The marsupial-placental mammal dichotomy revisited: new morphological data and the relevance of geography on evolutionary patterns of diversity and disparity

SANCHEZ-VILLAGRA M.; GEIGER, M.; FORASIEPI, A.M.

Edinburgh

Simposio; 61th ANNUAL SYMPOSIUM OF VERTEBRATE PALAEONTOLOGY AND COMPARATIVE ANATOMY; 2013

Placentals occupy a larger morphospace and are taxonomically more diverse than marsupials, even considering the rich ecomorphological diversity of fossils. This contrasting evolutionary pattern has been coupled with biases introduced by marsupials? developmental features, including altriciality and functional requirements around birth and postnatal life. For example, unlike placentals, marsupials maintain many epiphyseal growth plates separated throughout life, most likely the derived state. The relevance of life history features in imposing constraints on morphological evolution is at best speculative. There are numerous cases of circumvention of developmental biases, such as the autopodial specializations of marsupial moles and the ever growing sabre-tooth of thylacosmilids. These suggest that other factors produced the marsupial pattern of restricted morphospace. Phylogenetic and geographic data offer new insights on this issue. In the Cretaceous and Palaeogene faunas from North and South America, metatherians have been more than or as diverse as eutherians. There are no positive tests of competitive displacement of metatherians by eutherians. The diversification of Marsupialia, including the differentiation into its major clades occurred about 20 Ma, more recently than that of Placentalia. The geographic pattern of taxonomic and morphological diversity within Placentalia mirrors that of placentals versus marsupials: northern clades are more diverse (ca. 4,800 spp.) than southern ones (200 spp.) and include those that are outliers in taxonomic (rodents; bats) and ecomorphological (whales; bats) richness. The differential diversity and disparity among therians is more a reflection of ?opportunity? than a bias in the production of morphological variants during development in marsupials.