The Evolution of the Maxillary Canal in Probainognathia (Cynodontia, Synapsida): Reassessment of the Homology of the Infraorbital Foramen in Mammalian Ancestors

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.

Bienotheroides wucaiensis sp. nov., a new tritylodontid (Cynodontia, Mammaliamorpha) from the Late Jurassic Shishugou Formation of Xinjiang, China

The Shishugou Formation of the Middle to Late Jurassic in Xinjiang, China, has produced abundant tetrapod fossils including dinosaurs and tritylodontids. Bienotheroides is a genus of highly specialized tritylodontids, characterized by a short and wide snout, ventrally expanded zygomatic process, strongly reduced maxilla, short and flat basisphenoid, and maxillary teeth cusp formula 2-3-3. Here, we report a new tritylodontid, Bienotheroides wucaiensis sp. nov. from the lower Upper Jurassic level of the formation at Wucaiwan, Xinjiang, Northwest China, represented by a well-preserved cranium and associated postcranial skeleton. Micro-computed tomography and 3D reconstruction reveal the medial view of the cranial structure and the replacement upper incisors, providing new anatomical information of Bienotheroides. The absence of a septomaxilla in B. wucaiensis reveals the homoplastic evolution of this feature within tritylodontids, as it remains in basal mammaliaforms but is lost in later descendants.

Revisiting the neuroanatomy of Massetognathus pascuali (Eucynodontia: Cynognathia) from the early Late Triassic of South America using Neutron Tomography

This paper analyzes the paleoneurology (cranial endocast and maxillary canal) of Massetognathus pascuali, an iconic non-mammaliaform cynodont from the early Late Triassic of South America, using Neutron Tomography. The application of neutron tomography holds the potential for uncovering more refined anatomical and quantitative data. The newly examined cranial endocast shows a forebrain with a tubular shape without an interhemispheric fissure, presence of a pineal body (with a closed parietal foramen), and a marked unossified zone. In comparison with a smaller, putatively juvenile specimen previously studied (PVL 4016), the new endocast exhibits a similar degree of encephalization, indicating little change in relative brain size between both ontogenetic stages. In the context of cynognathian brain evolution, M. pascuali maintained a low encephalization quotient, typical of early cynognathians, contrasting with the higher values of some Late Triassic taxa. The maxillary canal of M. pascuali is described here for the first time. It is considerably ramified, although slightly less than in some early cynognathians, following the general pattern of non-probainognathians and suggesting the absence of a flexible rhinarium or mobile vibrissae. By integrating endocast data with the maxillary canal, this study offers enhanced insights into the neurosensory ecology of M. pascuali, thereby deepening our understanding of its biology and ecological interactions.

Cranial osteology and neuroanatomy of the late Permian reptile Milleropsis pricei and implications for early reptile evolution

Millerettidae are a group of superficially lizard-like Permian stem reptiles originally hypothesized as relevant to the ancestry of the reptile crown group, and particularly to lepidosaurs and archosaurs. Since the advent of cladistics, millerettids have typically been considered to be more distant relatives of crown reptiles as the earliest-diverging parareptiles and therefore outside of 'Eureptilia'. Despite this cladistic consensus, some conspicuous features of millerettid anatomy invite reconsideration of their relationships. We provide a detailed description of the late Permian millerettid Milleropsis pricei using synchrotron X-ray phase-contrast micro-computed tomography focusing on the cranial anatomy of three individuals known from a burrow aggregation. Our data reveal a suite of neuroanatomical features Milleropsis shares with neodiapsids that are absent both in other 'parareptiles' and in early diverging groups of 'eureptiles'. Traits shared between Milleropsis and neodiapsids include: the presence of a tympanic emargination on the quadrate, quadratojugal and squamosal, the loss of epipterygoid contribution to the basicranial articulation suggesting a more kinetic palatoquadrate, the absence of a sphenethmoid and the pathway of the abducens nerve through the braincase. Our findings suggest that the early reptile neurocranium, a region poorly sampled in phylogenetic analyses due to relative visual inaccessibility and poor preservation, has the potential to inform the phylogenetic relationships of early reptiles.

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.

Jurassic fossil juvenile reveals prolonged life history in early mammals

Living mammal groups exhibit rapid juvenile growth with a cessation of growth in adulthood1. Understanding the emergence of this pattern in the earliest mammaliaforms (mammals and their closest extinct relatives) is hindered by a paucity of fossils representing juvenile individuals. We report exceptionally complete juvenile and adult specimens of the Middle Jurassic docodontan Krusatodon, providing anatomical data and insights into the life history of early diverging mammaliaforms. We used synchrotron X-ray micro-computed tomography imaging of cementum growth increments in the teeth2-4 to provide evidence of pace of life in a Mesozoic mammaliaform. The adult was about 7 years and the juvenile 7 to 24 months of age at death and in the process of replacing its deciduous dentition with its final, adult generation. When analysed against a dataset of life history parameters for extant mammals5, the relative sequence of adult tooth eruption was already established in Krusatodon and in the range observed in extant mammals but this development was prolonged, taking place during a longer period as part of a significantly longer maximum lifespan than extant mammals of comparable adult body mass (156 g or less). Our findings suggest that early diverging mammaliaforms did not experience the same life histories as extant small-bodied mammals and the fundamental shift to faster growth over a shorter lifespan may not have taken place in mammaliaforms until during or after the Middle Jurassic.

New evidence from high-resolution computed microtomography of Triassic stem-mammal skulls from South America enhances discussions on turbinates before the origin of Mammaliaformes

The nasal cavity of living mammals is a unique structural complex among tetrapods, acquired along a series of major morphological transformations that occurred mainly during the Mesozoic Era, within the Synapsida clade. Particularly, non-mammaliaform cynodonts document several morphological changes in the skull, during the Triassic Period, that represent the first steps of the mammalian bauplan. We here explore the nasal cavity of five cynodont taxa, namely Thrinaxodon, Chiniquodon, Prozostrodon, Riograndia, and Brasilodon, in order to discuss the main changes within this skull region. We did not identify ossified turbinals in the nasal cavity of these taxa and if present, as non-ossified structures, they would not necessarily be associated with temperature control or the development of endothermy. We do, however, notice a complexification of the cartilage anchoring structures that divide the nasal cavity and separate it from the brain region in these forerunners of mammals.

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.

A new early-diverging probainognathian cynodont and a revision of the occurrence of cf. Aleodon from the Chañares Formation, northwestern Argentina: New clues on the faunistic composition of the latest Middle-?earliest Late Triassic Tarjadia Assemblage Zone

The Chañares Formation (Ischigualasto-Villa Unión Basin) is worldwide known by its exquisitely preserved fossil record of latest Middle-to-early Late Triassic tetrapods, including erpetosuchids, "rauisuchians," proterochampsids, gracilisuchids, dinosauromorphs, pterosauromorphs, kannemeyeriiform dicynodonts, and traversodontid, chiniquodontid and probainognathid cynodonts, coming from the Tarjadia (bottom) and Massetognathus-Chanaresuchus (top) Assemblage Zones of its lower member. Regarding cynodonts, most of its profuse knowledge comes from the traditional layers discovered by Alfred Romer and his team in the 1960s that are now enclosed in the Massetognathus-Chanaresuchus Assemblage Zone (AZ). In this contribution we focus our study on the probainognathian cynodonts discovered in levels of the Tarjadia Assemblage Zone. We describe a new chiniquodontid cynodont with transversely broad postcanine teeth (Riojanodon nenoi gen. et sp. nov.) which is related to the genus Aleodon. In addition, the specimen CRILAR-Pv 567 previously referred to cf. Aleodon is here described, compared, and included in a phylogenetic analysis. It is considered as an indeterminate Aleodontinae nov., a clade here proposed to included chiniquodontids with transversely broad upper and lower postcanines, by having a cuspidated sectorial labial margin and a lingual platform that is twice broader than a lingual cingulum. Cromptodon mamiferoides, from the Cerro de Las Cabras Formation (Cuyo Basin), was also included in the phylogenetic analysis and recovered as an Aleodontinae. The new cynodont and the record of Aleodontinae indet. reinforce the faunal differentiation between the Tarjadia and Massetognathus-Chanaresuchus Assemblage Zones, in the lower member of the Chañares Formation, and inform on the diverse chiniquodontid clade with both sectorial and transversely broad postcanine teeth.

Skull anatomy and paleoneurology of a new traversodontid from the Middle-Late Triassic of Brazil

Traversodontidae, a clade of gomphodont cynodonts, thrived during the Middle and Late Triassic, displaying a wide geographical distribution. During fieldwork in 2009, a new specimen was discovered in Ladinian/early Carnian stratigraphic layers in southern Brazil. Here, we describe this specimen and propose a new taxon closely related to Traversodon stahleckeri (Traversodontinae) but displaying a unique combination of traits (e.g., presence of a poorly developed suborbital process, mesiodistal length of the paracanine fossa similar to the length of the canine, short diastema between the fourth incisor and the upper canine, and coronoid process not entirely covering the distalmost lower postcanine). Furthermore, the endocranial anatomy of the new taxon was examined. The reconstruction of the cranial endocast revealed paleoneurological features consistent with non-Gomphodontosuchinae traversodontids. These features include the presence of a pineal body (but the absence of an open parietal foramen). These recent findings contribute significantly to our understanding of the evolutionary history and cranial anatomy of Middle-Late Triassic traversodontids, shedding light on the diversity and adaptations of non-mammaliaform cynodonts.

Endocranial anatomy of the early prozostrodonts (Eucynodontia: Probainognathia) and the neurosensory evolution in mammal forerunners

Prozostrodon brasiliensis and Therioherpeton cargnini are non-mammaliaform cynodonts that lived ~233 million years ago (late Carnian, Late Triassic) in western Gondwana. They represent some of the earliest divergent members of the clade Prozostrodontia, which includes "tritheledontids", tritylodontids, "brasilodontids", and mammaliaforms (including Mammalia as crown group). Here, we studied the endocranial anatomy (cranial endocast, nerves, vessels, ducts, ear region, and nasal cavity) of these two species. Our findings suggest that during the Carnian, early prozostrodonts had a brain with well-developed olfactory bulbs, expanded cerebral hemispheres divided by the interhemispheric sulcus, and absence of an unossified zone and pineal body. The morphology of the maxillary canal represents the necessary condition for the presence of facial vibrissae. A slight decrease in encephalization is observed at the origin of the clade Prozostrodontia. This new anatomical information provides evidence for the evolution of endocranial traits of the first prozotrodonts, a Late Triassic lineage that culminated in the origin of mammals.

Derived faunivores are the forerunners of major synapsid radiations

Evolutionary radiations generate most of Earth's biodiversity, but are there common ecomorphological traits among the progenitors of radiations? In Synapsida (the mammalian total group), 'small-bodied faunivore' has been hypothesized as the ancestral state of most major radiating clades, but this has not been quantitatively assessed across multiple radiations. To examine macroevolutionary patterns in a phylogenetic context, we generated a time-calibrated metaphylogeny ('metatree') comprising 1,888 synapsid species from the Carboniferous through the Eocene (305-34 Ma) based on 269 published character matrices. We used comparative methods to investigate body size and dietary evolution during successive synapsid radiations. Faunivory is the ancestral dietary regime of each major synapsid radiation, but relatively small body size is only established as the common ancestral state of radiations near the origin of Mammaliaformes in the Late Triassic. The faunivorous ancestors of synapsid radiations typically have numerous novel characters compared with their contemporaries, and these derived traits may have helped them to survive faunal turnover events and subsequently radiate.

Craniodental anatomy in Permian-Jurassic Cynodontia and Mammaliaformes (Synapsida, Therapsida) as a gateway to defining mammalian soft tissue and behavioural traits

Mammals are diagnosed by more than 30 osteological characters (e.g. squamosal-dentary jaw joint, three inner ear ossicles, etc.) that are readily preserved in the fossil record. However, it is the suite of physiological, soft tissue and behavioural characters (e.g. endothermy, hair, lactation, isocortex and parental care), the evolutionary origins of which have eluded scholars for decades, that most prominently distinguishes living mammals from other amniotes. Here, we review recent works that illustrate how evolutionary changes concentrated in the cranial and dental morphology of mammalian ancestors, the Permian-Jurassic Cynodontia and Mammaliaformes, can potentially be used to document the origin of some of the most crucial defining features of mammals. We discuss how these soft tissue and behavioural traits are highly integrated, and how their evolution is intermingled with that of craniodental traits, thus enabling the tracing of their previously out-of-reach phylogenetic history. Most of these osteological and dental proxies, such as the maxillary canal, bony labyrinth and dental replacement only recently became more easily accessible-thanks, in large part, to the widespread use of X-ray microtomography scanning in palaeontology-because they are linked to internal cranial characters. This article is part of the theme issue 'The mammalian skull: development, structure and function'.

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.

A new cynodont from the Upper Triassic Los Colorados Formation (Argentina, South America) reveals a novel paleobiogeographic context for mammalian ancestors

Probainognathia is a derived lineage of cynodonts which encompass Mammalia as their crown-group. The rich record of probainognathians from the Carnian of Argentina contrasts with their Norian representation, with only one named species. Here we describe a new probainognathian, Tessellatia bonapartei gen. et sp. nov., from the Norian Los Colorados Formation of the Ischigualasto-Villa Unión Basin of Argentina. The new taxon, represented by a partial cranium with associated lower jaws, was analyzed through neutron and X-rays micro-tomography (μCT). The high-resolution neutron μCT data allowed the identification of a unique character combination, including features inaccessible through traditional techniques. We constructed the largest phylogenetic data matrix of non-mammalian cynodonts. The new species and its sister taxon, the Brazilian Therioherpeton cargnini, are recovered as probainognathians, closely related to Mammaliamorpha. We conducted the first quantitative paleobiogeographic analysis of non-mammalian cynodonts, focusing in probainognathians. The results indicate that Probainognathia and Mammaliamorpha originated in southwestern Gondwana (in the Brazilian Paraná Basin), which was an important center of diversification during the Triassic. Finally, the Chinese Lufeng Basin is identified as the ancestral area of Mammaliaformes. These new findings, besides adding to the knowledge of the poorly represented Norian cynodonts from the Los Colorados Formation, are significant to improve our understanding of probainognathian diversity, evolution, and paleobiogeographic history.

New tools suggest a middle Jurassic origin for mammalian endothermy: Advances in state-of-the-art techniques uncover new insights on the evolutionary patterns of mammalian endothermy through time: Advances in state-of-the-art techniques uncover new insights on the evolutionary patterns of mammalian endothermy through time

We suggest that mammalian endothermy was established amongst Middle Jurassic crown mammals, through reviewing state-of-the-art fossil and living mammal studies. This is considerably later than the prevailing paradigm, and has important ramifications for the causes, pattern, and pace of physiological evolution amongst synapsids. Most hypotheses argue that selection for either enhanced aerobic activity, or thermoregulation was the primary driver for synapsid physiological evolution, based on a range of fossil characters that have been linked to endothermy. We argue that, rather than either alternative being the primary selective force for the entirety of endothermic evolution, these characters evolved quite independently through time, and across the mammal family tree, principally as a response to shifting environmental pressures and ecological opportunities. Our interpretations can be tested using closely linked proxies for both factors, derived from study of fossils of a range of Jurassic and Cretaceous mammaliaforms and early mammals.

A new specimen provides insights into the anatomy of Irajatherium hernandezi, a poorly known probainognathian cynodont from the Late Triassic of southern Brazil

Irajatherium hernandezi is a poorly known non-mammaliaform cynodont from the Late Triassic of southern Brazil. A new specimen of this cynodont was found in recent fieldwork to the type-locality, the Linha São Luiz site (Candelária Sequence), providing new insights into the anatomy of this mammalian forerunner. This specimen comprises a partial skull preserving the left canine, two left and three right postcanines, and an isolated exoccipital; the left dentary with the canine and postcanines; a fragment of the right dentary; the proximal portion of the left partial humerus; the right scapula; and indeterminate fragments. Based on new material, it is here suggested that I. hernandezi presents: a rostrum broad and short, possibly long as the temporal region; three foramina on the lateral surface of the maxilla, that could correspond to the external openings of the rostral alveolar, infraorbital, and zygomaticofacial canals; a slender zygomatic arch and an absent postorbital bar; a posteriorly wide temporal fossa; a long secondary palate, slightly surpassing the level of the last postcanine tooth; the cerebral hemispheres of the cranial endocast divided by a median sulcus; the scapular blade long and straight, and the postscapular fossa absent in lateral aspect. Finally, I. hernandezi and other tritheledontids were included in a phylogenetic analysis of Eucynodontia. The analysis recovered unresolved relationships for ictidosaurs/tritheledontids, nested within a polytomy with Tritylodontidae and a clade composed by Pseudotherium argentinus, Botucaraitherium belarminoi, Brasilodon quadrangularis, and Mammaliaformes.

Mammalian face as an evolutionary novelty

The anterior end of the mammalian face is characteristically composed of a semimotile nose, not the upper jaw as in other tetrapods. Thus, the therian nose is covered ventrolaterally by the "premaxilla," and the osteocranium possesses only a single nasal aperture because of the absence of medial bony elements. This stands in contrast to those in other tetrapods in whom the premaxilla covers the rostral terminus of the snout, providing a key to understanding the evolution of the mammalian face. Here, we show that the premaxilla in therian mammals (placentals and marsupials) is not entirely homologous to those in other amniotes; the therian premaxilla is a composite of the septomaxilla and the palatine remnant of the premaxilla of nontherian amniotes (including monotremes). By comparing topographical relationships of craniofacial primordia and nerve supplies in various tetrapod embryos, we found that the therian premaxilla is predominantly of the maxillary prominence origin and associated with mandibular arch. The rostral-most part of the upper jaw in nonmammalian tetrapods corresponds to the motile nose in therian mammals. During development, experimental inhibition of primordial growth demonstrated that the entire mammalian upper jaw mostly originates from the maxillary prominence, unlike other amniotes. Consistently, cell lineage tracing in transgenic mice revealed a mammalian-specific rostral growth of the maxillary prominence. We conclude that the mammalian-specific face, the muzzle, is an evolutionary novelty obtained by overriding ancestral developmental constraints to establish a novel topographical framework in craniofacial mesenchyme.

Earliest evidence of herd-living and age segregation amongst dinosaurs

Sauropodomorph dinosaurs dominated the herbivorous niches during the first 40 million years of dinosaur history (Late Triassic-Early Jurassic), yet palaeobiological factors that influenced their evolutionary success are not fully understood. For instance, knowledge on their behaviour is limited, although herding in sauropodomorphs has been well documented in derived sauropods from the Late Jurassic and Cretaceous. Here we report an exceptional fossil occurrence from Patagonia that includes over 100 eggs and skeletal specimens of 80 individuals of the early sauropodomorph Mussaurus patagonicus, ranging from embryos to fully-grown adults, with an Early Jurassic age as determined by high-precision U-Pb zircon geochronology. Most specimens were found in a restricted area and stratigraphic interval, with some articulated skeletons grouped in clusters of individuals of approximately the same age. Our new discoveries indicate the presence of social cohesion throughout life and age-segregation within a herd structure, in addition to colonial nesting behaviour. These findings provide the earliest evidence of complex social behaviour in Dinosauria, predating previous records by at least 40 My. The presence of sociality in different sauropodomorph lineages suggests a possible Triassic origin of this behaviour, which may have influenced their early success as large terrestrial herbivores.

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.

Constraints on the deformation of the vibrissa within the follicle

Nearly all mammals have a vibrissal system specialized for tactile sensation, composed of whiskers growing from sensor-rich follicles in the skin. When a whisker deflects against an object, it deforms within the follicle and exerts forces on the mechanoreceptors inside. In addition, during active whisking behavior, muscle contractions around the follicle and increases in blood pressure in the ring sinus will affect the whisker deformation profile. To date, however, it is not yet possible to experimentally measure how the whisker deforms in an intact follicle or its effects on different groups of mechanoreceptors. The present study develops a novel model to predict vibrissal deformation within the follicle sinus complex. The model is based on experimental results from a previous ex vivo study on whisker deformation within the follicle, and on a new histological analysis of follicle tissue. It is then used to simulate whisker deformation within the follicle during passive touch and active whisking. Results suggest that the most likely whisker deformation profile is “S-shaped,” crossing the midline of the follicle right below the ring sinus. Simulations of active whisking indicate that an increase in overall muscle stiffness, an increase in the ratio between deep and superficial intrinsic muscle stiffness, and an increase in sinus blood pressure will all enhance tactile sensitivity. Finally, we discuss how the deformation profiles might map to the responses of primary afferents of each mechanoreceptor type. The mechanical model presented in this study is an important first step in simulating mechanical interactions within whisker follicles.

Many mammals rely on whiskers as a mode of tactile sensation, especially when exploring in darkness. Active, rhythmic protraction and retraction of the whiskers, commonly referred to as “whisking,” is observed among many whisker specialist animals. During whisker-based sensing, forces and moments generated by external stimuli are transmitted to the base of the whisker shaft inside the follicle. Within the follicle, the interaction between the whisker’s deformation and the surrounding tissue determines how different groups of mechanoreceptors will deform, thereby transducing the mechanical signals into electrical signals. However, it is not yet possible to experimentally measure this interaction in vivo. We therefore created a mechanical model of the follicle sinus complex to simulate whisker deformation within the follicle resulting from external whisker deflection. Our results provide the first estimate of whisker shape as it deforms in the follicle, during both passive touch and active whisking. In turn, these shape estimates allow us to predict how the whisker will deform against different types of mechanoreceptors at different locations within the follicle. In addition, we find that both intrinsic muscle contraction and an increase in blood pressure will improve the tactile sensitivity of the whisker system.

Regionalization of the axial skeleton predates functional adaptation in the forerunners of mammals

The evolution of semi-independent modules is hypothesized to underlie the functional diversification of serially repeating (metameric) structures. The mammal vertebral column is a classic example of a metameric structure that is both modular, with well-defined morphological regions, and functionally differentiated. How the evolution of regions is related to their functional differentiation in the forerunners of mammals remains unclear. Here we gathered morphometric and biomechanical data on the presacral vertebrae of two extant species that bracket the synapsid-mammal transition and use the relationship between form and function to predict functional differentiation in extinct non-mammalian synapsids. The origin of vertebral functional diversity does not correlate with the evolution of new regions but appears late in synapsid evolution. This decoupling of regions from functional diversity implies that an adaptive trigger is needed to exploit existing modularity. We propose that the release of axial respiratory constraints, combined with selection for novel mammalian behaviours in Late Triassic cynodonts, drove the functional divergence of pre-existing morphological regions.