Description
Distinctive patterns of heterochronic change, namely paedomorphosis and peramorphosis, can have implications on evolution. It has been suggested that paedomorphosis and peramorphosis may cause limited or released developmental constraints, leading to low or high taxonomic diversity, respectively. Three of the four extant baleen whale lineages (Balaenopteridae, Neobalaenidae, and Eschrichtiidae) display either pattern of heterochronic growth (the fourth, Balaenidae, has not yet been studied) and all lineages display either high or low taxonomic diversity. A previous study used geometric morphometric (landmark) analyses in a study of cranial growth in Neobalaenidae (low taxonomic diversity) and Balaenopteridae (high taxonomic diversity). It was hypothesized that neobalaenid skulls exhibit paedomorphosis in contrast to peramorphosis in balaenopterid skulls. The level of diversity and heterochronic growth pattern is hypothesized to be correlated. Eschrichtiidae has low diversity and first born with a large braincase that continues to grow during the first three months after birth. The rostrum then begins to widen, lengthen and increase in curvature. Finally, the braincase and rostrum both grow rapidly as the individual reaches sexual maturity. Here, for the first time, eschrichtiid skull shape variation is analyzed and considered in the same ontogenetic (heterochronic) context as Tsai and Fordyce (2014). A morphometric analysis including 29 landmarks was conducted on an ontogenetic series of 13 extant gray whale (Eschrichtius robustus) skulls, on seven fossil Eschrichtiidae skulls, and on four balaenopterid skulls. In addition, an ontogenetic sequence analysis (OSA) on Eschrichtiidae lineage was also conducted. The ontogenetic sequence map collected from the extant individuals made it possible to determine the ontogenetic ages of the extinct eschrichtiids. This helped to better understand the heterochronic growth pattern within Eschrichtiidae. The OSA was plotted on the PCA of the complete skulls and partial skulls with the categories of adult, juvenile, neonate and fossil. The fossils plot closer, morphologically, to the extant juveniles, indicating that the extant adult Eschrichtiidae are taking on previously unseen traits. Thus, the results support Eschrichtius robustus has a peramorphic growth pattern relative to the Eschrichtiidae fossils and extant Balaenopteridae.
