The phylogeny of early amniotes and the affinities of Parareptilia and Varanopidae

Amniotes include mammals, reptiles and birds, representing 75% of extant
vertebrate species on land. They originated around 318 million years ago
(early late Carboniferous)and their early fossil record is central to
understanding the expansion of vertebrates in terrestrial ecosystems. We
present a new phylogenetic hypothesis that challenges the widely-accepted
consensus about early amniote evolution, based on parsimony analysis and
Bayesian inference of a new morphological dataset. We find a reduced
membership of the mammalian stem-lineage, which excludes varanopids. This
implies that evolutionary turnover of the mammalian stem-lineage during
the early/middle Permian transition (273 Ma) was more
abrupt than has previously been recognised. We also find
that Parareptilia are nested within Diapsida. This suggests that temporal
fenestration, a key structural innovation with important functional
implications, evolved fewer times than generally thought, but showed
highly variable morphology among early reptiles after its initial origin.
Our phylogeny also addresses controversiesover the affinities of
mesosaurids, the earliest known aquatic amniotes,which we recover as
early-diverging parareptiles.