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Abstract The stem lineage of Archosauria is populated by a diverse fossil record that remains notably understudied relative to the crown clade. Prominent among these specimens is a beautifully preserved skull of the early mid-Triassic rhynchosaur Mesosuchus browni [Iziko South African Museum (SAM) 6536], whose phylogenetic position has considerable influence on patterns of pan-archosaurian cranial evolution. We used high-resolution, micro-computed tomography to re-examine the anatomy of this specimen, building on previous studies that were either limited to external observations or restricted to the braincase. A digital segmentation of the cranial elements and primary neurovascular canals of SAM-PK-6536 allows for expanded character scoring and constitutes a foundation for future comparative insights. Our data support the phylogenetically oldest instance of a pneumatized maxilla in a pan-archosaur, bringing the record of antorbital pneumatization into closer alignment with that of the neurocranium. The nasal cavity and primary palate of Mesosuchus includes a complex septomaxilla, a novel element anterior to the vomer, and is likely to have supported a well-developed vomeronasal system. The evolution of this system is discussed in terms of both phylogenetic pattern and how the skeletal architecture of Mesosuchus and other fossils could inform the signalling dynamics that pattern the vomeronasal system during development. 

Abstract Saxochelys gilbertiis a baenid turtle from the Late Cretaceous Hell Creek Formation of the United States of America known from cranial, shell, and other postcranial material. Baenid turtles are taxonomically diverse and common fossil elements within Late Cretaceous through Eocene faunas. Detailed anatomical knowledge is critical to understanding the systematics and morphological evolution of the group. This is particularly important as baenids represent an important group of continental vertebrates that survived the mass extinction event associated with the Cretaceous/Paleogene boundary. High-resolution micro-computed tomography scanning of the holotype skull reveals additional anatomical details for the already well-knownSaxochelys gilberti. This includes the revision of some anatomical statements from the original description, but also detailed knowledge on internal anatomical features of the braincase and the description of a well-preserved axis (cervical vertebra 2). Our new detailed description and previous work on the shell and postcrania makeSaxochelysone of the best-described, nearly complete baenid turtles, which are often only known from either isolated shell or cranial material. A revised phylogenetic analysis confirms the position ofSaxochelys gilbertias a derived baenid (Eubaeninae) more closely related toBaena arenosathan toEubaena cephalica. 

Denazinemys nodosais a Late Cretaceous representative of the North American turtle clade Baenidae diagnosed, among others, by a shell surface texture consisting of raised welts. We provide a detailed description of a partial skeleton from the late Campanian Kaiparowits Formation of Utah, USA, including bone-by-bone analysis of its cranium based on images obtained using micro-computed tomography. A revised phylogenetic analysis confirms placement ofDenazinemys nodosaclose toEubaena cephalicaandBoremysspp. within the clade Eubaeninae. Comparison with a second skull from the Kaiparowits Formation previously assigned toDenazinemys nodosaquestions its referral to this taxon. An assortment of specimens from the Early to Late Campanian of Mexico and the USA had previously been referred toDenazinemys nodosabased on shell surface texture alone, even though this characteristic is known to occur in other baenids. Our review of all available material concludes thatDenazinemys nodosais currently only known from the Late Campanian of New Mexico and Utah. 

The origin of turtles and their uniquely shelled body plan is one of the longest standing problems in vertebrate biology. The unfulfilled need for a hypothesis that both explains the derived nature of turtle anatomy and resolves their unclear phylogenetic position among reptiles largely reflects the absence of a transitional fossil record. Recent discoveries have dramatically improved this situation, providing an integrated, time-calibrated model of the morphological, developmental, and ecological transformations responsible for the modern turtle body plan. This evolutionary trajectory was initiated in the Permian (>260 million years ago) when a turtle ancestor with a diapsid skull evolved a novel mechanism for lung ventilation. This key innovation permitted the torso to become apomorphically stiff, most likely as an adaption for digging and a fossorial ecology. The construction of the modern turtle body plan then proceeded over the next 100 million years following a largely stepwise model of osteological innovation.