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Fossil records of Brasilodon date back 225 million years, predating Morganucodon, the previously confirmed first mammal, by approximately 20 million years
· The earliest known extinct mammal has been identified using fossil dental records
· The tiny animal existed at the same time as some of the oldest dinosaurs and sheds light on the evolution of modern mammals
· Scientists from the Natural History Museum and King’s College London contributed to this international collaboration led by the Federal University of Rio Grande do Sul (UFRGS) in Porto Alegre
New research has identified the fossil dental records of the oldest known mammal - Brasilodon quadrangularis - a small ‘shrew-like’ animal that measured around 20cm in length and had two sets of teeth.
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 ‘hard tissues’, mineralised bone and teeth that do fossilise, for alternative clues.
The dental records date 225 million years ago (Late Triassic/Norian), 25 million years after the Permian-Triassic mass extinction event that led to the extinction of roughly 70% of terrestrial vertebrate families. Morganucodon is usually considered the first mammal but its oldest fossils, only represented by isolated teeth, date from around 205 million years ago.
Dr Martha Richter, Scientific Associate at the Museum and senior author on the paper says, ‘Comparative studies with recent mammal dentitions and tooth replacement modes suggest that this was a placental, relatively short-lived animal. 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.
Brasilodon is the oldest extinct vertebrate with two successive sets of teeth which includes only one set of replacements, also known as a 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 dental replacement pattern occurs with the same temporal and morphological pattern that is a key feature of mammals. This differs from that of reptiles who regenerate new teeth multiple times during their lives, the ‘many for one’ replacement also known as poliphyodonty.
Diphyodonty is a complex and unique phenomenon that along with tooth replacement 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. It has also been shown to be linked to endothermy and even placentation (live birth) and fur.
Brasilodon existed at the same time as the oldest known dinosaurs and probably lived in burrows like the shrews today.
This new research pushes back the origin of diphyodonty in Brasilodon and mammals with related biological traits by 20 million years and illuminates the debate about the rise of mammals in deep time.
Prof Moya Meredith Smith, contributing author and Emeritus Professor of Evolution and Development of Dentoskeletal Anatomy at King’s College London says, ‘The evidence from how the dentition was built over developmental time is crucial and definitive to show that Brasilodons were mammals. 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.’
Dr Martha Richter concludes, ‘This research is a collaboration between Brazilian and British scientists, who brought together their expertise on skull development, dental anatomy, physiology and histology to interpret the juvenal and adult fossils of the extinct species Brasilodon quadragularis.’
The paper Diphyodont tooth replacement of Brasilodon - a Late Triassic eucynodont that 2 challenges the time of origin of mammals is published in the Journal of Anatomy.
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How to cite this paper:
Cabreira, S.F., Schultz, C.L., da Silva, L.R., Lora, L.H.P., Pakulski, C. & do Rêgo, R.C.B. et al. (2022) Diphyodont tooth replacement of Brasilodon—A Late Triassic eucynodont that challenges the time of origin of mammals. Journal of Anatomy, 00, 1–17. Available from: https://doi.org/10.1111/joa.13756
Sergio F. Cabreiraa, Cesar L. Schultzb, Lúcio R. da Silvaa, Luiz Henrique Puricelli Lorac, Cristiane Pakulskid, Rodrigo C. B. do Rêgoe, Marina B. Soaresb,f, Moya Meredith Smithg,h and Martha Richterg*
Associação Sul Brasileira de Paleontologia. Av. Vicente Pigatto, 305, Faxinal do Soturno, RS, Brazil.
Universidade Federal do Rio Grande do Sul, Instituto de Geociências, Campus do Vale Av. Bento Gonçalves, 9500 - Porto Alegre - RS - Brasil.
Porto Alegre, Brazil.
Avenida Irmão José Otão, 170/801, 90.035-060, Porto Alegre, RS, Brazil.
Rua Engenheiro João Luderitz, 204, 91130-050, Porto Alegre, RS, Brazil.
Universidade Unilasalle, Av. Vitor Barreto, 2288, Canoas, RS, Brazil.
Universidade Federal do Rio de Janeiro, Departamento de Geologia e Paleontologia, RJ, Brazil.
Earth Sciences Department, Natural History Museum, SW7 5BD London, UK.
Centre for Craniofacial and Regenerative Biology, King's College London, UK.