Why the Long Face? Kangaroos and Wallabies Follow the Same ‘Rule’ of Cranial Evolutionary Allometry (CREA) as Placentals
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Among closely related species, larger mammals tend to have a longer face and proportionally smaller braincase. This putative ‘rule’ in mammalian macroevolution has been proposed for the first time in 2013 based on 3D geometric morphometrics of antelopes, fruit bats, tree squirrels and mongooses. To firmly demonstrate that this trend holds as a ‘rule’ requires expanding the analysis in more lineages and other mammalian orders: if supported in most groups, it may indeed become a new evolutionary ‘rule’ besides famous ones such as Bergmann’s and Allen’s. In this study, using statistical shape analysis and both standard and comparative methods on a sample of kangaroos, wallabies and other macropodine marsupials, we show that the ‘big size-long face’ pattern is indeed found also outside the placentals. This provides support to the hypothesis of an important role of size-related shape changes (i.e., allometry) in the origin of the exceptional disparity of mammals, that, only in terms of size, span more orders of magnitude than any other animal: from 3 to 4 g of a tiny bat to more than 100 tons in blue whales.
KeywordsComparative methods Geometric morphometrics Macroevolution Macropodines Marsupials 3D landmarks
We thank Robert Meredith (University of California, Riverside) for kindly helping us with the phylogenetic background, including the nexus file used to do the comparative analyses. Curators and collection managers at museums throughout Australia allowed access to their collections. AC is grateful to Krish Seetah (Stanford University), the Lang Fellowship and the Departments of Archaeology and Anthropology of Stanford University, for providing the stimulating academic environment in which this study was finalized, and to SYNTHESYS, an EC-funded Project for an integrated European infrastructure for natural history collections, for supporting both the previous study on CREA in placentals and its follow up in 2015. Many thanks also to Dean Adams (Indiana University), Emma Sherratt (University of New England) and Mark Collyer (Western Kentucky University) for their help with Geomorph and the interpretation of the differences in the phylogenetic regressions using MorphoJ and Geomorph. Finally, we are also in debt to the Editor-in-Chief and an anonymous reviewer for suggesting to better address the issues of sexual dimorphism and sampling error: thanks to their input, we added several analyses, which greatly improved the quality of the study and helped to strengthen its main conclusions.
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