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

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

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

Anillo ACT 172099, Red Paleontológica U. Chile

Hearing is one of the key senses for modern cetaceans (Neoceti) to hunting and communication, adapted for either infrasonic (Mysticeti) or ultrasonic (Odontoceti) frequencies. Among odontocetes, the Platanistoidea comprises a single extant riverine representative (Platanista gangetica) but numerous extinct marine species from the late Oligocene onward. Platanista is a critically endangered odontocete species that possess unique morphological characteristics, but their evolutionary patterns remain mostly unknown. Studying extinct platanistoids' hearing abilities might contribute a piece to the complex evolutionary puzzle of this group to understand its drastic diversity reduction. Thus, we describe for the first time the inner ear morphology of 6 late Oligocene?early Miocene extinct marine platanistoids from New Zealand and Patagonia (Argentina). These species represent the most diverse moment in the evolutionary history of Platanistoidea. In this study, we hypothesized that extinct marine platanistoids lacked a specialized inner ear like P. gangetica and thus, their morphology and inferred hearing abilities were more similar to extant marine odontocetes. 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 and statistical analyses to inner ear models of 7 platanistoids, 2 stem odontocetes, 9 extant odontocetes, and 1 archaeocete species (n=21). We did not find a ?typical? platanistoid cochlea but rather a disparate range of high-frequency hearing morphologies in the group, supporting an early-acquired specialized underwater hearing ability in odontocetes. Notocetus and Platanista share a loosely coiled and wide cochlea, and a low number of turns that are widely separated. Stem odontocete Prosqualodon australis and platanistoid Otekaikea huata are the only species that possess a tympanal recess, of yet unknown function. Aondelphis talen?s inner ear morphology indicates it had lower high-frequency hearing than other platanistoids. As expected, Platanista has the most derived cochlear morphology and is always distant in the morphospace from its sister genus Zarhachis, adding to evidence that it is an outlier within the group. Inner ear morphology, pneumatized maxillary crests, and pterygoid sinus system, among other unique characteristics, would have ultimately allowed the survival of Platanista to the present day. New fossil platanistoids, particularly from the middle?late Miocene onward, will help test these hypotheses.