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Abstract The bony labyrinth of the petrosal bone, a distinctive feature of mammal skulls, is often identified in micro‐computed tomography imaging to infer species' physiological and ecological traits. When done as part of a comparative study, one individual specimen is normally considered representative of a species, and intraspecific variation is considered low. Yet tests of intraspecific variability have been performed on few species and on limited morphological traits. Studies of intraspecific variability are not only valuable to help us assess the need for multiple specimens in comparative work, but relative levels of variability can also be used to reveal insights into a trait's functional significance. In this study, we report measurements of intraspecific variation on two cetaceans with vastly different auditory specializations, a low‐frequency specialized mysticete and an echolocating odontocete. We examine the internal structures of the cochlea in beluga and bowhead whales and relate this to their hearing abilities. Overall levels of intraspecific variability are higher in the bowhead than the beluga, reflecting the more specialized auditory system of the latter. However, the levels of variation differ through the length of the cochlea (base to apex) and these appear to reflect known frequency specializations of the species, with the bowhead having lower variation in some measurements at the low‐frequency apical end than the beluga. 

Abstract Therian mammals possess numerous unique morphological features in their auditory system. Many of these have been well studied in attempts to understand their functional importance, and some aspects of cochlear morphology have been shown to correlate with hearing ability across taxa. Among the unique features of the therian inner ear is the presence of bony structures supporting the sensory epithelia. While these have been implicated in the development of extended high-frequency hearing, their functional role remains unclear. In this study, I use micro-computed tomography (μCT) imaging to take quantitative measurements of the bony spiral laminae and cochlear scalae through the length of the basilar membrane. I present an analysis of the variation showing how these change from the cochlear base to the apex, and how they differ between clades. I find considerable variation in the structures between different clades, particularly in the area of the cochlear scalae which show differences between terrestrial and aquatic species. These results provide a broad survey demonstrating a diversity that could be further exploited to explore functional significance in more detail. A cursory comparison shows correlation between various morphological measures and hearing ability, with the dimensions of the basilar membrane appearing the most useful to make predictions. 

Toothed whales (odontocetes) make use of high-frequency sounds to echolocate, differing significantly from their sister group baleen whales (mysticetes), which make use of low-frequency sound for long-distance communication. This divergence in auditory ability has led to considerable speculation as to how hearing functioned in the ancestral archaeocetes, and when the specializations of modern species arose. Numerous studies have attempted to infer auditory capabilities from morphological correlates valid in modern species. Here, we build upon these previous methods with a focus on cochlear structures that have well-understood links to function. We combine this with information on the sound conduction apparatus to chart the evolutionary trajectory of cetacean hearing. Our results suggest an initial move toward low-frequency specialization in early Eocene cetaceans, which coincides with the appearance of new sound conduction pathways. This paved the way for the later movement toward higher-frequency hearing in protocetids; however, the ultra-high- and low-frequency hearing specializations of both modern cetacean clades evolved after their divergence. We use these data to test the hypotheses that evolutionary brain size increases in cetaceans were related to the origin of high-frequency echolocation. We show that no shift in relative brain size coincides with any changes toward high-frequency perception. However, this does not rule out a role for other changes in hearing ability such as some simple forms of echolocation, similar to that suggested for hippopotamuses or bowhead whales, which may have been present in even the earliest cetaceans. 

The raoellid artiodactyl Indohyus indirae is known from northern Pakistan and northwestern India, with substantial skeletal material found in the Sindkhatudi locality near Kalakot in Kashmir. Eight specimens from this locality, two of which exhibit deciduous dentitions, offer invaluable insights into cranial osteology of Indohyus indirae. We present a comprehensive examination of the cranial osteology of Indohyus, highlighting detailed unique features using this material. These specimens, though flattened dorsoventrally and mediolaterally, allow for anatomical observations. Previous literature has extensively employed these specimens for phylogenetic analyses indicating that Indohyus is a close relative of cetaceans. These findings are partly based on the material described here. Notably, the cranial morphology of Indohyus differs in several aspects from that of most early and middle Eocene artiodactyls. For instance, the rostrum of Indohyus is longer than that of most middle Eocene artiodactyls, resembling Eocene cetaceans. The positioning of the nasal opening above the incisors is similar to terrestrial artiodactyls. However, the ventral wall of the nasopharyngeal duct does not extend to the ear region, which distinguishes it from Eocene cetaceans. Moreover, the frontal bone is thick and has a concave profile, forming a dorsal shield for the laterally facing orbits. Furthermore, the tympanic bone has an involucrum, a feature that characterizes all cetaceans. These cranial features may be indicative of specific cranial functions and, given their similarity in some regards to Eocene cetaceans, could be related to the land-to-water transition. Further research, including explicit functional studies, is required to investigate these hypotheses.