Evidence for convergent evolution of ultrasonic hearing in toothed whales (Cetacea: Odontoceti)

Abstract

Toothed whales (Cetacea: Odontoceti) are the most diverse group of modern cetaceans, originating during the Eocene/Oligocene transition approximately 38 Ma. All extant odontocetes echolocate; a single origin for this behaviour is supported by a unique facial source for ultrasonic vocalizations and a cochlea adapted for hearing the corresponding echoes. The craniofacial and inner ear morphology of Oligocene odontocetes support a rapid (less than 5 Myr) early evolution of echolocation. Although some cranial features in the stem odontocetes Simocetus and Olympicetus suggest an ability to generate ultrasonic sound, until now, the bony labyrinths of taxa of this grade have not been investigated. Here, we use µCT to examine a petrosal of a taxon with clear similarities to Olympicetus avitus . Measurements of the bony labyrinth, when added to an extensive dataset of cetartiodactyls, resulted in this specimen sharing a morphospace with stem whales, suggesting a transitional inner ear. This discovery implies that either the lineage leading to this Olympicetus ­-like taxon lost the ability to hear ultrasonic sound, or adaptations for ultrasonic hearing evolved twice, once in xenorophids and again on the stem of the odontocete crown group. We favour the latter interpretation as it matches a well-documented convergence of craniofacial morphology between xenorophids and extant odontocetes.

Wonky whales: the evolution of cranial asymmetry in cetaceans

“…Additional episodes of rapid change were found in the Mid-Late Oligocene, a period of rapid evolution in cranial asymmetry in odontocetes. These results support studies suggesting that biosonar, the signature adaptation of odontocetes, and associated asymmetry were acquired at or soon after the origin of this clade [ 4 – 6 , 39 ].…”

“…4 ). This result adds further evidence to the idea that xenorophids and other odontocetes iteratively evolved specialisations for the production of high-frequency sounds necessary for echolocation [ 4 – 6 , 39 ]. The distinct cranial morphology (and by inference, distinct soft tissue morphology) found in xenorophids (e.g.…”

“…Mysticetes do not echolocate iv. All extant odontocetes echolocate [ 39 ] Echolocation-frequency model (‘echo-freq’) Categorising by echolocation in the extant odontocetes and sound production in the extant mysticetes i. Data on frequency specifics is not available for fossils ii.…”

What are the limits on whale ear bone size? Non-isometric scaling of the cetacean bulla

“…The bulla combines with the periotic to form the tympanoperiotic complex ( Mead & Fordyce, 2009 ). The tympanoperiotic complex is highly diagnostic for taxonomic and phylogenetic research ( Ekdale, Berta & Demere, 2011 ; Ekdale & Racicot, 2015 ), and it is readily preserved in the fossil record, providing a marker of acoustic evolution ( Churchill et al, 2016 ; Park, Fitzgerald & Evans, 2016 ; Park et al, 2019 ; Mourlam & Orliac, 2017 ; Racicot, Darroch & Kohno, 2018 ; Racicot et al, 2019 ). Thus, this anatomical unit is useful for studying allometric patterns in cetacean evolutionary history.…”

Peer Review #2 of "Intraspecific variation in the cochleae of harbour porpoises (Phocoena phocoena) and its implications for comparative studies across odontocetes (v0.1)"

“…Nevertheless, in many studies this assumption is not (or cannot be) tested because of logistical constraints, for example, where using multiple specimens for each species is not feasible due to time or cost restrictions, or because samples are limited. Palaeontological studies, for example, commonly use a single individual as representative of a whole population or species because of the rarity of fossils (Ekdale & Racicot, 2015;Ekdale, 2016;Park et al, 2017a;Gonzales, Malinzak & Kay 2018;Marx et al, 2018;Racicot et al, 2019).…”