Morphological phylogenies stand in a major conflict with molecular hypotheses regarding
the phylogeny of Cryptodira, the most diverse and widely distributed clade of extant
turtles. However, molecular hypotheses are often considered a better estimate of phylogeny
given that it is more consistent with the stratigraphic and geographic distribution
of extinct taxa. That morphology fails to reproduce the molecular topology partly
originates from problematic character polarization due to yet another contradiction
around the composition of the cryptodiran stem lineage. Extinct sinemydids are one
of these problematic clades: they have been either placed among stem-cryptodires,
stem-chelonioid sea turtles, or even stem-turtles. A new sinemydid from the Early
Cretaceous Jehol Biota (Yixian Formation, Barremian-Early Aptian) of China, Xiaochelys
ningchengensis gen. et sp. nov., allows for a reassessment of the phylogenetic position
of Sinemydidae. Our analysis indicates that sinemydids mostly share symplesiomorphies
with sea turtles and their purported placement outside the crown-group of turtles
is an artefact of previous datasets. The best current phylogenetic estimate is therefore
that sinemydids are part of the stem lineage of Cryptodira together with an array
of other Jurassic to Cretaceous taxa. Our study further emphasises the importance
of using molecular scaffolds in global turtle analyses.
