Convergent dental adaptations in pseudo-tribosphenic and tribosphenic mammals

Tribosphenic or 'grinding' molars seem to have evolved twice in mammals: once in the descendants of marsupials and placentals and once in a group of mammals whose descendants include the monotremes or egg-laying mammals. A new specimen of the latter, from the Middle Jurassic of Inner Mongolia, has teeth with very advanced morphology compared with the primitive nature of the rest of its body. This highlights the diversity of the ancient mammals — these fossil beds also yielded a beaver-like swimming mammal and an extremely archaic gliding mammal. A 'pseudotribosphenic' mammal from the Middle Jurassic whose teeth have a very advanced morphology for mammals of such an early date compared with the primitive nature of the rest of its body, is described. The find confirms the previously unexpected diversity of the most ancient mammals, as the same fossil beds (in Inner Mongolia) had previously revealed remains of a beaver-like swimming mammal. Tribosphenic molars of basal marsupials and placentals are a major adaptation, with the protocone (pestle) of the upper molar crushing and grinding in the talonid basin (mortar) on the lower molar1,2,3,4. The extinct pseudo-tribosphenic mammals have a reversed tribosphenic molar in which a pseudo-talonid is anterior to the trigonid, to receive the pseudo-protocone of the upper molar. The pseudo-protocone is analogous to the protocone, but the anteriorly placed pseudo-talonid is opposite to the posterior talonid basin of true tribosphenic mammals5,6,7. Here we describe a mammal of the Middle Jurassic period with highly derived pseudo-tribosphenic molars but predominantly primitive mandibular and skeletal features, and place it in a basal position in mammal phylogeny. Its shoulder girdle and limbs show fossorial features similar to those of mammaliaforms and monotremes, but different compared with those of the earliest-known Laurasian tribosphenic (boreosphenid) mammals. The find reveals a much greater range of dental evolution in Mesozoic mammals than in their extant descendants, and strengthens the hypothesis of homoplasy of ‘tribosphenic-like’ molars among mammals.