Trionychid (softshell) turtles have a peculiar bauplan, which includes shell reductions
and cranial elongation. Despite a rich fossil record dating back to the Early Cretaceous,
the evolutionary origin of the trionychid bauplan is poorly understood, as even old
fossils show great anatomical similarities to extant species. Documenting structural
detail of fossil trionychids may help resolve the evolutionary history of the group.
Here, we study the cranial and mandibular anatomy of Plastomenus thomasii using mu
CT scanning. Plastomenus thomasii belongs to the Plastomenidae, a long-lived (Santonian-Eocene)
clade with uncertain affinities among trionychid subclades. The skulls of known plastomenids
are characterized by unusual features otherwise not known among trionychids, such
as extremely elongated, spatulate mandibular symphyses. We use anatomical observations
for updated phylogenetic analyses using both parsimony and Bayesian methods. There
is strong support across methods for stem-cyclanorbine affinities for plastomenids.
The inclusion of stratigraphic data in our Bayesian analysis indicates that a range
of Cretaceous Asian fossils including Perochelys lamadongensis may be stem-trionychids,
suggesting that many features of trionychid anatomy evolved prior to the appearance
of the crown group. Divergence time estimates from Bayesian tip-dating for the origin
of crown Trionychia (134.0 Ma) and Pan-Trionychidae (123.8 Ma) constrain the evolutionary
time span during which the trionychid bauplan has evolved to a range of < 11 million
years. Bayesian rate estimation implies high morphological rates during early softshell
turtle evolution. If correct, plastomenids partially fill the stratigraphic gap which
results from shallow divergence times of crown cyclanorbines during the late Eocene.
