Surprising connection discovered between prehistoric dinosaurs and mammals in their teeth


Gorgonopsian were the first saber-toothed animals. Their canines extended up to 5 inches. Credit: CCA 3.0 / Dmitry Dogdanov

When most people think of ferocious, knife-like teeth on prehistoric creatures they imagine Smilodon, better known as the saber-toothed tiger. But in the world of dinosaurs, theropods are known to have knife-like teeth with serrated cutting edges that are used to bite and rip their prey. And until recently, the complex composition of tissues that gave rise to these terrifying teeth was considered unique to these carnivorous dinosaurs.

In a paper published December 16, 2020, in Biology Letters, lead author Megan Whitney, a postdoctoral fellow in the Department of Organismic and Evolutionary Biology, Harvard University, examined thin fossil slices of gorgonopsian teeth and discovered a similarly complex arrangement of tissues that made the steak-knife-like teeth in theropods.

Gorgonopsian Canine Scan

Thin portion of a partial gorgonopsian canine under polarized light. The teeth can be seen on the right side of this specimen. Credit: Megan Whitney

Gorgonopsians are a group of synapses from the mid-late Permian 270-252 million years ago. These animals, like other synapsids, are considered the forerunners of mammals and fall within the lineage that eventually produced mammals. “These animals were the apex predators of their time and are characterized by their saber-like canine teeth that can reach up to 13 cm in length,” Whitney said.

Previous studies of theropod dinosaurs revealed a complex composition of tissues, made of both enamel and dentin, that formed the serrations on their teeth. This complex arrangement was considered unique to theropod dinosaurs. But no one had ever cut a thin section of a gorgonopsian tooth before to examine the serrations.

Gorgonopsian Saber Tooth Canine

A complete saber-toothed gorgonopsian from Zambia. This specimen contains both the crown (top) and the root (bottom) of the tooth. Credit: Megan Whitney

Inspired, Whitney and co-authors combined their expertise in paleohistology (the study of the microstructure of fossilized skeletal tissues) and examined thin sections of fossils from three synapses from three different time periods to test a theory of the structure of the serrations in these group. . “We were surprised to find theropod-like serrations in gorgonopsians,” Whitney said. “We wanted to see how other carnivorous synapsids made their teeth, so we looked at an older synapsid [Dimetrodon] and a younger mammalian synapsid [Smilodon]. “

Gorgonopsian, Dimetrodon, and Smilodon are all synapsids and, like theropods, were apex predators of their day and had serrated, knife-like teeth (i.e. ziphodonty). Dimetrodon is one of the first synapses during the Cisural period, approximately 295 to 272 million years ago; Dimetrodon is often incorrectly described as a dinosaur. Smilodon lived in America during the Pleistocene 2.5 million to 10,000 years ago. “All of these animals fall along the mammal line, which is different from the dinosaur reptile line,” Whitney said. “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 mammal ancestors, so she examined the gorgonopsian specimens collected during ongoing, extensive fieldwork in Zambia, where many of these animals 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, and Kirstin Brink, Assistant Professor in the Department of Geological Sciences, University of Manitoba, has brought expertise in dental histology and the animals included in this study.

The thin sections revealed that the Gorgonopsian serrations are composed of tightly packed serrations made of both enamel and dentin, the same complex arrangement of tissues previously attributed to theropod dinosaurs and considered unique to them. “What is surprising is that the type of serrations in gorgonopsians is more like that of the carnivorous dinosaurs of the Mesozoic era, ”said LeBlanc. “It means that this unique type of incisor evolved first in the lineage that led to mammals, and later evolved independently in dinosaurs.”

“The fact that we only see this type of perforation evolving in carnivorous animals is significant,” said Brink. “The tiny microstructures hidden in the teeth provide significant benefit to the tooth, strengthening the teeth and helping them stay in the mouth longer, which in turn allows the animal to eat more efficiently.”

Gorgonopsian Serrations enlarged

Both are enlarged images of the serrations under polarized light. Gorgonopsian serrations are made of both enamel (thinner, lighter tissue to the right) and dentin (thicker tissue to the left) and an interdental crease (black central structure that is a crease between the serrations). This specific arrangement allows more serrations to be tightly packed along the tooth and each toothing becomes more resistant to wear. Credit: Megan Whitney

While gorgonopsians share this trait with theropod dinosaurs, they actually share more traits with other synapsids such as Dimetrodon and people. “These animals converged on a similar tooth-tooth morphology because of the functional benefits, not because they are close relatives,” Whitney said. “In this case, it probably has to do with the fact that animals really put a lot of wear on their teeth. And so independently they have been able to form a serration that can withstand the repeated forces it takes to eat, because food is important. So there is a lot of selection that works on teeth. “

Gorgonopsians were a diverse group with body sizes ranging from the size of a medium-sized dog to a bear, and Whitney notes that although the specimens sampled had this type of morphology, it remains possible that there is a diversity of tooth types to match. the diversity of gorgonopsians.

Reference: “Convergent tooth adaptations in hypercarnivorous synapsid and dinosaur teeth” by MR Whitney, ARH LeBlanc, AR Reynolds, KS Brink, December 15, 2020, Biology Letters.
DOI: 10.1098 / rsbl.2020.0750