Paleontologists have examined the fossilized lower jaws of Brasilodon quadrangularis, a mouse-sized creature that lived in Brazil during the Late Triassic epoch, some 225 million years ago. The analysis of the different growth stages showing tooth development in each of the Brasilodon fossils provided evidence that these were the remains of a mammal. Previously, the earliest accepted record in geological time of mammals was 205 million years ago.
Mammalian glands, which produce milk and feed the young of mammals today, have not been preserved in any fossils found to date.
Therefore, scientists have had to rely on fossilized bone and teeth for alternative clues.
The dental records date 225 million years ago, 25 million years after the Permian-Triassic mass extinction event that led to the extinction of roughly 70% of terrestrial vertebrate families.
A tiny animal called Morganucodon is usually considered the first mammal but its oldest fossils, only represented by isolated teeth, date from around 205 million years ago.
Examining the dentitions of Brasilodon quadrangularis, Dr. Martha Richter from the Natural History Museum, London, and colleagues discovered evidence of only one set of replacement teeth — a key feature of mammals known as diphyodonty.
“The first set starts developing during the embryonic stage and a second and last set of teeth develops once the animal is born,” the researchers explained.
“By contrast, reptilian dentitions are different, especially in that replacement is ‘many for one’ (polyphyodonty), in which each tooth site has tooth regeneration many times over the lifetime of a reptile to replace damaged ones.”
Diphyodonty is a complex and unique phenomenon that, with tooth replacement, also involves profound, time-controlled changes to the skull anatomy, for instance, the closure of the secondary palate (the roof of the mouth) that allows the young to suckle, while breathing at the same time.
“Comparative studies with recent mammal dentitions and tooth replacement modes suggest that Brasilodon was a placental, relatively short-lived animal,” Dr. Richter said.
“Dated at 225.42 million years old, this is the oldest known mammal in the fossil record contributing to our understanding of the ecological landscape of this period and the evolution of modern mammals.”
“The evidence from how the dentition was built over developmental time is crucial and definitive to show that brasilodons were mammals,” added Professor Moya Meredith Smith, a researcher at King’s College London.
“Our paper raises the level of debate about what defines a mammal and shows that it was a much earlier time of origin in the fossil record than previously known.”
The paper appears this week in the Journal of Anatomy.
Sergio F. Cabreira et al. Diphyodont tooth replacement of Brasilodon – A Late Triassic eucynodont that challenges the time of origin of mammals. Journal of Anatomy, published online September 5, 2022; doi: 10.1111/joa.13756
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