Occipital ossification of balaenopteroid mysticetes.

The bones of the posterior portion of the mammalian skull often exhibit incomplete ossification of the joints between the bones at the time of birth, with complete ossification at some point after birth. The sequence of ossification of these joints in mysticetes can be used to characterize the relative age in the calf and early juvenile ontogenetic stages. This study examined occipital joints ossification of 38 dry prepared neonate specimens in four mysticete species from two families (Eschrichtiidae: Eschrichtius robustus; Balaenopteridae: Balaenoptera acutorostrata, Balaenoptera physalus, and Megaptera novaeangliae). Each of the joints responsible for the fusion of the occiput were examined and rated for degree of ossification. The cranial ossification analysis indicates that E. robustus calves have open occipital joints until ~6 months of age and are born at a less mature stage than closely related balaenopterids. All of the species followed the same sequence of ossification: basioccipital/exoccipital joint, followed by the basioccipital/basisphenoid joint, and completed by the supraoccipital/exoccipital joint.