Reevaluating Body Size in Fossil Physeteroids Using Cranial Proportions

Fossil physeteroids exhibited substantially greater diversity in body size and feeding ecology than their extant relatives, yet body-length estimates for many taxa remain dependent on incomplete skeletons and proportional scaling from a limited number of reference species. This study reassesses body-size estimates in fossil physeteroids using a cranial proportion-based framework centered on bizygomatic width (BZW) and condylobasal length (CBL). The reference body lengths of Zygophyseter varolai and Brygmophyseter shigensis were reevaluated using published vertebral measurements, preserved skeletal length, and scale-bar-based skeletal data, and were conservatively set at approximately 6.5 m and 6.0 m, respectively. These revised reference values were then used to generate updated body-length estimates for a broad sample of extinct physeteroids. Revised estimates indicate that large-bodied physeteroids were relatively uncommon. Among the non-kogiid genera examined, only seven reached or exceeded 6 m in estimated total length, whereas Livyatan and Eophyseter were the only fossil genera estimated to exceed 9.5 m. Macroraptorial physeteroids ranged from approximately 4.0–4.3 m in Acrophyseter spp. to approximately 15–16 m in the giant Livyatan melvillei. The results further suggest that large body size evolved under multiple ecological regimes. Some large-bodied physeteroids were macroraptorial predators, whereas others show adaptations consistent with suction feeding or predation on smaller prey. The temporal distribution of giant forms is therefore more consistent with repeated, independent increases in body size than with a single evolutionary trend toward gigantism. However, incomplete preservation and uncertainty in the body lengths of reference taxa remain important limitations. The framework presented here provides a more consistent basis for reassessing body-size diversity in fossil physeteroids and can be refined as more complete cranial and postcranial material becomes available.