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News on Marsupial diversity

September 9, 2013

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

Marcelo R. Sánchez-Villagra, Madeleine Geiger & Analía M. Forasiepi
Paläontologisches Institut und Museum der Universität Zürich, Switzerland.

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 metatharians 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.

TL;DR: Basically, marsupials are not restricted morphologically as previously thought, and therefore are not “inferior” to placentals.

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