New insights into the early morphological evolution of sea turtles by re-investigation of , a three-dimensionally preserved fossil stem chelonioid from the Campanian of Alberta, Canada.

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The early evolution of Pan-Chelonioidea (sea turtles) is poorly understood. This is in part due to the rarity of undeformed skulls of definitive early stem chelonioids. In this work, we redescribe the holotype of using µCT scans and segmentations of the skull. This fossil is the best 3D preserved skull of any Campanian sea turtle, and includes partial "soft tissue" preservation. is morphologically similar but clearly distinct from spp., and both show a mosaic of plesiomorphic and derived chelonioid features. The internal cranial anatomy documents the presence of derived characters in that are absent in spp., such as the loss of the epipterygoids and the rod-like shape of the rostrum basisphenoidale. Among the numerous plesiomorphic characters is the presence of a splenial bone, which was unnoticed before. An updated phylogenetic analysis retrieves as a non-protostegid early stem chelonioid in a slightly more crownward position than . Our phylogeny includes macrobaenids and protostegids as pan-chelonioids, and we find unorthodox results for dermochelyids. Thus, although provides important new insights into the early morphological evolution of sea turtles, much work remains to be done. As a completely 3D preserved specimen, we included into a recent landmark-based skull shape dataset of turtles. Morphospace analysis reveals an intermediate position between cryptodires and crown chelonioids. Based on these data, we also predict that was still capable of neck retraction, constraining the loss of this trait to more crownward pan-chelonioids.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1186/s13358-024-00323-8.