PaleoBios

The temporalis is an important muscle used in biting and mastication, and whose morphology is strongly influenced by feeding ecology. Although the anatomy of this muscle and its relation to feeding function are well-studied in terrestrial mammals, few studies have examined its variation in whales. Our study focuses on quantifying the area of attachment for the temporalis muscle within the temporal fossa, calculating two metrics of comparison: A temporal fossa index (TFI) which is a size corrected measure of temporalis muscle attachment area, and a corrected temporal fossa index (CTFI), which also corrects for cranial telescoping. We calculated TFI and CTFI scores for 72 species of extant odontocetes as well as 37 species of extinct whale, including archaeocetes and toothed mysticetes. We statistically tested for differences related to diet and prey capture method using ANOVA. We then performed ancestral character state reconstruction (ACSR) for both metrics.

We found no significant differences in TFI scores for diet, however both grip-and-tear and snap feeding taxa had significantly larger TFI scores than suction or ram feeding odontocetes. The ACSR found major decreases in TFI at the base of the Neoceti, Odontoceti, and just prior to the evolution of the crown group, relating to the gradual loss of mastication. When we corrected for telescoping, CTFI scores increase within mysticetes and stem odontocetes, before decreasing again within crown odontocetes. This suggests that as telescoping increased, some compensation was needed to account for the reduced intertemporal region. We find evidence of macropredatory behavior for Basilosaurus, Coronodon, Ankylorhiza, Livyatan, and possibly Atocetus. Hyper-longirostrine taxa such as eurhinodelphinids possess unusually low TFI scores, as does the bizarre prognathous porpoise Semirostrum, highlighting their unique feeding styles. Our study reveals a complex interplay between the reduction in mastication, increase in cranial telescoping, and prey capture specialization. It also introduces a useful and simple metric which can be used to infer feeding ecology in fossil whales.