A new study reports the discovery of a complex tissue arrangement in early mammals that led to having serrated teeth - a feature previously thought unique to carnivorous dinosaurs.
People usually associate ferocious, blade-like teeth with the prehistoric saber-toothed tiger, or the Smilodon. However, in dinosaurs, theropods are also known for having similar teeth with serrated cutting edges used for biting and ripping apart flesh from prey.
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Led by Megan Whitney, a postdoc fellow in Harvard's Department of Organismic and Evolutionary Biology, the new study examined thin fossil slices of tooth samples from gorgonopsia - an extinct group of saber-toothed early mammals. The new study details the discovery of a similar complex arrangement of tissues that made the serrations in therapod teeth.
The Same Serrated Teeth in Early Mammals
The samples used in the study belonged to gorgonopsians, a group under synapsids - also referred to as proto-mammals - from the middle to late Permian period, about 270 to 252 million years ago. Both gorgonopsians, as well as most of the synapsids, are considered distant ancestors to modern mammals.
"These animals were the apex predators of their day and are characterized by their saber-like canine that could extend up to 13 cm long," Whitney noted.
Previous studies of theropod teeth revealed complex tissue arrangements, composed of both enamel and dentine, that formed the unique serrations. Since no other research found the same serrations and tissue arrangements in other prehistoric creatures, it was thought to be unique to these dinosaurs.
Whitney and her co-authors turned to their expertise in paleohistology, or the science of analyzing microstructures in fossilized tissues, to examine thin sections of three synapsid fossil teeth, each from a different time period. This procedure was done to test a theory of serrated teeth in this group of animals. She was joined by Aaron Leblanc from the University of Alberta, Ashley Reynolds from the University of Toronto, and Kirsten Brink, from the University of Manitoba.
"We were surprised to find theropod-like serrations in gorgonopsians," Whitney said, adding that they were interested in checking whether other carnivorous synapsids had the same feature. It prompted researchers to include an older species, Dimetrodon, and a younger mammalian species, Smilodon.
"What's surprising is that the type of serrations in gorgonopsians are more like those of the meat-eating dinosaurs from the Mesozoic era," LeBlanc said, explaining that this kind of tooth first evolved in the proto-mammal lineage before independently developing in dinosaurs.
Mammalian Ancestors Closer to Humans than Dinosaurs
All three species in the study were synapsids and similar to theropod dinosaurs, were apex predators with ziphodonty: serrated and knife-life teeth. The older Dimetrodon lived during the Cisuralian (Early/ Lower Permian), 295 to 272 million years ago. As a non-mammalian synapsid, it has a large sail on its back, which led to it being mistaken as a dinosaur despite going extinct some 40 million years before the first dinosaurs.
Meanwhile, the Smilodon or saber-toothed tiger is an early mammal that roamed the Americas during the Pleistocene epoch, between 250 million years ago to 10,000 years ago. Researchers noted that these animals used in the study were fall closer to the mammal-line, a separate evolutionary track from the reptile line that included dinosaurs. Whitney noted that the three species were "more closely related to humans than to dinosaurs."
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