Evolution of facial innervation in anomodont therapsids (Synapsida): Insights from X-ray computerized microtomography

Bone microstructure of the basal anomodont Suminia getmanovi supports its arboreal lifestyle

The paleohistology of Permo-Triassic anomodonts has been extensively studied and, independent of phylogeny, body size and lifestyle, reflects a pattern of rapid growth indicated by a woven-parallel complex. Moreover, anomodonts uniformly show a relative bone cortical thickness (RBT) exceeding 30% and a medullary cavity generally filled by trabeculae. Here, we investigate the paleohistology of the basal anomodont Suminia getmanovi from the Permian of Russia, which has been hypothesized as one of the earliest arboreal tetrapods. Osteohistology and skeletal proportions reveal that our sample comprises at least two late juvenile to early subadult individuals, exhibiting well-vascularized and mostly uninterrupted woven-parallel complex or parallel-fibered tissues, suggesting relatively high growth rates, consistent with other anomodonts. However, all elements of Suminia present an open medullary cavity virtually free of bony trabeculae and a RBT lower than 18%. The microanatomy of Suminia thus differs from all other anomodonts studied so far, including its closest relative Galeops, as well as more basal synapsids that also tend to show higher RBT values and/or a medullary territory obstructed by trabeculae. Compared to extant climbers, which possess thinner bone walls and lower compactness than their terrestrial and aquatic relatives, the bone architecture of Suminia further supports its arboreal lifestyle.

Anatomy of the maxillary canal of Riograndia guaibensis (Cynodontia, Probainognathia)-A prozostrodont from the Late Triassic of southern Brazil

Investigating the evolutionary trajectory of synapsid sensory and cephalic systems is pivotal for understanding the emergence and diversification of mammals. Recent studies using CT-scanning to analyze the rostral foramina and maxillary canals morphology in fossilized specimens of probainognathian cynodonts have contributed to clarifying the homology and paleobiological interpretations of these structures. In the present work, μCT-scannings of three specimens of Riograndia guaibensis, an early Norian cynodont from southern Brazil, were analyzed and revealed an incomplete separation between the lacrimal and maxillary canals, with points of contact via non-ossified areas. While the maxillary canal exhibits a consistent morphological pattern with other Prozostrodontia, featuring three main branches along the lateral region of the snout, the rostral alveolar canal in Riograndia displays variability in the number of extra branches terminating in foramina on the lateral surface of the maxilla, showing differences among individuals and within the same skull. Additionally, pneumatization is observed in the anterior region of the skull, resembling similar structures found in reptiles and mammals. Through this pneumatization, certain branches originating from the maxillary canal extend to the canine alveolus. Further investigation is warranted to elucidate the functionality of this structure and its occurrence in other cynodont groups.

Synapsids and sensitivity: Broad survey of tetrapod trigeminal canal morphology supports an evolutionary trend of increasing facial tactile specialization in the mammal lineage

The trigeminus nerve (cranial nerve V) is a large and significant conduit of sensory information from the face to the brain, with its three branches extending over the head to innervate a wide variety of integumentary sensory receptors, primarily tactile. The paths of the maxillary (V2) and mandibular (V3) divisions of the trigeminus frequently transit through dedicated canals within the bones of the upper and lower jaws, thus allowing this neuroanatomy to be captured in the fossil record and be available to interpretations of sensory ability in extinct taxa. Here, we use microCT and synchrotron scans from 38 extant and fossil species spanning a wide phylogenetic sample across tetrapods to investigate whether maxillary and mandibular canal morphology can be informative of sensory biology in the synapsid lineage. We found that in comparison to an amphibian and sauropsid outgroup, synapsids demonstrate a distinctive evolutionary pattern of change from canals that are highly ramified near the rostral tip of the jaws to canals with increasingly simplified morphology. This pattern is especially evident in the maxillary canal, which came to feature a shortened infraorbital canal terminating in a single large infraorbital foramen that serves as the outlet for branches of V2 that then enter the soft tissues of the face. A comparison with modern analogues supports the hypothesis that this morphological change correlates to an evolutionary history of synapsid-specific innovations in facial touch. We interpret the highly ramified transitional form found in early nonmammalian synapsids as indicative of enhanced tactile sensitivity of the rostrum via direct or proximal contact, similar to tactile specialists such as probing shorebirds and alligators that possess similar proliferative ramifications of the maxillary and mandibular canals. The transition toward a simplified derived form that emerged among Mid-Triassic prozostrodont cynodonts and is retained among modern mammals is a unique configuration correlated with an equally unique and novel tactile sensory apparatus: mobile mystacial whiskers. Our survey of maxillary and mandibular canals across a phylogenetic and ecological variety of tetrapods highlights the morphological diversity of these structures, but also the need to establish robust form-function relationships for future interpretations of osteological correlates for sensory biology.

A comprehensive phylogeny and revised taxonomy of Diadectomorpha with a discussion on the origin of tetrapod herbivory

Among terrestrial tetrapods, the origin of herbivory marked a key evolutionary event that allowed for the evolution of modern terrestrial ecosystems. A 100 Ma gap separates the oldest terrestrial tetrapods and the first undisputed herbivorous tetrapods. While four clades of early tetrapod herbivores are undisputed amniotes, the phylogenetic position of Diadectomorpha with respect to Amniota has long been controversial. Given that the origin of herbivory coincides with the oldest amniotes, and obligate herbivory is unknown within amphibians, this suggests that a key adaptation necessary to evolve obligate herbivory is unique to amniotes. Historically, phylogenetic analyses have found Diadectomorpha as the sister-group to amniotes, but recent analyses recover Diadectomorpha as sister-group to Synapsida, within Amniota. We tested whether diadectomorphs are amniotes by updating the most recent character-taxon matrix. Specifically, we added new characters from the lower jaw and added diadectomorph taxa, resulting in a dataset of 341 characters and 61 operational taxonomic units. We updated the description of five diadectomorph jaws using microcomputed tomography data. Our majority-rule consensus places Diadectomorpha as sister-group to Synapsida; other methods do not recover this relationship. We revise diadectomorph taxonomy, erecting a new species from the early Permian Bromacker locality, Germany, and a new genus to accommodate 'Diadectes' sanmiguelensis.

Early synapsids neurosensory diversity revealed by CT and synchrotron scanning

Non-mammaliaform synapsids (NMS) represent the closest relatives of today's mammals among the early amniotes. Exploring their brain and nervous system is key to understanding how mammals evolved. Here, using CT and Synchrotron scanning, we document for the first time three extreme cases of neurosensory and behavioral adaptations that probe into the wide range of unexpected NMS paleoneurological diversity. First, we describe adaptations to low-frequency hearing and low-light conditions in the non-mammalian cynodont Cistecynodon parvus, supporting adaptations to an obligatory fossorial lifestyle. Second, we describe the uniquely complex and three-dimensional maxillary canal morphology of the biarmosuchian Pachydectes elsi, which suggests that it may have used its cranial bosses for display or low-energy combat. Finally, we introduce a paleopathology found in the skull of Moschognathus whaitsi. Since the specimen was not fully grown, this condition suggests the possibility that this species might have engaged in playful fighting as juveniles-a behavior that is both social and structured. Additionally, this paper discusses other evidence that could indicate that tapinocephalid dinocephalians were social animals, living and interacting closely with one another. Altogether, these examples evidence the wide range of diversity of neurological structures and complex behavior in NMS.

The origin and evolution of Cynodontia (Synapsida, Therapsida): Reassessment of the phylogeny and systematics of the earliest members of this clade using 3D-imaging technologies

The origin of cynodonts, the group ancestral to and including mammals, is one of the major outstanding problems in therapsid evolution. One of the most troubling aspects of the cynodont fossil record is the lengthy Permian ghost lineage between the latest possible divergence from its sister group Therocephalia and the first appearance of definitive cynodonts in the late Permian. The absence of cynodonts and dominance of therocephalians in middle Permian strata has led some workers to argue that cynodonts evolved from within therocephalians, rendering the latter paraphyletic, but more recent analyses support the reciprocal monophyly of Cynodontia and Therocephalia. Furthermore, although a fundamental dichotomy in the derived subclade Eucynodontia is well-supported in cynodont phylogeny, the relationships of more stemward cynodonts from the late Permian and Early Triassic are unresolved. Here, we provide a re-evaluation of the phylogeny of Eutheriodontia (Cynodontia + Therocephalia) and an assessment of character evolution within the group. Using computed tomographic data derived from extensive sampling of the earliest known (late Permian and Early Triassic) cynodonts and selected exemplars of therocephalians and later (Middle Triassic onwards) cynodonts, we describe novel aspects of the endocranial anatomy of these animals. These data were incorporated into a new phylogenetic data set including a comprehensive sample of early cynodonts. Our phylogenetic analyses support some results previously recovered by other authors, but recover therocephalians as paraphyletic with regards to cynodonts, with cynodonts and eutherocephalians forming a clade to the exclusion of the "basal therocephalian" families Lycosuchidae and Scylacosauridae. Though both conservatism and homoplasy mark the endocranial anatomy of early non-mammalian cynodonts, we were able to identify several new endocranial synapomorphies for eutheriodont subclades and recovered generally better-supported topologies than previous analyses using primarily external craniodental characters.

Nasal turbinates of the dicynodont Kawingasaurus fossilis and the possible impact of the fossorial habitat on the evolution of endothermy

The nasal region of the fossorial anomodont Kawingasaurus fossilis was virtually reconstructed from neutron-computed tomographic data and compared with the terrestrial species Pristerodon mackayi and other nonmammalian synapsids. The tomography of the Kawingasaurus skull reveals a pattern of maxillo-, naso-, fronto- and ethmoturbinal ridges that strongly resemble the mammalian condition. On both sides of the nasal cavity, remains of scrolled maxilloturbinals were preserved that were still partially articulated with maxilloturbinal ridges. Furthermore, possible remains of the lamina semicircularis as well as fronto- or ethmoturbinals were found. In Kawingasaurus, the maxilloturbinal ridges were longer and stronger than in Pristerodon. Except for the nasoturbinal ridges, no other ridges in the olfactory region and no remains of turbinates were recognized. This supports the hypothesis that naso-, fronto-, ethmo- and maxilloturbinals were a plesiomorphic feature of synapsids, but due to their cartilaginous nature in most taxa were, in almost all cases, not preserved. The well-developed maxilloturbinals in Kawingasaurus were probably an adaptation to hypoxia-induced hyperventilation in the fossorial habitat, maintaining the high oxygen demands of Kawingasaurus' large brain. The surface area of the respiratory turbinates in Kawingasaurus falls into the mammalian range, which suggests that they functioned as a countercurrent exchange system for thermoregulation and conditioning of the respiratory airflow. Our results suggest that the environmental conditions of the fossorial habitat led to specific sensory adaptations, accompanied by a pulse in brain evolution and of endothermy in cistecephalids, ~50 million years before the origin of endothermy in the mammalian stem line. This supports the Nocturnal Bottleneck Theory, in that we found evidence for a similar evolutionary scenario in cistecephalids as proposed for early mammals.

The maxillary canal of the titanosuchid Jonkeria (Synapsida, Dinocephalia)

The maxillary canal of the titanosuchid dinocephalian Jonkeria is described based on digitised serial sections. We highlight that its morphology is more like that of the tapinocephalid Moschognathus than that of Anteosaurus. This is unexpected given the similarities between the dentition of Jonkeria and Anteosaurus (i.e., presence of a canine) and the fact that the branching pattern of the maxillary canal in synapsids usually co-varies with dentition. Hypotheses to account for similarities between Jonkeria and Moschognathus (common ancestry, function in social signalling or underwater sensing) are discussed. It is likely that the maxillary canal carries a strong phylogenetic signal, here supporting the clade Tapinocephalia.

At the root of the mammalian mind: The sensory organs, brain and behavior of pre-mammalian synapsids

All modern mammals are descendants of the paraphyletic non-mammaliaform Synapsida, colloquially referred to as the "mammal-like reptiles." It has long been assumed that these mammalian ancestors were essentially reptile-like in their morphology, biology, and behavior, i.e., they had a small brain, displayed simple behavior, and their sensory organs were unrefined compared to those of modern mammals. Recent works have, however, revealed that neurological, sensory, and behavioral traits previously considered typically mammalian, such as whiskers, enhanced olfaction, nocturnality, parental care, and complex social interactions evolved before the origin of Mammaliaformes, among the early-diverging "mammal-like reptiles." In contrast, an enlarged brain did not evolve immediately after the origin of mammaliaforms. As such, in terms of paleoneurology, the last "mammal-like reptiles" were not significantly different from the earliest mammaliaforms. The abundant data and literature published in the last 10 years no longer supports the "three pulses" scenario of synapsid brain evolution proposed by Rowe and colleagues in 2011, but supports the new "outside-in" model of Rodrigues and colleagues proposed in 2018, instead. As Mesozoic reptiles were becoming the dominant taxa within terrestrial ecosystems, synapsids gradually adapted to smaller body sizes and nocturnality. This resulted in a sensory revolution in synapsids as olfaction, audition, and somatosensation compensated for the loss of visual cues. This altered sensory input is aligned with changes in the brain, the most significant of which was an increase in relative brain size.

Evolution of the therian face through complete loss of the premaxilla

The anatomical framework of the jawbones is highly conserved among most of the Osteichthyes, including the tetrapods. However, our recent study suggested that the premaxilla, the rostralmost upper jaw bone, was rearranged during the evolution of therian mammals, being replaced by the septomaxilla at least in the lateral part. In the present study, to understand more about the process of evolution from the ancestral upper jaw to the therian face, we re-examined the development of the therian premaxilla (incisive bone). By comparing mouse, bat, goat, and cattle fetuses, we confirmed that the therian premaxilla has dual developmental origins, the lateral body and the palatine process. This dual development is widely conserved among the therian mammals. Cell-lineage-tracing experiments using Dlx1-CreER^T2 mice revealed that the palatine process arises in the ventral part of the premandibular domain, where the nasopalatine nerve distributes, whereas the lateral body develops from the maxillary prominence in the domain of the maxillary nerve. Through comparative analysis using various tetrapods, we concluded that the palatine process should not be considered part of the ancestral premaxilla. It rather corresponds to the anterior region of the vomerine bone of nonmammalian tetrapods. Thus, the present findings indicate that the true premaxilla was completely lost during the evolution of the therian mammals, resulting in the establishment of the unique therian face as an evolutionary novelty. Reconsideration of the homological framework of the cranial skeleton based on the topographical relationships of the ossification center during embryonic development is warranted.

The topography of diet: Orientation patch count predicts diet in turtles

Use of quantitative morphological methods in biology has increased with the availability of 3D digital data. Rotated orientation patch count (OPCr) leverages such data to quantify the complexity of an animal's feeding surface, and has previously been used to analyze how tooth complexity signals diet in squamates, crocodilians, and mammals. These studies show a strong correlation between dental complexity and diet. However, dietary prediction using this technique has not been tested on the feeding structures of edentulous (toothless) taxa. This study is the first to test the applicability of OPCr to the triturating surface morphology of a beaked clade. Fifty-five turtle specimens, 42 of which preserved both the skull and rhamphotheca, were categorized into dietary categories based on the food sources comprising 90% or 60% of their diets. Photogrammetric models of each specimen were read into molaR, producing OPCr results. Comparison of bone and rhamphotheca OPCr values shows no significant difference in complexity, implying that bone can suffice for predicting diet from morphology when keratin is absent. Carnivorous taxa have significantly lower OPCr values than herbivorous or omnivorous taxa, showing that feeding surface complexity in edentulous animals varies with diet similarly to tooth complexity in toothed taxa. Comparison of bone OPCr values by family shows that Testudinidae (tortoises) are more complex than Cheloniidae (sea turtles) and Chelydridae (snapping turtles), but that Cheloniidae and Chelydridae are not significantly different from each other. We therefore find that OPCr can be used to differentiate between carnivores and other dietary categories in edentulous taxa.

Histology and μCT reveal the unique evolution and development of multiple tooth rows in the synapsid Endothiodon

Several amniote lineages independently evolved multiple rows of marginal teeth in response to the challenge of processing high fiber plant matter. Multiple tooth rows develop via alterations to tooth replacement in captorhinid reptiles and ornithischian dinosaurs, but the specific changes that produce this morphology differ, reflecting differences in their modes of tooth attachment. To further understand the mechanisms by which multiple tooth rows can develop, we examined this feature in Endothiodon bathystoma, a member of the only synapsid clade (Anomodontia) to evolve a multi-rowed marginal dentition. We histologically sampled Endothiodon mandibles with and without multiple tooth rows as well as single-rowed maxillae. We also segmented functional and replacement teeth in µ-CT scanned mandibles and maxillae of Endothiodon and several other anomodonts with 'postcanine' teeth to characterize tooth replacement in the clade. All anomodonts in our sample displayed a space around the tooth roots for a soft tissue attachment between tooth and jaw in life. Trails of alveolar bone indicate varying degrees of labial migration of teeth through ontogeny, often altering the spatial relationships of functional and replacement teeth in the upper and lower jaws. We present a model of multiple tooth row development in E. bathystoma in which labial migration of functional teeth was extensive enough to prevent resorption and replacement by newer generations of teeth. This model represents another mechanism by which multiple tooth rows evolved in amniotes. The multiple tooth rows of E. bathystoma may have provided more extensive contact between the teeth and a triturating surface on the palatine during chewing.

Cranial anatomy of Bolotridon frerensis, an enigmatic cynodont from the Middle Triassic of South Africa, and its phylogenetic significance

The cynodont fauna of the Trirachodon-Kannemeyeria Subzone of the Middle Triassic Cynognathus Assemblage Zone (AZ) is almost exclusively represented by taxa belonging to the clade Eucynodontia. However, there is one basal (non-eucynodont) cynodont known to have survived into this assemblage: the enigmatic Bolotridon frerensis. BSPG 1934-VIII-7 represents by far the most extensive specimen of B. frerensis, consisting of a partial skull with occluded lower jaw. The specimen was initially described by Broili & Schröder (1934), but their description was limited to surface details of the skull and the dental morphology. Here, by using a computed tomographic (CT) reconstruction, we redescribe this specimen, providing novel information on its palatal and internal anatomy. New endocranial characters recognized for this taxon include ridges in the nasal cavity indicating the presence of cartilaginous respiratory turbinals. New data obtained from the CT scan were incorporated into the most recently published data matrix of early non-mammalian cynodonts to test the previously unstable phylogenetic position of Bolotridon. Our phylogenetic analyses recovered Bolotridon as the sister-taxon of Eucynodontia, a more crownward position than previously hypothesized.

A re-assessment of the oldest therapsid Raranimus confirms its status as a basal member of the clade and fills Olson's gap

The non-mammalian therapsids comprise a paraphyletic assemblage of Permian-Jurassic synapsids closely related to mammals that includes six major clades of largely unresolved phylogenetic affinity. Understanding the early evolutionary radiation of therapsids is complicated by a gap in the fossil record during the Roadian (middle Permian) known as Olson's gap. Because of its early stratigraphic occurrence and its primitive features, Raranimus dashankouensis, from the Dashankou fauna (Rodian), Qingtoushan Formation (China), is currently considered the best candidate to fill this gap. However, it is known from only a single specimen, an isolated snout, which limits the amount of usable phylogenetic characters to reconstruct its affinities. In addition, understanding of the stratigraphy of the Qingtoushan Formation is poor. Here, we used CT scanning techniques to digitally reconstruct the bones and trigeminal canals of the snout of Raranimus in 3D. We confirm that Raranimus shares a high number of synapomorphies with more derived therapsids and is the only therapsid known so far to display a "pelycosaur"-like maxillary canal bearing a long caudal alveolar canal that gives off branches at regular intervals. This plesiomorphic feature supports the idea that Raranimus is basal to other therapsids.

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.

Endocranial morphology of the Brazilian Permian dicynodont Rastodon procurvidens (Therapsida: Anomodontia)

Dicynodontia is a major clade of terrestrial tetrapods that greatly diversified during the Permian and Triassic periods, reaching a worldwide distribution. In this study, the endocranial cavity of the Brazilian Permian dicynodont Rastodon procurvidens is described based on a digital endocast obtained using digital imaging (X-ray computed tomography) and 3D modeling. It was possible to reconstruct the brain, olfactory bulbs, inner ear, some neurovascular canals, cranial nerves, the nasal cavity, and the maxillary recesses. The endocast of R. procurvidens preserves a typical plesiomorphic morphology of non-mammaliaform therapsids, being predominantly tubular and displaying a relatively short and robust hindbrain. Encephalization quotients (EQs) were calculated for R. procurvidens, resulting in EQs of 0.09 ± 0.03 and 0.13 ± 0.05 (Jerison's EQ and Manger's EQ, respectively). Finally, some biological implications of the endocast morphology were inferred for R. procurvidens. Its inner ear is especially small, and its orientation implies a slightly downturned head posture in life. Furthermore, the presence of uncompressed maxillary recesses in R. procurvidens indicates a correlation between the enlargement of the recesses and the reduction of the tusks, also seen in other dicynodonts with reduced tusks.

A geometric morphometric approach to the analysis of skull shape in Triassic dicynodonts (Therapsida, Anomodontia) from South America

Dicynodont therapsids were a major component of the Permo-Triassic terrestrial ecosystems across Pangea and have been regarded as specialized herbivores. In South America, the group was represented by several taxa of the clade Kannemeyeriiformes spanning from the Middle to the Late Triassic. In order to evaluate if cranial differences among taxa are potentially related to differences in feeding function, we performed a geometric morphometric analysis on 28 South American dicynodont crania. We digitized 19 cranial landmarks and conducted generalized Procrustes analysis, principal component analysis (PCA), principal component analysis between groups (bg-PCA), and a branch weighted squared-change parsimony approach. Phylogenetic inertia was not a significant driver of cranial shape evolution in the group, whereas PCA and bg-PCA support that major morphological shape differences are concentrated in the preorbital region (relative length of the snout and width of the caniniform process), in the position of quadrate condyle in relation to the caniniform process, and in the increase in the intertemporal surface area. In this context, tusked Dinodontosaurus, "Kannemeyeria," and Vinceria have relatively smaller adductor attachment areas and input moment arm than younger taxa lacking tusks, such as Ischigualastia, Stahleckeria, and Jachaleria. Differences in cranial morphology in later dicynodonts reflect modifications in feeding mechanics, probably due to changes in food resources (vegetation) in their habitats toward the end of the Triassic.