A study of a mouse advances the origin of mammals 20 million years

09/07/2022 Act. at 15:44 CEST The analysis has been carried out on fossils of Brasilodon quadrangularis, a species that lived 225 million years ago and had been cataloged as a reptile An examination of the jaws in fossils of Brasilodon quadrangularis , a mouse of about 20 centimeters that lived 225 million years ago, delays the time of the origin of mammals. The analysis of the different stages of growth that show the development of the teeth in each of the fossils showed that they were the remains of a mammal. Previously, the oldest accepted record in geologic time for mammals is 205 million years ago. This new research suggests a much earlier origin of mammals by 20 million years. “Evidence for how the dentition was built during the time of development is crucial and definitive in proving that brazilodonts were mammals. Our paper raises the level of the debate about what defines a mammal and shows that it was a much earlier time of origin.” earlier in the fossil record than previously known,” says Moya Meredith Smith, a contributing author and Emeritus Professor of Evolution and Development of Dentoskeletal Anatomy at King’s College London. Mammalian glands, which produce milk and feed mammalian young today, have not been preserved in any fossil found to date. Therefore, scientists have had to rely on “hard tissues,” mineralized bone, and fossilizing teeth for alternative clues. Examining dentitions found in Brasilodon quadrangularis fossils from southern Brazil, dating to around 225 million years ago (Late Triassic/Norian), the research team uncovered evidence of a single set of replacement teeth. This is a key feature of mammals known as diphodonty. The first set begins to develop during the embryonic stage and a second and final set of teeth develops once the animal is born. In contrast, reptile dentitions are different, especially as replacement is “many for one” (polyphodontia), in which each tooth has regeneration of teeth many times during a reptile’s life to replace damaged ones. Diphyodontia is a complex and unique phenomenon that, with tooth replacement, also involves profound and time-controlled changes in the anatomy of the skull, for example, the closure of the secondary palate (the roof of the mouth) that allows the young suck, while breathing at the same time. “This research is a collaboration between Brazilian and British scientists, who brought together their expertise in skull development, dental anatomy, physiology and histology to interpret juvenile and adult fossils of the extinct species Brasilodon quadragularis,” says Dr. Martha Richter, scientist associated. at the Museum of Natural History and lead author of the article. Brasilodon existed at the same time as the oldest known dinosaurs and probably lived in burrows like shrews today. The new research pushes back the origin of difodonty in Brasilodon, and thus also in mammals, with related biological traits by 20 million years and illuminates the debate about the rise of mammals in deep time. The finding is reported in the Journal of Anatomy.