Early origins of divergent patterns of morphological evolution on the mammal and reptile stem-lineages

The origin of amniotes 320 million years ago signalled independence from
water in vertebrates and was closely followed by divergences within the
mammal and reptile stem lineages (Synapsida and Reptilia). Early members
of both groups had highly similar morphologies, being superficially
‘lizard-like’ forms with many plesiomorphies. However, the extent to which
they might have exhibited divergent patterns of evolutionary change, with
potential to explain the large biological differences between their living
members, is unresolved. We use a new, comprehensive phylogenetic dataset
to quantify variation in rates and constraints of morphological evolution
among Carboniferous–early Permian amniotes. We find evidence for an early
burst of evolutionary rates, resulting in the early origins of
morphologically distinctive subgroups that mostly persisted through the
Cisuralian. Rates declined substantially through time, especially in
reptiles. Early reptile evolution was also more constrained compared to
early synapsids, exploring a more limited character state space.
Postcranial innovation in particular was important in early synapsids,
potentially related to their early origins of large body size. In
contrast, early reptiles predominantly varied the temporal region,
suggesting disparity in skull and jaw kinematics, and foreshadowing the
variability of cranial biomechanics seen in reptiles today. Our results
demonstrate that synapsids and reptiles underwent an early divergence of
macroevolutionary patterns. This laid the foundation for subsequent
evolutionary events and may be critical in understanding the substantial
differences between mammals and reptiles today. Potential explanations
include an early divergence of developmental processes or of ecological
factors, warranting cross-disciplinary investigation.