Why the Long Face? Kangaroos and Wallabies Follow the Same ‘Rule’ of Cranial Evolutionary Allometry (CREA) as Placentals

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.