Cetacean Skull Telescoping Brings Evolution of Cranial Sutures into Focus

Abstract

Many modifications to the mammalian bauplan associated with the obligate aquatic lives of cetaceans—fusiform bodies, flukes, flippers, and blowholes—are evident at a glance. But among the most strikingly unusual and divergent features of modern cetacean anatomy are the arrangements of their cranial bones: (1) bones that are situated at opposite ends of the skull in other mammals are positioned close together, their proximity resulting from (2) these bones extensively overlapping the bones that ordinarily would separate them. The term “telescoping” is commonly used to describe the odd anatomy of modern cetacean skulls, yet its usage and the particular skull features to which it refers vary widely. Placing the term in historical and biological context, this review offers an explicit definition of telescoping that includes the two criteria enumerated above. Defining telescoping in this way draws attention to many specific biological questions that are raised by the unusual anatomy of cetacean skulls; highlights the central role of sutures as the locus for changes in the sizes, shapes, mechanical properties, and connectivity of cranial bones; and emphasizes the importance of sutures in skull development and evolution. The unusual arrangements of cranial bones and sutures referred to as telescoping are not easily explained by what is known about cranial development in more conventional mammals. Discovering the evolutionary‐developmental processes that produce the extensive overlap characteristic of cetacean telescoping will give insights into both cetacean evolution and the “rules” that more generally govern mammalian skull function, development, and evolution. Anat Rec, 302:1055–1073, 2019. © 2019 Wiley Periodicals, Inc.

Prenatal developmental sequence of the skull of minke whales and its implications for the evolution of mysticetes and the teeth‐to‐baleen transition

“…In the neurocranium, all sutures are closed at birth, except those between the occipital elements (supraoccipital/exoccipital, exoccipital/basioccipital, basioccipital/basisphenoid) (Walsh & Berta, ). Therefore, analyzing in detail the timing of suture closure prenatally would probably provide more information to understand the differences in development that influence the skull anatomy in adult mysticetes and odontocetes, given that the ossification sequence of the bone elements appears mostly conserved (Roston & Roth, ).…”

A Study of the Head during Prenatal and Perinatal Development of Two Fetuses and One Newborn Striped Dolphin (Stenella coeruleoalba, Meyen 1833) Using Dissections, Sectional Anatomy, CT, and MRI: Anatomical and Functional Implications in Cetaceans and Terrestrial Mammals

“…Telescoping refers to the overlap of the incisive and maxilla bones and the retraction of nasal bones on top of the frontal bone, as well as to the reduction of the temporal fossa and the rostral displacement of some muscles [3,12,28]. However, in the dolphin's snout, there is a link between the vomer, maxilla, and incisive bones.…”

“…In both suborders of living cetaceans, the skull has been highly modified by changes in feeding apparatus and the elimination or reduction of many structures [1]. The relationship of the bones in the skull to one another is altered due to the caudal migration of the nasal opening, a process known as telescoping [2][3][4]. In addition, differences occur in the location of the external nasal passages and the structure of the middle and the inner ear.…”

“…During these stages (end of the fetal and all the perinatal period), we can obtain valuable data on the species' ontogeny, so we concur with [7] about the many applications in the fields of biology and animal medicine. One of these applications could be determining an organ's development during the fetal period, helping researchers to calculate approximately the time of fetal development in odontocetes based on anatomical changes during gestation, similar to the Carnegie system designed for the human fetus [20] or for terrestrial mammals [3]. So far, we can only estimate cetacean parameters such as the gestation time by using a mathematical formula in Stenella longirostris [21], calculate the time of parturition using ultrasonography in Bottlenose dolphins [22], or estimate the adult's age through dental growing lines in striped dolphins [23].…”

Evolutionary phenome-genome analysis of cranial suture closure in mammals

“…This continuous measure provided 3 1 7 an amenable variable to perform the phylogenetic path analysis. We omitted from our analyses the 3 1 8 sagittal suture in the orca and the dolphin, because cetaceans never form this suture in the first place 3 1 9 due to the expansion of the occipital bone (Roston & Roth, 2019). comparisons require as input two discrete groups of species (Muntané et al, 2018).…”