Reduction of olfactory and respiratory turbinates in the transition of whales from land to sea: the semiaquatic middle Eocene Aegyptocetus tarfa

Abstract

Ethmoturbinates, nasoturbinates, and maxilloturbinates are well developed in the narial tract of land‐dwelling artiodactyls ancestral to whales, but these are greatly reduced or lost entirely in modern whales. Aegyptocetus tarfa is a semiaquatic protocetid from the middle Eocene of Egypt. Computed axial tomography scans of the skull show that A. tarfa retained all three sets of turbinates like a land mammal. It is intermediate between terrestrial artiodactyls and aquatic whales in reduction of the turbinates. Ethmoturbinates in A. tarfa have 26% of the surface area expected for an artiodactyl. These have an olfactory function and indicate that early whales retained a sense of smell in the transition from land to sea. Maxilloturbinates in A. tarfa have 6% of the surface area expected for an artiodactyl. These have a respiratory function and their markedly reduced size suggests that rapid inhalation and exhalation was already more important than warming and humidifying air, in contrast to extant land mammals. Finally, the maxilloturbinates of A. tarfa, although greatly reduced, still show some degree of similarity to those of artiodactyls, supporting the phylogenetic affinity of cetaceans and artiodactyls based on morphological and molecular evidence.

Convergent evolution of olfactory and thermoregulatory capacities in small amphibious mammals

“…Our evidence for the enlargement of respiratory turbinals in amphibious mammals is consistent with histological studies showing a thickening of the epithelium of respiratory turbinals in amphibious shrews (38) and of the bony structures of the respiratory turbinals in extinct aquatic mammals (50,51). Our results also show a gradient of increasing relative surface area of respiratory turbinals with greater aquatic specialization.…”

Turbinal bones are still one of the last frontiers of the tetrapod skull: hypotheses, challenges and perspectives

“…With the popularization of the CT technology many studies quantified the surface area of turbinals (e.g., Van Valkenburgh et al 2004Green et al 2012;Martinez et al 2018Martinez et al , 2020Martinez et al , 2023aLundeen and Kirk 2019;Wagner and Ruf 2019;Peri et al 2020;Lundeen and Kay 2022;Melin et al 2022). As often with quantitative approaches, the methodology in the data acquisition as well as in the use of the data may impact the results.…”

“…Over the course of the evolution of cetaceans the relative size of their olfactory organs was hypothesized to mirror their land-to-water transition and thus follow a reduction (Farnkopf 2022). Olfactory turbinals as well as potential maxilloturbinal and nasoturbinal have been recovered in several extinct cetaceans (e.g., Stromer 1903;Uhen 2004;Geisler et al 2005;Nummela et al 2006;Pihlström 2008;Fahlke et al 2011;Godfrey et al 2013;Berta et al 2014;Peri et al 2020). Berta et al (2014) hypothesized that the reduction in turbinal bones among cetaceans probably occurred during the Eocene or early Oligocene.…”

“…Berta et al (2014) hypothesized that the reduction in turbinal bones among cetaceans probably occurred during the Eocene or early Oligocene. Available data on olfactory turbinals are often incomplete and too damaged to properly test their potential reduction but Peri et al (2020) documented a potential reduction of olfactory turbinals in the middle Eocene protocetid Aegyptocetus tarfa as compared to terrestrial Artiodactyla. Some shallow-diving tetrapods acquired a suite of adaptations referred to as bone mass increase, which is understood as taking part in underwater trim and buoyancy control (Domning and de Buffrénil 1991).…”

“…Pinnipeds (seals and walrus) are also known for their efficient moisture conservation, which has also been interpreted as an adaptation to salty environments (Lester and Costa 2006;Van Valkenburgh et al 2011). To our knowledge, among aquatic species, only cetaceans present a reduction or an absence of respiratory turbinals whereas an Eocene protocetid may feature a reduction of the respiratory and olfactory turbinals as compared to terrestrial Artiodactyla (Peri et al 2020). In some mysticetes, a maxilloturbinal was documented whereas it presents some variation including some vestigial forms in some odontocetes (reviewed in Berta et al 2014).…”