Figure 6.

Phylogenetic tree plotting the positions of relative delay in cortical neurogenesis, to determine whether delay in onset of cortical neurogenesis, associated with a taxonomic grade shift up in neocortical volume, had occurred at single or multiple times in mammalian phylogeny. Bold pluses and minuses mark branches with delays described in this dataset, and which also have species with increased adult cortical volume, while the unmarked ones represent branches where only relative cortical volume differences in adults are known (Reep et al., 2007). Increases in relative cortical volume appear to have occurred at least three times in mammals, in Metatheria (various marsupials), in the branch giving rise to carnivores, various ungulates, and dolphins, and in primates. It cannot be determined from these data as yet whether a large cortex is the “basal” state, and reduced cortical volume has been selected for in glires, bats, shrews, and Afrotheria, or the reverse. Phylogeny from Song et al. (2012).