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What are the intrinsic factors responsible for shaping mammalian biodiversity? This question is highly relevant to discussion of the two groups of therian mammals alive today, marsupials (kangaroos, possums, etc.) and placentals (humans, bats, whales, etc.). Despite arising at the same time, marsupials have never achieved the taxonomic or morphologic diversity of their sister group, the placentals (Lillegraven 1975; Kirsch 1977; Sears 2004; Cooper and Steppan 2010). To explain this phenomenon, scientists hypothesized that the evolution of marsupials had been constrained relative to that of placentals as a result of marsupials’ unique mode of reproduction (Lillegraven 1975; Klima 1987; Sanchez-Villagra and Maier 2003).
Subsequent research confirmed that marsupials are less morphologically diverse and specialized than placentals (Sears 2004; Cooper and Steppan 2010; Kelly and Sears 2011a). Sears (2004) found that the shoulder girdles of living and extinct adult marsupials are less diverse than those of adult placentals, and adult marsupial scapulae are less morphologically diverse than adult marsupial pelves, as predicted by the marsupial constraint. Cooper and Steppan (2010) and Kelly and Sears (2011a) found that this pattern extended to forelimb versus hind limb comparisons in living mammals. Sears (2004) also linked this reduction in morphological diversity to a reduction in morphologic variation during development as a result of the functional requirements on the marsupial newborn.
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