Testing heterochrony: Connecting skull shape ontogeny and evolution of feeding adaptations in baleen whales

Abstract

Ontogeny plays a key role in the evolution of organisms, as changes during the complex processes of development can allow for new traits to arise. Identifying changes in ontogenetic allometry – the relationship between skull shape and size during growth – can reveal the processes underlying major evolutionary transformations. Baleen whales (Mysticeti, Cetacea) underwent major morphological changes in transitioning from their ancestral raptorial feeding mode to the three specialised filter feeding modes observed in extant taxa. Heterochronic processes have been implicated in the evolution of these feeding modes, and their associated specialised cranial morphologies, but their role has never been tested with quantitative data. Here, we quantified skull shapes ontogeny and reconstructed ancestral allometric trajectories using 3D geometric morphometrics and phylogenetic comparative methods on sample representing modern mysticetes diversity. Our results demonstrate that Mysticeti, while having a common developmental trajectory, present distinct cranial shapes from early in their ontogeny corresponding to their different feeding ecologies. Size is the main driver of shape disparity across mysticetes. Disparate heterochronic processes are evident in the evolution of the group: skim feeders present accelerated growth relative to the ancestral nodes, while Balaenopteridae have overall slower growth, or paedomorphosis. The grey whale is the only taxon with a relatively faster rate of growth in this group, which might be connected to its unique benthic feeding strategy. Reconstructed ancestral allometries and related skull shapes indicate that extinct taxa used less specialized filter feeding modes, a finding broadly in line with the available fossil evidence.

Morphological description of spontaneous pelvic fin loss in a neotropical cichlid fish

“…If this slower development especially affects the pelvic fins, this could restrict the probability of pleiotropic effects on either musculoskeletal structures or organ systems. However, heterochronic changes, atavism, and vestiges of pelvic girdles in some other animals (e.g., snakes and whales) have also been found (Hall, 2010; Lanzetti et al, 2023; Sherratt et al, 2019; Stiassny, 1991; Werneburg & Sánchez‐Villagra, 2015), and suggest that the timing of pelvic fin development can play a role in their loss. It might be that structures that form later in development are often the ones that are more readily lost during evolution and are also those traits that commonly show limited pleiotropic effects on other anatomy.…”