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Thalattosuchian crocodylomorphs were a ubiquitous component of shallow marine ecosystems during the Jurassic and Early Cretaceous. Alas, their origins remain a mystery. Here we describe three specimens from the Sinemurian (and possibly Early Pliensbachian) of the UK: a partial cranial rostrum, a series of cervical vertebrae, and two dorsal vertebrae adhered with matrix. These specimens are amongst the oldest known thalattosuchian fossils, with the partial cranial rostrum being the oldest known non-neothalattosuchian thalattosuchian. This partial cranial rostrum has a unique combination of rostral characters never seen before in any crocodylomorph, and helps to elucidate early thalattosuchian internal rostrum evolution, suggesting that the reduction in thalattosuchian paranasal sinuses was not related to either the reorganization of rostral neurovasculature seen in later diverging taxa or the increased cancellous bone microstructure. Based on our CT sample, a shift in cranial bone microstructure occurred in the Eoneustes  + Metriorhynchidae subclade, one that coincided with the enlargement of the salt glands and decoupling of the external antorbital fenestra from the paranasal sinuses. Without extensive histological sampling we cannot determine whether the shift to an obligate aquatic lifestyle occurred prior to the evolution of Metriorhynchidae. [ABSTRACT FROM AUTHOR]

During major evolutionary transitions, groups develop radically new body plans and radiate into new habitats. A classic example is cetaceans which evolved from terrestrial ancestors to become pelagic swimmers. In doing so, they altered their air-filled sinuses, transitioning some of these spaces to allow for fluctuations in air capacity and storage via soft tissue borders. Other tetrapods independently underwent land-to-sea transitions, but it is unclear if they similarly changed their sinuses. We use computed tomography to study sinus changes in thalattosuchian crocodylomorphs that transformed from land-bound ancestors to become the only known aquatic swimming archosaurs. We find that thalattosuchian braincase sinuses reduced over their transition, similar to cetaceans, but their snout sinuses counterintuitively expanded, distinct from cetaceans, and that both trends were underpinned by high evolutionary rates. We hypothesize that aquatic thalattosuchians were ill suited to deep diving by their snout sinuses, which seem to have remained large to help drain their unusual salt glands. Thus, although convergent in general terms, thalattosuchians and cetaceans were subject to different constraints that shaped their transitions to water. Thalattosuchians attained a stage similar to less pelagic transitional forms in the cetacean lineage (late protocetid-basilosaurid) but did not become further specialized for ocean life., Competing Interests: The authors declare no competing interests., (© 2024 The Author(s).)

Birds and crocodiles show radically different patterns of brain development, and it is of interest to compare these to determine the pattern of brain growth expected in dinosaurs. Here we provide atlases of 3D brain (endocast) reconstructions for Alligator mississippiensis (alligator) and Struthio camelus (ostrich) through ontogeny, prepared as digital restorations from CT scans of stained head and dry skull specimens. Our morphometric analysis confirms that ostrich brains do not change significantly in shape during postnatal growth, whereas alligator brains unfold from a cramped bird-like shape in the hatchling to an elongate, straight structure in the adult. We confirm that birds exhibit paedomorphic dinosaur endocranial traits such as retaining an enlarged and compact brain shape in the adult, whereas crocodiles show peramorphic traits where the brain elongates with growth as the skull elongates. These atlases of ontogenetic stages of modern bird and crocodilian endocrania provide a basis for comparison of non-avian dinosaur endocasts and consideration of the divergence of the "avian" and "crocodilian" modes of brain development and heterochronic change on phylogenies., (© 2020 American Association for Anatomy.)