Trigeminal Nerve Morphology inAlligator mississippiensisand Its Significance for Crocodyliform Facial Sensation and Evolution

Morphological disparity and structural performance of the dromaeosaurid skull informs ecology and evolutionary history

Non-avialan theropod dinosaurs had diverse ecologies and varied skull morphologies. Previous studies of theropod cranial morphology mostly focused on higher-level taxa or characteristics associated with herbivory. To better understand morphological disparity and function within carnivorous theropod families, here we focus on the Dromaeosauridae, 'raptors' traditionally seen as agile carnivorous hunters.We applied 2D geometric morphometrics to quantify skull shape, performed mechanical advantage analysis to assess the efficiency of bite force transfer, and performed finite element analysis to examine strain distribution in the skull during biting. We find that dromaeosaurid skull morphology was less disparate than most non-avialan theropod groups. Their skulls show a continuum of form between those that are tall and short and those that are flat and long. We hypothesise that this narrower morphological disparity indicates developmental constraint on skull shape, as observed in some mammalian families. Mechanical advantage indicates that Dromaeosaurus albertensis and Deinonychus antirrhopus were adapted for relatively high bite forces, while Halszkaraptor escuilliei was adapted for high bite speed, and other dromaeosaurids for intermediate bite forces and speeds. Finite element analysis indicates regions of high strain are consistent within dromaeosaurid families but differ between them. Average strain levels do not follow any phylogenetic pattern, possibly due to ecological convergence between distantly-related taxa.Combining our new morphofunctional data with a re-evaluation of previous evidence, we find piscivorous reconstructions of Halszkaraptor escuilliei to be unlikely, and instead suggest an invertivorous diet and possible adaptations for feeding in murky water or other low-visibility conditions. We support Deinonychus antirrhopus as being adapted for taking large vertebrate prey, but we find that its skull is relatively less resistant to bite forces than other dromaeosaurids. Given the recovery of high bite force resistance for Velociraptor mongoliensis, which is believed to have regularly engaged in scavenging behaviour, we suggest that higher bite force resistance in a dromaeosaurid taxon may reflect a greater reliance on scavenging rather than fresh kills.Comparisons to the troodontid Gobivenator mongoliensis suggest that a gracile rostrum like that of Velociraptor mongoliensis is ancestral to their closest common ancestor (Deinonychosauria) and the robust rostra of Dromaeosaurus albertensis and Deinonychus antirrhopus are a derived condition. Gobivenator mongoliensis also displays a higher jaw mechanical advantage and lower resistance to bite force than the examined dromaeosaurids, but given the hypothesised ecological divergence of troodontids from dromaeosaurids it is unclear which group, if either, represents the ancestral condition. Future work extending sampling to troodontids would therefore be invaluable and provide much needed context to the origin of skull form and function in early birds. This study illustrates how skull shape and functional metrics can discern non-avialan theropod ecology at lower taxonomic levels and identify variants of carnivorous feeding.

Braincase and neuroanatomy of the lagerpetid Dromomeron gregorii (Archosauria, Pterosauromorpha) with comments on the early evolution of the braincase and associated soft tissues in Avemetatarsalia

The anatomy of the braincase and associated soft tissues of the lagerpetid Dromomeron gregorii (Archosauria: Avemetatarsalia) from the Late Triassic of the United States is here described. This corresponds to the first detailed description of cranial materials of Lagerpetidae, an enigmatic group of Late Triassic (c. 236-200 Million years ago) animals that are the closest known relatives of pterosaurs, the flying reptiles. The braincase of D. gregorii is characterized by the presence of an anteriorly elongated laterosphenoid and a postparietal, features observed in stem-archosaurs but that were still unknown in early members of the avian lineage of archosaurs. Using micro-computed tomography (CT-scan data), we present digital reconstructions of the brain and endosseous labyrinth of D. gregorii. The brain of D. gregorii exhibits a floccular lobe of the cerebellum that projects within the space of the semicircular canals. The semicircular canals are relatively large when compared to other archosauromorphs, with the anterior canal exhibiting a circular shape. These features of the sensory structures of D. gregorii are more similar to those of pterosaurs than to those of other early avemetatarsalians. In sum, the braincase anatomy of D. gregorii shows a combination of plesiomorphic and apomorphic features in the phylogenetic context of Archosauria and suggests that the still poorly understood early evolution of the braincase in avemetatarsalians is complex, with a scenario of independent acquisitions and losses of character states.

Neuroanatomy and pneumaticity of the extinct Malagasy "horned" crocodile Voay robustus and its implications for crocodylid phylogeny and palaeoecology

Voay robustus, the extinct Malagasy "horned" crocodile, was originally considered to be the only crocodylian representative in Madagascar during most part of the Holocene. However, Malagasy crocodylian remains have had confused taxonomic attributions and recent studies have underlined that Crocodylus and Voay populations coexisted on the island for at least 7500 years. Here, we describe the inner braincase anatomy of Voay robustus using x-ray computed tomography on four specimens, to provide new anatomical information that distinguishes Voay from Crocodylus, especially features of the brain endocast and the paratympanic sinuses. Geometric morphometric analyses are performed on 3D models of the internal organs to compare statistically Voay with a subset of extant Crocodylidae. Following these comparisons, we build an endocranial morphological matrix to discuss the proposed phylogenetic affinities of Voay with Osteolaeminae from an endocranial point of view. Additionally, we discuss the use of internal characters in systematic studies and find that they can have a major impact on morphological analyses. Finally, new radiocarbon data on Voay and subfossil Crocodylus specimens are recovered between 2010 and 2750 cal BP, which confirm the cohabitation of the two species in the same area for a long period of time. We thus assess several extinction scenarios, and propose a slightly different ecology of Voay compared to Crocodylus, which could have allowed habitat partitioning on the island. Our approach complements information obtained from previous molecular and morphological phylogenies, as well as previous radiocarbon dating, together revealing past diversity and faunal turnovers in Madagascar.

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.

The neuroanatomy and pneumaticity of Hamadasuchus (Crocodylomorpha, Peirosauridae) from the Cretaceous of Morocco and its paleoecological significance for altirostral forms

We describe the endocranial structures of Hamadasuchus, a peirosaurid crocodylomorph from the late Albian-Cenomanian Kem Kem group of Morocco. The cranial endocast, associated nerves and arteries, endosseous labyrinths, and cranial pneumatization, as well as the bones of the braincase of a new specimen, are reconstructed and compared with extant and fossil crocodylomorphs, which represent different lifestyles. Cranial bones of this specimen are identified as belonging to Hamadasuchus, with close affinities with Rukwasuchus yajabalijekundu, another peirosaurid from the 'middle' Cretaceous of Tanzania. The endocranial structures are comparable to those of R. yajabalijekundu but also to baurusuchids and sebecids (sebecosuchians). Paleobiological traits of Hamadasuchus, such as alert head posture, ecology, and behavior are explored for the first time, using quantitative metrics. The expanded but narrow semi-circular canals and enlarged pneumatization of the skull of Hamadasuchus are linked to a terrestrial lifestyle. Continuing work on the neuroanatomy of supposedly terrestrial crocodylomorphs needs to be broadened to other groups and will allow to characterize whether some internal structures are affected by the lifestyle of these organisms.

Description and phylogenetic relationships of a new species of Torvoneustes (Crocodylomorpha, Thalattosuchia) from the Kimmeridgian of Switzerland

Metriorhynchids are marine crocodylomorphs found across Jurassic and Lower Cretaceous deposits of Europe and Central and South America. Despite being one of the oldest fossil families named in paleontology, the phylogenetic relationships within Metriorhynchidae have been subject to many revisions over the past 15 years. Herein, we describe a new metriorhynchid from the Kimmeridgian of Porrentruy, Switzerland. The material consists of a relatively complete, disarticulated skeleton preserving pieces of the skull, including the frontal, prefrontals, right postorbital, nasals, maxillae, right premaxillae and nearly the entire mandible, and many remains of the axial and appendicular skeleton such as cervical, dorsal, and caudal vertebrae, ribs, the left ischium, the right femur, and the right fibula. This new specimen is referred to the new species Torvoneustes jurensis sp. nov. as part of the large-bodied macrophagous tribe Geosaurini. Torvoneustes jurensis presents a unique combination of cranial and dental characters including a smooth cranium, a unique frontal shape, acute ziphodont teeth, an enamel ornamentation made of numerous apicobasal ridges shifting to small ridges forming an anastomosed pattern toward the apex of the crown and an enamel ornamentation touching the carina. The description of this new species allows to take a new look at the currently proposed evolutionary trends within the genus Torvoneustes and provides new information on the evolution of this clade.

Modified skulls but conservative brains? The palaeoneurology and endocranial anatomy of baryonychine dinosaurs (Theropoda: Spinosauridae)

The digital reconstruction of neurocranial endocasts has elucidated the gross brain structure and potential ecological attributes of many fossil taxa, including Irritator, a spinosaurine spinosaurid from the "mid" Cretaceous (Aptian) of Brazil. With unexceptional hearing capabilities, this taxon was inferred to integrate rapid and controlled pitch-down movements of the head that perhaps aided in the predation of small and agile prey such as fish. However, the neuroanatomy of baryonychine spinosaurids remains to be described, and potentially informs on the condition of early spinosaurids. Using micro-computed tomographic scanning (μCT), we reconstruct the braincase endocasts of Baryonyx walkeri and Ceratosuchops inferodios from the Wealden Supergroup (Lower Cretaceous) of England. We show that the gross endocranial morphology is similar to other non-maniraptoriform theropods, and corroborates previous observations of overall endocranial conservatism amongst more basal theropods. Several differences of unknown taxonomic utility are noted between the pair. Baryonychine neurosensory capabilities include low-frequency hearing and unexceptional olfaction, whilst the differing morphology of the floccular lobe tentatively suggests less developed gaze stabilisation mechanisms relative to spinosaurines. Given the morphological similarities observed with other basal tetanurans, baryonychines likely possessed comparable behavioural sophistication, suggesting that the transition from terrestrial hypercarnivorous ancestors to semi-aquatic "generalists" during the evolution of Spinosauridae did not require substantial modification of the brain and sensory systems.

Ecomorphological patterns in trigeminal canal branching among sauropsids reveal sensory shift in suchians

The vertebrate trigeminal nerve is the primary mediator of somatosensory information from nerve endings across the face, extending nerve branches through bony canals in the face and mandibles, terminating in sensory receptors. Reptiles evolved several extreme forms of cranial somatosensation in which enhanced trigeminal tissues are present in species engaging in unique mechanosensory behaviors. However, morphology varies by clade and ecology among reptiles. Few lineages approach the extreme degree of tactile somatosensation possessed by crocodylians, the only remaining members of a clade that underwent an ecological transition from the terrestrial to semiaquatic habitat, also evolving a specialized trigeminal system. It remains to be understood how trigeminal osteological correlates inform how adaptations for enhanced cranial sensation evolved in crocodylians. Here we identify an increase in sensory abilities in Early Jurassic crocodylomorphs, preceding the transitions to a semiaquatic habitat. Through quantification of trigeminal neurovascular canal branching patterns in an extant phylogenetic bracket we quantify and identify morphologies associated with sensory behaviors in representative fossil taxa, we find stepwise progression of increasing neurovascular canal density, complexity, and distribution from the primitive archosaurian to the derived crocodilian condition. Model-based inferences of sensory ecologies tested on quantified morphologies of extant taxa with known sensory behaviors indicate a parallel increase in sensory abilities among pseudosuchians. These findings establish patterns of reptile trigeminal ecomorphology, revealing evolutionary patterns of somatosensory ecology.

Anatomy and relationships of the early diverging Crocodylomorphs Junggarsuchus sloani and Dibothrosuchus elaphros

The holotype of Junggarsuchus sloani, from the Shishugou Formation (early Late Jurassic) of Xinjiang, China, consists of a nearly complete skull and the anterior half of an articulated skeleton, including the pectoral girdles, nearly complete forelimbs, vertebral column, and ribs. Here, we describe its anatomy and compare it to other early diverging crocodylomorphs, based in part on CT scans of its skull and that of Dibothrosuchus elaphros from the Early Jurassic of China. Junggarsuchus shares many features with a cursorial assemblage of crocodylomorphs, informally known as "sphenosuchians," whose relationships are poorly understood. However, it also displays several derived crocodyliform features that are not found among most "sphenosuchians." Our phylogenetic analysis corroborates the hypothesis that Junggarsuchus is closer to Crocodyliformes, including living crocodylians, than are Dibothrosuchus and Sphenosuchus, but not as close to crocodyliforms as Almadasuchus and Macelognathus, and that the "Sphenosuchia" are a paraphyletic assemblage. D. elaphros and Sphenosuchus acutus are hypothesized to be more closely related to Crocodyliformes than are the remaining non-crocodyliform crocodylomorphs, which form several smaller groups but are largely unresolved.

Neuroanatomy of the mekosuchine crocodylian Trilophosuchus rackhami Willis, 1993

Although our knowledge on crocodylomorph palaeoneurology has experienced considerable growth in recent years, the neuroanatomy of many crocodylomorph taxa has yet to be studied. This is true for Australian taxa, where thus far only two crocodylian crocodylomorphs have had aspects of their neuroanatomy explored. Here, the neuroanatomy of the Australian mekosuchine crocodylian Trilophosuchus rackhami is described for the first time, which significantly increases our understanding on the palaeoneurology of Australian crocodylians. The palaeoneurological description is based on the taxon's holotype specimen (QMF16856), which was subjected to a μCT scan. Because of the exceptional preservation of QMF16856, most neuroanatomical elements could be digitally reconstructed and described in detail. Therefore, the palaeoneurological assessment presented here is hitherto the most in‐depth study of this kind for an extinct Australian crocodylomorph. Trilophosuchus rackhami has a brain endocast with a distinctive morphology that is characterized by an acute dural peak over the hindbrain region. While the overall morphology of the brain endocast is unique to T. rackhami, it does share certain similarities with the notosuchian crocodyliforms Araripesuchus wegeneri and Sebecus icaeorhinus. The endosseous labyrinth displays a morphology that is typical for crocodylians, although a stand‐out feature is the unusually tall common crus. Indeed, the common crus of T. rackhami has one of the greatest height ratios among crocodylomorphs with currently known endosseous labyrinths. The paratympanic pneumatic system of T. rackhami is greatly developed and most similar to those of the extant crocodylians Osteolaemus tetraspis and Paleosuchus palpebrosus. The observations on the neuroanatomy of T. rackhami are also discussed in the context of Crocodylomorpha. The comparative palaeoneurology reinforces previous evaluations that the neuroanatomy of crocodylomorphs is complex and diverse among species, and T. rackhami has a peculiar neuromorphology, particularly among eusuchian crocodyliforms.

Dispatches from the age of crocodiles: New discoveries from ancient lineages

Crocodilians inspire researchers and the public alike with their explosive hunting methodologies, distinct craniofacial and dental morphology, and resplendent fossil record. This special issue highlights recent advances in the biology and paleontology of this fascinating lineage of vertebrates. The authors in this volume bring crocodylians and their extinct ancestors to life using a variety of approaches including fieldwork, imaging, 3D modeling, developmental biology, physiological monitoring, dissection, and a host of other comparative methods. Our journey begins with early crocodylomorphs from the Triassic, carries us through the radiation of crocodyliforms during the rest of the Mesozoic Era, and finally celebrates the diversification development and biology of extant crocodylians. Crocodyliform science has grown appreciably the past few decades. New fossil species and genetic evidence continue to keep phylogenies and our understanding of relationships wavering in key places of the tree such as the relationships of the extinct marine thalattosuchians as well as still living species like gharials. The application of imaging approaches and 3D modeling to both preserved tissues as well as living specimens is now revealing patterns in brain and lung evolution and function, growth strategies, and feeding and locomotor behaviors across the lineage. Comparative anatomical studies are offering new data on genitals, cephalic venous drainage and thoracoabdominal pressures. The new discoveries found here only reveal there is far more work to be done to understand the biology and behavior responsible for the great radiation extinct suchians and their crocodylian descendants experienced during their conquest of Mesozoic and Tertiary ecosystems.

Ontogeny of the trigeminal system and associated structures in Alligator mississippiensis

From the appearance of the vertebrate head, the trigeminal system has played a role in behavioral and ecological adaptation. The trigeminal nerve is the primary cranial somatosensory nerve, also innervating the jaw muscles. In crocodylians, the trigeminal nerve plays a role in modulating the high bite force and unique integumentary sensation. In association with these behaviors, crocodylians are known for large trigeminal nerves, a high volume of trigeminal-innervated musculature, and densely packed, specialized sensory receptors. These innovations also occurred in concert with a restructuring of the lateral braincase wall. These morphologies have previously been investigated in phylogenetic and evolutionary contexts, but an ontogenetic, whole-system investigation of trigeminal tissue and associated musculature, cartilage, and bone is lacking, as is an understanding of developmental timing of morphologies significant to hypotheses of homology. Here, we use contrast-enhanced computed tomography imaging to provide description and analysis of the trigeminal system in an ontogenetic series of Alligator mississippiensis from embryonic to adult form. We explore growth rates and allometric relationships of structures and discuss the significance to hypotheses of homology. We find a high growth rate and allometric trajectory of the trigeminal nerve in comparison to other cranial nerves, likely associated with the large volume of trigeminal musculature and high densities of sensory receptors. We identify a similar trend in the pterygoideus dorsalis muscle, the highest contributor to bite force. We narrow ontogenetic timing of features related to the trigeminal topological paradigm and the undeveloped epipterygoid. Overall, we provide a basis for understanding trigeminal development in crocodylians, which upon comparison across reptiles will reveal ontogenetic origins of morphological variation.

Microstructure of the Bonnethead Shark (Sphyrna tiburo) Olfactory Rosette

Synopsis

The unusual shape of sphyrnid (hammerhead shark) heads has led to many functional hypotheses of potential sensory advantages and enhanced olfactory performance. Recent investigations into the flow of water within the sphyrnid olfactory chamber demonstrate that this complex structure exhibits a differential pressure system between the 2 nares that induces flow through the chamber. This leads to differential fluid velocities in different parts of the olfactory chamber. Particularly, lamellae at the medial end of the olfactory chamber experience a near-stagnant recirculation of water. The objectives of this study were to (1) describe the microstructure of the olfactory rosette of bonnethead sharks (Sphyrna tiburo) and (2) given the variability of water flow within the sphyrnid olfactory rosette, investigate differences of individual lamellae based on their positioning within the rosette. Specifically, we investigated degree of secondary folding, percent sensory area, and relative surface along the lateral-to-medial gradient. Both degree of secondary folding and percent sensory area may serve as proxies for olfactory sensitivity, providing connectivity between area devoted to sensitivity and water flow within the olfactory organ. We found that bonnethead sharks exhibited similar lamellar morphology to other shark species. We also described the projection of the olfactory nerve layer through an individual lamella. Additionally, we found that lamellae within the medial portion of the organ, which experience slower water velocities, had less secondary lamellar folds and less sensory area. These findings imply that these areas may be less sensitive. Future work should test for sensitivity differences within the rosette along the lateral-to-medial gradient.

Spanish

La forma inusual de las cabezas de los esfírnidos (tiburones martillo) ha llevado a muchas hipótesis funcionales de posibles ventajas sensoriales y unas mejores capacidades olfativas. Las investigaciones recientes sobre el flujo de agua dentro del órgano olfativo de los esfírnidos, demuestran que esta estructura compleja exhibe un sistema de presión diferente entre las dos fosas nasales que induce el flujo en el órgano. Esto conduce a velocidades de fluido diferentes en distintas partes del órgano olfativo. En particular, las láminas en el extremo medial del órgano olfativo experimentan una recirculación de agua casi estancada. Los objetivos de este estudio fueron 1) describir la microestructura de la roseta olfativa de los tiburones cabeza de pala (Sphyrna tiburo) y 2) considerando la variabilidad del flujo de agua dentro de la roseta olfativa de los esfírnidos, investigar las diferencias de las laminillas individuales, basadas en su posición dentro de la roseta. Específicamente, hemos investigado el grado de plegamiento secundario, el porcentaje del área sensorial y el área relativa de superficie a lo largo del gradiente de lateral a medial. El grado de plegamiento secundario y el porcentaje del área sensorial pueden servir como indicadores de la sensibilidad olfativa, proporcionando conectividad entre el área dedicada a la sensibilidad y el flujo de agua dentro del órgano olfativo. Descubrimos que los tiburones cabeza de pala exhibían una morfología laminar similar a la de otras especies de tiburones. También hemos descrito la proyección del estrato del nervio olfativo dentro de una lámina individual. Además, encontramos que las laminillas dentro de la porción medial del órgano que experimentan velocidades de agua más lentas, tenían menos pliegues laminares secundarios y una menor área sensorial. Estos hallazgos implican que estas áreas pueden ser menos sensitivas. El trabajo futuro debería evaluar las diferencias de sensibilidad dentro de la roseta a lo largo del gradiente de lateral a medial.

German

Die ungewöhnliche Kopfform der Sphyrniden (Hammerhaie) hat schon zu vielen funktionellen Hypothesen bezüglich möglicher sensorischer Vorteile und verbesserter olfaktorischer Leistung geführt. Kürzlich veröffentlichte Studien zur Wasserströmung innerhalb der olfaktorischen Kammern von Sphyrniden zeigen, dass diese komplexe Struktur unterschiedliche Drucksysteme zwischen den beiden Nasenlöchern erzeugt, welches eine Strömung durch die Nasenkammer erzeugt. Dies wiederum führt zu unterschiedlichen Flüssigkeitsströmungen in verschiedenen Abschnitten der olfaktorischen Kammer. Besonders bei den Lamellen am medialen Ende der olfaktorischen Kammer gibt es eine fast schon stillstehende Rezirkulation von Wasser. Die Ziele dieser Studie waren 1) das Beschreiben der Mikrostruktur der olfaktorischen Rosette des Schaufelnasen-Hammerhais (Sphyrna tiburo) und 2) wollten wir, aufgrund der Variabilität der Wasserströmung innerhalb der olfaktorischen Rosette der Sphyrniden, die Unterschiede von individuellen Lamellen basierend auf ihrer unterschiedlichen Position innerhalb der Rosette untersuchen. Wir untersuchten den Grad an sekundären Falten, den Prozentsatz an sensorischer Fläche und die relative Oberfläche entlang dem lateral-zu-medialem Gradienten. Sowohl der Grad an sekundären Falten wie auch der Prozentsatz an sensorischer Fläche mögen als Annäherung für die olfaktorische Sensibilität dienen, weil sie für eine Verbindung zwischen der Fläche, die dem Geruchssinn und der Strömung zwischen den olfaktorischen Organen sorgt. Wir fanden, dass die Schaufelnasen-Hammerhaie eine ähnliche lamellare Morphologie zeigen wie andere Hai-Arten. Wir beschreiben auch wie der Geruchsnerv durch eine individuelle Lamelle verläuft. Weiter fanden wir, dass die Lamellen innerhalb des mittleren Teils des Organs, welches geringe Strömungsgeschwindigkeiten erfährt, weniger sekundäre lamellare Falten enthält und weniger sensorische Fläche. Diese Entdeckungen implizieren, dass diese Bereiche weniger sensibel sind auf Gerüche. Zukünftige Arbeiten sollten die unterschiedlichen Sensibilitäten innerhalb der Rosette entlang des lateral-medialem Gradienten testen.

New frontiers in imaging, anatomy, and mechanics of crocodylian jaw muscles

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.

Neurological examination in healthy adult inland bearded dragons (Pogona vitticeps)

OBJECTIVE

To evaluate neurological tests and expected results in inland bearded dragons (Pogona vitticeps) and generate recommendations for bearded dragon-specific neurological examination.

ANIMALS

26 healthy adult inland bearded dragons.

PROCEDURES

A complete neurological examination utilizing tests described in both mammals and reptiles was performed on each lizard, and test feasibility and outcome were recorded.

RESULTS

Tests with poor feasibility included oculocardiac reflex (successfully completed in 62% [16/26] of animals) and voluntary ambulation and swallowing by use of a food item (0% [0/26] of animals). Tests with outcomes considered abnormal in mammals but attributable to normal bearded dragon behavior included head position (head tilt present in 12% [3/26]) and head movement (head bob present in 4% [1/26]). Many tests had absent or inconsistent outcomes, including menace response (present in 19% [5/26]), proprioceptive positioning (present in 4% [1/26] in the thoracic limbs and 0% [0/26] in the pelvic limbs), vent reflex (present in 27% [7/26]), and myotatic reflexes (biceps present in 8% [2/26]; patellar, gastrocnemius, and triceps present in 0% [0/26]). Extensor postural thrust was absent in all successfully tested animals, but a novel reflex termed the caudal thoracic extensor reflex was noted instead in all observed animals (100% [21/21]).

CLINICAL RELEVANCE

Tests with poor feasibility or inconsistent outcomes should have low priority or be excluded from neurological examinations of inland bearded dragons. Normal behaviors should be considered for head position and movement. A bearded dragon-specific neurological examination protocol derived from these findings is described and recommended in order to decrease stress and improve neurolocalization.

An exceptional neurovascular system in abelisaurid theropod skull: New evidence from Skorpiovenator bustingorryi

Abelisaurids were one of the most successful theropod dinosaurs during Cretaceous times. They are featured by numerous derived skull traits, such as heavily ornamented bones, short and tall snout, and a strongly thickened cranial roof. Furthermore, nasals are distinctive on having two distinct nasal patterns: strongly transversely convex and heavily sculptured (e.g., Carnotaurus), and transversely concave, with marked bilateral crests and poorly sculptured surfaces (e.g., Rugops). Independently of the pattern, some abelisaurid nasals (e.g., Rugops) show a distinctive row of large foramina on the dorsal surface, which were in general associated to skin structures (scales). Skorpiovenator bustingorryi is a derived abelisaurid coming from the upper Cretaceous beds of northwestern Patagonia, represented by an almost complete skeleton including a well-preserved skull. Particularly, the skull of Skorpiovenator shows nasal bones characterized by being transversely concave, rimmed by lateral crests and with a conspicuous row of foramina on the dorsal surface. But more interesting is that the skull roof also exhibits a row of large foramina that seem to be continuous with the previous nasal foramina. CT scans made on the skull corroborates a novel feature within theropods: the nasal foramina on the external surface are linked to an internal canal that runs across the nasal bones. We compared this feature with CT scans of Carnotaurus and revealed that it also possess an internal system as in Skorpiovenator, but being notably smaller. The symmetry and disposition of the foramina in the nasal and skull roof bones of Skorpiovenator would indicate a neurovascular correlate (i.e., blood vessels and nerves), probably to the lateral nasal and supraorbital vessels and the trigeminal nerve. The biological significance of such neurovascular system can be conjectured from several hypotheses. A possible one involves an enhanced blood volume in these bones linked to a zone of thermal exchange, which may help avoid overheat of encephalic tissues. Another plausible hypothesis takes into account the presence of display skin structures in which blood volume nourished the mineralized skin, which would have a role in intraspecific communication. However, other more speculative explanations should not be discarded such as a correlation with integumentary sensory organs.

Anatomy and development of skull-neck boundary structures in the skeleton of the extant crocodylian Alligator mississippiensis

A system-by-system approach dominates morphological and evolutionary study; however, some structures that are better understood within the context of an interface between two systems or traditional units remain less well understood. As part of a larger goal to clarify aspects of skull-neck boundary evolution, we herein describe the morphology and development of the occiput and atlas-axis complex in the crocodylian Alligator mississippiensis. We apply micro-computed tomography scanning, clearing and double staining, and histological analyses to skull-neck boundary structures at three stages of development (embryonic stage 22, 23, and hatchling). Regions of ossification that could possibly pertain to a postparietal were found adjacent to the parietal bone and supraoccipital; however, these were not deemed convincing and are considered part of the supraoccipital. Within the atlas-axis complex, the proatlas appears as two discrete cartilaginous elements in Stage 22 that ossify together at Stage 23. Posterior to the proatlas, the atlas-axis complex is composed of two centra, each with cervical ribs ventrally and neural arches dorsally that begin ossifying at Stage 23. Histology and clearing and staining of Stages 22 and 23 embryos reveal a discrete atlas intercentrum applied to the ventral part of the occipital condyle of the skull. Posterior to this is a cartilage that appears to be a co-chondrified atlas pleurocentrum, axis intercentrum, and axis pleurocentrum. Ossification of this cartilaginous structure produces discrete atlas inter- and pleurocentra, as well as a singular axis centrum. Together these data are discussed with reference to clarifying historical discrepancies concerning elements at the crocodylian skull-neck boundary.

The neuroanatomy of Zulmasuchus querejazus (Crocodylomorpha, Sebecidae) and its implications for the paleoecology of sebecosuchians

The endocranial structures of the sebecid crocodylomorph Zulmasuchus querejazus (MHNC 6672) from the Lower Paleocene of Bolivia are described in this article. Using computed tomography scanning, the cranial endocast, associated nerves and arteries, endosseous labyrinths, and cranial pneumatization are reconstructed and compared with those of extant and fossil crocodylomorphs, representative of different ecomorphological adaptations. Z. querejazus exhibits an unusual flexure of the brain, pericerebral spines, semicircular canals with a narrow diameter, as well as enlarged pharyngotympanic sinuses. First, those structures allow to estimate the alert head posture and hearing capabilities of Zulmasuchus. Then, functional comparisons are proposed between this purportedly terrestrial taxon, semi-aquatic, and aquatic forms (extant crocodylians, thalattosuchians, and dyrosaurids). The narrow diameter of the semicircular canals but expanded morphology of the endosseous labyrinths and the enlarged pneumatization of the skull compared to other forms indeed tend to indicate a terrestrial lifestyle for Zulmasuchus. Our results highlight the need to gather new data, especially from altirostral forms in order to further our understanding of the evolution of endocranial structures in crocodylomorphs with different ecomorphological adaptations.

Rostral neurovasculature indicates sensory trade-offs in Mesozoic pelagic crocodylomorphs

Metriorhynchoid thalattosuchians were a marine clade of Mesozoic crocodylomorphs that evolved from semi-aquatic, "gharial"-like species into the obligately pelagic subclade Metriorhynchidae. To explore whether the sensory and physiological demands of underwater life necessitates a shift in rostral anatomy, both in neurology and vasculature, we investigate the trigeminal innervation and potential somatosensory abilities of metriorhynchoids by digitally segmenting the rostral neurovascular canals in CT scans of 10 extant and extinct crocodyliforms. The dataset includes the terrestrial, basal crocodyliform Protosuchus haughtoni, two semi-aquatic basal metriorhynchoids, four pelagic metriorhynchids and three extant, semi-aquatic crocodylians. In the crocodylian and basal metriorhynchoid taxa, we find three main neurovascular channels running parallel to one another posteroanteriorly down the length of the snout, whereas in metriorhynchids there are two, and in P. haughtoni only one. Crocodylians appear to be unique in their extensive trigeminal innervation, which is used to supply the integumentary sensory organs (ISOs) involved with their facial somatosensory abilities. Crocodylians have a far higher number of foramina on the maxillary bones than either metriorhynchoids or P. haughtoni, suggesting that the fossil taxa lacked the somatosensory abilities seen in extant species. We posit that the lack of ISO osteological correlates in metriorhynchoids is due to their basal position in Crocodyliformes, rather than a pelagic adaptation. This is reinforced by the hypothesis that extant crocodyliforms, and possibly some neosuchian clades, underwent a long "nocturnal bottleneck"-hinting that their complex network of ISOs evolved in Neosuchia, as a sensory trade-off to compensate for poorer eyesight.

Braincase anatomy of the Paleocene crocodyliform Rhabdognathus revealed through high resolution computed tomography

Dyrosaurids were highly specialized, largely marine, relatives of living crocodylians, and one of the few archosaur lineages to survive the K-Pg extinction. Dyrosaurids lived during the Cretaceous to the Eocene and represent a unique combination of morphology and ecology not seen in living crocodylians. Little is known about their endocranial anatomy, leaving many questions about their neurosensory adaptations unaddressed. Recently, µCT (micro-computed tomography) scans were made of a well-preserved skull of Rhabdognathus, a Paleocene dyrosaurid from Mali. This marks the first time the braincase and neurosensory features of a dyrosaurid have been examined using CT. We focus our attention to three specific internal structures: the cranial endocast; the inner ear; and the paratympanic sinuses. The cranial endocast of Rhabdognathus revealed novel features including a unique conformation of its paratympanic system, a prominent dorsal venous system that communicates with the external skull table, extremely enlarged tympanic vestibules that meet at the midline of the endocranium, a prominent spherical cerebrum, and elongate olfactory tracts accounting for half the total endocast length. The bizarre laterally facing lateral Eustachian foramen of dyrosaurids is now understood to be a complex fossa including both a ventrally directed lateral Eustachian foramen and a laterally directed foramen for the basioccipital diverticulum. A novel median pterygopharyngeal canal was discovered connecting the pharynx to the adductor chamber. These revelations require a reinterpretation of the associated external foramina visible on the posterior of the skull in dyrosaurids and potentially their close relatives the pholidosaurids. The olfactory tract terminates in an enlarged olfactory region possessing complex bony projections—a unique morphology perhaps serving to increase surface area for olfaction. The inner ear of Rhabdognathus exhibits characteristics seen in both Pelagosaurus and Gavialis. The vestibule is spherical, as in Gavialis, but is significantly expanded. The semicircular canals are enlarged but pyramidal in shape as in the thalattosuchian Pelagosaurus. The proportion of the cochlear length to total endosseous labyrinth height is roughly 0.5 in Rhabdognathus implying that the hearing capabilities resemble that of thalattosuchians. A suite of expanded sense organs (e.g., bony olfactory lamina; hypertrophied vestibule of the inner ear), and the clear expansion of the cerebrum to a more symmetrical and spherical shape suggest that dyrosaurids possess neuroanatomical modifications facilitating an agile predatory near-shore ecology.

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.

Cretaceous origins of the vibrotactile bill-tip organ in birds

Some probe-foraging birds locate their buried prey by detecting mechanical vibrations in the substrate using a specialized tactile bill-tip organ comprising mechanoreceptors embedded in densely clustered pits in the bone at the tip of their beak. This remarkable sensory modality is known as 'remote touch', and the associated bill-tip organ is found in probe-foraging taxa belonging to both the palaeognathous (in kiwi) and neognathous (in ibises and shorebirds) clades of modern birds. Intriguingly, a structurally similar bill-tip organ is also present in the beaks of extant, non-probing palaeognathous birds (e.g. emu and ostriches) that do not use remote touch. By comparison with our comprehensive sample representing all orders of extant modern birds (Neornithes), we provide evidence that the lithornithids (the most basal known palaeognathous birds which evolved in the Cretaceous period) had the ability to use remote touch. This finding suggests that the occurrence of the vestigial bony bill-tip organ in all modern non-probing palaeognathous birds represents a plesiomorphic condition. Furthermore, our results show that remote-touch probe foraging evolved very early among the Neornithes and it may even have predated the palaeognathous-neognathous divergence. We postulate that the tactile bony bill-tip organ in Neornithes may have originated from other snout tactile specializations of their non-avian theropod ancestors.

Paleoneurology of Baurusuchus (Crocodyliformes: Baurusuchidae), ontogenetic variation, brain size, and sensorial implications

Knowledge on crocodyliform paleoneurology has significantly improved with development of computed tomography. However, studies so far have been able to reconstruct brain endocasts based only on single specimens for each taxon. Here for the first time, we reconstructed brain endocasts for multiple fossil specimens of the same crocodyliform taxon (Baurusuchus), consisting of complete skulls of two medium sized specimens, one large adult, and a late juvenile. In addition, we were able to reconstruct the inner ear anatomy of a fragmentary skull using microtomography. We present estimates of brain size using simple models, based on modern Crocodylia, able to adapt brain to endocranial cavity ratios to expected ontogenetic variation instead of using fixed ratios. We also analyzed relative brain sizes, olfactory ratios, facial sensation, alert head posture, best hearing frequencies, and hearing range. The calculated endocranial volumes showed that they can be greatly altered by taphonomic processes, altering both total and partial endocranial volumes. Reconstructed endocasts are compatible with different degrees of occupation along the endocranial cavity and some of their characteristics might be useful as phylogenetic characters. The relative brain size of Baurusuchus seems to be small in comparison to modern crocodilians. Sensorial abilities were somewhat similar to modern crocodilians and hearing ranges and best mean frequencies remarkably similar to modern taxa, whereas olfactory ratio values are a little higher. Differing from its modern relatives, Baurusuchus hypothesized alert head posture is compatible with a terrestrial habit.

Quantifying neurovascular canal branching patterns reveals a shared crocodylian arrangement

Highly branched dendritic structures are common in nature and often difficult to quantify and therefore compare. Cranial neurovascular canals, examples of such structures, are osteological correlates for somatosensory systems and have been explored only qualitatively. Adaptations of traditional stream-ordering methods are applied to representative structures derived from computed tomography-scan data. Applying these methods to crocodylian taxa, this clade demonstrates a shared branching pattern and exemplifies the comparative utility of these methods. Additionally, this pattern corresponds with current understanding of crocodylian sensory abilities and behaviors. The method is applicable to many taxa and anatomical structures and provides evidence for morphology-based hypotheses of sensory and physiological evolution.

A 3D ontogenetic atlas of Alligator mississippiensis cranial nerves and their significance for comparative neurology of reptiles

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.

The phylogenetics of Teleosauroidea (Crocodylomorpha, Thalattosuchia) and implications for their ecology and evolution

Teleosauroidea was a clade of ancient crocodylomorphs that were a key element of coastal marine environments during the Jurassic. Despite a 300-year research history and a recent renaissance in the study of their morphology and taxonomy, macroevolutionary studies of teleosauroids are currently limited by our poor understanding of their phylogenetic interrelationships. One major problem is the genus Steneosaurus, a wastebasket taxon recovered as paraphyletic or polyphyletic in phylogenetic analyses. We constructed a newly updated phylogenetic data matrix containing 153 taxa (27 teleosauroids, eight of which were newly added) and 502 characters, which we analysed under maximum parsimony using TNT 1.5 (weighted and unweighted analyses) and Bayesian inference using MrBayes v3.2.6 (standard, gamma and variation). The resulting topologies were then analysed to generate comprehensive higher-level phylogenetic hypotheses of teleosauroids and shed light on species-level interrelationships within the clade. The results from our parsimony and Bayesian analyses are largely consistent. Two large subclades within Teleosauroidea are recovered, and they are morphologically, ecologically and biogeographically distinct from one another. Based on comparative anatomical and phylogenetic results, we propose the following major taxonomic revisions to Teleosauroidea: (1) redefining Teleosauridae; (2) introducing one new family and three new subfamilies; (3) the resurrection of three historical genera; and (4) erecting seven new generic names and one new species name. The phylogeny infers that the Laurasian subclade was more phenotypically plastic overall than the Sub-Boreal-Gondwanan subclade. The proposed phylogeny shows that teleosauroids were more diverse than previously thought, in terms of morphology, ecology, dispersal and abundance, and that they represented some of the most successful crocodylomorphs during the Jurassic.

Mandibular nerve block in juvenile Nile crocodile: a cadaveric study

OBJECTIVE

To develop a technique for performing the mandibular nerve block in Nile crocodiles.

STUDY DESIGN

Experimental cadaveric study.

ANIMALS

A total of 16 juvenile Nile crocodile heads.

METHODS

To study the course of the mandibular nerve, one head was dissected. Computed tomography (CT) examination was performed in two heads to identify useful landmarks. Thereafter, a hypodermic needle was inserted through the external mandibular fenestra of 17 hemimandibles (13 heads), and a mixture of methylene blue and iohexol was injected. Injection volumes were 0.5 (n = 7) and 1.0 mL (n = 10) for hemimandibles < 15 and ≥ 15 cm long, respectively. Iohexol spread and nerve staining with methylene blue were assessed with CT and anatomical dissection, respectively. Data were analysed with one-sample t test or Mann-Whitney U test. Significance was set at p < 0.05.

RESULTS

Both anatomical dissection and imaging confirmed the external mandibular fenestra as a useful anatomical landmark for needle insertion. The CT images acquired after needle positioning confirmed that its tip was located on the medial bony mandibular surface formed by the fusion of the angular and coronoid bones in 100% cases. In all the hemimandibles, the rostrocaudal spread of contrast was > 23 mm. The length of the stained mandibular nerve in the temporal region and of the stained medial branch of the mandibular nerve, as well as the dorsoventral and mediolateral spread of iohexol, was greater in group 1.0 than in group 0.5 (p < 0.001). The caudal spread of iohexol was greater in group 1.0 than in group 0.5 (p = 0.01).

CONCLUSIONS AND CLINICAL RELEVANCE

The technique developed in this study is feasible. Both injection volumes resulted in staining of the mandibular nerve. The spread of contrast in the anatomical region of interest may result in successful sensory block.

Neuroanatomy of the spinosaurid Irritator challengeri (Dinosauria: Theropoda) indicates potential adaptations for piscivory

Spinosauridae, a theropod group characterized by elongated snouts, conical teeth, enlarged forelimbs, and often elongated neural spines, show evidence for semiaquatic adaptations and piscivory. It is currently debated if these animals represent terrestrial carnivores with adaptations for a piscivorous diet, or if they largely lived and foraged in aquatic habitats. The holotype of Irritator challengeri, a nearly complete skull from the late Early Cretaceous Santana Formation of northeastern Brazil, includes one of the few preserved spinosaurid braincases and can provide insights into neuroanatomical structures that might be expected to reflect ecological affinities. We generated digital models of the neuroanatomical cavities within the braincase, using computer tomography (CT) data. The cranial endocast of Irritator is generally similar to that of other non-maniraptoriform theropods, with weakly developed distinctions of hindbrain and midbrain features, relatively pronounced cranial flexures and relatively long olfactory tracts. The endosseous labyrinth has a long anterior semicircular canal, a posteriorly inclined common crus and a very large floccular recess fills the area between the semicircular canals. These features indicate that Irritator had the ability for fast and well-controlled pitch-down head movements. The skull table and lateral semicircular canal plane are strongly angled to one another, suggesting a downward angling of approximately 45° of the snout, which reduces interference of the snout with the field of vision of Irritator. These neuroanatomical features are consistent with fast, downward snatching movements in the act of predation, such as are needed for piscivory.

A rostral neurovascular system in the mosasaur Taniwhasaurus antarcticus

Mosasaurs were a cosmopolitan group of marine squamate reptiles that lived during the Late Cretaceous period. Tylosaurinae mosasaurs were characterized for having an edentulous rostrum anterior to the premaxillary teeth. External morphology of the snout of the tylosaurine Taniwhasaurus antarcticus from the Upper Cretaceous beds at James Ross Island (Antarctic Peninsula) shows a complex anatomy with diverse large foramina and bone sculpture. A computed tomography scan of the Taniwhasaurus rostrum revealed a complex internal neurovascular system of branched channels in the anteriormost part of the snout. Systems like this are present in extant aquatic vertebrates such as cetaceans and crocodiles to aid them with prey detection, and are inferred to have functioned in a similar manner for several extinct reptile clades such as plesiosaurs and ichthyosaurs. Thus, it is probable that Taniwhasaurus also was able to detect prey with an enhanced neural system located in its rostrum. This condition may be more widespread than previously thought among mosasaurs and other marine reptiles.

Anatomy and Ontogeny of the Mandibular Symphysis in Alligator mississippiensis

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 of Alligator mississippiensis using 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 of in ovo development. 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 the Alligator mandibular 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.

New Reconstruction of Cranial Musculature in Ornithischian Dinosaurs: Implications for Feeding Mechanisms and Buccal Anatomy

The charismatic and diverse ornithischian dinosaurs exhibited some of the most extreme examples of cranial anatomy, inspiring decades of investigation into their muscular anatomy. Current ornithischian jaw muscle reconstructions, although parsimonious, pose concerns of small adductor muscles and caudally displaced insertions relative to mandibular proportions. Here, craniomandibular material of ornithischian genera spanning all subclades is reexamined for osteological correlates indicative of intracranial and oral soft tissues. M. adductor mandibulae externus (mAME) has traditionally been reconstructed as solely inserting along the caudal margin of the coronoid process for jaw closure. Here, a new mAME reconstruction is proposed in derived ornithischians, with the superficial-most mAME layer reconstructed as a rostrolabial expansion of muscle, exiting the cranium rostroventrally beneath a unique, laterally flaring jugal and inserting along the lateral surface of the coronoid process and its rostrally extending, shelf-like labial dentary ridge (LDR). Through previous dental microwear and morphological studies, ceratopsians, hadrosaurids, and ankylosaurs are known to have implemented a major palinal feeding component in their jaw motions, unlike other primarily basal ornithischians. This rostral fan-like extension of muscle in these derived clades would create a greater mandibular support system and mechanical advantage along the labial margin of the jaw, cradling the entire mandible while lifting it up into occlusion and retracting it. In hadrosaurids and ankylosaurs, this rostrolabially expanding muscle also acts in medial rotation of the dentaries about their long axes. With these new reconstructions, the notion of a novel, unparsimonious "cheek" muscle is rejected, with further discussion of plausible buccal soft tissues. Anat Rec, 2018. © 2018 Wiley Periodicals, Inc. Anat Rec, 303:347-362, 2020. © 2018 American Association for Anatomy.

Assessing Soft-Tissue Shrinkage Estimates in Museum Specimens Imaged With Diffusible Iodine-Based Contrast-Enhanced Computed Tomography (diceCT)

The increased accessibility of soft-tissue data through diffusible iodine-based contrast-enhanced computed tomography (diceCT) enables comparative biologists to increase the taxonomic breadth of their studies with museum specimens. However, it is still unclear how soft-tissue measurements from preserved specimens reflect values from freshly collected specimens and whether diceCT preparation may affect these measurements. Here, we document and evaluate the accuracy of diceCT in museum specimens based on the soft-tissue reconstructions of brains and eyes of five bats. Based on proxies, both brains and eyes were roughly 60% of the estimated original sizes when first imaged. However, these structures did not further shrink significantly over a 4-week staining interval, and 1 week in 2.5% iodine-based solution yielded sufficient contrast for differentiating among soft-tissues. Compared to six "fresh" bat specimens imaged shortly after field collection (not fixed in ethanol), the museum specimens had significantly lower relative volumes of the eyes and brains. Variation in field preparation techniques and conditions, and long-term storage in ethanol may be the primary causes of shrinkage in museum specimens rather than diceCT staining methodology. Identifying reliable tissue-specific correction factors to adjust for the shrinkage now documented in museum specimens requires future work with larger samples.

Molecular anatomy of the alligator dorsal telencephalon

The evolutionary relationships of the mammalian neocortex and avian dorsal telencephalon (DT) nuclei have been debated for more than a century. Despite their central importance to this debate, nonavian reptiles remain underexplored with modern molecular techniques. Reptile studies harbor great potential for understanding the changes in DT organization that occurred in the early evolution of amniotes. They may also help clarify the specializations in the avian DT, which comprises a massive, cell-dense dorsal ventricular ridge (DVR) and a nuclear dorsal-most structure, the Wulst. Crocodilians are phylogenetically and anatomically attractive for DT comparative studies: they are the closest living relatives of birds and have a strikingly bird-like DVR, but they also possess a highly differentiated reptile cerebral cortex. We studied the DT of the American alligator, Alligator mississippiensis, at late embryonic stages with a panel of molecular marker genes. Gene expression and cytoarchitectonic analyses identified clear homologs of all major avian DVR subdivisions including a mesopallium, an extensive nidopallium with primary sensory input territories, and an arcopallium. The alligator medial cortex is divided into three components that resemble the mammalian dentate gyrus, CA fields, and subiculum in gene expression and topography. The alligator dorsal cortex contains putative homologs of neocortical input, output, and intratelencephalic projection neurons and, most notably, these are organized into sublayers similar to mammalian neocortical layers. Our findings on the molecular anatomy of the crocodilian DT are summarized in an atlas of the alligator telencephalon.

Comparative morphology of snake (Squamata) endocasts: evidence of phylogenetic and ecological signals

Brain endocasts obtained from computed tomography (CT) are now widely used in the field of comparative neuroanatomy. They provide an overview of the morphology of the brain and associated tissues located in the cranial cavity. Through anatomical comparisons between species, insights on the senses, the behavior, and the lifestyle can be gained. Although there are many studies dealing with mammal and bird endocasts, those performed on the brain endocasts of squamates are comparatively rare, thus limiting our understanding of their morphological variability and interpretations. Here, we provide the first comparative study of snake brain endocasts in order to bring new information about the morphology of these structures. Additionally, we test if the snake brain endocast encompasses a phylogenetic and/or an ecological signal. For this purpose, the digital endocasts of 45 snake specimens, including a wide diversity in terms of phylogeny and ecology, were digitized using CT, and compared both qualitatively and quantitatively. Snake endocasts exhibit a great variability. The different methods performed from descriptive characters, linear measurements and the outline curves provided complementary information. All these methods have shown that the shape of the snake brain endocast contains, as in mammals and birds, a phylogenetic signal but also an ecological one. Although phylogenetically related taxa share several similarities between each other, the brain endocast morphology reflects some notable ecological trends: e.g. (i) fossorial species possess both reduced optic tectum and pituitary gland; (ii) both fossorial and marine species have cerebral hemispheres poorly developed laterally; (iii) cerebral hemispheres and optic tectum are more developed in arboreal and terrestrial species.

Archosauriform endocranial morphology and osteological evidence for semiaquatic sensory adaptations in phytosaurs

The examination of endocranial data of archosauriforms has led to advances on the evolution of body size, nerve pathways, and sensory abilities. However, much of that research has focused on bird-line archosaurs, resulting in a skewed view of Archosauria. Phytosauria, a hypothesized sister taxon to or early-branching member of Archosauria, provides a potential outgroup condition. Most previous phytosaur endocranial studies were executed without the use of modern technology and focused on derived members of Phytosauria. We present a comparative CT examination of the internal cranial anatomy of Wannia scurriensis, the most basal known parasuchid phytosaur. Wannia scurriensis shows some overall similarity with extant crocodylians and derived phytosaurs in general endocranial shape, a large hypophyseal fossa, and trigeminal (CN V) innervation, but as a whole, the endocast has noticeable differences to crocodylians and other phytosaurs. The pineal region is expanded dorsally as in other phytosaurs but also laterally (previously unrecognized). CN V exits the pons in a more dorsal position than in Parasuchus hislopi, Machaeroprosopus mccauleyi, or Smilosuchus gregorii. Wannia scurriensis also exhibits a larger hypophyseal fossa relative to brain size than observed in P. hislopi or S. gregorii, which may indicate more rapid growth. The well-preserved semicircular canals have lateral canals that are angled more anteroventrally than in derived phytosaurs. Extensive facial innervation from the large CN V indicates increased rostrum sensitivity and mechanoreceptive abilities as in Alligator mississippiensis. These endocranial similarities among phytosaurs and with Alligator indicate conserved ecological and functional results of an aquatic lifestyle, and highlight a need for further exploration of endocranial anatomy among Archosauriformes.

The biochronology and palaeobiogeography of Baru (Crocodylia: Mekosuchinae) based on new specimens from the Northern Territory and Queensland, Australia

New records of the Oligo–Miocene mekosuchine crocodylian, Baru, from Queensland and the Northern Territory are described. Baru wickeni and Baru darrowi are accepted as valid species in the genus and their diagnoses are revised. Both species are present in Queensland and the Northern Territory but are restricted in time, with B. wickeni known from the late Oligocene and B. darrowi from the middle Miocene. The broad geographic distributions and restricted time spans of these species indicate that this genus is useful for biochronology. The record of B. wickeni from the Pwerte Marnte Marnte Local Fauna in the Northern Territory establishes that the species inhabited the north-western margin of the Lake Eyre Basin (LEB) drainage system. More southerly Oligo–Miocene sites in the LEB contain only one crocodylian species, Australosuchus clarkae. The Pwerte Marnte Marnte occurrence of B. wickeni indicates that the separation of Baru and Australosuchus did not correspond with the boundaries of drainage basins and that palaeolatitude was a more likely segregating factor.

Complex neuroanatomy in the rostrum of the Isle of Wight theropod Neovenator salerii

The discovery of large, complex, internal canals within the rostra of fossil reptiles has been linked with an enhanced tactile function utilised in an aquatic context, so far in pliosaurids, the Cretaceous theropod Spinosaurus, and the related spinosaurid Baryonyx. Here, we report the presence of a complex network of large, laterally situated, anastomosing channels, discovered via micro-focus computed tomography (μCT), in the premaxilla and maxilla of Neovenator, a mid-sized allosauroid theropod from the Early Cretaceous of the UK. We identify these channels as neurovascular canals, that include parts of the trigeminal nerve; many branches of this complex terminate on the external surfaces of the premaxilla and maxilla where they are associated with foramina. Neovenator is universally regarded as a ‘typical’ terrestrial, predatory theropod, and there are no indications that it was aquatic, amphibious, or unusual with respect to the ecology or behaviour predicted for allosauroids. Accordingly, we propose that enlarged neurovascular facial canals shouldn’t be used to exclusively support a model of aquatic foraging in theropods and argue instead that an enhanced degree of facial sensitivity may have been linked with any number of alternative behavioural adaptations, among them defleshing behaviour, nest selection/maintenance or social interaction.

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.

Neural and endocranial anatomy of Triassic phytosaurian reptiles and convergence with fossil and modern crocodylians

Phytosaurs are a clade of large, carnivorous pseudosuchian archosaurs from the Late Triassic with a near cosmopolitan distribution. Their superficial resemblance to longirostrine (long-snouted) crocodylians, such as gharials, has often been used in the past to infer ecological and behavioural convergence between the two groups. Although more than thirty species of phytosaur are currently recognised, little is known about the endocranial anatomy of this clade. Here, we describe the endocranial anatomy (including the brain, inner ear, neurovascular structures and sinus systems) of the two non-mystriosuchine phytosaurs Parasuchus angustifrons (=“Paleorhinus angustifrons”) and Ebrachosuchus neukami from the Late Triassic of Germany based on digital reconstructions. Results show that the endocasts of both taxa are very similar to each other in their rostrocaudally elongate morphology, with long olfactory tracts, weakly demarcated cerebral regions and dorsoventrally short endosseous labyrinths. In addition, several sinuses, including large antorbital sinuses and prominent dural venous sinuses, were reconstructed. Comparisons with the endocranial anatomy of derived phytosaurs indicate that Phytosauria is united by the presence of elongate olfactory tracts and longitudinally arranged brain architecture—characters which are also shared with Crocodyliformes. However, a substantial morphological variability is observed in the cephalic and pontine flexure and the presence of a pineal organ across the different phytosaur species. These results suggest that the endocranial anatomy in Phytosauria generally follows a plesiomorphic pattern, with moderate variation within the clade likely resulting from divergent sensory and behavioural adaptations.

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.

A new species of Allodaposuchus (Eusuchia, Crocodylia) from the Maastrichtian (Late Cretaceous) of Spain: phylogenetic and paleobiological implications

Background. The Late Cretaceous is a keystone period to understand the origin and early radiation of Crocodylia, the group containing all extant lineages of crocodilians. Among the taxa described from the latest Cretaceous of Europe, the genus Allodaposuchus is one of the most common but also one of the most controversial. However, because of its fragmentary record, several issues regarding its phylogenetic emplacement and its ecology remain unsolved or unknown. The discovery of a single specimen attributed to Allodaposuchus, represented by both cranial and postcranial remains, from the Casa Fabà site (Tremp Basin, NE Spain) in the lower red unit of the Tremp Fm. (early Maastrichtian, Late Cretaceous) offers a unique opportunity to deepen in the phylogenetic relationships of the group and its ecological features.

Methods. The specimen is described in detail, and CT scan of the skull is performed in order to study the endocranial morphology as well as paratympanic sinuses configuration. In addition, myological and phylogenetic analyses are also carried out on the specimen for to shed light in ecological and phylogenetic issues, respectively.

Results. The specimen described herein represents a new species, Allodaposuchus hulki sp. nov., closely related to the Romanian A. precedens. The CT scan of the skull revealed an unexpected paratympanic sinuses configuration. Allosaposuchus hulki exhibits an “anterodorsal tympanic sinus” not observed in any other extant or extinct crocodilian. The caudal tympanic recesses are extremely enlarged, and the expanded quadratic sinus seems to be connected to the middle-ear channel. Phylogenetic analyses confirm the emplacement of the informal taxonomic group ‘Allodaposuchia’ at the base of Crocodylia, being considered the sister group of Borealosuchus and Planocraniidae.

Discussion. Although this is a preliminary hypothesis, the unique paratympanic configuration displayed by A. hulki suggests that it could possess a high-specialized auditory system. Further, the large cranial cavities could help to reduce the weight of the cranium. Concerning the postcranial skeleton, Allodaposuchus hulki shows massive and robust vertebrae and forelimb bones, suggesting it could have a bulky body. The myological study performed on the anterior limb elements supports this interpretation. In addition, several bone and muscular features seem to point at a semi-erected position of the forelimbs during terrestrial locomotion. Taking all the above results into consideration, it seems plausible to suggest that A. hulki could conduct large incursions out of the water and have a semi-terrestrial lifestyle.

Are vibrissae viable sensory structures for prey capture in northern elephant seals, Mirounga angustirostris?

Little is known about the tactics northern elephant seals (NES) use to capture prey due to the difficulties in observing these animals underwater. NES forage on vertically migrating prey at depths >500 m during day and at night where light levels are negligible. Although NES have increased visual sensitivity in deep water, vision is likely a limited sensory modality. Still images of NES foraging show that the mystacial vibrissae are protracted before prey capture. As a representative phocid, harbor seals can follow hydrodynamic trails using their vibrissae, and are highly sensitive to water velocity changes. In lieu of performance data, vibrissal innervation can be used as a proxy for sensitivity. Although comparative data are few, seals average 1,000 to 1,600 axons per vibrissa (five to eight times more than terrestrial mammals). To test the hypothesis that NES have increased innervation as other pinnipeds, vibrissae from the ventral-caudal mystacial field from nine individuals were sectioned and stained for microstructure (trichrome) and innervation (Bodian silver stain). Follicles were tripartite and consisted of lower and upper cavernous sinuses separated by a ring sinus containing an asymmetrical ringwulst. The deep vibrissal nerve penetrated the follicular capsule at the base, branched into several bundles, and coursed through the lower cavernous sinus to the ring sinus. Axons in the ring sinus terminated in the ringwulst and along the inner conical body. NES averaged 1,584 axons per vibrissa. The results add to the growing body of evidence that phocids, and perhaps all pinnipeds, possess highly sensitive mystacial vibrissae that detect prey.

Innervation patterns of sea otter (Enhydra lutris) mystacial follicle-sinus complexes

Sea otters (Enhydra lutris) are the most recent group of mammals to return to the sea, and may exemplify divergent somatosensory tactile systems among mammals. Therefore, we quantified the mystacial vibrissal array of sea otters and histologically processed follicle-sinus complexes (F - SCs) to test the hypotheses that the number of myelinated axons per F - SC is greater than that found for terrestrial mammalian vibrissae and that their organization and microstructure converge with those of pinniped vibrissae. A mean of 120.5 vibrissae were arranged rostrally on a broad, blunt muzzle in 7-8 rows and 9-13 columns. The F-SCs of sea otters are tripartite in their organization and similar in microstructure to pinnipeds rather than terrestrial species. Each F-SC was innervated by a mean 1339 ± 408.3 axons. Innervation to the entire mystacial vibrissal array was estimated at 161,313 axons. Our data support the hypothesis that the disproportionate expansion of the coronal gyrus in somatosensory cortex of sea otters is related to the high innervation investment of the mystacial vibrissal array, and that quantifying innervation investment is a good proxy for tactile sensitivity. We predict that the tactile performance of sea otter mystacial vibrissae is comparable to that of harbor seals, sea lions and walruses.