Asia belonging to the closely related Old World river otter genus, Lutra. Geneticists disagreed with the paleontologists, as their own studies of modern otter genetics placed the origin of Lontra several million years earlier in the Pliocene of North America. A paleontologist’s knowledge can be limited by what the fossil record provides, so we appreciate and oftentimes benefit from the knowledge provided by other disciplines, such as genetics. It helps, of course, to eventually find the “missing” data to corroborate the geneticists’ claims, and with an age of 3.8 million years in age, Hagerman’s new otter does just that.

Hagerman’s otters. Size comparison of Lontra weiri jaw (top) to Hagerman's other and much larger otter, Satherium piscinarium (bottom). Scale bar is noted as 5 mm.

NPS Photo

Lontra weiri was similar in size to the smallest of today’s otters, the Asian small-clawed otter and the South American marine otter, which typically weigh less than 10 lb., and so was about the size of a house cat. Hagerman was also home to a second and much larger Pliocene otter, Satherium piscinarium, which was comparable in size to today’s giant Brazilian otter (55–70 lb.) or around the size of a chocolate lab. These otters were sympatric at Hagerman, and both were likely piscivorous (fish-eaters) but would have utilized different-sized prey, limiting competition between them.

So far, Lontra weiri is only known from the Hagerman Fossil Beds. These Beds were managed and protected by the Bureau of Land Management at the time of the otter’s original discovery (collection) and are today further protected as a National Park Service Monument. Had these lands not been recognized and protected for future generations, had we not ensured that the fossils were properly collected and curated at a museum and made accessible to scientists, we may have forever lost this important key to understanding otter evolution. Imagine what other amazing finds are waiting to be discovered in museum collections and in the ancient grounds of our own country