A new spalacolestine mammal from the Early Cretaceous Jehol Biota and implications for the morphology, phylogeny, and palaeobiology of Laurasian 'symmetrodontans'

摘要

Symmetrodontans' are extinct mammals characterized by having a reversed-triangle molar pattern in which three main cusps define a triangular molar crown. This dental morpholgy has been regarded as being intermediate between the triconodont' tooth and the tribosphenic pattern characterizing therians; it is a key feature in taxonomy of Mesozoic mammals and one to understand mammalian evolution and palaeobiology. Here we report a new genus and species of symmetrodontan' mammal, Lactodens sheni, from the Early Cretaceous Jehol Biota, represented by a partial skeleton with dentary and upper and lower teeth with dental morphologies well-preserved. The new species has a dental formula of three upper incisors, one canine, three premolars, and six molars/three lower incisors, one canine, five premolars and six lower molars, double-rooted canines, extremely low-crowned and transversely thin premolars, and acute angled molars. The dental morphologies of molars and peculiar deciduous premolars are similar to those of Spalacolestes from North America. The associated upper and lower dentitions from one individual animal helped to clarify tooth identification of some spalacotheriids represented only by fragmentary material. Phylogenetic analyses indicate a close relationship of the new species to North American spalacolestines and faunal interchanges between Eurasia and North America, thus supporting the notion that small-bodied spalacotheriids were diverse and had a pan-Laurasian distribution during the Early Cretaceous. Absence of the Meckelian groove suggests acquisition of the definitive mammalian middle ear in spalacolestines, and deciduous canines and premolars in the slim and extremely long dentary imply a faunivorous diet.