Middle Eocene to Late Eocene
Eastern Asia; North America
An Inside Look
In October and November 2001, paleontologists from the San Diego Natural History Museum collected beautifully preserved brontothere fossil remains from a construction site near Rancho del Oro Road in Oceanside. The scientists excavated both adult and juvenile brontothere bones from a thick sandstone stratum which was also loaded with other fossils.
A juvenile brontothere skeleton was the same species as the adult. This skeleton would have been complete had the scraper not taken part of it away before the paleontologists spotted it.
How can paleontologists determine whether bones from a small animal are a juvenile specimen, or actually some other species? They look for such things as small deciduous teeth, which would eventually be replaced with adult teeth as the animal grows into adulthood.
From this site, a skull, articulated feet, ribs, and forelimb bones from a baby brontothere, as well as the lower jaws and skull from an adult brontothere, were excavated. These fossils represent the best examples of Eocene brontotheres known from California.
In Our Region
Fossils of Parvicornus were found in Oceanside, California, near Rancho del Oro Road, and in Carlsbad near El Camino Real.
Brontotheres are extinct odd-toed ungulates and are distant evolutionary "cousins" of living rhinoceroses. Parvicornus was thick-bodied and stood about 5-6 feet (2 meters) tall at the shoulder. Other brontotheres like Brontotherium grew to 8 feet (2.5 meters). Many brontotheres featured prominent horns on their heads.
Some species of brontotheres developed large, strangely shaped horns on their heads as larger and larger species evolved throughout the Eocene and early Oligocene. However, the medium-sized brontothere, Parvicornus, was an earlier species that only had a very small horn, really more just a bump, near each eye.
The horns of brontotheres were actually not true horns, such as modern rhinos display. Rhino horns are covered with fibrous keratin, a protein that makes up our fingernails and hair. Brontothere horns were made of bone rather than keratin, and brontotheres that evolved later than Parvicornus developed very large prong-like horns on their heads. These bony extensions of the skull were probably covered with skin.
In addition to the horns, brontotheres had very robust neck vertebrae. Scientists think this suggests that they may have engaged in a head-butting, aggressive type of behavior. The horns may have played a protective function for the animal's brain and skull when butting heads with another.
Despite the fact that modern-day rhinos can be very aggressive with their horns, they do not engage in head-butting, but males use their horns for display and for goring male opponents. Horns help both male and female rhinos defend their territories.
Parvicornus was a browser. Plant leaves and twigs provided abundant food for browsers such as Parvicornus.
Turner, Alan. 2004. Prehistoric Mammals. Washington, DC: National Geographic Society.
Text: Margaret Dykens and Lynett Gillette
Illustration: Jim Melli
Model photograph: François Gohier