A NEW SYMMETRODONT MAMMAL FROM CHINA AND ITS IMPLICATIONS FOR MAMMALIAN EVOLUTION

A new symmetrodont mammal has been discovered in the Mesozoic era (Lat e Jurassic or Early Cretaceous period) of Liaoning Province, China, Ar chaic therian mammals, including symmetrodonts, are extinct relatives of the living marsupial and placental therians. However, these archaic therians have been mostly documented by fragmentary fossils. This new fossil taxon, represented by a nearly complete postcranial skeleton a nd a partial skull with dentition, is the best-preserved symmetrodont mammal yet discovered. It provides a new insight into the relationship s of the major lineages of mammals and the evolution of the mammalian skeleton. Our analysis suggests that this new taxon represents a part of the early therian radiation before the divergence of living marsupi als and placentals; that therians and multituberculates are more close ly related to each other than either group is to other mammalian linea ges; that archaic therians lacked the more parasagittal posture of the forelimb of most living therian mammals; and that archaic therians, s uch as symmetrodonts, retained the primitive feature of a finger-like promontorium (possibly with a straight cochlea) of the non-therian mam mals. The fully coiled cochlea evolved later in more derived therian m ammals, and is therefore convergent to the partially coiled cochlea of monotremes.