Evolution of cranial telescoping in echolocating whales (Cetacea: Odontoceti)

Abstract

Odontocete (echolocating whale) skulls exhibit extreme posterior displacement and overlapping of facial bones, here referred to as retrograde cranial telescoping. To examine retrograde cranial telescoping across 40 million years of whale evolution, we collected 3D scans of whale skulls spanning odontocete evolution. We used a sliding semilandmark morphometric approach with Procrustes superimposition and PCA to capture and describe the morphological variation present in the facial region, followed by Ancestral Character State Reconstruction (ACSR) and evolutionary model fitting on significant components to determine how retrograde cranial telescoping evolved. The first PC score explains the majority of variation associated with telescoping and reflects the posterior migration of the external nares and premaxilla alongside expansion of the maxilla and frontal. The earliest diverging fossil odontocetes were found to exhibit a lesser degree of cranial telescoping than later diverging but contemporary whale taxa. Major shifts in PC scores and centroid size are identified at the base of Odontoceti, and early burst and punctuated equilibrium models best fit the evolution of retrograde telescoping. This indicates that the Oligocene was a period of unusually high diversity and evolution in whale skull morphology, with little subsequent evolution in telescoping.

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

“…; Churchill et al. ). Baleen whales possess an apomorphic structure, the infraorbital process of the maxilla, which probably does not allow for the complete overlap of the maxilla on the frontal as seen in toothed whales (Deméré et al.…”

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

“…The retention of reduced turbinates in some archaic odontocetes displaying limited telescopy and external bony nares that still do not reach the vertex of the skull (Churchill et al. ) lends some support to this hypothesis.…”

Evidence for convergent evolution of ultrasonic hearing in toothed whales (Cetacea: Odontoceti)

“…Although posterior expansion of the nasal process of the maxilla is an odontocete synapomorphy, it is fairly small in some taxa, most notably Archaeodelphis patrius [22]. When the size of this process, as well as posterior migration of the external nares, are mapped on odontocete phylogeny, there is clear convergence in these characters between xenorophids and the odontocete crown group [3,23]. If correlations between facial muscles and the size of the process, as seen in extant odontocetes, apply to extinct species, then this implies that some aspects of the MLDB complex and associated muscles evolved in parallel in both clades alongside the previously noted parallel evolution of retrograde cranial telescoping [23].…”