OBJECT OF THE MONTH - JUNE 2006
Fossil Land Tortoise (Stylemys nebrascensis)
Fossilized tortoise found in the sediments of the White River Group, Dawes County, Nebraska, in 1980.
Turtle or tortoise? It can be very confusing, especially when they belong to the same taxonomic order of Chelonia, which in Greek means tortoise! Basically, tortoises live on land and have short strong legs which they use for support and to dig burrows for protection. Turtles live in water and have webbed feet to help them swim. They will occasionally venture on to land for short periods of time (for example, to lay their eggs), but they will always stay close to water.
The fossilized carapace (shell) and plastron (the underside), shown here, belong to a dry land tortoise known as Stylemys nebrascensis, which lived during the Oligocene Epoch between 34 and 24 million years ago. During the Oligocene, the global climate was cooling and there was a pronounced difference among climates around the globe. In the United States, the environment was changing from closed subtropical forests to more open savannah-like habitats.
Stylemys nebrascensis was very common during the Oligocene and many fossilized carapaces are found in the western United States, particularly in Nebraska and South Dakota. However it is rare to find a juvenile tortoise like the one shown here, and it is even rarer to find a tortoise carapace with a skull. Vertebrate Paleontologists study the length of the carapace and the growth rings on the scutes to help determine the age of tortoises and turtles.
The carapace is a modified rib cage and is also part of the vertebral column (backbone). If you look closely at the photograph you may notice that the pattern on the carapace is comparable to the pattern found on today's tortoises. Tortoises and turtles are unique among terrestrial vertebrates as the pectoral and pelvic girdles (or hip) are located inside the rib cage. The legs are flat to fit with the carapace.
Like modern tortoises, Stylemys nebrascensis did not possess teeth. Its diet, therefore, was limited to low growing shrubs, grasses, cacti and insects. This tortoise also had modified jaw muscles that allowed it to retract its head into the carapace. Tortoises and turtles which retract their heads in to the carapace are known as Cryptodires (hidden neck), but not all of them have modified jaw muscles.
Before this tortoise was fossilized, the carcass may have been transported (possibly by water or scavengers) and then rapidly buried by sediment. Unfortunately, during the transportation process the skull was separated from the rest of the body and was not found with the carapace, limbs and other bones.
