When most of us consider ferocious, blade-like teeth prehistoric creatures they picture Smilodon, know as the saber-toothed tiger. However, at the world of dinosaurs, theropods are well-known for getting blade-like teeth with serrated cutting edges used for biting and ripping their prey. And until recently, the intricate arrangement of tissues that gave rise to those frightening teeth has been considered exceptional to these meat-eating dinosaurs.
In a paper published at Biology Letters, Lead author Megan Whitney, postdoctoral fellow in the Department of Organismic and Evolutionary Biology, Harvard University, analyzed thin fossil slices of gorgonopsian’s teeth and discovered comparable complicated arrangement of cells which produced the steak-knife-like serrations in theropods.
Gorgonopsians are a set of synapsids from the middle-late Permian 270-252 million years back. These animals, as with other synapsids, are regarded as the forerunners of mammals and fall within the lineage that eventually gave rise to mammals. “These animals were the apex predators of their day and are characterized by their sabre-like canine that could extend up to 13 cm long,” said Whitney.
Past studies of theropod dinosaurs discovered a complicated arrangement of tissues made of both enamel and dentine that formed the serrations on their teeth. This intricate arrangement was believed unique to theropod dinosaurs. But nobody had ever made a thin section of a gorgonopsian tooth before to examine the serrations.
Inspired, Whitney and co-authors combined their expertise in Paleohistology (the analysis of the microstructure of fossilized skeletal tissues) and examined thin sections of fossils from three synapsids from three distinct time intervals to examine a concept of the structure of the serrations in this group. “We were surprised to find theropod-like serrations in gorgonopsians,” said Whitney. “We wanted to see how other carnivorous synapsids had made their serrations, so we looked at an older synapsid [Dimetrodon] and a younger, mammalian synapsid [Smilodon].”
Gorgonopsian, Dimetrodon, and Smilodon are all synapsids and such as theropods have been apex predators of the daily life and had plump, knife-like teeth (i.e. ziphodonty). Dimetrodon is one of the earliest synapsids throughout the Cisuralian period around 295 to 272 million decades back; Dimetrodon is often mistakenly called a dinosaur. Smilodon lived in the Americas during the Pleistocene epoch 2.5 million to 10,000 years ago. “All of these animals fall along the mammal-line which is divergent from the reptile line with dinosaurs,” said Whitney. “In fact, these three animals are more closely related to humans than to dinosaurs.”
Whitney’s PhD focused on the teeth of gorgonopsians and other forerunners of mammals so she analyzed the gorgonopsian specimens that were gathered from continuing, extensive fieldwork in Zambia in which many of these creatures are found. Co-authors Aaron LeBlanc, postdoctoral fellow in the Department of Biological Sciences, University of Alberta, Ashley Reynolds, PhD candidate in the Department of Ecology and Evolutionary Biology, University of Toronto, also Kirstin Brink, assistant professor at the Department of Geological Sciences, University of Manitoba, led experience in dental histology and the creatures included in this study.
The thin sections showed that the gorgonopsian serrations are composed of tightly-packed serrations made of both enamel and dentinethe same complex arrangement of cells that had formerly been attributed to theropod dinosaurs and believed unique to them.
“What’s surprising is that the type of serrations in gorgonopsians are more like those of the meat-eating dinosaurs from the Mesozoic era,” said LeBlanc. “It means that this unique type of cutting tooth evolved first in the lineage leading to mammals, only to later evolve independently in dinosaurs.”
“The fact that we only see this type of serration evolve in meat-eating animals is significant,” said Brink. “The tiny microstructures hidden inside the teeth offer a significant advantage to the tooth, strengthening the serrations and helping them last longer in the mouth, which in turn helps the animal eat efficiently.”
Though gorgonopsians share this trait with theropod dinosaurs, they Really share more characteristics with different synapsids like Dimetrodon and humans. “These animals converged to a similar tooth serration morphology because of the operational benefits, not because they are close relatives to one another,” said Whitney. “In this case, it likely has something to do with the fact that animals were actually putting a great deal of wear and tear on their own teeth. And so independently they’ve been able to create a serration that will withstand the repeated forces necessary to eat since eating is important. So, there’s a good deal of selection acting on teeth”
Gorgonopsians were a diverse group with body sizes that ranged from the size of a medium-sized canine to a bear and Whitney notes that although the specimens sampled had this sort of morphology, it remains possible that there’s a diversity of teeth serration types that would suit the diversity of gorgonopsians.
Related Journal Article: https://royalsocietypublishing.org/doi/10.1098/rsbl.2020.0750