ETSU Scientist Identifies the Oldest and Largest Fossil Kangaroo Rat

Take a stroll through a desert in western North America, and you might spot a tiny hopping rodent with enormous ears. Kangaroo rats and kangaroo mice are well-adapted for life in dry habitats, but their distant ancestors were not. Now, thanks to a new fossil discovered in Oregon, scientists have a clearer picture of how and why these charismatic critters evolved their unique lifestyle.

This fossil was identified as a new species in a study published in the journal PeerJ. The study was led by Dr. Joshua Samuels, associate professor in the ETSU Department of Geosciences and curator at the Gray Fossil Site & Museum.

The new species has been named Aurimys xeros, a name that means “ear mouse from dry habitats.” This species is known from a single fossil individual consisting of a nearly complete skull and a partial foot. The fossils were uncovered at the John Day Fossil Beds in Oregon, at a site that preserves an ecosystem from the Early Miocene Epoch, about 23 million years old. Aurimys is the oldest known kangaroo rat, and thus it represents an early stage in the evolution of these animals.

The fossil skull of the new species, Aurimys xeros. Photo courtesy of Joshua Samuels.

Modern kangaroo rats are bipedal, hopping around on two feet like kangaroos, and they have very large ears. These are key adaptations that help them survive in flat, arid landscapes.

“Since there is not much cover in open habitats,” Samuels explains, “they must use their excellent hearing and erratic hopping movements to help them escape from predators like rattlesnakes and owls.”

Like modern kangaroo rats, Aurimys had very large ears, as evidenced by its large “auditory bullae” (the bony components of the ears), but it does not appear to have been bipedal. In modern kangaroo rats and other bipeds, the spine enters the skull from underneath, but in Aurimys, that entrance is in the back of the skull, like in quadrupedal animals. Overall, the skull of Aurimys has a mixture of some features that are similar to modern kangaroo rats and others that are more like their distant ancestors.

Dr. Joshua Samuels on a fossil-finding excursion in Oregon. Photo courtesy of Joshua Samuels.

Aurimys is also the largest known species of kangaroo rat. Modern kangaroo rats can get as big as twelve inches long (about half of that is tail), but Aurimys was about one-third larger than that. It’s possible that this extra bulk served as added protection against danger.

“The size may reflect the fact some of the main predators of kangaroo rats, rattlesnakes, were just showing up in the fossil record at this time,” Samuels says.

This early stage of kangaroo rat evolution also coincides with a global climate shift. During the Early Miocene, habitats around the world, including in Oregon, were becoming cooler and dryer.

“As climate changed, forests started to decline in many places, and were replaced by more open environments like deserts, steppe, and grasslands,” Samuels says. “Kangaroo rats thrive in those habitats today, and are now some of the most common mammals in arid parts of western North America.”

Today, kangaroo rats are probably the mammals most specialized for life in dry desert environments. Photo courtesy of Joshua Samuels.

Samuels JX, Calede JJ-M, Hunt, Jr. RM. 2023. The earliest dipodomyine heteromyid in North America and the phylogenetic relationships of geomorph rodents. PeerJ 11:e14693