biostudies-literatureMenon JCLSwiss National Science Foundation27https://www.ebi.ac.uk/biostudies/studies/S-EPMC11245440biostudies-literatureUnknown143(1)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 <i>Nichollsemys baieri</i> 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. <i>Nichollsemys</i> is morphologically similar but clearly distinct from <i>Toxochelys</i> spp., and both show a mosaic of plesiomorphic and derived chelonioid features. The internal cranial anatomy documents the presence of derived characters in <i>Nichollsemys baieri</i> that are absent in <i>Toxochelys</i> spp., such as the loss of the epipterygoids and the rod-like shape of the rostruSwiss journal of palaeontologyNew insights into the early morphological evolution of sea turtles by re-investigation of <i>Nichollsemys baieri</i>, a three-dimensionally preserved fossil stem chelonioid from the Campanian of Alberta, Canada.PMC11245440200021202019207377Menon JCLEvers SWBrinkman DBHermanson GJoyce WGfalseNew insights into the early morphological evolution of sea turtles by re-investigation of <i>Nichollsemys baieri</i>, a three-dimensionally preserved fossil stem chelonioid from the Campanian of Alberta, Canada.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 <i>Nichollsemys baieri</i> 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. <i>Nichollsemys</i> is morphologically similar but clearly distinct from <i>Toxochelys</i> spp., and both show a mosaic of plesiomorphic and derived chelonioid features. The internal cranial anatomy documents the presence of derived characters in <i>Nichollsemys baieri</i> that are absent in <i>Toxochelys</i> spp., such as the loss of the epipterygoids and the rod-like shape of the rostru2024-01-01T00:00:00Z20242025-04-18T21:42:47.589Z2025-02-19T02:34:28.254ZS-EPMC112454403900695110.1186/s13358-024-00323-8