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Abstract Cranial nerves are key features of the nervous system and vertebrate body plan. However, little is known about the anatomical relationships and ontogeny of cranial nerves in crocodylians and other reptiles, hampering understanding of adaptations, evolution, and development of special senses, somatosensation, and motor control of cranial organs. Here we share three dimensional (3D) models an of the cranial nerves and cranial nerve targets of embryonic, juvenile, and adult American Alligators (Alligator mississippiensis) derived from iodine‐contrast CT imaging, for the first time, exploring anatomical patterns of cranial nerves across ontogeny. These data reveal the tradeoffs of using contrast‐enhanced CT data as well as patterns in growth and development of the alligator cranial nervous system. Though contrast‐enhanced CT scanning allows for reconstruction of numerous tissue types in a nondestructive manner, it is still limited by size and resolution. The position of alligator cranial nerves varies little with respect to other cranial structures yet grow at different rates as the skull elongates. These data constrain timing of trigeminal and sympathetic ganglion fusion and reveal morphometric differences in nerve size and path during growth. As demonstrated by these data, alligator cranial nerve morphology is useful in understanding patterns of neurological diversity and distribution, evolution of sensory and muscular innervation, and developmental homology of cranial regions, which in turn, lead to inferences of physiology and behavior. 

Abstract Notosuchia is a clade of crocodyliforms that was highly successful and diverse in the Cretaceous of Gondwana.Araripesuchus gomesiiis a small notosuchian from the Early Cretaceous of Brazil that belongs to Uruguaysuchidae, one of the subgroups of notosuchians that first radiated, during the Aptian–Albian. Here we present a finite element analysis ofA. gomesiibased on a model reconstructed from CT scans and performed using published bone properties for crocodiles. The adductor musculature and their respective attachment areas were reconstructed based on Extant Phylogenetic Bracket. Different functional scenarios were tested applying an estimated 158 N bite force: unilateral bite, bilateral bite, pullback, head‐shake, and head‐twist. The results obtained were compared with those ofAlligator mississippiensis, one of its closest living relatives. In the different simulations, the skull and lower jaws ofAraripesuchussuffers more stress in the head‐shake movement, followed by the unilateral and pullback bites with stress focalized in the premaxillary region. In contrast, the head‐twist is the one with smaller stress values.Araripesuchuspossess an oreinirostral skull that may provide greater overall resistance in the different scenarios on average, unlikeAlligatorthat has a platyrostral skull with less resistance to dorsoventral mechanical loads. Previous hypotheses that consideredA. gomesiias omnivorous coupled with our results, its small size, and likely limited bite force, suggest this taxon probably fed on small prey and other trophic items that could catch and handle entirely with its mouth, such as insects and small vertebrates. 

Abstract Jaw muscles are key features of the vertebrate feeding apparatus. The jaw musculature is housed in the skull whose morphology reflects a compromise between multiple functions, including feeding, housing sensory structures, and defense, and the skull constrains jaw muscle geometry. Thus, jaw muscle anatomy may be suboptimally oriented for the production of bite force. Crocodylians are a group of vertebrates that generate the highest bite forces ever measured with a flat skull suited to their aquatic ambush predatory style. However, basal members of the crocodylian line (e.g.,Prestosuchus) were terrestrial predators with plesiomorphically tall skulls, and thus the origin of modern crocodylians involved a substantial reorganization of the feeding apparatus and its jaw muscles. Here, we reconstruct jaw muscles across a phylogenetic range of crocodylians and fossil suchians to investigate the impact of skull flattening on muscle anatomy. We used imaging data to create 3D models of extant and fossil suchians that demonstrate the evolution of the crocodylian skull, using osteological correlates to reconstruct muscle attachment sites. We found that jaw muscle anatomy in early fossil suchians reflected the ancestral archosaur condition but experienced progressive shifts in the lineage leading to Metasuchia. In early fossil suchians, musculus adductor mandibulae posterior and musculus pterygoideus (mPT) were of comparable size, but by Metasuchia, the jaw musculature is dominated by mPT. As predicted, we found that taxa with flatter skulls have less efficient muscle orientations for the production of high bite force. This study highlights the diversity and evolution of jaw muscles in one of the great transformations in vertebrate evolution. 

ABSTRACT The attachments of jaw muscles are typically implicated in the evolution and shape of the dorsotemporal fenestra on the skull roof of amniotes. However, the dorsotemporal fenestrae of many archosaurian reptiles possess smooth excavations rostral and dorsal to the dorsotemporal fossa which closely neighbors the dorsotemporal fenestra and jaw muscle attachments. Previous research has typically identified this region, here termed the frontoparietal fossa, to also have attachment surfaces for jaw‐closing muscles. However, numerous observations of extant and extinct archosaurs described here suggest that other tissues are instead responsible for the size and shape of the frontoparietal fossa. This study reviewed the anatomical evidence that support soft‐tissue hypotheses of the frontoparietal fossa and its phylogenetic distribution among sauropsids. Soft‐tissue hypotheses (i.e., muscle, pneumatic sinus, vascular tissues) were analyzed using anatomical, imaging andin vivothermography techniques within a phylogenetic framework using extant and extinct taxa to determine the inferential power underlying the reconstruction of the soft tissues in the skull roofs of dinosaurs, pseudosuchians, and other reptiles. Relevant anatomical features argue for rejection of the default hypothesis—that the fossa was muscular—due to a complete lack of osteological correlates reflective of muscle attachment. The most‐supported inference of soft tissues is that the frontoparietal fossa contained a large vascular structure and adipose tissue. Despite the large sizes and diverse morphologies of these fossae found among dinosaur taxa, these data suggest that non‐avian dinosaurs had the anatomical foundation to support physiologically significant vascular devices and/or vascular integumentary structures on their skull roofs. Anat Rec, 303:1060–1074, 2020. © 2019 Wiley Periodicals, Inc. 

Abstract New imaging and biomechanical approaches have heralded a renaissance in our understanding of crocodylian anatomy. Here, we review a series of approaches in the preparation, imaging, and functional analysis of the jaw muscles of crocodylians. Iodine‐contrast microCT approaches are enabling new insights into the anatomy of muscles, nerves, and other soft tissues of embryonic as well as adult specimens of alligators. These imaging data and other muscle modeling methods offer increased accuracy of muscle sizes and attachments without destructive methods like dissection. 3D modeling approaches and imaging data together now enable us to see and reconstruct 3D muscle architecture which then allows us to estimate 3D muscle resultants, but also measurements of pennation in ways not seen before. These methods have already revealed new information on the ontogeny, diversity, and function of jaw muscles and the heads of alligators and other crocodylians. Such approaches will lead to enhanced and accurate analyses of form, function, and evolution of crocodylians, their fossil ancestors and vertebrates in general. 

ABSTRACT The extinct nonavian dinosaurTyrannosaurus rex, considered one of the hardest biting animals ever, is often hypothesized to have exhibited cranial kinesis, or, mobility of cranial joints relative to the braincase. Cranial kinesis inT.rexis a biomechanical paradox in that forcefully biting tetrapods usually possess rigid skulls instead of skulls with movable joints. We tested the biomechanical performance of a tyrannosaur skull using a series of static positions mimicking possible excursions of the palate to evaluate Postural Kinetic Competency inTyrannosaurus. A functional extant phylogenetic bracket was employed using taxa, which exhibit measurable palatal excursions:Psittacus erithacus(fore–aft movement) andGekko gecko(mediolateral movement). Static finite element models ofPsittacus,Gekko, andTyrannosauruswere constructed and tested with different palatal postures using anatomically informed material properties, loaded with muscle forces derived from dissection, phylogenetic bracketing, and a sensitivity analysis of muscle architecture and tested in orthal biting simulations using element strain as a proxy for model performance. Extant species models showed lower strains in naturally occurring postures compared to alternatives. We found that fore–aft and neutral models ofTyrannosaurusexperienced lower overall strains than mediolaterally shifted models. Protractor muscles dampened palatal strains, while occipital constraints increased strains about palatocranial joints compared to jaw joint constraints. These loading behaviors suggest that even small excursions can strain elements beyond structural failure. Thus, these postural tests of kinesis, along with the robusticity of other cranial features, suggest that the skull ofTyrannosauruswas functionally akinetic. Anat Rec, 303:999–1017, 2020. © 2019 Wiley Periodicals, Inc. 

ABSTRACT Crocodylians evolved some of the most characteristic skulls of the animal kingdom with specializations for semiaquatic and ambush lifestyles, resulting in a feeding apparatus capable of tolerating high biomechanical loads and bite forces and a head with a derived sense of trigeminal‐nerve‐mediated touch. The mandibular symphysis accommodates these specializations being both at the end of a biomechanical lever and an antenna for sensation. Little is known about the anatomy of the crocodylian mandibular symphysis, hampering our understanding of form, function, and evolution of the joint in extant and extinct lineages. We explore mandibular symphysis anatomy of an ontogenetic series ofAlligator mississippiensisusing imaging, histology, and whole mount methods. Complex sutural ligaments emanating about a midline‐fused Meckel's cartilage bridge the symphysis. These tissues organize during days 37–42 ofin ovodevelopment. However, interdigitations do not manifest until after hatching. These soft tissues leave a hub and spoke‐like bony morphology of the symphyseal plate, which never fuses. Interdigitation morphology varies within the symphysis suggesting differential loading about the joint. Neurovascular canals extend throughout the mandibles to alveoli, integument, and bone adjacent to the symphysis. These features suggest theAlligatormandibular symphysis offers compliance in an otherwise rigid skull. We hypothesize a fused Meckel's cartilage offers stiffness in hatchling mandibles prior to the development of organized sutural ligaments and mineralized bone while offering a scaffold for somatic growth. The porosity of the dentaries due to neurovascular tissues likely allows transmission of sensory and proprioceptive information from the surroundings and the loaded symphysis. Anat Rec, 302:1696–1708, 2019. © 2019 American Association for Anatomy