This study demonstrates that the major families of lice began to radiate before the K–Pg boundary, and thus supports a Cretaceous diversification of many modern bird and mammal lineages. Our results are consistent with three independent lines of evidence—the insect fossil record, avian and mammalian molecular divergence estimates, and biogeographic data. Recent Mid-Cretaceous finds of fossilized book-louse family (‘Psocoptera’: Liposcelididae), the close relatives to parasitic lice, led Grimaldi & Engel [11
] to estimate that parasitic and non-parasitic louse lineages diverged from each other deep in the Cretaceous, approximately 100–145 Myr ago. Similar dates have been estimated for the radiation of modern bird and mammal orders—the contemporary hosts of parasitic lice [7
]. The biogeographic evidence from modern mammal and bird groups is also compelling. Phylogenetic studies suggest most basal and extant lineages of birds (e.g. like the ratites) and mammals (e.g. marsupials) have a Gondwanan distribution, leading many to conclude that modern bird and mammal orders had a pre-Tertiary diversification before the final break-up of Gondwana 80–160 Myr ago [22
Another long debated question is whether birds or mammals are the ancestral host for parasitic lice? Simple character state reconstruction across the louse phylogeny infer mammals as the ancestral host of the clade containing Ischnocera, Rhynchophthirina and Anoplura; and birds as the ancestral host for Amblycera. However, reconstructing the alternative host for these clades requires only a single additional step. Other lines of evidence suggest birds may have played a more prominent role in the diversification of lice. Chewing lice of birds are species-rich and ubiquitous; there are approximately 3000 species recognized, parasitizing all but 16 of the 173 extant bird families. This broad distribution across birds coupled with ancient diversification point towards a long history of association with birds. Only Anoplura (sucking lice) appear to have undergone a major radiation on mammals. The timing of the diversification of sucking lice in the Late Cretaceous (approx. 75 Ma) and subsequent radiation soon after the K–Pg boundary is in agreement with mammalian evolutionary history [7
] and a recent study focused solely on the Anoplura [19
]. Interestingly, the diversification of extant chewing lice parasitizing mammals postdates the K–Pg boundary, in contrast to the Cretaceous diversification of their avian infesting relatives. This suggests parasitic chewing lice were first acquired by birds and later diversified on mammals. The distribution of lice on mammals also supports this hypothesis. Chewing lice parasitizing mammals are species-poor and patchily distributed across mammalian families. Although anopluran divergence dates suggest a radiation that was largely contemporaneous with their hosts, sucking lice are even more patchily distributed across mammals, parasitizing only 20 per cent of all mammalian species.
Parasitic lice use their host's pelage or plumage as both a habitat and a food resource and host specificity can be attributed to phenotypic variation of host hair and feathers [25
]. It seems reasonable to assume that the evolution of this fundamental host resource is closely associated with the evolution of parasitic lice. The oldest definitive fossil feathers are still those of Archaeopteryx
from the Late Jurassic and fossilized hair is not known until the Late Palaeocene (roughly 55 Ma). By this time, both feathers and hair were modern in form and microscopic detail [27
], but there is evidence to suggest these characters predate their first definitive occurrence in the fossil record [28
]. Given the diversification times presented here and the early origin of feathers, parasitic lice probably infested feathered theropod dinosaurs.