Hearing the puzzle: the inner ear evolution of platanistoidea (Cetacea: Odontoceti)

MARIANA VIGLINO; CARLOS MAXIMILIANO GAETÁN; MÓNICA. R. BUONO; R. EWAN FORDYCE; TRAVIS PARK

Virtual Meeting

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

Hearing is one of the key senses for modern cetaceans (Neoceti) to hunting andcommunication, adapted for either infrasonic (Mysticeti) or ultrasonic (Odontoceti)frequencies. Among odontocetes, the Platanistoidea comprises a single extant riverinerepresentative (Platanista gangetica) but numerous extinct marine species from the lateOligocene onward. Platanista is a critically endangered odontocete species that possessunique morphological characteristics, but their evolutionary patterns remain mostly unknown.Studying extinct platanistoids' hearing abilities might contribute a piece to the complexevolutionary puzzle of this group to understand its drastic diversity reduction. Thus, wedescribe for the first time the inner ear morphology of 6 late Oligocene–early Miocene extinctmarine platanistoids from New Zealand and Patagonia (Argentina). These species representthe most diverse moment in the evolutionary history of Platanistoidea. In this study, wehypothesized that extinct marine platanistoids lacked a specialized inner ear like P. gangeticaand thus, their morphology and inferred hearing abilities were more similar to extant marineodontocetes. Based on microCT scans and 3D models, we took 15 measurements, 3 ratios,and estimated their low-frequency limit. Then, we applied 3D geometric morphometric andstatistical analyses to inner ear models of 7 platanistoids, 2 stem odontocetes, 9 extantodontocetes, and 1 archaeocete species (n=21). We did not find a “typical” platanistoidcochlea but rather a disparate range of high-frequency hearing morphologies in the group,supporting an early-acquired specialized underwater hearing ability in odontocetes. Notocetusand Platanista share a loosely coiled and wide cochlea, and a low number of turns that arewidely separated. Stem odontocete Prosqualodon australis and platanistoid Otekaikea huataare the only species that possess a tympanal recess, of yet unknown function. Aondelphistalen’s inner ear morphology indicates it had lower high-frequency hearing than otherplatanistoids. As expected, Platanista has the most derived cochlear morphology and isalways distant in the morphospace from its sister genus Zarhachis, adding to evidence that itis an outlier within the group. Inner ear morphology, pneumatized maxillary crests, andpterygoid sinus system, among other unique characteristics, would have ultimately allowedthe survival of Platanista to the present day. New fossil platanistoids, particularly from themiddle–late Miocene onward, will help test these hypotheses.*Project supported by CSI, SAREM, MCZ-Harvard, SMM, ANPCyT-PICT 2015-0792,MSCA-IF 748167/ECHO, ERC Starting Grant 677774/TEMPO and LeverhulmeRPG-2019-323.