Decelerated dinosaur skull evolution with the origin of birds.

The evolutionary radiation of birds has produced incredible morphological variation, including a huge range of skull form and function. Investigating how this variation arose with respect to non-avian dinosaurs is key to understanding how birds achieved their remarkable success after the Cretaceous-Paleogene extinction event. Using a high-dimensional geometric morphometric approach, we quantified the shape of the skull in unprecedented detail across 354 extant and 37 extinct avian and non-avian dinosaurs. Comparative analyses reveal fundamental differences in how skull shape evolved in birds and non-avian dinosaurs. We find that the overall skull shape evolved faster in non-avian dinosaurs than in birds across all regions of the cranium. In birds, the anterior rostrum is the most rapidly evolving skull region, whereas more posterior regions-such as the parietal, squamosal, and quadrate-exhibited high rates in non-avian dinosaurs. These fast-evolving elements in dinosaurs are strongly associated with feeding biomechanics, forming the jaw joint and supporting the jaw adductor muscles. Rapid pulses of skull evolution coincide with changes to food acquisition strategies and diets, as well as the proliferation of bony skull ornaments. In contrast to the appendicular skeleton, which has been shown to evolve more rapidly in birds, avian cranial morphology is characterised by a striking deceleration in morphological evolution relative to non-avian dinosaurs. These results may be due to the reorganisation of skull structure in birds-including loss of a separate postorbital bone in adults and the emergence of new trade-offs with development and neurosensory demands. Taken together, the remarkable cranial shape diversity in birds was not a product of accelerated evolution from their non-avian relatives, despite their frequent portrayal as an icon of adaptive radiations.

Inner ear sensory system changes as extinct crocodylomorphs transitioned from land to water.

Major evolutionary transitions, in which animals develop new body plans and adapt to dramatically new habitats and lifestyles, have punctuated the history of life. The origin of cetaceans from land-living mammals is among the most famous of these events. Much earlier, during the Mesozoic Era, many reptile groups also moved from land to water, but these transitions are more poorly understood. We use computed tomography to study changes in the inner ear vestibular system, involved in sensing balance and equilibrium, as one of these groups, extinct crocodile relatives called thalattosuchians, transitioned from terrestrial ancestors into pelagic (open ocean) swimmers. We find that the morphology of the vestibular system corresponds to habitat, with pelagic thalattosuchians exhibiting a more compact labyrinth with wider semicircular canal diameters and an enlarged vestibule, reminiscent of modified and miniaturized labyrinths of other marine reptiles and cetaceans. Pelagic thalattosuchians with modified inner ears were the culmination of an evolutionary trend with a long semiaquatic phase, and their pelagic vestibular systems appeared after the first changes to the postcranial skeleton that enhanced their ability to swim. This is strikingly different from cetaceans, which miniaturized their labyrinths soon after entering the water, without a prolonged semiaquatic stage. Thus, thalattosuchians and cetaceans became secondarily aquatic in different ways and at different paces, showing that there are different routes for the same type of transition.

Ontogenetic variation in the crocodylian vestibular system.

Crocodylians today live in tropical to subtropical environments, occupying mostly shallow waters. Their body size changes drastically during ontogeny, as do their skull dimensions and bite forces, which are associated with changes in prey preferences. Endocranial neurosensory structures have also shown to change ontogenetically, but less is known about the vestibular system of the inner ear. Here we use 30 high-resolution computed tomography (CT) scans and three-dimensional geometric morphometrics to investigate the size and shape changes of crocodylian endosseous labyrinths throughout ontogeny, across four stages (hatchling, juvenile, subadult and adult). We find two major patterns of ontogenetic change. First, the labyrinth increases in size during ontogeny, with negative allometry in relation to skull size. Second, labyrinth shape changes significantly, with hatchlings having shorter semicircular canal radii, with thicker diameters and an overall dorsoventrally shorter labyrinth than those of more mature individuals. We argue that the modification of the labyrinth during crocodylian ontogeny is related to constraints imposed by skull growth, due to fundamental changes in the crocodylian braincase during ontogeny (e.g. verticalisation of the basicranium), rather than changes in locomotion, diet, or other biological functions or behaviours.

Endocast structures are reliable proxies for the sizes of corresponding regions of the brain in extant birds.

Endocasts are increasingly relied upon to examine avian brain evolution because they can be used across extant and extinct species. The endocasts of birds appear to be relatively faithful representatives of the external morphology of their brains, but it is unclear how well the size of a surface feature visible on endocasts reflects the volume of the underlying brain region. The optic lobe and the Wulst are two endocast structures that are clearly visible on the external surface of avian endocasts. As they overlie two major visual regions of the brain, the optic tectum and hyperpallium, the surface areas of the optic lobe and Wulst, respectively, are often used to infer visual abilities. To determine whether the surface area of these features reflects the volume of the underlying brain regions, we compared the surface areas of the optic lobes and Wulsts from digital endocasts with the volumes of the optic tecta and hyperpallia from the literature or measured from histological series of brains of the same species. Regression analyses revealed strong, statistically significant correlations between the volumes of the brain regions and the surface areas of the overlying endocast structures. In other words, the size of the hyperpallium and optic tectum can be reliably inferred from the surface areas of the Wulst and optic lobe, respectively. This validation opens the possibility of estimating brain-region volumes for extinct species in order to gain better insights in their visual ecology. It also emphasizes the importance of adopting a quantitative approach to the analysis of endocasts in the study of brain evolution.

Open data and digital morphology.

Over the past two decades, the development of methods for visualizing and analysing specimens digitally, in three and even four dimensions, has transformed the study of living and fossil organisms. However, the initial promise that the widespread application of such methods would facilitate access to the underlying digital data has not been fully achieved. The underlying datasets for many published studies are not readily or freely available, introducing a barrier to verification and reproducibility, and the reuse of data. There is no current agreement or policy on the amount and type of data that should be made available alongside studies that use, and in some cases are wholly reliant on, digital morphology. Here, we propose a set of recommendations for minimum standards and additional best practice for three-dimensional digital data publication, and review the issues around data storage, management and accessibility.

Cranial joint histology in the mallard duck (Anas platyrhynchos): new insights on avian cranial kinesis.

The evolution of avian cranial kinesis is a phenomenon in part responsible for the remarkable diversity of avian feeding adaptations observable today. Although osteological, developmental and behavioral features of the feeding system are frequently studied, comparatively little is known about cranial joint skeletal tissue composition and morphology from a microscopic perspective. These data are key to understanding the developmental, biomechanical and evolutionary underpinnings of kinesis. Therefore, here we investigated joint microstructure in juvenile and adult mallard ducks (Anas platyrhynchos; Anseriformes). Ducks belong to a diverse clade of galloanseriform birds, have derived adaptations for herbivory and kinesis, and are model organisms in developmental biology. Thus, new insights into their cranial functional morphology will refine our understanding of avian cranial evolution. A total of five specimens (two ducklings and three adults) were histologically sampled, and two additional specimens (a duckling and an adult) were subjected to micro-computed tomographic scanning. Five intracranial joints were sampled: the jaw joint (quadrate-articular); otic joint (quadrate-squamosal); palatobasal joint (parasphenoid-pterygoid); the mandibular symphysis (dentary-dentary); and the craniofacial hinge (a complex flexion zone involving four different pairs of skeletal elements). In both the ducklings and adults, the jaw, otic and palatobasal joints are all synovial, with a synovial cavity and articular cartilage on each surface (i.e. bichondral joints) ensheathed in a fibrous capsule. The craniofacial hinge begins as an ensemble of patent sutures in the duckling, but in the adult it becomes more complex: laterally it is synovial; whereas medially, it is synostosed by a bridge of chondroid bone. We hypothesize that it is chondroid bone that provides some of the flexible properties of this joint. The heavily innervated mandibular symphysis is already fused in the ducklings and remains as such in the adult. The results of this study will serve as reference for documenting avian cranial kinesis from a microanatomical perspective. The formation of: (i) secondary articular cartilage on the membrane bones of extant birds; and (ii) their unique ability to form movable synovial joints within two or more membrane bones (i.e. within their dermatocranium) might have played a role in the origin and evolution of modern avian cranial kinesis during dinosaur evolution.

Vascular patterns in the heads of crocodilians: blood vessels and sites of thermal exchange.

Extant crocodilians are a highly apomorphic archosaur clade that is ectothermic, yet often achieve large body sizes that can be subject to higher heat loads. Therefore, the anatomical and physiological roles that blood vessels play in crocodilian thermoregulation need further investigation to better understand how crocodilians establish and maintain cephalic temperatures and regulate neurosensory tissue temperatures during basking and normal activities. The cephalic vascular anatomy of extant crocodilians, particularly American alligator (Alligator mississippiensis) was investigated using a differential-contrast, dual-vascular injection technique and high resolution X-ray micro-computed tomography (µCT). Blood vessels were digitally isolated to create representations of vascular pathways. The specimens were then dissected to confirm CT results. Sites of thermal exchange, consisting of the oral, nasal, and orbital regions, were given special attention due to their role in evaporative cooling and cephalic thermoregulation in other diapsids. Blood vessels to and from sites of thermal exchange were studied to detect conserved vascular patterns and to assess their ability to deliver cooled blood to neurosensory tissues. Within the orbital region, both the arteries and veins demonstrated consistent branching patterns, with the supraorbital, infraorbital, and ophthalmotemporal vessels supplying and draining the orbit. The venous drainage of the orbital region showed connections to the dural sinuses via the orbital veins and cavernous sinus. The palatal region demonstrated a vast plexus that comprised both arteries and veins. The most direct route of venous drainage of the palatal plexus was through the palatomaxillary veins, essentially bypassing neurosensory tissues. Anastomotic connections with the nasal region, however, may provide an alternative route for palatal venous blood to reach neurosensory tissues. The nasal region in crocodilians is probably the most prominent site of thermal exchange, as it offers a substantial surface area and is completely surrounded by blood vessels. The venous drainage routes from the nasal region offer routes directly to the dural venous sinuses and the orbit, offering evidence of the potential to directly affect neurosensory tissue temperatures. The evolutionary history of crocodilians is complex, with large-bodied, terrestrial, and possibly endothermic taxa that may have had to deal with thermal loads that likely provided the anatomical building-blocks for such an extensive vascularization of sites of thermal exchange. A clear understanding of the physiological abilities and the role of blood vessels in the thermoregulation of crocodilians neurosensory tissues is not available but vascular anatomical patterns of crocodilian sites of thermal exchange indicate possible physiological abilities that may be more sophisticated than in other extant diapsids.

Best practices for digitally constructing endocranial casts: examples from birds and their dinosaurian relatives.

The rapidly expanding interest in, and availability of, digital tomography data to visualize casts of the vertebrate endocranial cavity housing the brain (endocasts) presents new opportunities and challenges to the field of comparative neuroanatomy. The opportunities are many, ranging from the relatively rapid acquisition of data to the unprecedented ability to integrate critically important fossil taxa. The challenges consist of navigating the logistical barriers that often separate a researcher from high-quality data and minimizing the amount of non-biological variation expressed in endocasts - variation that may confound meaningful and synthetic results. Our purpose here is to outline preferred approaches for acquiring digital tomographic data, converting those data to an endocast, and making those endocasts as meaningful as possible when considered in a comparative context. This review is intended to benefit those just getting started in the field but also serves to initiate further discussion between active endocast researchers regarding the best practices for advancing the discipline. Congruent with the theme of this volume, we draw our examples from birds and the highly encephalized non-avian dinosaurs that comprise closely related outgroups along their phylogenetic stem lineage.

Diffusible iodine-based contrast-enhanced computed tomography (diceCT): an emerging tool for rapid, high-resolution, 3-D imaging of metazoan soft tissues.

Morphologists have historically had to rely on destructive procedures to visualize the three-dimensional (3-D) anatomy of animals. More recently, however, non-destructive techniques have come to the forefront. These include X-ray computed tomography (CT), which has been used most commonly to examine the mineralized, hard-tissue anatomy of living and fossil metazoans. One relatively new and potentially transformative aspect of current CT-based research is the use of chemical agents to render visible, and differentiate between, soft-tissue structures in X-ray images. Specifically, iodine has emerged as one of the most widely used of these contrast agents among animal morphologists due to its ease of handling, cost effectiveness, and differential affinities for major types of soft tissues. The rapid adoption of iodine-based contrast agents has resulted in a proliferation of distinct specimen preparations and scanning parameter choices, as well as an increasing variety of imaging hardware and software preferences. Here we provide a critical review of the recent contributions to iodine-based, contrast-enhanced CT research to enable researchers just beginning to employ contrast enhancement to make sense of this complex new landscape of methodologies. We provide a detailed summary of recent case studies, assess factors that govern success at each step of the specimen storage, preparation, and imaging processes, and make recommendations for standardizing both techniques and reporting practices. Finally, we discuss potential cutting-edge applications of diffusible iodine-based contrast-enhanced computed tomography (diceCT) and the issues that must still be overcome to facilitate the broader adoption of diceCT going forward.

Edentulism, beaks, and biomechanical innovations in the evolution of theropod dinosaurs.

Maniraptoriformes, the speciose group of derived theropod dinosaurs that ultimately gave rise to modern birds, display a diverse and remarkable suite of skeletal adaptations. Apart from the evolution of flight, a large-scale change in dietary behavior appears to have been one of the main triggers for specializations in the bauplan of these derived theropods. Among the different skeletal specializations, partial or even complete edentulism and the development of keratinous beaks form a recurring and persistent trend in from the evolution of derived nonavian dinosaurs. Therizinosauria is an enigmatic maniraptoriform clade, whose members display these and other osteological characters thought to be correlated with the shift from carnivory to herbivory. This makes therizinosaurians prime candidates to assess the functional significance of these morphological characters. Based on a highly detailed biomechanical model of Erlikosaurus andrewsi, a therizinosaurid from the Upper Cretaceous of Mongolia, different morphological configurations incorporating soft-tissue structures, such as a keratinous rhamphotheca, are evaluated for their biomechanical performance. Our results indicate that the development of beaks and the presence of a keratinous rhamphotheca would have helped to dissipate stress and strain, making the rostral part of the skull less susceptible to bending and displacement, and this benefit may extend to other vertebrate clades that possess rhamphothecae. Keratinous beaks, paralleled by edentulism, thus represent an evolutionary innovation developed early in derived theropods to enhance cranial stability, distinct to postulated mass-saving benefits associated with the origin of flight.

Thought for food: the endothermic brain hypothesis.

The evolution of whole-body endothermy occurred independently in dinosaurs and mammals and was associated with some of the most significant neurocognitive shifts in life's history. These included a 20-fold increase in neurons and the evolution of new brain structures, supporting similar functions in both lineages. We propose the endothermic brain hypothesis, which holds that elaborations in endotherm brains were geared towards increasing caloric intake through efficient foraging. The hypothesis is grounded in the intrinsic coupling of cognition and organismic self-maintenance. We argue that coevolution of increased metabolism and new forms of cognition should be jointly investigated in comparative studies of behaviors and brain anatomy, along with studies of fossil species. We suggest avenues for such research and highlight critical open questions.

Avialan-like brain morphology in Sinovenator (Troodontidae, Theropoda).

Many modifications to the skull and brain anatomy occurred along the lineage encompassing non-avialan theropod dinosaurs and modern birds. Anatomical changes to the endocranium include an enlarged endocranial cavity, relatively larger optic lobes that imply elevated visual acuity, and proportionately smaller olfactory bulbs that suggest reduced olfactory capacity. Here, we use micro-computed tomographic (µCT) imaging to reconstruct the endocranium and its neuroanatomical features from an exceptionally well-preserved skull of Sinovenator changii (Troodontidae, Theropoda). While its overall morphology resembles the typical endocranium of other troodontids, Sinovenator also exhibits unique endocranial features that are similar to other paravian taxa and non-maniraptoran theropods. Landmark-based geometric morphometric analysis on endocranial shape of non-avialan and avialan dinosaurs points to the overall brain morphology of Sinovenator most closely resembling that of Archaeopteryx, thus indicating acquisition of avialan-grade brain morphology in troodontids and wide existence of such architecture in Maniraptora.

Soft tissues influence nasal airflow in diapsids: Implications for dinosaurs.

The nasal passage performs multiple functions in amniotes, including olfaction and thermoregulation. These functions would have been present in extinct animals as well. However, fossils preserve only low-resolution versions of the nasal passage due to loss of soft-tissue structures after death. To test the effects of these lower resolution models on interpretations of nasal physiology, we performed a broadly comparative analysis of the nasal passages in extant diapsid representatives, e.g., alligator, turkey, ostrich, iguana, and a monitor lizard. Using computational fluid dynamics, we simulated airflow through 3D reconstructed models of the different nasal passages and compared these soft-tissue-bounded results to similar analyses of the same airways under the lower-resolution limits imposed by fossilization. Airflow patterns in these bony-bounded airways were more homogeneous and slower flowing than those of their soft-tissue counterparts. These data indicate that bony-bounded airway reconstructions of extinct animal nasal passages are far too conservative and place overly restrictive physiological limitations on extinct species. In spite of the diverse array of nasal passage shapes, distinct similarities in airflow were observed, including consistent areas of nasal passage constriction such as the junction of the olfactory region and main airway. These nasal constrictions can reasonably be inferred to have been present in extinct taxa such as dinosaurs.

Orbital soft tissues, bones, and allometry: Implications for the size and position of crocodylian eyes.

Although the visual system of crocodylians has attracted interest regarding optical parameters and retinal anatomy, fundamental questions remain about the allometry of the eyeball and whether such scaling is the same across all crown groups of crocodylians. In addition, anatomy and identities of adnexal soft tissues that interact with the visual system are not well understood in many cases. We used contrast-enhancing iodine stain and high-resolution micro-computed tomography to assess the anatomy of orbital soft tissues, including extraocular muscles and glands, in crocodylians. We also used regression analysis to estimate the allometric relationship between the bony orbit and eyeball across Alligator mississippiensis and Crocodylus niloticus for the first time. Results revealed tight, negatively allometric relationships between the bony orbit and eyeball. Notably, the eyes of C. niloticus were larger for a given orbit size than the eyes of A. mississippiensis, although the slope of the relationship was no different between these two crown crocodylian groups. Among the findings from our anatomical study, new details were uncovered about the homologies of muscles of the abducens complex. In particular, M. rectus lateralis of crocodylians is revealed to have a more complex form than previously appreciated, being adhered to the tendon of the nictitating membrane, which may be apomorphic for Crocodylia. Our calculation of the orbit-eyeball allometric relationship and study of the adnexal soft tissues of the crocodylian visual system, in combination with previous work by other teams in other crown saurian clades, is a critical, formerly missing, piece in the Extant Phylogenetic Bracket for restoring the visual apparatus of extinct crocodyliforms and other archosauriform groups.

Evidence for a novel cranial thermoregulatory pathway in thalattosuchian crocodylomorphs.

Thalattosuchian crocodylomorphs were a diverse clade that lived from the Early Jurassic to the Early Cretaceous. The subclade Metriorhynchoidea underwent a remarkable transition, evolving from semi-aquatic ambush predators into fully aquatic forms living in the open oceans. Thalattosuchians share a peculiar palatal morphology with semi-aquatic and aquatic fossil cetaceans: paired anteroposteriorly aligned grooves along the palatal surface of the bony secondary palate. In extant cetaceans, these grooves are continuous with the greater palatine artery foramina, arteries that supply their oral thermoregulatory structures. Herein, we investigate the origins of thalattosuchian palatal grooves by examining CT scans of six thalattosuchian species (one teleosauroid, two early-diverging metriorhynchoids and three metriorhynchids), and CT scans of eleven extant crocodylian species. All thalattosuchians had paired osseous canals, enclosed by the palatines, that connect the nasal cavity to the oral cavity. These osseous canals open into the oral cavity via foramina at the posterior terminus of the palatal grooves. Extant crocodylians lack both the external grooves and the internal canals. We posit that in thalattosuchians these novel palatal canals transmitted hypertrophied medial nasal vessels (artery and vein), creating a novel heat exchange pathway connecting the palatal vascular plexus to the endocranial region. Given the general hypertrophy of thalattosuchian cephalic vasculature, and their increased blood flow and volume, thalattosuchians would have required a more extensive suite of thermoregulatory pathways to maintain stable temperatures for their neurosensory tissues.

Cranial biomechanics of Diplodocus (Dinosauria, Sauropoda): testing hypotheses of feeding behaviour in an extinct megaherbivore.

Sauropod dinosaurs were the largest terrestrial herbivores and pushed at the limits of vertebrate biomechanics and physiology. Sauropods exhibit high craniodental diversity in ecosystems where numerous species co-existed, leading to the hypothesis that this biodiversity is linked to niche subdivision driven by ecological specialisation. Here, we quantitatively investigate feeding behaviour hypotheses for the iconic sauropod Diplodocus. Biomechanical modelling, using finite element analysis, was used to examine the performance of the Diplodocus skull. Three feeding behaviours were modelled: muscle-driven static biting, branch stripping and bark stripping. The skull was found to be 'over engineered' for static biting, overall experiencing low stress with only the dentition enduring high stress. When branch stripping, the skull, similarly, is under low stress, with little appreciable difference between those models. When simulated for bark stripping, the skull experiences far greater stresses, especially in the teeth and at the jaw joint. Therefore, we refute the bark-stripping hypothesis, while the hypotheses of branch stripping and/or precision biting are both consistent with our findings, showing that branch stripping is a biomechanically plausible feeding behaviour for diplodocids. Interestingly, in all simulations, peak stress is observed in the premaxillary-maxillary 'lateral plates', supporting the hypothesis that these structures evolved to dissipate stress induced while feeding. These results lead us to conclude that the aberrant craniodental form of Diplodocus was adapted for food procurement rather than resisting high bite forces.

The first occurrence of an avian-style respiratory infection in a non-avian dinosaur.

Other than repaired fractures, osteoarthritis, and periosteal reaction, the vertebrate fossil record has limited evidence of non-osseous diseases. This difficulty in paleontological diagnoses stems from (1) the inability to conduct medical testing, (2) soft-tissue pathologic structures are less likely to be preserved, and (3) many osseous lesions are not diagnostically specific. However, here reported for the first time is an avian-style respiratory disorder in a non-avian dinosaur. This sauropod presents irregular bony pathologic structures stemming from the pneumatic features in the cervical vertebrae. As sauropods show well-understood osteological correlates indicating that respiratory tissues were incorporated into the post-cranial skeleton, and thus likely had an 'avian-style' form of respiration, it is most parsimonious to identify these pathologic structures as stemming from a respiratory infection. Although several extant avian infections produce comparable symptoms, the most parsimonious is airsacculitis with associated osteomyelitis. From actinobacterial to fungal in origin, airsacculitis is an extremely prevalent respiratory disorder in birds today. While we cannot pinpoint the specific infectious agent that caused the airsacculitis, this diagnosis establishes the first fossil record of this disease. Additionally, it allows us increased insight into the medical disorders of dinosaurs from a phylogenetic perspective and understanding what maladies plagued the "fearfully great lizards".

'Ear stones' in crocodylians: a cross-species comparative and ontogenetic survey of otolith structures.

The vestibular system of the inner ear is a crucial sensory organ, involved in the sensation of balance and equilibrium. It consists of three semicircular canals that sense angular rotations of the head and the vestibule that detects linear acceleration and gravity. The vestibule often contains structures, known as the otoliths or 'ear stones'. Otoliths are present in many vertebrates and are particularly well known from the fossil record of fish, but surprisingly have not been described in detail in most tetrapods, living or extinct. Here, we present for the first time a survey of the otoliths of a broad sample of extant crocodylian species, based on computed tomography scans. We find that otoliths are present in numerous crocodylian species of different growth stages, and they continue to increase in size during ontogeny, with positive allometry compared to skull length. Our results confirm that otoliths are a common component of the crocodylian vestibular system, and suggest they play an important role in sensory detection. Otoliths are likely common, but overlooked, constituents of the inner ear in tetrapods, and a broader study of their size, shape and distribution promises insight into sensory abilities.

New digital braincase endocasts of two species of Desmatosuchus and neurocranial diversity within Aetosauria (Archosauria: Pseudosuchia).

In the present contribution we revise, figure, and redescribe several isolated braincases of the iconic aetosaur Desmatosuchus from the Placerias Quarry locality, Chinle Formation, Arizona, United States. The detailed study of the isolated braincases from the UCMP collection allowed us to assign them at the species-level and recognize two species of Desmatosuchus for the Placerias Quarry: D. spurensis and D. smalli. The former can be distinguished from the latter by the presence of a transverse sulcus on the parietals, deep median pharyngeal recess on the basisphenoid, almost no gap between the basal tubera and the basipterygoid processes, and the exoccipitals meeting at the midline. The presence of D. smalli at the Placerias Quarry has not been previously reported. Based on the braincases UCMP 27408, 27410, 27407, three new brain endocasts were developed through CT scan images, reconstructing the most complete endocranial casts known for an aetosaur, including the encephalon, cranial nerves, inner ear, and endocranial vasculature. The cranial endocasts also exhibited some differences between both species of Desmatosuchus, with D. spurensis having a distinguishable dural expansion and markedly asymmetric anterior and posterior semicircular canals of the labyrinth. Additionally, the combination of osteological features and the endocranial casts allowed us to identify and discuss the presence of an ossified orbitosphenoid on the anteriormost region of the braincase among aetosaurs. Furthermore, we were able to reinterpret some of the observations made by previous authors on the endocast of the holotype of Desmatosuchus spurensis (UMMP VP 7476) and provide some insight into their neurosensory capabilities.