![]() ![]() The four lineages (cranial landmarks shown in Figs 1 and 2 and defined in Table 1 samples described in Table 2) occupy distinct, parallel regions of the first three axes of form space (96.1, 1.7 and 1.0% of total variance, respectively) ( Fig. That the pattern is broadly similar across four orders of mammals, whose divergence started soon after (if not before) the extinction of dinosaurs, suggests that it might be a ‘rule’ with few exceptions in the adaptive radiations of placentals. Larger animals are generally long-faced and small ones have proportionally larger braincases. The study confirms that a pervasive effect of size-related craniofacial shape variation, strongly reminiscent of that found within species during ontogeny, occurs across adults of closely related species of placentals. ![]() ![]() The groups have a range of cranial sizes more than twofold within the groups and tenfold among them. These four lineages all originated in the Miocene (~15–25 million years ago) and have their most recent common ancestor in the deepest part of the Eutherian tree 15, 16, 17. To investigate the answers to these questions, we assessed size-related craniofacial changes using statistical shape analysis on three-dimensional (3D) anatomical landmarks in four speciose, ecomorphologically and phylogenetically disparate groups of placental mammals: antelopes (Cetarctiodactyla: Antilopinae and Cephalophinae) fruit bats (Chiroptera: Pteropodinae) African mongooses (Carnivora: Herpestinae) African tree squirrels (Rodentia: Sciurinae). The evidence of a size-related trend from brachycephaly to dolichocephaly in the ontogeny raises intriguing questions about a similar trend in macroevolution: is there a general species-level correlation between size and dolichocephaly? Might the correlation actually be a common occurrence among closely related species across a wide range of placental orders? The face subsequently grows faster than the braincase, making the head less globular and the jaws more prominent. Human babies, as well as most, if not all, other juvenile placental mammals, have small faces relative to big and comparatively round heads. For instance, in papionins, mandrils and baboons the evolution of bigger sizes and longer faces has followed similar, although non-identical, allometric trajectories, as members of each clade became less arboreal and incorporated a wider variety of foods in their diet compared with their closest relatives, the hard-fruit-eating and short-faced mangabeys 14.Ĭraniofacial elongation is a common aspect of postnatal growth in mammals. Allometry thus acts in evolution both as a constraint by reducing the range of directions of shape change, and as an accelerator by producing pronounced phenotypic differences along lines of least evolutionary resistance 12, 13. Consequently, small differences in the genetic regulation of growth factors can, by altering size, indirectly but readily affect shape. Integration probably explains why the proportional length and orientation of the cranial base and face account for a large amount of variation in dogs and humans 11. Indeed, without integration, fundamental functions could be compromised as size changes during ontogeny and in evolution 9, 10. Allometry is known to be a major component of covariation in the mammalian cranium 7 and contributes to its integration and modularity 8. How the profound variety of skull forms was generated from such a seemingly conservative system is a fundamental and largely open evolutionary question.Ĭorrelated variation, known as allometry when it results in proportional changes relative to size, may facilitate the rapid origin of differences among closely related species 2, 4, 6. Despite the scale of differences between major, and even minor, clades, the covariance structure in the mammalian skull seems to be highly conserved 2, 3, 4, 5. The morphological disparity in their cranial shape is extraordinary, as every visitor to a natural history museum appreciates. Mammals range in size from minute shrews to gigantic whales 1.
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