The mammal fauna in the Early Cretaceous Jehol Biota: implications for diversity and biology of Mesozoic mammals

Eleven species belonging to five major groups of mammals (multituberculates, eutriconodontans, 'symmetrodontans', metatherians and eutherians) have been described from the Jehol Biota, Liaoning, China. These fossils came from three horizons of the Yixian Formation: Lujiatun (lowest), Jianshangou and Dawahgzhangzi (highest) beds. Ages and correlations of these beds are still in debate, but are generally accepted as Early Cretaceous. Biostratigraphic distributions of some Jehol mammals corroborate the Early Cretaceous age of the biota. Many species are represented by skulls and articulated skeletons, although in most cases a species is known only from a single holotype that is squashed. These fossils furnish a wealth of morphological data for Mesozoic mammals, from which large character data sets have been amassed to generate competing higher-level phylogenetic hypotheses of mammals. The Early Cretaceous divergence of eutherians is first documented as skull and skeletal fossils. The Jehol mammals indicate a diverse mammal fauna in which species range from 25 g to 14 000 g in body masses, have insectivorous, omnivorous and carnivorous diets (as reflected by their dentitions and by stomach content), and acquire scansorial, possibly arboreal and terrestrial habits (as inferred from articulated skeletons). The well-preserved material helps to clarify some anatomical uncertainties in the study of early mammals, such as an ossified Meckel's cartilage as the occupant for the internal groove on the lower jaw of some Mesozoic species and a dental formula I3-C1-P3-M4/i2-c1-p2-3-m5 for gobiconodontids as suggested by dentitions of several Jehol eutriconodontans. Evidence from cranial specimens of Jehol eutriconodontans also disfavours the brain-expansion model for the detachment of middle car ossicles from the dentary during evolution of mammals. Copyright (c) 2006 John Wiley & Sons, Ltd.