From previous studies, scientists knew that Thryohyrax, along with numerous other early hyracoid relatives, had an enlarged opening on the inside of its lower jaws. The feature is known as an internal mandibular fenestra.
But knowledge essentially stopped there, Simons said. Part of the problem was that today's hyraxes lack such skeletal features, so they offered little insight, he said. Also, relatively few Thryohyrax fossils had been found for study.
Simons solved the supply problem as he collected fossils at the Fayum site, located about two hours' drive southwest of Cairo. For several decades, Simons visited the site in search of early ape and monkey fossils, as part of his main research focus on the evolution of primates leading up to humans. During these explorations in the "Badlands of Egypt," his group collected enough specimens of Thyrohyrax to allow a systematic analysis of its jaws.
"Paleontologists have known of the internal mandibular fenestra among fossil hyraxes since early last century, but its significance was poorly understood," Simons said.
In order to gain answers, the Duke team measured tooth samples from the newly available fossils.
"We looked first at molar teeth, which are used for grinding food, and found that all of them in our specimens were identical, so we were pretty sure they were from the same species," DeBlieux said. "But only half of the fossils had the inflated jaws. This meant the trait was possessed by only one sex -- but which one? Previous research, he said, suggested such jaw structures likely were found in females.
The researchers then analyzed incisor teeth, located in the front of the jaw and used for biting. In modern hyraxes, males have larger incisors than females. Among the fossils analyzed, measurements showed that those with inflated jaws had larger incisors, DeBlieux said. This observation led the researchers to infer that the animals were males.