Lokiceratops rangiformis gen. et sp. nov. (Ceratopsidae: Centrosaurinae) from the Campanian Judith River Formation of Montana reveals rapid regional radiations and extreme endemism within centrosaurine dinosaurs

The Late Cretaceous of western North America supported diverse dinosaur assemblages, though understanding patterns of dinosaur diversity, evolution, and extinction has been historically limited by unequal geographic and temporal sampling. In particular, the existence and extent of faunal endemism along the eastern coastal plain of Laramidia continues to generate debate, and finer scale regional patterns remain elusive. Here, we report a new centrosaurine ceratopsid, Lokiceratops rangiformis, from the lower portion of the McClelland Ferry Member of the Judith River Formation in the Kennedy Coulee region along the Canada-USA border. Dinosaurs from the same small geographic region, and from nearby, stratigraphically equivalent horizons of the lower Oldman Formation in Canada, reveal unprecedented ceratopsid richness, with four sympatric centrosaurine taxa and one chasmosaurine taxon. Phylogenetic results show that Lokiceratops, together with Albertaceratops and Medusaceratops, was part of a clade restricted to a small portion of northern Laramidia approximately 78 million years ago. This group, Albertaceratopsini, was one of multiple centrosaurine clades to undergo geographically restricted radiations, with Nasutuceratopsini restricted to the south and Centrosaurini and Pachyrostra restricted to the north. High regional endemism in centrosaurs is associated with, and may have been driven by, high speciation rates and diversity, with competition between dinosaurs limiting their geographic range. High speciation rates may in turn have been driven in part by sexual selection or latitudinally uneven climatic and floral gradients. The high endemism seen in centrosaurines and other dinosaurs implies that dinosaur diversity is underestimated and contrasts with the large geographic ranges seen in most extant mammalian megafauna.

Cranial and postcranial anatomy of a juvenile baurusuchid (Notosuchia, Crocodylomorpha) and the taxonomical implications of ontogeny

Baurusuchidae comprises a clade of top-tier terrestrial predators and are among the most abundant crocodyliforms found in the Adamantina Formation, Bauru Basin, Brazil (Campanian-Maastrichtian). Here, we provide a detailed description of the cranial and postcranial osteology and myology of the most complete juvenile baurusuchid found to date. Although the preservation of juvenile individuals is somewhat rare, previously reported occurrences of baurusuchid egg clutches, a yearling individual, and larger, but skeletally immature specimens, comprise a unique opportunity to track anatomical changes throughout their ontogenetic series. Its cranial anatomy was resolved with the aid of a three-dimensional model generated by the acquisition of computed tomography data, and its inferred adductor mandibular musculature was compared to that of mature specimens in order to assess possible ontogenetic shifts. A subsequent phylogenetic analysis included the scoring of Gondwanasuchus scabrosus, the smallest baurusuchid species known to date, to evaluate its phylogenetic relations relative to a known juvenile. We find considerable differences between juveniles and adults concerning skull ornamentation and muscle development, which might indicate ontogenetic niche partitioning, and also anatomical and phylogenetic evidence that G. scabrosus corresponds to a young semaphoront lacking mature cranial features.

An extinct deep-snouted Alligator species from the Quaternary of Thailand and comments on the evolution of crushing dentition in alligatorids

Fossil Alligator remains from Asia are critical for tracing the enigmatic evolutionary origin of the Chinese alligator, Alligator sinensis, the only living representative of Alligatoridae outside the New World. The Asian fossil record is extremely scarce and it remains unknown whether A. sinensis is an anagenetic lineage or alternatively, extinct divergent species were once present. We provide a detailed comparative description of a morphologically highly distinct Alligator skull from the Quaternary of Thailand. Several autapomorphic characters warrant the designation of a new species. Alligator munensis sp. nov. shares obvious derived features with A. sinensis but autapomorphies imply a cladogenetic split, possibly driven by the uplift of the southeastern Tibetan plateau. The presence of enlarged posterior alveoli in Alligator munensis is most consistent with a reversal to the alligatorine ancestral condition of having crushing dentition, a morphology strikingly absent among living alligatorids. Crushing dentition has been previously considered to indicate an ecological specialisation in early alligatorines that was subsequently lost in Alligator spp. However, we argue that there is yet no evidence for crushing dentition reflecting an adaptation for a narrower niche, while opportunistic feeding, including seasonal utilisation of hard-shelled preys, is a reasonable alternative interpretation of its function.

How Triceratops got its face: An update on the functional evolution of the ceratopsian head

Ceratopsian dinosaurs arguably show some of the most extravagant external cranial morphology across all Dinosauria. For over a century, ceratopsian dinosaurs have inspired a multitude of cranial functional studies as more discoveries continued to depict a larger picture of the enormous diversity of these animals. The iconic horns and bony frills in many ceratopsians portray a plethora of shapes, sizes, and arrangements across taxa, and their overall feeding apparatus show the development of unique specializations previously unseen in large herbivores. Here, I give a brief updated review of the many functional studies investigating different aspects of the ceratopsian head. The functional role of the horns and bony frills is explored, with an overview of studies investigating their potential for weaponization or defense in either intraspecific or anti-predatory combat, among other things. A review of studies pertaining to the ceratopsian feeding apparatus is also presented here, with analyses of studies exploring their beak and snout morphology, dentition and tooth wear, cranial musculature with associated skull anatomy, and feeding biomechanics.

Cranial anatomy of the mekosuchine crocodylian Trilophosuchus rackhami Willis, 1993

One of the best-preserved crocodylian fossil specimens from the Cenozoic of Australia is the holotype of the mekosuchine Trilophosuchus rackhami, from the middle Miocene (13.56 ± 0.67 Ma) Ringtail Site at Riversleigh, northwestern Queensland. Although lacking most of the snout, the holotype skull of T. rackhami (QMF16856) has an exceptionally well-preserved cranium. Micro-CT scanning of the holotype has allowed for all the preserved cranial bones to be digitally disarticulated, facilitating an unprecedented insight into the cranial anatomy of not just T. rackhami, but any mekosuchine. Trilophosuchus rackhami was a small-bodied crocodylian and one of the most morphologically distinct mekosuchines, characterized by a unique combination of cranial characteristics several of which are exclusive to the species. Fossil material that is definitively referrable to the species T. rackhami is currently known solely from the middle Miocene Ringtail Site. However, an isolated parietal from Hiatus Site at Riversleigh demonstrates that Trilophosuchus also occurred during the late Oligocene (~25 Ma), extending the range of the genus by more than 10 million years. The new description of T. rackhami also allowed for a reevaluation of its phylogenetic relationships. Our results reaffirm the placement of T. rackhami as a member of Mekosuchinae within the subclade Mekosuchini. In all analyses, Mekosuchinae was consistently found to be monophyletic and part of the larger crocodylian clade Longirostres. However, the assignment of Mekosuchinae as a subset of Crocodylidae is brought into question, suggesting that the status of Mekosuchinae as a subfamily should be reconsidered.

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.

The effects of skull flattening on suchian jaw muscle evolution

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.

Paratympanic sinuses in juvenile Alligator

Crocodylia has an extensive epithelial pneumatic space in the middle ear, paratympanic sinus system. Although fossil and extant crocodylian paratympanic sinus systems have been studied recently using the computed tomography (CT) and three-dimensional (3D) reconstruction data, due to the soft tissue nature of the pneumatic system and presence of its surrounding soft tissue structures, some boundaries, and definitions of each extension remain ambiguous. We describe the comprehensive paratympanic sinus system in posthatched alligator using soft tissue enhanced CT data with 3D reconstructions. The data are compared to the available data to discuss the ontogenetic pattern in alligator. We introduce further divided entities of the pneumatic system based on their associated bony and soft tissue structures and epithelial membrane and clarify the pneumatic terminologies. We then re-visit the potential homology of the paratympanic sinus in Archosauria. Epithelial boundaries of the ventral portion of the pneumatic system from the histological data suggest that the dual origin of the basioccipital diverticulum derived from the tympanic sinus and basicranial diverticulum medially. The presence of the epithelial boundary and pneumatic changes in ontogeny suggests that the middle ear may function differently in developmental stages. Lastly, a morphogenetic tree is constructed to help future work of comparative developmental studies of the paratympanic sinus system between crocodiles and birds.

Cranial muscle reconstructions quantify adaptation for high bite forces in Oviraptorosauria

Oviraptorosaurians are an unusual and probably herbivorous group of theropod dinosaurs that evolved pneumatised crania with robust, toothless jaws, apparently adapted for producing a strong bite. Using 3D retrodeformed skull models of oviraptorid oviraptorosaurians Citipati, Khaan, and Conchoraptor, along with the earliest diverging oviraptorosaurian, Incisivosaurus, we digitally reconstruct jaw adductor musculature and estimate bite force to investigate cranial function in each species. We model muscle length change during jaw opening to constrain optimal and maximum gape angles. Results demonstrate oviraptorids were capable of much stronger bite forces than herbivorous theropods among Ornithomimosauria and Therizinosauria, relative to body mass and absolutely. Increased bite forces in oviraptorid oviraptorosaurians compared to the earliest diverging oviraptorosaurian result from expanded muscular space and different cranial geometry, not changes in muscular arrangement. Estimated optimal and maximum possible gapes are much smaller than published estimates for carnivorous theropods, being more similar to the herbivorous therizinosaurian theropod Erlikosaurus and modern birds. Restrictive gape and high bite force may represent adaptation towards exploiting tough vegetation, suggesting cranial function and dietary habits differed between oviraptorids and other herbivorous theropods. Differences in the relative strength of jaw adductor muscles between co-occurring oviraptorids may be a factor in niche partitioning, alongside body size.

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

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

The contribution of ontogenetic growth trajectories on the divergent evolution of the crocodylian skull table

To explore shape variability among crocodylian skull tables, an analysis using geometric morphometric methods is conducted with the inclusion of extant and fossil taxa. Skull tables are variable and the differences likely play a role in hydrodynamics, species recognition, and biomechanical adaptations. Comparisons of allometric change within taxa are explored revealing that adults significantly diverge from juvenile skull table morphologies in most species and these changes happen in a stereotyped way. In all analyses, adults of the smallest extant taxa plot alongside the juveniles of related taxa and heterochrony may explain the maintenance of these morphologies into adulthood. When landmarks representing the supratemporal fenestrae are included, longirostrine taxa are broadly separated from one another due to variation in the size of the supratemporal fenestrae. The hypotheses of previous studies suggesting that the size of the supratemporal fenestrae is influenced by snout length-with longer snouts corresponding to larger fenestrae-must be re-evaluated. Although species of the crocodyloids Tomistoma and Euthecodon approach or exceed the length of the snout in gavialoids, their supratemporal fenestrae are proportionally smaller-this suggests a phylogenetic constraint in crocodyloids regardless of snout length.

Braincase anatomy of extant Crocodylia, with new insights into the development and evolution of the neurocranium in crocodylomorphs

Present-day crocodylians exhibit a remarkably akinetic skull with a highly modified braincase. We present a comprehensive description of the neurocranial osteology of extant crocodylians, with notes on the development of individual skeletal elements and a discussion of the terminology used for this project. The quadrate is rigidly fixed by multiple contacts with most braincase elements. The parabasisphenoid is sutured to the pterygoids (palate) and the quadrate (suspensorium); as a result, the basipterygoid joint is completely immobilized. The prootic is reduced and externally concealed by the quadrate. It has a verticalized buttress that participates in the canal for the temporal vasculature. The ventrolateral processes of the otoccipitals completely cover the posteroventral region of the braincase, enclose the occipital nerves and blood vessels in narrow bony canals and also provide additional sutural contacts between the braincase elements and further consolidate the posterior portion of the crocodylian skull. The otic capsule of crocodylians has a characteristic cochlear prominence that corresponds to the lateral route of the perilymphatic sac. Complex internal structures of the otoccipital (extracapsular buttress) additionally arrange the neurovascular structures of the periotic space of the cranium. Most of the braincase elements of crocodylians are excavated by the paratympanic pneumatic sinuses. The braincase in various extant crocodylians has an overall similar structure with some consistent variation between taxa. Several newly observed features of the braincase are present in Gavialis gangeticus and extant members of Crocodylidae to the exclusion of alligatorids: the reduced exposure of the prootic buttress on the floor of the temporal canal, the sagittal nuchal crest of the supraoccipital projecting posteriorly beyond the postoccipital processes and the reduced paratympanic pneumaticity. The most distinctive features of the crocodylian braincase (fixed quadrate and basipterygoid joint, consolidated occiput) evolved relatively rapidly at the base of Crocodylomorpha and accompanied the initial diversification of this clade during the Late Triassic and Early Jurassic. We hypothesize that profound rearrangements in the individual development of the braincases of basal crocodylomorphs underlie these rapid evolutionary modifications. These rearrangements are likely reflected in the embryonic development of extant crocodylians and include the involvement of neomorphic dermal anlagen in different portions of the developing chondrocranium, the extensive ossification of the palatoquadrate cartilage as a single expanded quadrate and the anteromedial inclination of the quadrate.

Reevaluation of the cranial osteology and phylogenetic position of the early crocodyliform Eopneumatosuchus colberti, with an emphasis on its endocranial anatomy

Eopneumatosuchus colberti Crompton and Smith, 1980, known from a single partial skull, is an enigmatic crocodylomorph from the Lower Jurassic Kayenta Formation. In spite of its unique morphology, an exceptionally pneumatic braincase, and presence during a critical time period of crocodylomorph evolution, relatively little is known about this taxon. Here, we redescribe the external cranial morphology of E. colberti, present novel information on its endocranial anatomy, evaluate its phylogenetic position among early crocodylomorphs, and seek to better characterize its ecology. Our examination clarifies key aspects of cranial suture paths and braincase anatomy. Comparisons with related taxa (e.g., Protosuchus haughtoni) demonstrate that extreme pneumaticity of the braincase may be more widespread in protosuchids than previously appreciated. Computed tomography scans reveal an endocranial morphology that resembles that of other early crocodylomorphs, in particular the noncrocodyliform crocodylomorph Almadasuchus figarii. There are, however, key differences in olfactory bulb and cerebral hemisphere morphology, which demonstrate the endocranium of crocodylomorphs is not as conserved as previously hypothesized. Our phylogenetic analysis recovers E. colberti as a close relative of Protosuchus richardsoni and Edentosuchus tienshanensis, contrasting with previous hypotheses of a sister group relationship with Thalattosuchia. Previous work suggested the inner ear has some similarities to semi-aquatic crocodyliforms, but the phylogenetic placement of E. colberti among protosuchids with a terrestrial postcranial skeletal morphology complicates paleoecological interpretation.

Reconstruction of the adductor chamber and predicted bite force in pterodactyloids (Pterosauria)

The reconstruction of jaw muscles is critical in establishing potential cranial functions; however, myological studies of extinct groups that have no descendants are difficult to perform and test. This is particularly true for pterosaurs, a group of extinct flying reptiles that present a plethora of cranial morphologies, suggesting different functions and feeding habits. Here we present a first attempt to reconstruct the adductor musculature of the pterodactyloid skull in detail, using osteological correlates and the extant phylogenetic bracketing method. Using these reconstructions, we estimate bite force for nine selected species and investigate implications for potential dietary habits.

3D skull modelling and description of a new baurusuchid (Crocodyliformes, Mesoeucrocodylia) from the Late Cretaceous (Bauru Basin) of Brazil

Baurusuchidae is one of the most diverse groups of South American notosuchians, unambiguously recorded in Late Cretaceous deposits of Brazil and Argentina. The group is characterized by a reduced tooth formula, a lateromedially compressed rostrum, and a verticalized quadrate, representing one of the top predators of their faunas. Historically, skull morphology is the most employed tool to investigate the relationships of baurusuchids, as most of the species have been primarily based on cranial remains. The present study describes a new baurusuchid species from the Bauru Basin of Brazil, based on the first tridimensional digital reconstruction of individualized skull bones for Notosuchia, and discusses its phylogenetic position within the group. The new species differs from all the other known baurusuchids by a depression on the posterior portion of the nasal bearing a crest, an infraorbital crest of the jugal that extends until the anterior margin of the lacrimal, the dorsal surface of the frontal lacking a longitudinal crest or depression, and the lateral convexity of the squamosal prongs participating in the occipital wall. The new taxon is consistently positioned as sister to the remaining baurusuchines, with Aplestosuchus sordidus and Stratiotosuchus maxhechti, as successive sister-taxa to a monophyletic Baurusuchus (Ba. albertoi, Ba. Salgadoensis, and Ba. pachecoi). Our updated phylogenetic analysis helps to differentiate the two major Baurusuchidae lineages, Baurusuchinae and Pissarrachampsinae. Yet, the new species shares morphological features with both groups, suggesting the occurrence of "Zones of Variability" in the radiation of Baurusuchidae.

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.

Cranial and endocranial anatomy of a three-dimensionally preserved teleosauroid thalattosuchian skull

Thalattosuchians represent one of the several independent transitions into the marine realm among crocodylomorphs. The extent of their aquatic adaptations ranges from the semiaquatic teleosauroids, superficially resembling extant gharials, to the almost cetacean-like pelagic metriorhynchids. Understanding the suite of osteological, physiological, and sensory changes that accompanied this major transition has received increased attention, but is somewhat hindered by a dearth of complete three-dimensionally preserved crania. Here, we describe the cranial and endocranial anatomy of a well-preserved three-dimensional specimen of Macrospondylus bollensis from the Toarcian of Yorkshire, UK. The trigeminal fossa contains two similar-sized openings separated by a thin lamina of prootic, a configuration that appears unique to a subset of teleosauroids. Macrospondylus bollensis resembles other thalattosuchians in having pyramidal semicircular canals with elongate cochlear ducts, enlarged carotid canals leading to an enlarged pituitary fossa, enlarged orbital arteries, enlarged endocranial venous sinuses, reduced pharyngotympanic sinuses, and a relatively straight brain with a hemispherical cerebral expansion. We describe for the first time the olfactory region and paranasal sinuses of a teleosauroid. A relatively large olfactory region suggests greater capacity for airborne olfaction in teleosauroids than in the more aquatically adapted metriorhynchoids. Additionally, slight swellings in the olfactory region suggest the presence of small salt glands of lower secretory capacity than those of metriorhynchoids. The presence of osteological correlates for salt glands in a teleosauroid corroborates previous hypotheses that these glands originated in the common ancestor of Thalattosuchia, facilitating their rapid radiation into the marine realm.

First record of a tomistomine crocodylian from Australia

Based on the known fossil record, the majority of crocodylians from the Cenozoic Era of Australia are referred to the extinct clade Mekosuchinae. The only extant crocodylians in Australia are two species of Crocodylus. Hence, the viewpoint that Crocodylus and mekosuchines have been the only crocodylians inhabiting Australia during the Cenozoic has remained largely undisputed. Herein we describe Australia's first tomistomine crocodylian, Gunggamarandu maunala gen. et sp. nov., thus challenging the notion of mekosuchine dominance during most of the Cenozoic. The holotype specimen of Gunggamarandu maunala derives from the Pliocene or Pleistocene of south-eastern Queensland, marking the southern-most global record for Tomistominae. Gunggamarandu maunala is known from a large, incomplete cranium that possesses a unique combination of features that distinguishes it from other crocodylians. Phylogenetic analyses place Gunggamarandu in a basal position within Tomistominae, specifically as a sister taxon to Dollosuchoides from the Eocene of Europe. These results hint at a potential ghost lineage between European and Australian tomistomines going back more than 50 million years. The cranial proportions of the Gunggamarandu maunala holotype specimen indicate it is the largest crocodyliform yet discovered from Australia.

Morphology of the temporal skull region in tetrapods: research history, functional explanations, and a new comprehensive classification scheme

The morphology of the temporal region in the tetrapod skull traditionally has been a widely discussed feature of vertebrate anatomy. The evolution of different temporal openings in Amniota (mammals, birds, and reptiles), Lissamphibia (frogs, salamanders, and caecilians), and several extinct tetrapod groups has sparked debates on the phylogenetic, developmental, and functional background of this region in the tetrapod skull. This led most famously to the erection of different amniote taxa based on the number and position of temporal fenestrae in their skulls. However, most of these taxa are no longer recognised to represent natural groupings and the morphology of the temporal region is not necessarily an adequate trait for use in the reconstruction of amniote phylogenies. Yet, new fossil finds, most notably of parareptiles and stem-turtles, as well as modern embryological and biomechanical studies continue to provide new insights into the morphological diversity of the temporal region. Here, we introduce a novel comprehensive classification scheme for the various temporal morphotypes in all Tetrapoda that is independent of phylogeny and previous terminology and may facilitate morphological comparisons in future studies. We then review the history of research on the temporal region in the tetrapod skull. We document how, from the early 19th century with the first recognition of differences in the temporal region to the first proposals of phylogenetic relationships and their assessment over the centuries, the phylogenetic perspective on the temporal region has developed, and we highlight the controversies that still remain. We also compare the different functional and developmental drivers proposed for the observed morphological diversity and how the effects of internal and external factors on the structure of the tetrapod skull have been interpreted.

Australia's prehistoric 'swamp king': revision of the Plio-Pleistocene crocodylian genus Pallimnarchus de Vis, 1886

The crocodylian fossil record from the Cenozoic of Australasia is notable for its rich taxonomic diversity, and is primarily represented by members of the clade Mekosuchinae. Reports of crocodylian fossils from Australia date back to the late nineteenth century. In 1886, Charles Walter de Vis proposed the name Pallimnarchus pollens for crocodylian fossils from southeast Queensland-the first binomen given to an extinct crocodylian taxon from Australia. Pallimnarchus has come to be regarded as a large, broad-snouted crocodylian from Australia's Plio-Pleistocene, and numerous specimens, few of which are sufficiently complete, have been assigned to it by several authors throughout the twentieth century. In the late 1990s, the genus was expanded to include a second species, Pallimnarchus gracilis. Unfortunately, the original syntype series described as Pallimnarchus pollens is very fragmentary and derives from more than one taxon, while a large part of the subsequently selected lectotype specimen is missing. Because descriptions and illustrations of the complete lectotype do not reveal any autapomorphic features, we propose that Pallimnarchus pollens should be regarded as a nomen dubium. Following this decision, the fossil material previously referred to Pallimnarchus is of uncertain taxonomic placement. A partial skull, formerly assigned to Pallimnarchus pollens and known as 'Geoff Vincent's specimen', possesses many features of diagnostic value and is therefore used as basis to erect a new genus and species-Paludirex vincenti gen. et sp. nov. A comprehensive description is given for the osteology of 'Geoff Vincent's specimen' as well as aspects of its palaeoneurology, the latter being a first for an extinct Australian crocodyliform. The newly named genus is characterized by a unique combination of premaxillary features such as a distinctive arching of the anterior alveolar processes of the premaxillae, a peculiar arrangement of the first two premaxillary alveoli and a large size disparity between the 3rd and 4th premaxillary alveoli. These features presently allow formal recognition of two species within the genus, Paludirex vincenti and Paludirex gracilis comb. nov., with the former having comparatively more robust rostral proportions than the latter. The Paludirex vincenti holotype comes from the Pliocene Chinchilla Sand of the Darling Downs, south-eastern Queensland, whereas the material assigned to Paludirex gracilis is from the Pleistocene of Terrace Site Local Fauna, Riversleigh, northwest Queensland. Phylogenetic analyses recover Paludirex vincenti as a mekosuchine, although further cladistic assessments are needed to better understand the relationships within the clade.

Convergent evolution and possible constraint in the posterodorsal retraction of the external nares in pelagic crocodylomorphs

Amongst Mesozoic marine reptiles, metriorhynchid crocodylomorphs were unique in evolving into pelagically adapted forms with little-to-no posterodorsal retraction of the external nares. Narial retraction is a common adaptation seen in sustained swimmers, notably occurring during cetacean evolution. Mesosaurids and the basalmost known members of ichthyosauriforms, thalattosaurians, saurosphargids, sauropterygians, pleurosaurids and mosasauroids had the external nares divided by an ossified bar, bound by multiple cranial bones and were positioned back from the tip of the rostrum. However, metriorhynchids evolved from taxa with a single external naris bound solely by the premaxilla, and positioned near the tip of an elongate rostrum. We posit that metriorhynchids were uniquely disadvantaged in evolving into sustained swimmers. Herein we describe three Late Jurassic metriorhynchid cranial rostra that display differing degrees of narial retraction. In our new phylogenetic analyses, the backwards migration of the narial fossa posterior margin occurred independently at least four times in Metriorhynchidae, whereas the backwards migration of the anterior margin only occurred twice. Although Rhacheosaurini share the backwards migration of the anterior and posterior narial margins, posterodorsal retraction occurred differently along three lineages. This culminated in the Early Cretaceous, where a rhacheosaurin evolved nares bound by the premaxilla and maxilla, and significantly posterodorsally retracted.

The braincase of

We describe in detail three braincases of the ankylosaur Bissektipelta archibaldi from the Late Cretaceous (Turonian) of Uzbekistan with the aid of computed tomography, segmentation, and 3D modeling. Bissektipelta archibaldi is confirmed as a valid taxon and attributed to Ankylosaurinae based on the results of a phylogenetic analysis. The topographic relationships between the elements forming the braincase are determined using a newly referred specimen with preserved sutures, which is an exceedingly rare condition for ankylosaurs. The mesethmoid appears to be a separate ossification in the newly referred specimen ZIN PH 281/16. We revise and discuss features of the neurocranial osteology in Ankylosauria and propose new diagnostic characters for a number of its subclades. We present a 3D model of the braincase vasculature of Bissektipelta and comment on vascular patterns of armored dinosaurs. A complex vascular network piercing the skull roof and the wall of the braincase is reported for ankylosaurs for the first time. We imply the presence of a lepidosaur-like dorsal head vein and the venous parietal sinus in the adductor cavity of Bissektipelta. We suggest that the presence of the dorsal head vein in dinosaurs is a plesiomorphic diapsid trait, and extant archosaur groups independently lost the vessel. A study of two complete endocranial casts of Bissektipelta allowed us to compare endocranial anatomy within Ankylosauria and infer an extremely developed sense of smell, a keen sense of hearing at lower frequencies (100–3000 Hz), and the presence of physiological mechanisms for precise temperature control of neurosensory tissues at least in derived ankylosaurids.

A high-resolution growth series of Tyrannosaurus rex obtained from multiple lines of evidence

Background

During the growth of complex multicellular organisms, chronological age, size and morphology change together in a hierarchical and coordinated pattern. Among extinct species, the growth of Tyrannosaurus rex has received repeated attention through quantitative analyses of relative maturity and chronological age. Its growth series shows an extreme transformation from shallow skulls in juveniles to deep skulls in adults along with a reduction in tooth count, and its growth curve shows that T. rex had a high growth rate in contrast to its closest relatives. However, separately, these sets of data provide an incomplete picture of the congruence between age, size, and relative maturity in this exemplar species. The goal of this work is to analyze these data sets together using cladistic analysis to produce a single hypothesis of growth that includes all of the relevant data.

Methods

The three axes of growth were analyzed together using cladistic analysis, based on a data set of 1,850 morphological characters and 44 specimens. The analysis was run in TNT v.1.5 under a New Technology search followed by a Traditional search. Correlation tests were run in IBM SPSS Statistics v. 24.0.0.0.

Results

An initial analysis that included all of the specimens recovered 50 multiple most parsimonious ontograms a series of analyses identified 13 wildcard specimens. An analysis run without the wildcard specimens recovered a single most parsimonious tree (i.e., ontogram) of 3,053 steps. The ontogram is composed of 21 growth stages, and all but the first and third are supported by unambiguously optimized synontomorphies. T. rex ontogeny can be divided into five discrete growth categories that are diagnosed by chronological age, morphology, and, in part, size (uninformative among adults). The topology shows that the transition from shallow to deep skull shape occurred between 13 and 15 years of age, and the size of the immediate relatives of T. rex was exceeded between its 15th and 18th years. Although size and maturity are congruent among juveniles and subadults, congruence is not seen among adults; for example, one of the least mature adults (RSM 2523.8) is also the largest and most massive example of the species. The extreme number of changes at the transition between juveniles and subadults shows that the ontogeny of T. rex exhibits secondary metamorphosis, analogous to the abrupt ontogenetic changes that are seen at sexual maturity among teleosts. These results provide a point of comparison for testing the congruence between maturity and chronological age, size, and mass, as well as integrating previous work on functional morphology into a rigorous ontogenetic framework. Comparison of the growth series of T. rex with those of outgroup taxa clarifies the ontogenetic trends that were inherited from the common ancestor of Archosauriformes.