A dental characteristic of mammals is the presence of two sets of teeth: milk teeth and adult teeth. In contrast, reptiles and fish can often regrow their teeth (if they have any) and go through multiple sets, as juveniles and adults.
Researchers analyzed three Brasilodon lower jawbones – a juvenile jawbone and two adult jawbones – from the Linha São Luiz site in southern Brazil, where the species was discovered decades ago alongside some of the earliest dinosaurs.
The team used the destructive technique of severing the jaws to see the teeth grow inside. Universities in southern Brazil have plenty of Brasilodon fossils, so the jaws weren’t too valuable to cut.
“What we found is that Brasilodon only molts once,” co-author Martha Richter, a science associate at the Natural History Museum in London, England, told Live Science. The presence of two sets of teeth makes Brasilodon a diphiodont – the name given to an animal with two sets of teeth – and suggests that Brasilodon is more closely related to mammals than to reptiles.
“We don’t have any example of a reptile that only has two sets of teeth,” Richter said. “Ancient reptiles constantly swapped their teeth at different rates. »
Studies show that diphiodont teeth are the result of a “genetic chain of events” that not only regulates the shape of the skull and teeth, but also bodily functions also associated with mammals, such as endothermy (the ability to regulate metabolically heat). body), lactation and hair growth, Richter said.
“They are all related to each other,” added the researcher. Brasilodon looked like a mammal, with a long tail and other shrew-like features. Positioning the species as the earliest known mammal would make it part of the evolutionary lineage that survived two mass extinction events, including the asteroid strike that wiped out non-avian dinosaurs in the late Cretaceous period ago. is 66 million years old and allowed mammals to diversify on land and at sea.
However, not everyone is convinced by the results of the study. “The study does not present data to support a change in the position of Brasilodon,” Simone Hoffmann, an associate professor at the New York Institute of Technology, USA, who specializes in evolutionary history, told Live Science. of the first mammals.