Convergent dental adaptations in pseudo-tribosphenic and tribosphenic mammals.
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 molar. 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 mammals. 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.
Luo
Zhe-Xi
ZX
Carnegie Museum of Natural History, Pittsburgh, Pennsylvania 15213, USA. luoz@carnegiemnh.org
Ji
Qiang
Q
Yuan
Chong-Xi
CX
D000818:Animals; D005075:Biological Evolution; D002681:China; D005580:Fossils; D049690:History, Ancient; D008322:Mammals / Q000033:anatomy & histology*; D008334:Mandible / Q000033:anatomy & histology; D008963:Molar / Q000033:anatomy & histology*; D010802:Phylogeny; D012863:Skeleton