Mammals lived alongside the first dinosaurs, claims a controversial study

The earliest known mammal was a tiny shrew-like animal that lived alongside the first dinosaurs 225 million years ago, delaying the appearance of mammals by about 20 million years, says a controversial new study, published in Anatomy diary.

An international team of researchers studied the fossil remains of an animal 20 centimeters long called Brasilodon quadrangularis and decided it was a mammal because during its life it developed two sets of teeth like most mammals, including humans. modern.

The authors state that Brasilodon is today the first mammal known to science, as it appears in the fossil record some 20 million years before Morganucodon, which was previously the first known animal to be considered a mammal.

However, the initial classification of mammals is complex, and a researcher who was not involved in the study told Live Science that neither Brasilodon nor Morganucodon are mammals and that both belong further down the evolutionary tree, despite their similar dentition. that of a mammal.

Mammary glands that produce milk, a feature that helps define mammals today, have not been found in the fossil record. Therefore, the researchers look for evidence of mammalian ancestry in mineralized bones and teeth.

A dental feature of mammals is the presence of two sets of teeth: milk teeth and adult teeth. Conversely, reptiles and fish can often regrow their teeth (if they have any) and go through multiple sets, such as juveniles and adults.

Researchers analyzed Brasilodon’s three lower jaws – one juvenile and two adult – from the Linha São Luiz site in southern Brazil, where the species was discovered decades ago along with some of the earliest dinosaurs.

The team used the destructive technique of dissecting the jaws to see the teeth developing inside them. Universities in southern Brazil have a lot of Brasilodon fossils, so the jaws weren’t too precious to cut.

“What we found is that Brasilodon only sheds its teeth once,” co-author Martha Richter, a scientific associate at the Natural History Museum in London, England, told Live Science. The presence of two sets of teeth makes Brasilodon a diiodon – the name given to an animal with two sets of teeth – and suggests that Brasilodon is more closely related to mammals than reptiles.

“We don’t have any example of a reptile that has only two sets of teeth,” Richter said. “The ancient reptiles were constantly exchanging their teeth at different speeds.”

Studies show that diiodontic teeth are the result of a “genetic chain of events” that regulates not only the shape of the skull and teeth, but also the bodily functions associated with mammals, such as endothermia (the ability to metabolically regulate the heat). of the body), breastfeeding and hair growth, Richter said.

“They are all connected 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 first known mammal would make it part of the evolutionary line that survived two mass extinction events, including an asteroid attack that wiped out non-avian dinosaurs at the end of the Cretaceous period 66 million years ago and allowed mammals to diversify on land and at sea.

However, not everyone is convinced by the study results. “The study presents no data to support a change in Brasilodon’s position,” Simone Hoffmann, an associate professor at the New York Institute of Technology, USA, who specializes in the evolutionary history of early mammals, told Live Science.

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

While this was common practice in the scientific community, he said, these groups should now be seen as separate. “Mammals are a clade that is further back in the evolutionary tree than mammals,” she said. “Mammals include the fossils that preceded mammals as well as mammals.”

Morganucodon is the oldest known mammalian form, while Brasilodon is typically placed as a twin group alongside mammalian forms in the evolutionary tree, or as a twin group of the next larger clade, mammals, according to Hoffmann.

Diodontic teeth are a known feature of mammalian forms, but while finding this feature in Brasilodon is “exciting,” that doesn’t necessarily mean it belongs to other mammalian forms, he 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 distinguishing feature of birds. We now know that feathers originated much earlier and are common in many dinosaurs. But just because Tyrannosaurus Rex has been reconstructed with feathers doesn’t make it a bird. “

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

through living science

Featured Image: Illustration / Wiley Anatomical Society

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Source: Olhar Digital