[MM] Akidolestes cifellii
Symmetrodontans
DESCRIBER Li and Luo, 2006
TIME Cretaceous Early Barremian Aptian
CLASS Mammalia
ORDER Incertae Sedis
FAMILY Spalacotheriidae
SUB FAMILY Spalacolestinae
FOSSILSITE Yixian Formation, Liaoning Province, China
INFO Akidolestes is interesting because its front half shares many traits with therian mammals (the relatively advanced group made up of marsupial and placental mammals). But its spine, pelvis, and hindlimb are similar to the egg-laying monotremes, which have a more sprawling gait. Scientists think that Akidolestes is more closely related to therians than monotremes, but that it re-evolved a backside similar to those of its distant ancestors.
Named as Akidolestes after its pointed rostrum, Akidolestes is about 4 inches (or 12 cm) long and estimated to weigh about 15 to 20 grams; it has triangular teeth cusps and shearing crests for feeding on insects and worms. The skull, fore-limb, and shoulder of Akidolestes show many advanced features of therians that the scientists conclude that is an extinct relative to the more modern therians including marsupials and placentals.
Akidolestes, however, is very unusual in the vertebral column, the pelvis and the hind-limb. The back-half of its skeleton is primitive and very similar to egg-laying monotreme mammals. Dr. Zhe-Xi Luo, curator of vertebrate paleontology at Carnegie Museum of Natural History, noted that \"This new fossil is a chimera of body structures of different kinds of mammals. Its front half resembles those of more derived marsupials and placentals, but its back half is unmistakably monotreme-like.\" Its skeleton is a mosaic of structures that are found either in modern monotremes, or in modern therians, but never together in one animal
Li and Luo (2006) A Cretaceous symmetrodont therian with some monotreme-like postcranial features Nature 439, 195-200
A new spalacotheriid mammal preserved with a complete postcranium and a partial skull has been discovered from the Yixian Formation of Liaoning, China. Spalacotheroid symmetrodonts are relatives to modern therians (combined group of marsupials and placentals) and are characterized by many skeletal apomorphies of therians. But unlike the closely related spalacotheroids and living therians, this new mammal revealed some surprisingly convergent features to monotremes in the lumbar vertebrae, pelvis and hindlimb. These peculiar features may have developed as functional convergence to locomotory features of monotremes, or the presence of lumbar ribs in this newly discovered mammal and their absence in its close relatives might be due to evolutionary developmental homoplasy. Analysis including this new taxon suggests that spalacotheroids evolved earlier in Eurasia and then dispersed to North America, in concordance with prevailing geodispersal patterns of several common mammalian groups during the Early Cretaceous period.