Morphology of the toothed mysticete Fucaia goedertorum and a reassessment of Aetiocetidae (Cetacea, Mysticeti)

Aetiocetids are a relatively diverse group of small toothed mysticetes that lived from the late Eocene through the late Oligocene in the North Pacific Ocean. They are characterized by a combination of morphological features that places them between earlier mysticetes (e.g., mammalodontids) and chaeomysticetes. Since the first aetiocetid was described in the 1960s, their significance in understanding mysticete evolution has been broadly discussed, with particular attention to aspects of their paleobiology (i.e., feeding strategies). However, morphological features that could offer additional insight into their paleobiology, like the tympanoperiotic complex, dentition, and postcranial elements, are still poorly understood due to incomplete specimens or partial descriptions. In this context, this study provides a detailed description of Fucaia goedertorum (LACM 131146), which represents one of the most completely preserved aetiocetid specimens, thus allowing for a better understanding of the cranial and postcranial morphology of this group. The craniomandibular and postcranial morphology of Fucaia goedertorum displays a unique combination of plesiomorphic and derived characters, such as proportionately large orbits, presence of upper and lower teeth, well-defined embrasure pits, a kinetic mandibular symphysis, multi-element sternum, flexible vertebral column and proportionately long flippers, among others. Our interpretations of the cranial and postcranial morphology of Fucaia goedertorum, suggest that it was a raptorial or suction-assisted raptorial feeder with high flexibility and maneuverability, more akin to what is observed in otariids. Combined with current interpretations proposed for other aetiocetids, we suggest that this group of early mysticetes displayed a broad range of feeding strategies.