Data from: Inner ear morphology of diadectomorphs and seymouriamorphs (Tetrapoda) uncovered by high‐resolution x‐ray microcomputed tomography, and the origin of the amniote crown group

The origin of amniotes was a key event in vertebrate evolution, enabling
tetrapods to break their ties with water and invade terrestrial
environments. Two pivotal clades of early tetrapods, the diadectomorphs
and the seymouriamorphs, have played an unsurpassed role in debates about
the ancestry of amniotes for over a century, but their skeletal morphology
has provided conflicting evidence for their affinities. Using
high-resolution X-ray microcomputed tomography, we reveal the
three-dimensional architecture of the well preserved endosseous labyrinth
of the inner ear in representative species belonging to both groups. Data
from the inner ear are coded in a new cladistic matrix of stem and
primitive crown amniotes. Both maximum parsimony and Bayesian inference
analyses retrieve seymouriamorphs as derived non-crown amniotes and
diadectomorphs as sister group to synapsids. If confirmed, this sister
group relationship invites re-examination of character polarity near the
roots of the crown amniote radiation. Major changes in the endosseous
labyrinth and adjacent braincase regions are mapped across the transition
from non-amniote to amniote tetrapods, and include: a ventral shift of the
cochlear recess relative to the vestibule and the semicircular canals;
cochlear recess (primitively housed exclusively within the opisthotic)
accommodated within both the prootic and the opisthotic; development of a
distinct fossa subarcuata. The inner ear of seymouriamorphs foreshadows
conditions of more derived groups, whereas that of diadectomorphs shows a
mosaic of plesiomorphic and apomorphic traits, some of which are
unambiguously amniote-like, including a distinct and pyramid-like cochlear
recess.