Oldest known animal with mammalian teeth discovered in Brazil

After about 10 million years in which the planet took to recover, the terrestrial fauna emerging from the Triassic period was initially not so different from what had preceded it. In what would become southern South America, large herbivorous synapsids known as dicynodonts became dominant.

These animals were not true mammals, however, in that they probably had some kind of beak instead of teeth and walked with a sprawling, lizard-like gait. They were probably preyed upon by large crocodile-like predatory animals called thecodons.

But scurrying around the feet of these relative giants was a much smaller animal.

All in the teeth

Originally discovered in the 1980s and known from a large number of near-complete specimens, it wasn’t until 2003 that scientists realized Brasilodon was its own species. Reaching around 20 centimeters in length, there was something enigmatic about this shrew-sized animal.

While its body and skull looked quite mammal-like, a first look at its teeth ultimately meant it was classified as an extinct “mammal-like reptile”.

But by re-examining these fossils and the possibility of cutting some of the jaw bones, the scientists re-examined that assessment.

“The critical, most important question was what kind of tooth replacement pattern occurred in this animal, because we know two things for sure,” says Martha. “First, all reptiles are polyphyodonts, we know this because there is not a single example in any line of fish, amphibians, or reptiles that was not originally a polyphyodont animal.”

“And second, almost all known mammals are diphyodonts.”

To put it simply, all reptiles have a type of tooth replacement in which they can replace them multiple times, known as polyphyodonty. In mammals, however, the ability to replace teeth is limited to only twice, which is called diphyodontia.

The results showed that young people Brasilodon the specimens had a single growing tooth, directly below teeth that had already emerged, proving that these animals were diphyodonts. As these fossils are about 20 million years older than any other definitive mammal, this suggests that the origin of this group dates back at least 225 million years.

This is not to say that this assessment is without controversy. Some suggest that Brasilodon is still a “mammalian-like reptile”, and that instead these results show that diphyodontia can no longer be used to define mammals. Martha, however, is not convinced.

“From modern studies, we know that diphyodontia is linked to many mammalian traits,” says Martha. “Diphyodontia has been widely studied and it is not just tooth replacement, but a phenomenon that involves the development of the skull and changes in physiology, for example endothermy.”

“The entire skull undergoes transformations to allow the sucking of milk.”

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