New insights into the early morphological evolution of sea turtles by re-investigation of Nichollsemys baieri, 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 Nichollsemys baieri 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. Nichollsemys is morphologically similar but clearly distinct from Toxochelys spp., and both show a mosaic of plesiomorphic and derived chelonioid features. The internal cranial anatomy documents the presence of derived characters in Nichollsemys baieri that are absent in Toxochelys 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 Nichollsemys baieri as a non-protostegid early stem chelonioid in a slightly more crownward position than Toxochelys latiremis. Our phylogeny includes macrobaenids and protostegids as pan-chelonioids, and we find unorthodox results for dermochelyids. Thus, although Nichollsemys baieri 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 Nichollsemys baieri 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 Nichollsemys baieri 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.

The cranial and postcranial morphology of Hutchemys rememdium and its impact on the phylogenetic relationships of Plastomenidae (Testudinata, Trionychidae).

Hutchemys rememdium is a poorly understood softshell turtle (Trionychidae) from the mid Paleocene of the Williston Basin of North America previously known only from postcranial remains. A particularly rich collection of previously undescribed material from the Tiffanian 4 North American Land Mammal Age (NALMA) of North Dakota is here presented consisting of numerous shells that document new variation, some non-shell postcrania, and cranial remains, which are described based on 3D models extracted from micro-CT data. Although the observed shell variation weakens previously noted differences with the younger species Hutchemys arctochelys from the Clarkforkian NALMA, the two taxa are still recognized as distinct. Parsimony and Bayesian phylogenetic analyses reaffirm the previously challenged placement of Hutchemys rememdium within the clade Plastomenidae, mostly based on novel observations of cranial characters made possible by the new material and the micro-CT data. The new topology supports the notion that the well-ossified plastron of plastomenids originated twice in parallel near the Cretaceous/Paleogene boundary, once in the Hutchemys lineage and once in the Gilmoremys/Plastomenus lineage. Hutchemys rememdium is notable for being the only documented species of trionychid in the mid Paleocene of the Williston Basin. The presence of multiple individuals in a carbonaceous claystone indicates this taxon lived in swamps and lakes and its expanded triturating surface suggests it had a durophagous diet. Supplementary Information: The online version contains supplementary material available at 10.1186/s13358-024-00315-8.

Diversity and biogeography of South American mud turtles elucidated by multilocus DNA sequencing (Testudines: Kinosternidae).

Kinosternon is the most speciose genus of extant turtles, with 22 currently recognized species, distributed across large parts of the Americas. Most species have small distributions, but K. leucostomum and K. scorpioides range from Mexico to South America. Previous studies have found discordance between mitochondrial and nuclear phylogenies in some kinosternid groups, with the current taxonomy following the nuclear-based results. Herein, based on extended molecular, geographic, and taxonomic sampling, we explore the phylogeographic structure and taxonomic limits for K. leucostomum and the K. scorpioides group and present a fossil-calibrated nuclear time tree for Kinosternon. Our results reveal contrasting differentiation patterns for the K. scorpioides group and K. leucostomum, despite overlapping distributions. Kinosternon leucostomum shows only shallow geographic divergence, whereas the K. scorpioides group is polyphyletic with up to 10 distinct taxa, some of them undescribed. We support the elevation of K. s. albogulare and K. s. cruentatum to species level. Given the deep divergence within the genus Kinosternon, we propose the recognition of three subgenera, Kinosternon, Cryptochelys and Thyrosternum, and the abandonment of the group-based classification, at least for the K. leucostomum and K. scorpioides groups. Our results show an initial split in Kinosternon that gave rise to two main radiations, one Nearctic and one mainly Neotropical. Most speciation events in Kinosternon occurred during the Quaternary and we hypothesize that they were mediated by both climatic and geological events. Additionally, our data imply that at least three South American colonizations occurred, two in the K. leucostomum group, and one in the K. scorpioides group. Additionally, we hypothesize that discordance between mitochondrial and nuclear phylogenetic signal is due to mitochondrial capture from an extinct kinosternine lineage.

Skull osteology, neuroanatomy, and jaw-related myology of the pig-nosed turtle Carettochelys insculpta (Cryptodira, Trionychia).

The osteology, neuroanatomy, and musculature are known for most primary clades of turtles (i.e., "families"), but knowledge is still lacking for one particular clade, the Carettochelyidae. Carettochelyids are represented by only one living taxon, the pig-nosed turtle Carettochelys insculpta. Here, we use micro-computed tomography of osteological and contrast-enhanced stained specimens to describe the cranial osteology, neuroanatomy, circulatory system, and jaw musculature of Carettochelys insculpta. The jaw-related myology is described in detail for the first time for this taxon, including m. zygomaticomandibularis, a muscular unit only found in trionychians. We also document a unique arterial pattern for the internal carotid artery and its subordinate branches and provide an extensive list of osteological ontogenetic differences. The present work provides new insights into the craniomandibular anatomy of turtles and will allow a better understanding of the evolutionary history of the circulatory system of trionychians and intraspecific variation among turtles.

Cranial and mandibular anatomy of Plastomenus thomasii and a new time-tree of trionychid evolution.

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 µCT 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. Supplementary Information: The online version contains supplementary material available at 10.1186/s13358-023-00267-5.

A pan-chelydrid, Chelydropsis aubasi sp. nov., from the Middle Eocene (MP 15, early Bartonian) of Chéry-Chartreuve, France.

Snapping turtles (Pan-Chelydridae) play an important role in modern ecosystems throughout North America, but their fossil record is notably poor. We here describe a new species of fossil pan-chelydrid, Chelydropsis aubasi, from the Middle Eocene (MP15, Bartonian) of Chéry-Chartreuve, Department of Aisne, France, based on a series of fragments that document most of the shell. The new species not only bridges the morphological gap between earlier pan-chelydrids from North America and later pan-chelydrids from Europe but also advances the arrival of the group from North America to Europe by at least 10 million years. Although pan-chelydrids are notably absent from Early and Middle Eocene localities throughout Europe, the new fossil gives credence to the idea that pan-chelydrids may have dispersed from North America to Europe in concert with other turtles during the Paleocene-Eocene Thermal Maximum, but remain undetected to date.

Cranial osteology, taxonomic reassessment, and phylogenetic relationships of the Early Cretaceous (Aptian-Albian) turtle Trinitichelys hiatti (Paracryptodira).

We describe the skull of the Early Cretaceous (Aptian-Albian) baenid turtle Trinitichelys hiatti using micro-computed tomography to provide new insights into the cranial anatomy of basal baenids and into the evolution of paracryptodires. We show that the validity of Trinitichelys hiatti vs Arundelemys dardeni still holds true, that the most basal known baenids for which skull material is known share an intriguing combination of features that are typical of either Pleurosternidae or Baenidae, and that the carotid system of Trinitichelys hiatti is intermediate to that of pleurosternids and more advanced baenids. Our expanded phylogenetic analysis confirms the traditional placement of Arundelemys dardeni, Lakotemys australodakotensis, and Trinitichelys hiatti as basal baenids, retrieves Helochelydridae along the stem of Baenoidea, but recovers Dinochelys whitei, Glyptops ornatus, Dorsetochelys typocardium, and Uluops uluops as basal branching Paracryptodira.

Independent origin of large labyrinth size in turtles.

The labyrinth of the vertebrate inner ear is a sensory system that governs the perception of head rotations. Central hypotheses predict that labyrinth shape and size are related to ecological adaptations, but this is under debate and has rarely been tested outside of mammals. We analyze the evolution of labyrinth morphology and its ecological drivers in living and fossil turtles, an understudied group that underwent multiple locomotory transitions during 230 million years of evolution. We show that turtles have unexpectedly large labyrinths that evolved during the origin of aquatic habits. Turtle labyrinths are relatively larger than those of mammals, and comparable to many birds, undermining the hypothesis that labyrinth size correlates directly with agility across vertebrates. We also find that labyrinth shape variation does not correlate with ecology in turtles, undermining the widespread expectation that reptilian labyrinth shapes convey behavioral signal, and demonstrating the importance of understudied groups, like turtles.

A review of helochelydrid shell material from late Albian to early Cenomanian greensands of Southern England, United Kingdom.

A number of helochelydrid turtle shell remains were recovered over the course of the 19th century from mid-Cretaceous sediments throughout Southern England, including the poorly figured and described types of Trachydermochelys phlyctaenus from the Cambridge Greensand of Cambridgeshire, Plastremys lata from the Upper Greensand of the Isle of Wight, and "Trachydermochelys" rutteri from the Melbury Sandstone of Dorset. A review of stratigraphic provenience suggests that all material originates from late Late Aptian portions of the Upper Greensand or early Early Cenomanian portions of the West Melbury Marly Chalk Formation, a relatively narrow time window geologically speaking. As described, Trachydermochelys phlyctaenus is a problematic taxon, because the most plausible type series is a chimera that includes two helochelydrid morphotypes in addition to protostegid remains. This conundrum is resolved through the designation of a lectotype. A review of all historic material confirms the presence of three English taxa distinct from Helochelydra danubina, a coeval taxon named from Germany. At least four helochelydrid taxa, therefore, occurred in western Europe during the Early to Late Cretaceous transition.

New insights into the cranial osteology of the Early Cretaceous paracryptodiran turtle Lakotemys australodakotensis.

Lakotemys australodakotensis is an Early Cretaceous paracryptodire known from two shells and a skull from the Lakota Formation of South Dakota, USA. Along with the Early Cretaceous Arundelemys dardeni and the poorly known Trinitichelys hiatti, Lakotemys australodakotensis is generally retrieved as an early branching baenid, but more insights into the cranial anatomy of these taxa is needed to obtain a better understanding of paracryptodiran diversity and evolution. Here, we describe the skull of Lakotemys australodakotensis using micro-computed tomography to provide the anatomical basis for future phylogenetic analyses that will be needed to investigate more precisely the intrarelationships of Paracryptodira. Preliminary comparisons reveal that the cranial anatomy of Lakotemys australodakotensis is very similar to that of the Aptian-Albian basal baenid Arundelemys dardeni, that both taxa exhibit a remarkable combination of derived characters found in baenodds and characters found in non-baenid paracryptodires, particularly Pleurosternidae, and that Lakotemys australodakotensis is the only known baenid to date to possess a canal for the palatine artery.

A redescription of the Late Jurassic (Tithonian) turtle Uluops uluops and a new phylogenetic hypothesis of Paracryptodira.

We study the Late Jurassic (Tithonian) turtle Uluops uluops using micro-computed tomography scans to investigate the cranial anatomy of paracryptodires, and provide new insights into the evolution of the internal carotid artery and facial nerve systems, as well as the phylogenetic relationships of this group. We demonstrate the presence of a canalis caroticus lateralis in Uluops uluops, the only pleurosternid for which a palatine artery canal can be confidently identified. Our phylogenetic analysis retrieves Uluops uluops as the earliest branching pleurosternid, Helochelydridae within Pleurosternidae, and Compsemydidae including Kallokibotion bajazidi within Baenidae, which suggests at least two independent losses of the palatine artery within paracryptodires. We expect future studies will provide additional insights into the evolution of the circulation system of paracryptodires, as well as clarifying relationships along the turtle stem. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s13358-021-00234-y.

The phylogenetic relationships of geoemydid turtles from the Eocene Messel Pit Quarry: a first assessment using methods for continuous and discrete characters.

The geoemydid turtles of the Eocoene Messel Pit Quarry of Hesse, Germany, are part of a rich Western European fossil record of testudinoids. Originally referred to as "Ocadia" kehreri and "Ocadia" messeliana, their systematic relationships remain unclear. A previous study proposed that a majority of the Western European geoemydids, including the Messel geoemydids, are closely related to the Recent European representatives of the clade Mauremys. Another study hypothesised that the Western European geoemydid fauna is more phylogenetically diverse, and that the Messel geoemydids are closely related to the East Asian turtles Orlitia and Malayemys. Here we present the first quantitative analyses to date that investigate this question. We use continuous characters in the form of ratios to estimate the placement of the Messel geoemydids in a reference tree that was estimated from molecular data. We explore the placement error obtained from that data with maximum likelihood and Bayesian methods, as well as linear parsimony in combination with discrete characters. We find good overall performance with Bayesian and parsimony analyses. Parsimony performs even better when we also incorporated discrete characters. Yet, we cannot pin down the position of the Messel geoemydids with high confidence. Depending on how intraspecific variation of the ratio characters is treated, parsimony favours a placement of the Messel fossils sister to Orlitia borneensis or sister to Geoemyda spengleri, with weak bootstrap support. The latter placement is suspect because G. spengleri is a phylogenetically problematic species with molecular and morphological data. There is even less support for placements within the Mauremys clade.

New interpretation of the cranial osteology of the Early Cretaceous turtle Arundelemys dardeni (Paracryptodira) based on a CT-based re-evaluation of the holotype.

Arundelemys dardeni is an Early Cretaceous paracryptodire known from a single, incomplete, but generally well-preserved skull. Phylogenetic hypotheses of paracryptodires often find Arundelemys dardeni as an early branching baenid. As such, it has a central role in understanding the early evolution of the successful clade Baenidae, which survived the Cretaceous-Paleogene mass extinction, as well as the diversification of Paracryptodira into its subclades, which recent research suggests to perhaps include helochelydrids, compsemydids, pleurosternids, and baenids. Computer tomography scans of the holotype material that were produced for the initial description of Arundelemeys dardeni reveal several errors in the initial anatomical description of the species, which we correct based on element-by-element segmentation. In addition, we provide entirely novel anatomical information, including descriptions of several previously undescribed cranial bones, the endosseous labyrinth, and the cranial scutes, the latter of which are unknown for most paracryptodires. We provide an interpretation of cranial scutes which homologizes the scutes of Arundelemys dardeni with those of other stem turtles.

Two turtles with soft tissue preservation from the platy limestones of Germany provide evidence for marine flipper adaptations in Late Jurassic thalassochelydians.

Late Jurassic deposits across Europe have yielded a rich fauna of extinct turtles. Although many of these turtles are recovered from marine deposits, it is unclear which of these taxa are habitually marine and which may be riverine species washed into nearby basins, as adaptations to open marine conditions are yet to be found. Two new fossils from the Late Jurassic of Germany provide unusually strong evidence for open marine adaptations. The first specimen is a partial shell and articulated hind limb from the Late Jurassic (early Tithonian) platy limestones of Schernfeld near Eichstätt, which preserves the integument of the hind limb as an imprint. The skin is fully covered by flat, polygonal scales, which stiffen the pes into a paddle. Although taxonomic attribution is not possible, similarities are apparent with Thalassemys. The second specimen is a large, articulated skeleton with hypertrophied limbs referable to Thalassemys bruntrutana from the Late Jurassic (early Late Kimmeridgian) platy limestone of Wattendorf, near Bamberg. Even though the skin is preserved as a phosphatic film, the scales are not preserved. This specimen can nevertheless be inferred to have had paddles stiffened by scales based on the pose in which they are preserved, the presence of epibionts between the digits, and by full morphological correspondence to the specimen from Schernfeld. An analysis of scalation in extant turtles demonstrated that elongate flippers stiffed by scales are a marine adaptation, in contrast to the elongate but flexible flippers of riverine turtles. Phylogenetic analysis suggests that Thalassemys bruntrutana is referable to the mostly Late Jurassic turtle clade Thalassochelydia. The marine adapted flippers of this taxon therefore evolved convergently with those of later clades of marine turtles. Although thalassochelydian fossils are restricted to Europe, with one notable exception from Argentina, their open marine adaptations combined with the interconnectivity of Jurassic oceans predict that the clade must have been even more wide-spread during that time.

A new pelomedusoid turtle, Sahonachelys mailakavava, from the Late Cretaceous of Madagascar provides evidence for convergent evolution of specialized suction feeding among pleurodires.

The Maevarano Formation in northwestern Madagascar has yielded a series of exceptional fossils over the course of the last three decades that provide important insights into the evolution of insular ecosystems during the latest Cretaceous (Maastrichtian). We here describe a new genus and species of pelomedusoid turtle from this formation, Sahonachelys mailakavava, based on a nearly complete skeleton. A phylogenetic analysis suggests close affinities of Sahonachelys mailakavava with the coeval Madagascan Sokatra antitra. These two taxa are the only known representatives of the newly recognized clade Sahonachelyidae, which is sister to the speciose clade formed by Bothremydidae and Podocnemidoidae. A close relationship with coeval Indian turtles of the clade Kurmademydini is notably absent. A functional assessment suggests that Sahonachelys mailakavava was a specialized suction feeder that preyed upon small-bodied invertebrates and vertebrates. This is a unique feeding strategy among crown pelomedusoids that is convergent upon that documented in numerous other clades of turtles and that highlights the distinct evolutionary pathways taken by Madagascan vertebrates.

A review of the carotid artery and facial nerve canal systems in extant turtles.

The cranial circulation and innervation systems of turtles have been studied for more than two centuries and extensively used to understand turtle systematics. Although a significant number of studies related to these structures exists, a broader comprehension of variation across the tree has been hindered by poor sampling and a lack of synthetic studies that addressed both systems together. We here provide new insights regarding the carotid circulation and facial nerve innervation systems in a broad set of extant turtles using CT (computed tomography) scans, which allow us to trace the canals these structures form in bone and understand the interaction between both systems. We document that the palatine artery, including the lateral carotid canal, is absent in all pleurodires and carettochelyids and was likely reduced or lost several times independently within Testudinoidea. We also highlight osteological correlates for the location of the mandibular artery. We finally summarize variation regarding the placement of the mandibular artery, location of the geniculate ganglion, placement of the hyomandibular and vidian nerves, and situations where we recommend caution when assessing canals in fossils. A morphometric study confirms that the relative sizes of the carotid canals are correlated with one another. Our results have the potential for building new phylogenetic characters and investigating the circulation systems of fossil taxa, which are expected to shed light on the evolution of the circulation system of turtles and clarify some unresolved relationships between fossil turtle clades.

Cranial osteology of the Early Cretaceous turtle Pleurosternon bullockii (Paracryptodira: Pleurosternidae).

Pleurosternon bullockii is a turtle from the Early Cretaceous of Europe known from numerous postcranial remains. Only one skull has so far been referred to the species. Pleurosternon bullockii belongs to a group of turtles called pleurosternids, which is thought to include several poorly known taxa from the Late Jurassic and Early Cretaceous of Europe and North America. Pleurosternids and baenids, a group of North American turtles that lived from the Late Cretaceous to the Eocene, define a clade called Paracryptodira. Additionally, Paracryptodira likely includes compsemydids, and, potentially, helochelydrids. Character support for Paracryptodira is relatively weak, and many global phylogenetic studies fail to support paracryptodiran monophyly altogether. Proposed paracryptodiran synapomorphies are largely cranial, despite the poor characterization of pleurosternid cranial material. In addition to their questionable monophyly, the global position of paracryptodires is debated. Early studies suggest crown-turtle affinities, but most phylogenies find them as stem-turtles, irrespective of their monophyly. Here, we document the cranial osteology of Pleurosternon bullockii with the use of three-dimensional models derived from segmenting high-resolution X-ray micro-computed tomography (CT) scans. Pleurosternon bullockii has a primitive basipterygoid region of the skull, but a cryptodire-like acustico-jugular region. A surprising number of similarities with pleurodires exist, particularly in the laterally expanded external process of the pterygoid and in the posterior orbital wall. Our observations constitute an important step toward a phylogenetic re-evaluation of Paracryptodira.

A revision of the pelomedusoid turtle Jainemys pisdurensis from the Late Cretaceous (Maastrichtian) Lameta Formation of India.

BACKGROUND: Jainemys pisdurensis comb. nov. is an extinct pleurodiran turtle from the Late Cretaceous (Maastrichtian) of India, previously referred to Carteremys and Shweboemys. The holotype, an eroded skull, had been collected near the village of Pisdura, south of Nagpur, in Maharashtra State, while all referred shell material originates from coeval sediments exposed at the nearby village of Dongargaon. Initial estimates believed this turtle to either be an early representative of Podocnemididae or a basal representative of Pelomedusoides. METHODS: We here figure and describe all specimens that had previously been referred to Jainemys pisdurensis comb. nov. We furthermore re-evaluate the validity of this fossil turtle and explore its phylogenetic relationships within Pleurodira. RESULTS: The holotype of Jainemys pisdurensis comb. nov. displays a morphology that differs substantially from that originally reported. Most notably, the palatines only have a minor contribution to the broad triturating surfaces but have a broad midline contact with each other, the pterygoids only have a midline contact of intermediate length and do not contact the opisthotics posteriorly, the basisphenoid is broad and short, and the opisthotics do not contribute to the flooring of the cavum acustico-jugulare. The referred shell material also displays a morphology different from that reported originally, in particular in that vertebral I does not contribute to the anterior margin of the carapace while the nuchal does. Phylogenetic analysis places the cranial material within the bothremydid clade Kurmademydini, while the shell material is placed in an unresolved polytomy at the base of this clade. Jainemys pisdurensis is confirmed to be a valid species of pleurodiran turtle, but the high diversity of coeval kurmademydines in India demands removal of the postcranial remains from this taxon. The realization that all valid species of Late Cretaceous (Maastrichtian) turtles from India form a clade supports the hypothesis that India was physically separated from the rest of Gondwana at this time.

New material of named fossil turtles from the Late Jurassic (late Kimmeridgian) of Wattendorf, Germany.

The newly discovered plattenkalk (platy limestone) locality of Wattendorf, southern Germany, has yielded a diverse fauna and flora dated to the base of the late Kimmeridgian, Late Jurassic. We here describe three fossil turtle specimens that were recovered during systematic excavations of a distinct, 15 cm thick package of plattenkalks by the Naturkunde-Museum Bamberg. The first specimen is a large shell of Achelonia formosa, a taxon that is based on material from the late Kimmeridgian of Cerin, France. The new specimen suggests synonymy with Enaliochelys chelonia from the late Kimmeridgian of the United Kingdom. The second is a near-complete skeleton of the enigmatic Tropidemys seebachi, which was previously known only from the late Kimmeridgian of Hannover, northern Germany. The third specimen is a partial skeleton of Eurysternum wagleri, which had previously been known only from the early Tithonian of the Solnhofen region, southern Germany. In addition to new anatomical insights, the new material provides further evidence for spatial links during the late Kimmeridgian between northern and southern Germany, France, and the United Kingdom and temporal link from the late Kimmeridgian to the early Tithonian. The prevalence of partial, though articulated specimens is suggestive of predation by an unknown large marine reptile.

A re-description of Sandownia harrisi (Testudinata: Sandownidae) from the Aptian of the Isle of Wight based on computed tomography scans.

Sandownidae is an enigmatic group of Cretaceous-Paleogene turtles with highly derived cranial anatomy. Although sandownid monophyly is not debated, relationships with other turtles remain unclear. Sandownids have been recovered in significantly different parts of the turtle tree: as stem-turtles, stem-cryptodires and stem-chelonioid sea turtles. Latest phylogenetic studies find sandownids as the sister-group of the Late Jurassic thalassochelydians and as stem-turtles. Here, we provide a detailed study of the cranial and mandibular anatomy of Sandownia harrisi from the Aptian of the Isle of Wight, based on high resolution computed tomography scanning of the holotype. Our results confirm a high number of anatomical similarities with thalassochelydians and particularly Solnhofia parsonsi, which is interpreted as an early member of the sandownid lineage. Sandownids + Solnhofia show many cranial modifications related to the secondary palate and a durophagous diet. Sandownia is additionally highly derived in features related to its arterial circulation and neuroanatomy, including the endosseous labyrinth. Our results imply rapid morphological evolution during the early history of sandownids. Sandownids likely evolved in central Europe from thalassochelydian ancestors during the Late Jurassic. The durophagous diet of sandownids possibly facilitated their survival of the Cretaceous/Paleogene mass extinction.