A Chinese triconodont mammal and mosaic evolution of the mammalian skeleton

Abstract

Here we describe a new triconodont mammal from the Late Jurassic/Early Cretaceous period of Liaoning, China. This new mammal is represented by the best-preserved skeleton known so far for triconodonts which form one of the earliest Mesozoic mammalian groups with high diversity. The postcranial skeleton of this new triconodont shows a mosaic of characters, including a primitive pelvic girdle and hindlimb but a very derived pectoral girdle that is closely comparable to those of derived therians. Given the basal position of this taxon in mammalian phylogeny, its derived pectoral girdle indicates that homoplasies (similarities resulting from independent evolution among unrelated lineages) are as common in the postcranial skeleton as they are in the skull and dentition in the evolution of Mesozoic mammals. Limb structures of the new triconodont indicate that it was probably a ground-dwelling animal.

A new mammal skull from the Lower Cretaceous of China with implications for the evolution of obtuse-angled molars and ‘amphilestid’ eutriconodonts

“…This morphology of the anterior dentition is seen in the 'amphilestid' Phascolotherium, but differs from the enlarged incisors and the reduced and procumbent canines of gobiconodontids (Kielan-Jaworowska et al 2004). The lower canine is large but conical in the Triconodontidae (Simpson 1929;Kielan-Jaworowska et al 2004) and conical, procumbent and small in the Jeholodentidae ( Ji et al 1999;Luo et al 2007a). …”

“…Phylogenetic analyses that include eutriconodonts tend to show only weak support for the group and a labile position among Mesozoic mammals (Ji et al 1999;Luo et al 2002;Meng et al 2003). As a result, the composition and arrangement of the Eutriconodonta have largely been considered provisional.…”

“…Two other families, the Gobiconodontidae and Jeholodentidae, have molars that are generally similar to amphilestids but are more robust in overall form. Gobiconodontids are known from the Early Cretaceous of Europe, Asia and North America (Kielan-Jaworowska et al 2004), and jeholodentids are only known from the Early Cretaceous of Asia (Ji et al 1999;Luo et al 2007a).…”

“…Jenkins & Schaff 1988;Ji et al 1999;Meng et al 2006;Luo et al 2007a,b). The new cranial and postcranial data have broadened our anatomic, taxonomic and palaeoecological views of Mesozoic mammals and led to major revisions of traditional hypotheses on phylogenetic relationships and morphological transformations, which previously hinged on fragmentary jaws and isolated teeth (e.g.…”

“…They have mainly been distinguished by postcanine characteristics. The Amphilestidae have molars with one dominant central cusp, and two smaller subequal cusps on either side; molars that occlude at the embrasure of opposing molars; lower molars that lack a tongue-in-groove interlock; and premolars that tend to be symmetrical (Simpson 1928;Ji et al 1999). They are known from the Middle Jurassic of Europe, the Late Jurassic of Asia, Africa and North America, the Early Cretaceous of Asia, and possibly the EarlyMiddle Jurassic of India and Middle Jurassic of South America (Kielan-Jaworowska et al 2004;Averianov et al 2005;Rougier et al 2007a,b).…”