By painstakingly measuring hundreds of specimens of a fossil mammal called Thyrohyrax, recovered from the famous fossil beds of Egypt's Fayum Province, the researchers determined that males of this now-extinct species -- and only males -- had oversized, swollen lower jaws shaped much like a banana. Further, the team speculated, the animals may have used the balloonlike structural chamber that shaped their bizarre jaws to produce sound.
If this speculation is correct, Thyrohyrax and its fossil relatives would be the only mammals found so far to use such a skeletal structure for producing sound, the researchers said. They added that some dinosaurs are thought to have used similar sound-producing mechanisms.
The researchers published their findings in the March issue of the Journal of Vertebrate Paleontology, which was released in mid-April. The study's lead author is Donald DeBlieux, a former research associate at the Duke Lemur Center (formerly the Primate Center) who is now at the Utah Geological Survey. Other team members from Duke include Elwyn Simons, head of the Division of Fossils at the Duke Lemur Center; Prithijit Chatrath, curator of the division; and Michael Baumrind, a former undergraduate anthropology student who initiated the research as a senior project. Yousry Attia of the Egyptian Geological Survey also participated in the study.
The team's research was supported by the National Science Foundation.
The banana-jawed hyraxes were much different in appearance from their modern counterparts, the scientists said.
"Hyraxes today are improbably cute-looking mammals that are about the size of rabbits, look much like guinea pigs and have a penchant for sunbathing," Simons said. "Seven species of hyraxes now inhabit a swathe from the Middle East to southern Africa."
From previous studies, scientists knew that Thyrohyrax, 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 Thyrohyrax 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.
But what purpose did those weird jaws serve?
According to Simons, today's hyraxes are relics of a time when Africa lacked most of the kinds of animals found there today. Current scientific thought holds that most modern mammal groups diversified and became larger only after a giant asteroid killed off the dinosaurs 65 million years ago.
Numerous ecological niches vacated by the annihilated dinosaurs were waiting to be filled. One group of creatures, called the Afrotheres -- "Beasts from Africa" -- seized the opportunity. It was from this group, Simons said, that today's hyraxes descended.
The Afrotheres, whose descendents include elephants and aardvarks as well as hyraxes, diversified into various niches and ultimately became Africa's dominant land animals.
As hyraxes diversified, the development in Thyrohyrax of a curved, swollen jaw containing a hollow chamber represented a particularly strange adaptation, DeBlieux said. Some other hyracoid species also showed a similar adaptation, but the skeletal trait was most pronounced in Thyrohyrax, he added.
"Our team had long speculated on the cavity's purpose, " Simons said. "One idea we proposed, for example, was that the chamber contained enlarged salivary glands.
"Our current findings narrow down the possibilities," he said. "Among the most plausible explanations is that the hollow jaw was a sound-producing device." The animals, he added, may have used the chamber to produce sounds as part of their courtship or mating displays.
According to DeBlieux, help in refining explanations behind this mammal's unusual jaw may come from studying dinosaur models. In particular, he said, a number of museums have built models of dinosaur sound-producing skeletal cavities that have enabled scientists to reconstruct the possible sounds made by them.
Now, DeBlieux would like to do the same for the fossil hyrax. "It would be really neat," he said, "to model their jaws so we could hear the sound these curious creatures actually made."
Journal of Vertebrate Paleontology