Mammals Lived Alongside Early Dinosaurs, Controversial Study Says

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.

Illustration of a Brasilodon (Image: Illustration/Wiley Anatomical Society)

“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.

1663148527 308 Mammals lived alongside the first dinosaurs according to a
Example of a shrew (Image: Udo Kieslisch/)

“Based on all the other studies, Brasilodon is not a mammal, it’s certainly not the oldest mammal, and it’s not even a form of mammal (mammals and their closest relatives). Hoffmann noted that the study authors use mammals and mammalian forms almost interchangeably in the study.

Although this has been common practice in the scientific community, she said, these groups should now be considered separate. “Mammals are a clade that is further back in the evolutionary tree than mammals,” he said. “Forms of mammals include fossils that preceded mammals as well as mammals. »

Morganucodon is the oldest known mammalian form, while Brasilodon is usually placed as a sister group alongside the mammalian forms in the evolutionary tree, or as a sister group to the next larger clade, the mammalomorphs, according to Hoffmann .

Diphiodont teeth are a known feature of mammalian forms, but while finding this feature in Brasilodon is “exciting,” it doesn’t necessarily indicate that it belongs to other mammalian forms, she said.

Animals inherit traits from their ancestors, but life is constantly evolving and changing to form new branches on the evolutionary tree, he continued. “Feathers were once considered a defining characteristic of birds. We now know that feathers appeared much earlier and are common to many dinosaurs. But just because Tyrannosaurus Rex was rebuilt to have feathers doesn’t mean it’s a bird. »

Richter acknowledged that the study’s findings may be disputed, noting that the classification of these early mammal groups is controversial and the conversation about how early mammals are classified is ongoing. “This article will help inform that discussion,” he added.

Via Live Science

Feature image: Illustration/Wiley Anatomical Society

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