Brasilestes stardusti is the name given to the oldest known mammal found in Brazil. He lived in what is now the northwest of the state of São Paulo at the end of the Mesozoic Era between 87 million and 70 million years ago. It is the only Brazilian mammal known to coexist with dinosaurs.
The discovery of Brasilestes was announced on May 30, 2018 by a team led by Max Langer, a professor at the School of Philosophy, Science and Literature of Ribeirão Preto of the University of São Paulo (FFCLRP-USP). Langer's team included colleagues at the Federal University of Goiás and the University of Campinas in Brazil, the La Plata Museum in Argentina and the Massachusetts Institute of Technology (MIT) in the United States.
Physically speaking, Brasilestes consists of a fossilized premolar tooth with a maximum crown length of 3.5 mm. "The tooth is small and incomplete: the roots are missing," said paleontologist Mariela Cordeiro de Castro, first author of the article recently published in Royal Society Open Science .
"Small but not small" Castro continued. "Although it is only 3.5 mm, the tooth of Brasilestes is three times larger than all the teeth of known Mesozoic mammals.During the time of the dinosaurs, most mammals were the size of mice." Brasilestes was much larger, about the size of an opossum. " 19659005] The name of the new species pays tribute to British rock star David Bowie, who died in January 2016, a month after the fossil was discovered. Brasilestes stardusti alludes to Ziggy Stardust, an extraterrestrial character created by Bowie for a 1972 album.
The research was supported by the São Paulo-FAPESP Research Foundation as part of the thematic project "The origin and ascent of dinosaurs in Gondwana (Early Triassic-Early Jurassic), "for which Langer is the principal investigator.
The fossilized tooth was found in a rocky outcrop of the Adamantina Formation in General Salgado, State of São Paulo. The rocks are in a field on a ranch called Fazenda Buriti.
"We were visiting Mesozoic outcrops when Júlio Marsola [another member of the team] insightful as a lynx, saw a small tooth protruding from a rock," Castro said. , professor at the Federal University of Goiás (UFG).
"General Salgado's deposits are well known, several mesozoic crocodiles have come out of them, the particular outcrop in which I found Brasilestes is interesting, with dozens of Mesozoic fragments, crocodile shells." I crouched down to look more closely. a small part of the outcrop to see if there were egg shells and I saw the tooth.If I had stayed outdoors like this for a few more days, the rain would have swept
"When I noticed what seemed to resemble the base of the two roots of the tooth [the roots themselves have broken off] I thought it must be a mammal. The laboratory analysis gave us the certainty that it is in fact a mammal. "
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While a simple 3.5 mm tooth, especially an incomplete one, may seem insufficient to describe a new species of mammal, in reality, Extinct mammals are often described on the basis of a single fossilized tooth.
This is because the teeth are the most durable part of the mammalian skeleton, after all, they have to resist the contrast. fish and reptiles, for example, develop new teeth continuously throughout their lives, in fact, the teeth of mammals are often the only skeletal remains that remain intact long enough to fossilize 19659005] The fact that a single Premolar is all that remains of Brasilestes and that it is incomplete, preventing the researchers from distinguishing with absolute confidence the group of mammals to which the species belonged. a therian, member of a large subclass of Mammalia that includes marsupials and placentals.
Although there is not enough evidence to support the inclusion of Brasilestes in any of the infractions, the researchers believe (but can not conclude categorically) that it was a placental mammal. If so, the fossil is unique.
Today, there are three major groups of mammals, namely, placentals, marsupials and monotremes. All three evolved during the Mesozoic Era. At that time, however, they were by no means the only groups of mammals. There were also multituberculates, which were common in the northern hemisphere, as well as typical groups of the southern hemisphere, such as Meridiolestides and Gondwanatherians, named for Gondwana, the ancient southern supercontinent that gave birth to Africa, South America, Australia, Antarctica and India. .
The first fossils of Mesozoic mammals were found in Argentine Patagonia in the early 1980s, and about 30 species are known. Until the announcement of Brasilestes, these were the only ones found in South America. None remotely resembles the small tooth found in Brazil.
"When I showed the fossil of Brasilestes to Edgardo Ortiz-Jaureguizar, a paleontologist at the Museum of La Plata, he was very surprised, he said he had never seen anything like that, and he immediately showed it to another specialist in the same. institution, Francisco Goin, who had the same reaction, Goin said that Brasilestes did not resemble any other Mesozoic mammal found in Argentina, therefore, in South America, "recalled Castro.
Among the 30-odd Argentine species of Mesozoic mammals, there are meridiolestides, Gondwanatherians and even some multituberculate suspects. There are no marsupials or placentals. The only fossils in these two groups found in South America date from after the mass extinction that annihilated the dinosaurs 66 million years ago in an event that marks the end of the Mesozoic and the beginning of the current geological era, the Cenozoic.
In 1993, José Reinaldo Bertini, professor at the State University of São Paulo (Unesp) in Rio Claro, announced the discovery of a mammal jaw fragment with one much smaller than the Brasilestes premolar tooth. However, Bertini did not publish a detailed study of the fossil and, therefore, could not name a new species.
"Brasilestes is not only the first Brazilian Mesozoic mammal described, but also one of the few Mesozoic mammals found in more central regions" Argentine fossils were found in geological formations in Patagonia, the southern tip of the continent ", said Langer.
"In addition, Brasilestes is different from everything previously found, suggesting that placental mammals probably inhabited South America between 87.8 million and 70 million years ago," said the coordinator of the thematic project FAPESP.
Even more surprising is the Mesozoic mammal with premolars that resemble more the Brazilian tooth lived on the other side of the world, in India, makes between 70 and 66 million years old, its name is Deccanolestes, and no other creature in the global fossil record is so similar to Brasilestes.  05] How could two members of the same lineage live separately in disconnected regions? About 100 million years ago, when South America and Africa had just been separated by the opening of the South Atlantic, India was separating from Gondwana and beginning to roam the Indian Ocean.
This implies that at least 100 million years ago A few years ago, the ancestors of Brasilestes and Deccanolestes inhabited the supercontinent of Gondwana. In other words, the lineage to which Brasilestes and Deccanoles belong is much older than the ages of their fossils: between 87 million and 70 million years ago for Brasilestes, and between 70 and 66 million for Deccanolestes.
"The discovery of Brasilestes raises many more questions than answers about the biogeography of South American Mesozoic mammals," said Langer. "Thanks to Brasilestes, we have realized that the mammalian history of Gondwana is more complex than we thought".
The finding triggers speculation about the origins of xenarthrans
This could lead to new hypotheses and new lines of research. Who knows, for example, if future research inspired by the discovery of Brasilestes will reveal the origin of a typical South American group, the xenarthrans, the order of armadillos, anteaters and sloths? Castro's main research interest, in fact, is the evolutionary history of xenarthrans.
"An interesting feature of the brazilian premolar is its supertiné enamel, which is only 20 micrometers thick." Brasilestes enamel is the thinnest of any Cretaceous mammal in the fossil record.Most Mesozoic mammals have enamel in the range of 100 to 300 micrometers, "said Castro.
"Dozens of known xenarthro species are alive now, hundreds have gone extinct, only three have enamel, the microstructure of Brasilestes, the premolar enamel is very similar to that of the nine-banded armadillo," said the researcher supported by FAPESP.
According to Castro, "molecular clock evidence suggests that the xenartran lineage began at least 85 million years ago, but the oldest armadillo fossils, found in Rio de Janeiro, are about 50 million years old."
Although it is intriguing to imagine Brasilestes as an ancient xenarthran, it is too early for such a claim.
The age and provenance of Brasilestes coincide with the molecular hypotheses of the origin of the xenarthra, but it would be premature to infer a taxonomic affinity in light of the morphological differences between the tooth of Brasilestes and the armadillo teeth, "said Castro. 19659005] Langer agreed: "We have a single fossil of Brasilestes. It is not enough to draw conclusions from the fossil record, "he said.
The fact that fossils of Mesozoic mammals were not found in Brazil before Brasilestes could mean that such fossils are rare or too fragile to be preserved" Who knows, someday we can find new fossils of Brasilestes that help us to better understand its history. It could take decades, "said Langer.
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Mariela C. Castro et al., A mammal from the Upper Cretaceous of Brazil and the first radioisotope age of the Bauru Group, Royal Society Open Science (2018). DOI: 10.1098 / rsos.180482
Royal Society Open Science