According to a controversial study, mammals coexisted with some of the earliest dinosaurs.
According to a controversial new study, the earliest known mammal was a tiny, shrewlike animal that lived alongside the first dinosaurs 225 million years ago, pushing back the appearance of mammals by about 20 million years.
An international team of researchers examined the fossilized remains of an 8-inch (20-centimeter) animal called Brasilodon quadrangularis and determined it was a mammal because it grew two sets of teeth over its lifetime, as do most mammals, including modern humans. According to the authors, B. quadrangularis is now the earliest mammal known to science, appearing in the fossil record about 20 million years before Morganucodon, which was previously thought to be the earliest known mammal.
However, early mammal classification is difficult, and according to a researcher who was not involved in the study, neither B. Despite their mammal-like dentition, neither quadrangularis nor Morganucodon are mammals and belong further back on the evolutionary tree.
Researchers look for evidence of mammal ancestry in mineralized bones and teeth because milk-producing mammary glands, which help define mammals today, have not been found in the fossil record. Mammalian dental characteristics include the presence of two sets of teeth: baby teeth and adult teeth. Reptiles and fish, on the other hand, can frequently regrow their teeth (if they have them) and go through multiple sets as juveniles and adults.
The researchers looked at three B. quadrangularis lower jaws — one juvenile jaw and two adult jaws — discovered at the Linha So Luiz site in southern Brazil decades ago alongside some of the earliest dinosaurs. The team used a destructive technique of sectioning the jaws to see the teeth developing inside them; southern Brazilian universities have a lot of B. quadrangularis fossils, the jaws were not too valuable to cut.
“What we discovered is that Brasilodon only changes its teeth once,” said co-author Martha Richter, a scientific associate at London’s Natural History Museum.
B. is distinguished by the presence of two sets of teeth. quadrangularis is a diphyodont — an animal with two sets of teeth — and suggests that B. Quadrangularis is related to mammals rather than reptiles.
“There are no examples of reptiles with only two sets of teeth,” Richter said. “The old reptiles’ teeth were constantly changing at different rates.”
According to Richter, studies show that diphyodont teeth are the result of a “genetic chain of events” that regulates not only the shape of the skull and teeth but also bodily functions associated with mammals, such as endothermy (the ability to metabolically regulate body heat), lactation, and fur growth. “They’re all connected,” she added.
B. quadrangularis resembled a mammal, with a long tail and other shrew-like characteristics. As the earliest known mammal, it would be part of an evolutionary line that survived two mass extinction events, including the asteroid strike that wiped out nonavian dinosaurs at the end of the Cretaceous period 66 million years ago, allowing mammals to diversify across land and sea. However, not everyone agrees with the study’s findings.
“The study presents no data to support a change in Brasilodon’s position,” said Simone Hoffmann, an associate professor at the New York Institute of Technology who specializes in the evolutionary history of early mammals.
“All other research indicates that Brasilodon is not a mammal, certainly not the oldest mammal, and is not even a mammaliaform (mammals and their closest relatives).”
In the study, the authors use the terms “mammals” and “mammaliaforms” almost interchangeably, according to Hoffmann. While this was once common practice in the scientific community, she believes these groups should now be considered distinct. “Mammaliaforms are a clade that is more distant on the evolutionary tree than mammals,” Hoffmann explained. “Mammaliaforms include both the fossils that led up to mammals and mammals themselves.”
The earliest known mammaliaform is Morganucodon, while B. According to Hoffmann, quadrangularis is typically placed on the evolutionary tree as a sister group to mammaliaforms or as a sister group to the next larger clade, mammaliamorphs.
Diphyodont teeth are a well-known feature of mammaliaforms, but it was discovered on B. Although quadrangularis is exciting, it does not necessarily imply that B. According to Hoffmann, quadrangularis belongs with other mammaliaforms. Animals inherit characteristics from their ancestors, but life is constantly evolving and changing, resulting in the formation of new branches on the evolutionary tree.
“Feathers were once thought to be a distinguishing feature of birds,” said Hoffmann. “We now know that feathers arose much earlier and were common in many dinosaurs, but just because T. rex has feathers does not make it a bird.”
Richter acknowledged that the study’s findings could be contested, noting that the classification of these early mammal groups is contentious and that the debate over how to classify the earliest mammals is ongoing.
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