The morphological diversity of the quadrate bone in squamate reptiles as revealed by high-resolution computed tomography and geometric morphometrics

We examined the morphological diversity of the quadrate bone in squamate
reptiles (i.e. lizards, snakes, amphisbaenians). The quadrate is the
principal splanchnocranial element involved in suspending the lower jaw
from the skull, and its shape is of particular interest because it is
potentially affected by several factors, such as phylogenetic history,
allometry, ecology, skull kinesis and hearing capabilities (e.g. presence
or absence of a tympanic ear). Due to its complexity, the quadrate bone is
also considered one of the most diagnostic elements in fragmentary fossil
taxa. We describe quadrates from 38 species spread across all major
squamate clades, using qualitative and quantitative (e.g. geometric
morphometrics) methods. We test for possible correlations between shape
variation and factors such as phylogeny, size, ecology and
presence/absence of a tympanum. Our results show that the shape of the
quadrate is highly evolutionarily plastic, with very little of the
diversity explained by phylogenetic history. Size variation (allometric
scaling) is similarly unable to explain much shape diversity in the
squamate quadrate. Ecology (terrestrial/fossorial/aquatic) and presence of
a tympanic ear are more significant, but together explain only about 20%
of the diversity observed. Other unexplored and more analytically complex
factors, such as skull biomechanics, likely play additional major roles in
shaping the quadrates of lizards and snakes.