Inner ear of two Miocene physeteroids (Cetacea, Odontoceti) from Patagonia (Argentina): evidence for an evolutionary stasis in sperm whale hearing morphology?

FLORENCIA PAOLUCCI; CARLOS MAXIMILIANO GAETÁN; MÓNICA R. BUONO; MARTA S. FERNÁNDEZ

Formato Virtual

Otro; 9th International Meeting on the Secondary Adaptation of Tetrapods to Life in Water Virtual Meeting; 2021

The Odontoceti are known for emitting and receiving ultrasonic sounds that are capturedinside the cochlea. In the last decades, the knowledge of cetaceans’ cochlea anatomy hasincreased due to the analyses of extant and extinct taxa, including physeteroids or spermwhales. Extant members of that clade are deep-diving animals and their ears are adapted tosupport great pressures. However, few studies have analysed the inner ear morphology of theearly extinct sperm whales, which can provide information about the acquisition of thesedeep diving specialized hearing abilities. In this contribution, we analyse for the first time theinner ear of two Miocene Patagonian sperm whales of Argentina. Two isolated periotics ofPhyseteroidea indet. (MLP 76-IX-5-1; Gran Bajo del Gualicho Formation, Miocene, andMPEF-PV-6074; Gaiman Formation, early Miocene) were scanned using MicroCT.Posteriorly, digital endocasts were reconstructed for each. The estimation of thelow-frequency hearing limit (LFL) was based on the radii ratio of the cochlea. Publishedillustrations of extant sperm whales endocasts were used for comparative purposes. Theanalyses of measures and ratios of the inner ear of the patagonian Miocene physeteroidsshows that their cochleae have intermediate values between extant sperm whale genera.Patagonian specimens have some values that are similar to Physeter macrocephalus (e.g.,number of turns, basal radii, cochlear), and others that are similar to Kogia spp. (e.g., basalratio, axial pitch). The comparison with available images of endocast models shows that thecochleae of Patagonian physeteroids resembles that of P. macrocephalus more than that ofKogia spp., even sharing the presence of a conspicuous tympanal recess. The LFL of MLP76-IX-5-1 and MPEF-PV-6074 are notably lower than in Kogia spp. with MLP 76-IX-5-1having the lowest LFL among the studied individuals, and MPEF-PV-6074 having an LFLintermediate between the extant genera. Nevertheless, we cannot exclude that the minordifferences between the estimated LFL of the patagonian specimens might be due totaphonomic processes. All these morphological comparisons show that, even thoughintermediate values are observed in the studied fossil specimens, the whole inner earmorphology shares more similarities with Physeter. These results agree with the cleardifferences also observed in the morphology of the tympano-periotic complex among kogiidsand non-kogiid physeteroids. This suggests that in physeteroids the hearing morphologicaland functional pattern was acquired early in their evolutionary history, with a subsequentevolutionary stasis over millions of years.11