The cochlea of the enigmatic pygmy right whale Caperea marginata informs mysticete phylogeny

Abstract

The pygmy right whale, Caperea marginata, is the least understood extant baleen whale (Cetacea, Mysticeti). Knowledge on its basic anatomy, ecology, and fossil record is limited, even though its singular position outside both balaenids (right whales) and balaenopteroids (rorquals + grey whales) gives Caperea a pivotal role in mysticete evolution. Recent investigations of the cetacean cochlea have provided new insights into sensory capabilities and phylogeny. Here, we extend this advance to Caperea by describing, for the first time, the inner ear of this enigmatic species. The cochlea is large and appears to be sensitive to low-frequency sounds, but its hearing limit is relatively high. The presence of a well-developed tympanal recess links Caperea with cetotheriids and balaenopteroids, rather than balaenids, contrary to the traditional morphological view of a close Caperea-balaenid relationship. Nevertheless, a broader sample of the cetotheriid Herpetocetus demonstrates that the presence of a tympanal recess can be variable at the specific and possibly even the intraspecific level.

Peer Review #2 of "A Miocene pygmy right whale fossil from Australia (v0.1)"

“…The axial pitch is also most similar to Caperea, whereas the cochlear slope value is higher than in all other mysticetes (Ekdale 2016;Park et al 2017a;Park et al 2017b), and more similar to that of Zygorhiza or even some odontocetes (Park et al 2016).…”

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

“…Overall, our results show that intraspecific variation of cochlea shape in P. phocoena is lower than interspecific variation of cochlea shape across odontocetes. Assuming that interspecific variation of P. phocoena is representative of patterns across odontocetes, this suggests we can still be reasonably confident in our conclusions when using n = 1 for comparative studies of cochlea morphology, as done in most previous studies (Ekdale & Racicot, 2015;Ekdale, 2016;Park, Fitzgerald & Evans, 2016;Park et al, 2017a;Park et al, 2017b). It is possible that P. phocoena is not representative of patterns across odontocetes.…”

“…Cochlear canal length (5) correlates with body size in odontocetes and is used as a quantitative measurement for comparison (Ketten, 1992a). Length of secondary spiral lamina (6) was measured to calculate percentage of extent of secondary spiral lamina (9), an indicator of the stiffness of the basilar membrane that is associated with high frequency hearing (Ekdale & Racicot, 2015;Park et al, 2017a). Basal ratio (7) and axial pitch (8) are two quantitative ways to examine cochlear shape and size for comparative studies (Racicot et al, 2016).…”

“…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).…”

A Miocene pygmy right whale fossil from Australia

“…The radii ratio (and thus also the low frequency hearing limit) could not be calculated, as damage prevented us from obtaining a basal radius. See Park et al (2017b) for a detailed description of all measurements and procedures.…”