Cranial Suture Closure Patterns in Sciuridae: Heterochrony and Modularity

Studies of sequence heterochrony in mammalian evolution have revealed differences in bone ossification between and within major clades. Sequences of late stage developmental events have been less well studied, and the relation of modularity to sequence heterochrony at these stages has not been explicitly tested. Here, the first data on cranial suture closure are provided for members of Sciuridae. Sequence heterochrony is quantified using the recently developed Parsimov-based genetic inference (PGi) algorithm to identify shifts in suture closure sequence, and modularity in heterochronic shifts is tested. Results indicate that suture closure pattern was quite variable among sciurids, and interspecific correspondence in closure sequence was generally lower than has been found for other rodents, and for carnivorans. A number of sequence heterochronies were detected for inclusive clades, and these were not randomly distributed but mainly concentrated among sutures that exhibited high rank variability and belonged to the orbit and cranial vault modules, suggesting that some regions of the cranium exhibited a greater capacity for variation in suture closure. Heterchronies were not detected for sutures belonging to the basicranium, or anterior orbit-nasal modules, both of which are recognized as highly integrated modules based on landmark data. Modularity of suture closure sequence was not significant for any modules following multiple-comparison correction, which contrasts with modularity that has been recovered in early stage, ossification sequence events among other mammals.