The contribution of ontogenetic growth trajectories on the divergent evolution of the crocodylian skull table

Abstract

To explore shape variability among crocodylian skull tables, an analysis using geometric morphometric methods is conducted with the inclusion of extant and fossil taxa. Skull tables are variable and the differences likely play a role in hydrodynamics, species recognition, and biomechanical adaptations. Comparisons of allometric change within taxa are explored revealing that adults significantly diverge from juvenile skull table morphologies in most species and these changes happen in a stereotyped way. In all analyses, adults of the smallest extant taxa plot alongside the juveniles of related taxa and heterochrony may explain the maintenance of these morphologies into adulthood.When landmarks representing the supratemporal fenestrae are included, longirostrine taxa are broadly separated from one another due to variation in the size of the supratemporal fenestrae. The hypotheses of previous studies suggesting that the size of the supratemporal fenestrae is influenced by snout length-with longer snouts corresponding to larger fenestrae-must be reevaluated. Although species of the crocodyloids Tomistoma and Euthecodon approach or exceed the length of the snout in gavialoids, their supratemporal fenestrae are proportionally smaller-this suggests a phylogenetic constraint in crocodyloids regardless of snout length.

Cranial anatomy of the mekosuchine crocodylianTrilophosuchus rackhamiWillis, 1993

“…The supratemporal fenestrae have variable outlines and sizes that differ between taxa (e.g., figure S1 .14 in Ristevski et al, 2020b ). The outline and size of the supratemporal fenestrae within a species is also subject to ontogenetic change (Cossette et al, 2021 ). Typically, hatchling and juvenile crocodylians have supratemporal fenestrae that are sub‐elliptical in outline and have their main axes diverging laterally (Kälin, 1933 ; Iordansky, 1973 ; Dodson, 1975 ; Clark & Norell, 1992 ; see figure 8 in Fernandez Blanco et al, 2018 ).…”

Dispatches from the age of crocodiles: New discoveries from ancient lineages

“…They found that a key series of rotations in the squamosal and other bones of the snout are likely primitive for crocodyliforms and responsible for the characteristic morphology of living species. Cossette et al (2022) employ morphometrics of multiple ontogenetic series of living crocodylians and fossil crocodylians to test for covariation between snout shape and supratemporal fenestra. They found fenestra shape has a more complicated relationship with snout shape than previously identified.…”

Phylogenetic evolutionary aspects of the vertebrate skull - an approach for teaching: vertebrate skull

“…In reptiles (figure 8) some bones no longer exist compared to amphibians. The postparietal bones, the elements of the orbital series, and the coronoids have joined the mandible (TORREY, 1966;COSSETE;GRASS;DEGUZMAN, 2021). Modern reptiles have two lateral openings on each side of the skull, except chelonians; however, mammals originated from one of the groups that had only one opening in the lateral portion of each side of the skull, the synapsids (ROMER, 1956;COSSETE;GRASS;DEGUZMAN, 2021).…”

“…An important aspect of reptiles is the association of the articular bone with the square that is part of the neurocranium. Except in snakes, the square bone is firmly associated with the scaly bone (TORREY, 1978;COSSETE;GRASS;DEGUZMAN, 2021). The skull of birds (figure 9) is similar to that of reptiles, except that they have a single condyle at the junction with the vertebral column (TORREY, 1978).…”