Mesozoic marine reptiles body temperatures + Marmoretta (Jurassic stem lepidosaur) tree-climbing adaptations + crocodylomorph resilience to mass extinctions + alligator dental microwear

[Ben Creisler]

Ben Creisler

bcre...@gmail.com

New reptile papers:

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Nicolas Séon, Peggy Vincent, Lene Liebe Delsett, Eve Poulallion, Guillaume Suan, Christophe Lécuyer, Aubrey Jane Roberts, François Fourel, Sylvain Charbonnier and Romain Amiot (2025)
Reassessment of body temperature and thermoregulation strategies in Mesozoic marine reptiles
Paleobiology (advance online publication)
DOI: https://doi.org/10.1017/pab.2025.2
https://www.cambridge.org/core/journals/paleobiology/article/reassessment-of-body-temperature-and-thermoregulation-strategies-in-mesozoic-marine-reptiles/5DB88592AC69236250A516904E20FEC4

Abstract:

Ichthyosauria, Plesiosauria, and Metriorhynchidae were apex predators in Mesozoic oceanic trophic networks. Previous stable oxygen isotope studies suggested that several taxa belonging to these groups were endothermic and that some of them were homeothermic organisms. However, these conclusions remain contentious owing to the associated uncertainties regarding the δ18O value and oxygen isotope fractionation relative to environmental seawater. Here, we present new bioapatite phosphate δ18O values (δ18Op) of Ichthyosauria, Plesiosauria, and Metriorhynchidae (Middle Jurassic to Early Cretaceous) recovered from mid- to high paleolatitudes to better constrain their thermophysiology and investigate the presence of regional heterothermies. The intraskeletal δ18Op variability failed to reveal distinct heterothermic patterns within any of the specimens, indicating either intrabody temperature homogeneity or an overriding diagenetic overprint of the original biological δ18Op bone record. Body temperature estimates have been reassessed from new and published δ18Op values of well-preserved isolated teeth, recently revised Mesozoic latitudinal δ18O oceanic gradients, and 18O-enrichment factors of fully aquatic air-breathing vertebrates. Our results confirm that Ichthyosauria were homeothermic endotherms (31°C to 41°C), while Plesiosauria were likely poikilothermic endotherms (27°C to 34°C). The new body temperature estimates of the Metriorhynchidae (25°C to 32°C) closely follow ambient temperatures and point to poikilothermic strategy with no or little endothermic ability. These results improve our understanding of Mesozoic marine reptile thermoregulation and indicate that due to their limited body temperature variations, the δ18Op values from Ichthyosauria fossil remains could be used as valuable archives of Mesozoic oceans δ18Osw values that may help improve paleoenvironmental and paleoclimatic reconstructions.

Non-technical Summary:

Some marine reptiles from the Mesozoic, such as ichthyosaurs, plesiosaurs, and metriorhynchids, were capable of reaching elevated body temperatures, and some could maintain body temperatures a few degrees above those of their marine environments, a characteristic similar to that observed in modern cetaceans. Nevertheless, the estimation of their body temperatures from the chemical oxygen signatures of their fossil remains (bones and teeth) is accompanied by uncertainties associated with the chemical oxygen signatures of the surrounding water and the mineralization processes of the bones and teeth. In this study, new data were collected from four ichthyosaurs, three plesiosaurs, and one metriorhynchid in order to gain a deeper understanding of the mechanisms by which these marine reptiles were able to maintain body temperatures higher than those of their environments. The chemical signatures of oxygen in the bones and teeth of the specimens did not exhibit any discernible patterns indicative of specific zones of heat production or loss, unlike what has been observed in modern marine vertebrates. Concurrently, we reassessed the estimated body temperatures of these marine reptiles, thereby corroborating the hypothesis that ichthyosaurs were homeothermic endotherms. Conversely, our new estimates suggest that plesiosaurs were likely poikilothermic endotherms, whereas metriorhynchids were probably also poikilothermic endotherms but with a limited capacity for heat production. Finally, the narrow range of body temperatures maintained by ichthyosaurs indicates that the oxygen chemical signatures of fossilized remains could serve as valuable markers for reconstructing variations in the oxygen isotope composition of the Mesozoic oceans, paving the way to enhance our understanding of the environment and climate of this period in Earth’s history.

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David P. Ford, Roger B. J. Benson, Elizabeth F. Griffiths and Susan E. Evans (2025)
Evidence for clinging arboreality in a Middle Jurassic stem lepidosaur
Proceedings of the Royal Society B: Biological Sciences 292(2045): 20250080
doi: https://doi.org/10.1098/rspb.2025.0080
https://royalsocietypublishing.org/doi/10.1098/rspb.2025.0080



Lepidosaurs are an ecologically diverse and speciose group with more than 11 000 living species (squamates and the tuatara). Stem lepidosaurs are known from the Early Triassic onwards, but primarily from very incomplete specimens. Therefore, we have little information on their ecological diversity or the ecological context of deep evolutionary divergences of Lepidosauria. Marmoretta oxoniensis, from the Middle Jurassic of the UK, is one of the most completely known candidate stem lepidosaurs. Previous studies proposed that it may have been semi-aquatic, based primarily on its abundance in marginal marine rocks. We show here that Marmoretta was adapted for climbing, based on the post-cranial anatomy of a partial skeleton, visualized using micro-computed tomography (µCT)—in particular, the steep angles of thoracic zygapophyses, ungual phalanx morphology and elongate penultimate manual phalanges that curve distoventrally along their lengths. Linear discriminant analysis of the partial hand, using a training dataset of hand skeleton measurements and habitat use in extant squamates, returns strong evidence for clinging arboreality and Marmoretta clusters among scansorial/arboreal iguanians in manus shape space. Evidence of arboreality in Marmoretta provides the first information about habitat use in a probable stem lepidosaur and illuminates the vertical structure of ecological communities of the mid-Mesozoic.

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Keegan M. Melstrom, Kenneth D. Angielczyk, Kathleen A. Ritterbush & Randall B. Irmis (2025)
For a while, crocodile: crocodylomorph resilience to mass extinctions
Palaeontology 68(2): e70005
doi:  https://doi.org/10.1111/pala.70005
https://onlinelibrary.wiley.com/doi/10.1111/pala.70005


Crocodylomorphs are the sole survivors of the pseudosuchian clade and have endured two mass extinctions. Despite being stereotyped as ‘living fossils’, the characteristics that facilitated their survival remain largely unknown, but trends in other clades, such as mammals, suggest that dietary ecology may play a key role in persisting during and after mass extinctions. To test the role of dietary ecology in the survival of crocodylomorphs, we use geometric and linear morphometrics to quantify the cranial shape of crocodylomorphs throughout the Mesozoic and into the Cenozoic. We then compare these data to living amniotes and combine the results with additional morphological proxies, which together facilitate the reconstruction of dietary ecology. We find that crocodylomorph cranial morphology exhibits a greater disparity during much of the Mesozoic Era than is seen today. Many extinct crocodylomorphs express morphologies that overlap with extant crocodylians and lepidosaurs, indicating a diverse range of diets, but convergence with extant mammals is exceptionally rare. Numerous crocodylomorphs from across geological time and phylogeny span a range of morphospace not occupied by sampled extant amniotes. Using these data, we suggest that a generalist dietary ecology is associated with mass extinction survivors, a trend previously observed in other clades, such as mammals. Terrestrial generalists appear to survive and diversify following the end-Triassic mass extinction, whereas semiaquatic generalists persist through the end-Cretaceous mass extinction. We propose that one reason for the success and longevity of the crocodylomorph clade is their remarkable dietary flexibility, a characteristic that is still observed in living crocodylians.

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K. Usami & M. O. Kubo (2025)
In-vitro puncture experiment using alligator teeth tracks the formation of dental microwear and its association with hardness of the diet
The Anatomical Record (advance online publication)
doi: https://doi.org/10.1002/ar.25659
https://anatomypubs.onlinelibrary.wiley.com/doi/10.1002/ar.25659
Free pdf:
https://anatomypubs.onlinelibrary.wiley.com/doi/epdf/10.1002/ar.25659



With the development of dental microwear texture analysis (DMTA), there has been an increasing application of DMTA for dietary estimation in extant and fossil reptiles, including dinosaurs. While numerous feeding experiments exist for herbivorous mammals, knowledge remains limited for carnivorous reptiles. This study aimed to qualitatively and quantitatively evaluate the formation of dental microwear through repeated puncture of different types of food using isolated teeth from the American alligator (Alligator mississippiensis) in an in-vitro experiment. Eleven isolated teeth were mounted on a force gauge, and each tooth sample was repeatedly punctured 200 times into sardines (tooth sample size, N = 6) and crayfish (N = 5). The tooth surfaces were scanned using a confocal laser microscope before, during, and after the experiment to track changes in the tooth surface. Additionally, the maximum force during puncture was measured with the force gauge. Examination of surface roughness parameters before and after the experiment revealed a significant increase at the tooth apexes for both types of food. Furthermore, the trials with crayfish increased microwear depth and density more than the sardine trials. There was a significant positive correlation between the total force experienced by each tooth and the changes in surface roughness parameters in the crayfish trials, indicating that greater force results in more dental wear. The findings of this study are significant as they complement existing feeding experiments and comparative studies of wild species with different diets, and they demonstrate the effectiveness of experimental approaches in understanding the formation mechanisms of dental microwear.