Stuart Sumida discovered some small bones in the Bromacker quarry in 1993, the same year that the holotype skeleton of Diadectes absitus was found. Dave Berman told me that when Stuart showed them to him, he couldn’t see anything because they were so small. Upon closer examination, Dave, Stuart, and Thomas Martens identified them as those of the captorhinomorph reptile Thuringothyris mahlendorffae. Thomas’ wife Stefani, whose maiden name is Mahlendorff, discovered the first specimen in the Bromacker in 1982, and Thomas and a colleague named it in her honor in a 1991 publication.
The fossil was exposed in several pieces of rock, which Thomas shipped to Carnegie Museum of Natural History (CMNH) along with the large block of rock containing Diadectes. I didn’t prepare the specimen until several years later, as other projects, including the Diadectes, overshadowed it. Once I began working on it, though, Dave and I realized that it was not Thuringothyris. Indeed, we had no idea what type of animal it was, and our puzzlement grew as I exposed more of it. The identity wasn’t revealed until I had uncovered some very unusual, tiny teeth, which under the high magnification of the preparation microscope appeared to have a bulbous cusp towering over a basin. They looked vaguely familiar to me, but because I couldn’t immediately put a name on them, I rushed to get Dave from his office. Once Dave saw the teeth, he realized that the specimen was a new genus and species in the rare, enigmatic reptile group Bolosauridae.
Until the discovery of Eudibamus cursoris, bolosaurids were represented in the fossil record by two genera, Bolosaurus and Belebey, which were based mainly on poorly preserved skull and fragmentary jaw fossils from Texas and Russia, respectively. Even though bolosaurids had been known since 1878, their relationship to other reptiles was not well understood. The nearly complete anatomy of Eudibamus allowed our team to determine that bolosaurids are the oldest member of the ancient group of reptiles called Parareptilia. This group has no living relatives, except possibly for turtles, a hypothesis that is highly debated by scientists.
When our study of the fossil began, we realized that Eudibamus was very different than other reptiles from that time. Proportions of the limbs and positions of the articulation surfaces on the upper and lower hind leg bones indicated that, in terms of posture, Eudibamus resembled a bow-legged human with a bad back instead of a typical sprawling reptile on four legs. It could stand and locomote on its hind legs in an upright posture (bipedal) with its legs held close together and in the same plane (parasagittal).
Dave was in constant phone communication with team member Dr. Robert Reisz (Professor, University of Toronto at Mississauga). One day Robert called Dave to ask if all the tail had been exposed, because he learned that modern lizards that are able to run bipedally have a long tail to help maintain their balance. The specimen was in Dave’s office and he immediately uncovered more of the tail and then let me finish the task. The tail was indeed very long and extended close to the edge of the block, which I had previously reduced in size. Additionally, we determined that the third, fourth, and fifth toes of the hind foot also were greatly elongated through lengthening of some of the individual toe bones, and that the first and second toes were extremely shortened by the reduction in size of individual toe bones. We hypothesized that when Eudibamus ran bipedally, it would rise on its toes, so that only the tips of the third, fourth, and fifth toes would contact the ground.
Eudibamus occurred at least 60 million years before other bipedal, parasagittally-running reptiles appeared in the fossil record. This is reflected in its scientific name, which is derived from the Greek “eu,” meaning original or primitive, and “dibamos,” meaning on two legs. “Cursoris” is Latin, meaning runner. Examples of other reptiles using this locomotion mode are the dinosaurs Allosaurus fragilis and Tyrannosaurus rex, which you can view in CMNH’s Dinosaurs in Their Time exhibition.
So, what was the advantage of being able to run bipedally instead of running on all four legs? Lengthening the hind leg and foot would greatly increase stride length, especially if only the tips of the toes contacted the ground, which is an efficient way to increase speed. Eliminating arm to ground contact while running removes forelimbs from the path of the long-striding hind legs. The bulbous teeth and jaw structure of Eudibamusindicate that it was herbivorous. It seems likely, then, that Eudibamus used its ability to sprint to avoid becoming a tasty meal for a pursuing predator.
One of our laments is that a fossil trackway preserving Eudibamus walking quadrupedally and then switching to a bipedal gait has yet to be found.
Next time you are at CMNH, make sure you see the cast of the fossil skeleton and a model of Eudibamus that are exhibited in the Fossil Frontiers display case in CMNH’s Dinosaurs in Their Time exhibition. Stay tuned for my next post, which will feature the herbivorous mammal-like reptile Martensius bromackerensis.
For those of you who would like to learn more about Eudibamus, here is a link to the 2000 Science publication in which it was described: https://science.sciencemag.org/content/290/5493/969.
Amy Henrici is Collection Manager in the Section of Vertebrate Paleontology at Carnegie Museum of Natural History. Museum employees are encouraged to blog about their unique experiences and knowledge gained from working at the museum.
The Bromacker Fossil Project Part VIII: Martensius bromackerensis, Honoring a Colleague