But today no bird, fish or reptile sports this extreme and ever-increasing bit of anatomy. Only mammals do, even though they weren’t the first toothed creatures. This is an ancient feature that predates dinosaurs, a new study has found.
“We were able to show that the first teeth were from animals that preceded modern mammals, called dicynodonts,” Ken Angelski, curator at Chicago’s Field Museum and author of the new study, said in a news release. “They are very strange animals.”
From the size of a rat to the size of an elephant, dicynodonts lived about 270 million to 201 million years ago. While their closest living relatives are mammals, they looked more reptilian, with a tortoise-shaped head.
Dysinodonts were the most abundant and diverse vertebrates before the rise of the dinosaurs, and they all had a pair of teeth protruding from the upper jaw.
tusks vs teeth
Before discovering exactly how teeth evolved, researchers had to define what a tooth was and how it differed from a tooth—something that was unclear.
They determined that a tooth should extend from the mouth, consist only of a substance called dentine and continue to grow throughout an animal’s life—even if it is damaged. Teeth are also formed from dentine. However, they are coated in enamel. This, along with their size, makes them durable, but once adult teeth grow in, not much can be done if they break. They don’t grow again.
“Enamel-coated teeth have a different evolutionary strategy than dentine-coated tusks — it’s a trade-off,” said Megan Whitney, a postdoctoral fellow in Harvard University’s Department of Biology and Evolutionary Biology. She was the lead author of the study.
The researchers then analyzed thin sections of 19 fossil teeth from dicynodonts, representing 10 different species found in South Africa, Antarctica, Zambia and Tanzania. They also used micro-computerized tomography scans to examine how the fossils were attached to the skull, and whether their roots showed evidence of continued growth. They found that some of the dicynodonts studied had true teeth, without enamel, while the rest had large teeth.
The scientists also found that there was no hard progression from non-tusk to tooth. Individual members of the dicynodont family evolved teeth independently at different times, and some never developed true teeth.
Whitney said, “I fully expected there to be a moment in dicynodont evolutionary history where tusks evolved because that is the simplest explanation. However, we found convergent evolution of teeth later in dicynodont evolution.” Convergent evolution occurs when similar features develop independently in different species or at different periods in time.
For teeth to evolve, they found that a flexible ligament connecting the teeth to the jaw was needed, as well as reduced rates of tooth replacement—a combination of characteristics uniquely found in modern mammals today.
“All of this goes a long way to give us a better understanding of the teeth we see in mammals today,” said Angelski, speaking about the research published in the journal Proceedings of the Royal Society b Biological Sciences.
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