After around 10 million years in which the planet took to recover, the emerging terrestrial fauna of the Triassic Period was initially not so different from what came before. In what would eventually become southern South America, large herbivorous synapsids known as dicynodonts became dominant.
These animals were not true mammals though, in that they likely had some sort of a beak in lieu of teeth and walked with a sprawling, lizard-like gait. They were likely preyed upon by large predatory crocodile-like animals called thecodonds.
But scurrying around the feet of these relative giants was a much smaller animal.
All in the teeth
Initially discovered in the 1980s and known from a large number of near-complete specimens, it would not be until 2003 that scientists realized Brasilodon was its own species. Reaching about 20 centimeters long, this shrew-sized animal had been something of an enigma.
While its body and skull looked fairly mammal-like, an initial look at its teeth eventually meant that it was classified as one of the extinct ‘mammal-like reptiles’.
But by looking again as these fossils, and the chance to cross section some of the jaw bones, has led to scientists re-examining this assessment.
‘The crucial, the most important question was what kind of dental replacement pattern occurred in this animal, because we know for sure two things,’ explains Martha. ‘Firstly, all reptiles are polyphyodonts, we know that because there isn’t a single example in a fish, amphibian or reptile lineage that is not originally a polyphyodont animal.’
‘And secondly, 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 just two times, known as diphyodonty.
The results showed that the young Brasilodon specimens had a single tooth growing up, directly under already erupted teeth, proving that these animals were diphyodonts. As these fossils are some 20 million years older than any other definitive mammal, it suggests that the origin of this group goes back to at least 225 million years ago.
That is not to say this assessment is without controversy. Some suggest that Brasilodon is still a ‘mammal-like reptile’, and that instead these results show that diphyodonty can no longer be used to define mammals. Martha, however, is not convinced by this.
‘From modern studies we know that diphyodonty is linked to a lot of mammalian traits,’ says Martha. ‘Diphyodonty has been extensively studied and it is not just about dental replacement, but a phenomenon that involves skull development and changes to the physiology, for instance, endothermy.’
‘The whole skull undergoes transformations to allow for milk sucking.’