534 FOSSIL VERTEBRATES/Mesozoic Mammals assigned to metatherians and eutherians Eutherians and metatherians can be distinguished by many characteristics of molars, anterior dentition, ear region, ankle bones, and wrist bones The Late Cretaceous metatherians can be recognized by the presence of three premolars and four molars (seven positions of postcanines), with a single replacement at the last premolar, an inflected angular process of the mandible, and its posterior shelf, the masseteric muscle fossa Other diagnostic features are from the wrist bones or carpals, such as enlargement of three carpal bones (the hamate, the triquetrum, and the scaphoid), enabling a more forceful hand grip These marsupiallike characteristics are supplemented by features of ankle bones or tarsals Metatherians can be recognized by features of the astragalar bone (or talus) and the heel bone (or calcaneus) The derived marsupiallike features in the ankle are correlated to a greater mobility between the tarsal bones and tibia and fibular at the upper ankle joint, and among the tarsals at the trans-tarsal joint These features contributed to a greater range of the medial and lateral movement of the hindfoot By contrast, the Early Cretaceous eutherians can be recognized by the presence of five premolars and three molars (eight positions of postcanines) and the presence of certain features related to the internal carotid artery and its branches in the petrosal bone of the ear region The ankle bones of the Cretaceous eutherians (with a narrow head and pulley-like upper joint in the astragalar bone) are different from those of all non-eutherian mammals The newly discovered Eomaia and Sinodelphys help to establish the ancestral anatomical conditions from which modern marsupials and placentals could have evolved, and provide evidence for the sequence of evolutionary acquisition of the marsupial-like features in the metatherian lineage and the acquisition of placental-like characters in the eutherian lineage Current evidence suggests that the foremost phylogenetic distinctions between metatherians (including marsupials) and eutherians (including placentals) reside in wrist and ankle anatomy, followed by the diagnostic characteristics of marsupial and placental dentitions, such as the reduced dental replacement related to specialized marsupial life history pattern, plus many characteristics in placental and marsupial molar crown morphology Cretaceous metatherians and eutherians also show remarkably diverse locomotory adaptations in the features of hands and hindfeet The phalangeal proportion of the finger bones and the shape of the claws suggest that both Eomaia and Sinodelphys were capable of branch walking and climbing on uneven substrates (Figure 3) The skeletons of many Late Cretaceous eutherians clearly show that these were terrestrial mammals Locomotory skeletal structures of the Cretaceous eutherians and metatherians indicate that their locomotory adaptations were diverse, and this may have facilitated diversification of early eutherians and metatherians into different ecological niches See Also Fossil Vertebrates: Palaeozoic Non-Amniote Tetrapods; Placental Mammals Mesozoic: End Cretaceous Extinctions Further Reading Allin EF and Hopson JA (1992) Evolution of the auditory system in Synapsida (‘‘mammal like reptiles’’ and primi tive mammals) as seen in the fossil record In: Webster DB, Fay RR, and Popper AN (eds.) The Evolutionary Biology of Hearing, pp 587 614 New York: Springer Verlag Cifelli RL (2001) Early mammalian radiations Journal of Paleontology 75: 1214 1226 Hopson JA (1994) Synapsid evolution and the radiation of non eutherian mammals In: Spencer RS (ed.) Major Features of Vertebrate Evolution, pp 190 219 Knox ville, TN: The Paleontological Society Kemp TS (1982) Mammal like Reptiles and the Origin of Mammals London: Academic Press Kielan Jaworowska Z, Cifelli RL, and Luo Z X (2004) Mammals from the Age of Dinosaurs: Origins, Evolution and Structure New York: Columbia University Press Luo ZX, Cifelli RL, and Kielan Jaworowska Z (2001) Dual origin of tribosphenic mammals Nature 409: 53 57 Luo ZX, Crompton AW, and Sun AL (2001) A new mammal from the Early Jurassic and evolution of mammalian characteristics Science 292: 1535 1540 Luo ZX, Kielan Jaworowska Z, and Cifelli RL (2002) In quest for a phylogeny of Mesozoic mammals Acta Palaeontologica Polonica 47: 78 Luo ZX, Ji Q, Wible JR, and Yuan CX (2003) An Early Cretaceous tribosphenic mammal and metatherian evolution Science 302: 1934 1940 Novacek MJ (1992) Mammalian phylogeny: shaking the tree Nature 356: 121 125 McKenna MC and Bell SK (1997) Classification of Mammals Above the Species Level New York: Columbia University Press Rowe TB (1993) Phylogenetic systematics and the early history of mammals In: Szalay FS, Novacek MJ, and McKenna MC (eds.) Mammal Phylogeny: Mesozoic Differ entiation, Multituberculates, Monotremes, Early Therians, and Marsupials, pp 129 145 New York: Springer Verlag Szalay FS (1994) Evolutionary History of the Marsupials and an Analysis of Osteological Characters Cambridge: Cambridge University Press Wible JR, Novacek MJ, and Rougier GW (2004) New data on the skull and dentition in the Mongolia Late Cret aceous eutherian mammal Zalambdalestes Bulletin of the American Museum of Natural History 281: 144