The so‐called foramen singulare in cetacean periotics is actually the superior vestibular area

Abstract

It is nearly 100 years ago that the “foramen singulare” was first identified in cetacean periotics. Since then, the “foramen singulare” has been recognized in periotics of many cetacean species, extant or extinct. Surprisingly, however, it has never been confirmed if the foramen singulare in cetacean periotics is really homologous to that in other mammals. It is known that in mammals including humans the posterior ampullary nerve, which innervates the posterior semicircular duct, passes through the foramen singulare. We use an X‐ray micro‐CT scan to examine endocasts of the bony labyrinth of the inner ear of cetacean periotics, showing that the osseous canal extending from the so‐called foramen singulare goes toward the anterior bony ampulla, meaning that the alleged foramen singulare in cetacean periotics is really the superior vestibular area, through which the utriculoampullary nerve enters. The transverse crest is quite significant to identify each quadrant of the fundus of the internal acoustic meatus, but in many cetacean species the transverse crest is poorly developed, almost imperceptible in some species, and this could have brought confusion into the interpretation over the superior vestibular area and the foramen singulare. The bony septum separating the cerebral aperture of the facial canal from the foramen singulare is not the transverse crest, but the perpendicular crest. The foramen singulare is not a distinct foramen separated from the inferior vestibular area. Instead, the true foramen singulare opens near the inferior vestibular area in the internal acoustic meatus in cetacean periotics.

A well‐preserved cranium from the Judith River Formation (Montana, USA) reveals the inner ear and neuroanatomy of a Campanian baenid turtle

“…Thus, turtle endocasts are relatively poor predictors of brain morphology (Edinger, 1929;Evers et al, 2019;Ferreira et al, 2022;Mautner et al, 2017;Paulina-Carabajal et al, 2013;Wyneken, 2001;Zangerl, 1960). This pattern also holds in certain other vertebrate taxa in which the brain volume does not fill most of the endocranial space, such as squamates (Allemand et al, 2023) and cetaceans (Boessenecker et al, 2017;Ichishima, Kawabe, & Sawamura, 2021;Racicot, 2022), but differs from the relatively good reflection of brain shape by endocast shape in archosaurs (Iwaniuk & Nelson, 2002;Watanabe et al, 2019), many mammals (e.g., De Miguel & Henneberg, 1998Haight & Nelson, 1987;Jerison, 1973;Macrini et al, 2007), and amphibians (Clement et al, 2021). In taxa for which the brain-to-endocranial cavity index is low, the endocast tends to be more cylindrical than the underlying brain (Hopson, 1979;Jerison, 1973;Watanabe et al, 2019).…”

Evolution of whale sensory ecology: Frontiers in nondestructive anatomical investigations

“…Although the impact of size differences of the different vestibular nerve branches to cetacean senses are yet to be determined, an intriguing study incorporating μCT along with iodine staining found that what was once known as the "foramen singulare" in cetacean periotics is actually the "superior vestibular area" (Ichishima et al, 2021). This work is a great example of a comparative analysis effectively incorporating new methods in CT to clarify homologies.…”

“…The ontogenetic development of cetaceans is an area where aspects of sensory system evolution can be further examined, and where inroads have been made using nondestructive imaging (Frainer et al, 2019;Ichishima, Kawabe, & Sawamura, 2021;Lanzetti, 2019;Lanzetti, Berta, & Ekdale, 2018;Thean, Kardjilov, & Asher, 2017;Yamato & Pyenson, 2015). The measurement techniques described above for the inner ear and brain can equally be applied to developing fetuses.…”

New cetacean fossils from the late Cenozoic of South Africa

“…All specimens are now housed in the Cenozoic Palaeontology Collections at Iziko South African Museum (SAM). Anatomical terminology follows Mead and Fordyce (2009) and Ichishima et al (2021). 1) and marked by a somewhat squared anterior border with a weakly developed anterolateral shelf.…”