Earliest mammal

The earliest mammal currently known to science is Brasilodon quadrangularis, which existed 225.42 million years ago, during the Late Triassic Period, in what is now Brazil. Presently known from fossilized teeth, skull and bone samples but no complete skeletons, it is believed to have been a small shrew-like creature, estimated to have measured 12–20 centimetres (4.7–7.9 inches) in total length and was probably insectivorous. The details around its ageing were published in the Journal of Anatomy on 5 September 2022.

Prior to the discovery and dating of this species' oldest confirmed remains, the earliest-known mammal was Adelobasileus cromptoni. It was formally described and named in 1990 by two New Mexico Museum of Natural History scientists after a partial skull had been obtained from rocks in what is now Texas, USA, and which dated back 225 million years ago. It probably resembled in outward appearance the modern-day tree shrews belonging to the taxonomic order Scandentia.

Significantly, B. quadrangularis only possessed two successive sets of teeth – a condition called diphyodonty. The first set (deciduous teeth) developed during the animal's embryonic development, and was replaced by a second set (adult teeth) once it was born. Its dental replacement pattern occurred with the same temporal and morphological pattern that is a key feature of mammals, and differs from that of reptiles, which undergo multiple dental replacements, i.e., polyphyodonty.

The study was a collaboration between Associação Sul Brasileira de Paleontologia, Universidade Federal do Rio Grande do Sul, Universidade Unilasalle, Universidade Federal do Rio de Janeiro (all Brazil), the Natural History Museum and Kings College London (both UK), led by Dr Sérgio Furtado Cabreira (Brazil).