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The strepsirhine cranium has received relatively little attention compared to that of its haplorhine sister clade. Where it has been subject to investigation, studies have tended to focus on a narrow range of species and/or include limited sample sizes. The current study investigates the evolution of the extant strepsirhine cranium using large sample sizes and a broad taxonomic range, focusing on modularity, allometry and phylogenetic signal to better understand the evolutionary pathways that have shaped its morphology. The data consist of 60 3D morphological landmarks, collected using a Microscribe MX, from the crania of 1633 adult strepsirhine specimens, across 20 genera and 30 species. The effect of reduced sample size on estimates of size and shape parameters was investigated for six species and found to be constant across taxa. Estimates of size parameters remained accurate, while estimates of shape parameters and of the angles between allometric trajectories became increasingly inaccurate as sample size was reduced. Further analyses were therefore limited to species with sample size above 20. Common patterns of modularity and allometry were found both within and across species. The cranium is best divided into two modules (face, neurocranium), and within those into a further six modules (face, orbit, oral, zygomatic, vault, base). Strepsirhines follow the general mammalian allometric pattern, with smaller taxa having a more paedomorphic appearance, although some differences were apparent between lorisiforms and lemuriforms. Strong phylogenetic signal is present in all cranial modules, as measured by Pagel's λ and Blomberg's K, with evidence that signal is strongly linked to size. Overall, the evolutionary pathways of the strepsirhine cranium are shown to be conserved, with comparative data suggestive of stabilising selection in extant lineages. Where species have diverged from the common pattern, this is attributed to selection for specialised diet or variation in activity pattern. |
