Non-mammalian Synapsids Research Articles

Broad similarities in shoulder muscle architecture and organization across two amniotes: implications for reconstructing non-mammalian synapsids.

The evolution of upright limb posture in mammals may have enabled modifications of the forelimb for diverse locomotor ecologies. A rich fossil record of non-mammalian synapsids holds the key to unraveling the transition from “sprawling” to “erect” limb function in the precursors to mammals, but a detailed understanding of muscle functional anatomy is a necessary prerequisite to reconstructing postural evolution in fossils. Here we characterize the gross morphology and internal architecture of muscles crossing the shoulder joint in two morphologically-conservative extant amniotes that form a phylogenetic and morpho-functional bracket for non-mammalian synapsids: the Argentine black and white tegu Salvator merianae and the Virginia opossum Didelphis virginiana. By combining traditional physical dissection of cadavers with nondestructive three-dimensional digital dissection, we find striking similarities in muscle organization and architectural parameters. Despite the wide phylogenetic gap between our study species, distal muscle attachments are notably similar, while differences in proximal muscle attachments are driven by modifications to the skeletal anatomy of the pectoral girdle that are well-documented in transitional synapsid fossils. Further, correlates for force production, physiological cross-sectional area (PCSA), muscle gearing (pennation), and working range (fascicle length) are statistically indistinguishable for an unexpected number of muscles. Functional tradeoffs between force production and working range reveal muscle specializations that may facilitate increased girdle mobility, weight support, and active stabilization of the shoulder in the opossum—a possible signal of postural transformation. Together, these results create a foundation for reconstructing the musculoskeletal anatomy of the non-mammalian synapsid pectoral girdle with greater confidence, as we demonstrate by inferring shoulder muscle PCSAs in the fossil non-mammalian cynodont Massetognathus pascuali.

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.

Investigation of a bone lesion in a gorgonopsian (Synapsida) from the Permian of Zambia and periosteal reactions in fossil non-mammalian tetrapods.

While only distantly related to mammals, the anatomy of Permian gorgonopsians has shed light on the functional biology of non-mammalian synapsids and on the origins of iconic 'mammal-like' anatomical traits. However, little is known of gorgonopsian behaviour or physiology, which would aid in reconstructing the paleobiological context in which familiar mammalian features arose. Using multi-modal imaging, we report a discrete osseous lesion in the forelimb of a late Permian-aged gorgonopsian synapsid, recording reactive periosteal bone deposition and providing insights into the origins and diversity of skeletal healing responses in premammalian synapsids. We suggest that the localized lesion on the anterolateral (preaxial) shaft of the left radius represents acute periostitis and, conservatively, most likely developed as a subperiosteal haematoma with subsequent bone deposition and limited internal remodelling. The site records an inner zone of reactive cortical bone forming irregular to radial bony spicules and an outer, denser zone of slowed subperiosteal bone apposition, all of which likely occurred within a single growing season. In surveys of modern reptiles-crocodylians, varanids-such haematomas are rare compared to other documented osteopathologies. The extent and rapidity of the healing response is reminiscent of mammalian and dinosaurian bone pathologies, and may indicate differing behaviour or bone physiology compared to non-dinosaurian reptiles. This report adds to a growing list of putative disease entities recognized in early synapsids and broadens comparative baselines for pathologies and the evolution of bone response to disease in mammalian forebears. This article is part of the theme issue 'Vertebrate palaeophysiology'.

The postcranial anatomy of Endothiodon bathystoma (Anomodontia, Therapsida)

ABSTRACT Dicynodonts were herbivorous non-mammalian synapsids that were relatively abundant on the landmass of Pangea from the Mid Permian to Late Triassic. The dicynodont genus Endothiodon is well known from late Permian strata of the Karoo Basin of South Africa. It is characterised by replacement waves of internal tooth rows on the premaxilla and dentary; longitudinal ridges from the premaxilla to the pineal crest; and a pineal foramen on a prominent boss. Endothiodon is well-represented by cranial and postcranial material in various South African museum collections. Recently, an almost complete skeleton of Endothiodon (SAM-PK-K011271) was recovered from the uppermost Pristerognathus Assemblage Zone of the Karoo Supergroup. It comprises the skull and most postcranial elements preserved in articulation. The present study identifies SAM-PK-K011271 as E. bathystoma and we show that it is the second largest E. bathystoma specimen known to date. Furthermore, the well-preserved skeletal remains enable us to present the first comprehensive description of its postcranial anatomy. Abbreviations Institutional abbreviations: AM: Albany Museum, Grahamstown, South Africa; CGS: Council for Geosciences, Pretoria, South Africa; ESI: Evolutionary Studies Institute, University of the Witwatersrand, Johannesburg, South Africa; ISI: Indian Statistical Institute, Kolkata, India; SAM: Iziko South African Museum, Cape Town, South Africa.Anatomical abbreviations: ang, angular; art, articular; ast, astragalus; br, M. brachialis; c, centrale; ca, calcaneum; cb, M. coracobrachialis; cbl, M. coracobrachialis longus; Cd, caudal vertebra; d, dentary; del, M. deltoideus; ent, entepidcondyle; ent f, entepicondylar foramen; f, frontal; fti, M. femorotibialis; H, height; ife, M. iliofemoralis; ifi, M. iliofibularis; it, M. incisura tibialis fossa; iti, M. iliotibialis; il, ilium; is, ischium; j, jugal; l, lacrimal; M., metacarpal; mf, mandibular fenestra; Mt., metatarsal; n, nasal; nc, neural canal; ns, neural spine; ol, olecranon; P., phalanx; pb, pineal boss; pec, M. pectoralis; pifi, M. puboischiofemoralis internus; piti, M. puboischiotibialis; pm, premaxilla; po, postorbital; poz, postzygapophyses; prz, prezygapophyses; pp, preparietal; psf, postspinal fossa; pu, pubis; q, quadrate; r con, radial condyle; ref. l, reflected lamina; S, sacral vertebra; sas, M. serratus anterior superficialis; sc, M. subcoracoideus; scs, M. subcoracoscapularis; sha, M. scapulohumeralis anterior; SL, skull length; spc, M. supracoracoideus; sq, squamosal; sup, supinator; SW, skull width; tha, M. triceps humeralis anterior; thm, M. triceps humeralis medialis; tp, transverse process; tri, M. triceps; W, width.

The many faces of synapsid cranial allometry

AbstractPrevious studies of cranial shape have established a consistent interspecific allometric pattern relating the relative lengths of the face and braincase regions of the skull within multiple families of mammals. In this interspecific allometry, the facial region of the skull is proportionally longer than the braincase in larger species. The regularity and broad taxonomic occurrence of this allometric pattern suggests that it may have an origin near the base of crown Mammalia, or even deeper in the synapsid or amniote forerunners of mammals. To investigate the possible origins of this allometric pattern, we used geometric morphometric techniques to analyze cranial shape in 194 species of nonmammalian synapsids, which constitute a set of successive outgroups to Mammalia. We recovered a much greater diversity of allometric patterns within nonmammalian synapsids than has been observed in mammals, including several instances similar to the mammalian pattern. However, we found no evidence of the mammalian pattern within Therocephalia and nonmammalian Cynodontia, the synapsids most closely related to mammals. This suggests that the mammalian allometric pattern arose somewhere within Mammaliaformes, rather than within nonmammalian synapsids. Further investigation using an ontogenetic series of the anomodontDiictodon felicepsshows that the pattern of interspecific allometry within anomodonts parallels the ontogenetic trajectory ofDiictodon.This indicates that in at least some synapsids, allometric patterns associated with ontogeny may provide a “path of least resistance” for interspecific variation, a mechanism that we suggest produces the interspecific allometric pattern observed in mammals.

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

In mammals, the infraorbital canal provides a passage for the infraorbital ramus of the maxillary branch of the trigeminal nerve. The infraorbital nerve ensures tactile sensitivity of the upper teeth and face between the eye and upper lip and, more significantly, the innervation of mystacial vibrissae (whiskers). In contrast, most non-mammalian synapsids display a more “reptilian-like” ancestral condition in which a long and ramified maxillary canal completely enclosed the infraorbital nerve along with other branches of the trigeminal nerve. The phylogenetic transition from the ancestral “reptilian-like” to the derived “mammal-like” condition has been hypothesized to occur at the base of the Probainognathia clade. Using μCT and synchrotron scanning, this study aims to document this transition in detail by focusing on a sample of non-mammalian probainognathian cynodonts and early mammaliaforms. We find that the mammalian condition is the result of a gradual shortening of the maxillary canal, which enabled the infraorbital nerve to ramify within the soft tissues of the face. Mobile whiskers became possible only after the mammalian infraorbital nerve had evolved, which suggest that these structures appeared in Probainognathus and more derived cynodonts. Finally a foramen located on the ventral margin of the lacrimal bone, which has been often homologized with the infraorbital foramen of derived Probainognathia and early Mammaliaformes, is most probably homologous to the mammalian zygomaticofacial foramen.

Morphological evolution in therocephalians breaks the hypercarnivore ratchet.

Large carnivorous mammals have been suggested to show a ratchet-like mode of morphological evolution. A limited number of specializations for hypercarnivory evolve repeatedly in multiple clades, with those lineages evolving such specialities being unable to retreat back along their evolutionary trajectory or jump between adaptive peaks. While it has been hypothesized that such mechanisms should have applied to the evolution of other terrestrial carnivores, the non-mammalian synapsid clade Therocephalia appears to defy this expectation. The earliest, basalmost members of this clade are large macropredators, and it is later that small carnivores appear, seemingly evolving from top-predator ancestors. In order to test this reading of therocephalian evolution, variation in rates of body size evolution were tested for and incorporated into an ancestral reconstruction. Similar studies were made of the evolution of discrete characters related to carnivory. All analyses indicate the ancestral therocephalian was a large macro-predator, with serrated teeth, elongated canines and robust lower jaws. Small sizes apparently evolve later. It is therefore suggested that the hypercarnivore ratchet is a feature of mammalian evolution.

Cranial anatomy of the gorgonopsian Cynariops robustus based on CT-reconstruction.

Gorgonopsia is one of the major clades of non-mammalian synapsids, and includes an array of large-bodied carnivores that were the top terrestrial predators of the late Permian. Most research on the clade has focused on these largest members; small-bodied gorgonopsians are relatively little-studied. Here, we redescribe a small gorgonopsian skull (MB.R.999) from the late Permian (Tropidostoma Assemblage Zone) of South Africa on the basis of neutron and synchrotron CT reconstructions, which yield new data on internal cranial morphology in Gorgonopsia. Because of the largely undistorted nature of MB.R.999, we were also able to reconstruct unossified areas such as the brain endocast and the otic labyrinth. MB.R.999 can be referred to the taxon Cynariops robustus based on its general skull proportions, postcanine tooth count, preparietal morphology, and vomerine morphology. We refer additional small gorgonopsian specimens from the Victoria West area to Cynariops robustus, and consider Cynarioides grimbeeki and Cynarioides laticeps to be synonymous with C. robustus. Inclusion of Cynariops in a phylogenetic analysis of Gorgonopsia recovers it within a large clade of African taxa, more closely related to Lycaenops and rubidgeines than Eriphostoma or Gorgonops.

Early shifts lead to big changes

Mammalian Evolution Mammals represent one of the most morphologically diverse taxonomic groups. One of the unique features underlying this diversity is variability of the spine, which facilitates everything from flexibility for speedy running and support for upright walking. Jones et al. studied a group ancestral to modern mammals—nonmammalian synapsids, or mammal-like reptiles. As forelimb function diversified, the spine developed distinct regions. These regions then differentiated further, leading to the highly varied mammalian forms we see today. Science , this issue p. [1249][1] [1]: /lookup/volpage/361/1249?iss=6408

Fossils reveal the complex evolutionary history of the mammalian regionalized spine.

A unique characteristic of mammals is a vertebral column with anatomically distinct regions, but when and how this trait evolved remains unknown. We reconstructed vertebral regions and their morphological disparity in the extinct forerunners of mammals, the nonmammalian synapsids, to elucidate the evolution of mammalian axial differentiation. Mapping patterns of regionalization and disparity (heterogeneity) across amniotes reveals that both traits increased during synapsid evolution. However, the onset of regionalization predates increased heterogeneity. On the basis of inferred homology patterns, we propose a "pectoral-first" hypothesis for region acquisition, whereby evolutionary shifts in forelimb function in nonmammalian therapsids drove increasing vertebral modularity prior to differentiation of the vertebral column for specialized functions in mammals.

Jurassic stem-mammal perinates and theorigin of mammalian reproduction and growth.

Transformations in morphology, physiology and behaviour along the mammalian stem lineage were accompanied by profound modifications to reproduction and growth, including the emergence of a reproductive strategy characterized by high maternal investment in a small number of offspring1,2 and heterochronic changes in early cranial development associated with the enlargement of the brain3. Because direct fossil evidence of these transitions is lacking, the timing and sequence of these modifications are unknown. Here we present what is, to our knowledge, the first fossil record of pre- or near-hatching young of any non-mammalian synapsid. A large clutch of well-preserved perinates of the tritylodontid Kayentatherium wellesi (Cynodontia, Mammaliamorpha) was found with a presumed maternal skeleton in Early Jurassic sediments of the Kayenta Formation. The single clutch comprises at least 38 individuals, well outside the range of litter sizes documented in extant mammals. This discovery confirms that production of high numbers of offspring represents the ancestral condition for amniotes, and also constrains the timing of a reduction in clutch size along the mammalian stem. Although tiny, the perinates have an overall skull shape that is similar to that of adults, with no allometric lengthening of the face during ontogeny. The only positive allometries are associated with the bones that support the masticatory musculature. Kayentatherium diverged just before a hypothesized pulse of brain expansion that reorganized cranial architecture at the base of Mammaliaformes4-6. The association of a high number of offspring and largely isometric cranial growth in Kayentatherium is consistent with a scenario in which encephalization-and attendant shifts in metabolism and development7,8-drove later changes to mammalian reproduction.

Endothiodon cf. bathystoma (Synapsida: Dicynodontia) bony labyrinth anatomy, variation and body mass estimates.

The semicircular canal (SC) system of the inner ear detects head angular accelerations and is essential for navigation and spatial awareness in vertebrates. Because the bony labyrinth encloses the membranous labyrinth SCs, it can be used as a proxy for animal behavior. The bony labyrinth of dicynodonts, a clade of herbivorous non-mammalian synapsids, has only been described in a handful of individuals and remains particularly obscure. Here we describe the bony labyrinth anatomy of three Endothiodon cf. bathystoma specimens from Mozambique based on digital reconstructions from propagation phase-contrast synchrotron micro-computed tomography. We compare these findings with the bony labyrinth anatomy of their close relative Niassodon. The bony labyrinths of Endothiodon and Niassodon are relatively similar and show only differences in the shape of the horizontal SCs and the orientation of the vertical SCs. When compared to extant mammals, Endothiodon and Niassodon have highly eccentric SCs. In addition, the Endothiodon SCs are nearly orthogonal. An eccentric and orthogonal SC morphology is consistent with a specialization in rapid head movements, which are typical of foraging or feeding behaviors. Furthermore, we estimate the body mass of these Endothiodon specimens at ~116 to 182 kg, based on the average SC radii calculated using a linear regression model optimized by the Amemiya Prediction Criterion. Our findings provide novel insights into the paleobiology of Endothiodon which are consistent with the peculiar feeding mechanism among dicynodonts presumed from their multiple postcanine toothrows.

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

Anomodontia was the most successful herbivorous clade of the mammalian stem lineage (non-mammalian synapsids) during the late Permian and Early Triassic. Among anomodonts, Dicynodontia stands apart because of the presence of an osseous beak that shows evidence of the insertion of a cornified sheath, the ramphotheca. In this study, fourteen anomodont specimens were microCT-scanned and their trigeminal canals reconstructed digitally to understand the origin and evolution of trigeminal nerve innervation of the ramphotheca. We show that the pattern of innervation of the anomodont "beak" is more similar to that in chelonians (the nasopalatine branch is enlarged and innervates the premaxillary part of the ramphotheca) than in birds (where the nasopalatine and maxillary branches play minor roles). The nasopalatine branch is noticeably enlarged in the beak-less basal anomodont Patranomodon, suggesting that this could be an anomodont or chainosaur synapomorphy. Our analyses suggest that the presence or absence of tusks and postcanine teeth are often accompanied by corresponding variations of the rami innervating the caniniform process and the alveolar region, respectively. The degree of ossification of the canal for the nasal ramus of the ophthalmic branch also appears to correlate with the presence of a nasal boss. The nasopalatine canal is absent from the premaxilla in the Bidentalia as they uniquely show a large plexus formed by the internal nasal branch of the maxillary canal instead. The elongated shape of this plexus in Lystrosaurus supports the hypothesis that the rostrum evolved as an elongation of the subnarial region of the snout. Finally, the atrophied and variable aspect of the trigeminal canals in Myosaurus supports the hypothesis that this genus had a reduced upper ramphotheca.

Evidence for convergent evolution of a neocortex-like structure in a late Permian therapsid.

The special sensory, motor, and cognitive capabilities of mammals mainly depend upon the neocortex, which is the six-layered cover of the mammalian forebrain. The origin of the neocortex is still controversial and the current view is that larger brains with neocortex first evolved in late Triassic Mammaliaformes. Here, we report the earliest evidence of a structure analogous to the mammalian neocortex in a forerunner of mammals, the fossorial anomodont Kawingasaurus fossilis from the late Permian of Tanzania. The endocranial cavity of Kawingasaurus is almost completely ossified, which allowed a less hypothetical virtual reconstruction of the brain endocast to be generated. A parietal foramen is absent. A small pit between the cerebral hemispheres is interpreted as a pineal body. The inflated cerebral hemispheres are demarcated from each other by a median sulcus and by a possible rhinal fissure from the rest of the endocast. The encephalization quotient estimated by using the method of Eisenberg is 0.52, which is 2-3 times larger than in other nonmammalian synapsids. Another remarkable feature are the extremely ramified infraorbital canals in the snout. The shape of the brain endocast, the extremely ramified maxillary canals as well as the small frontally placed eyes suggest that special sensory adaptations to the subterranean habitat such as a well developed sense of touch and binocular vision may have driven the parallel evolution of an equivalent of the mammalian neocortex and a mammal-like lemnothalamic visual system in Kawingasaurus. The gross anatomy of the brain endocast of Kawingasaurus supports the Outgroup Hypothesis, according to which the neocortex evolved from the dorsal pallium of an amphibian-like ancestor, which receives sensory projections from the lemnothalamic pathway. The enlarged brain as well as the absence of a parietal foramen may be an indication for a higher metabolic rate of Kawingasaurus compared to other nonmammalian synapsids.

What did the "Unossified zone" of the non-mammalian therapsid braincase house?

Most nonmammalian synapsids possess a mid-dorsal depression in the brain cavity known as the "unossified zone." It remains obscure which structures this zone contained, and, as candidates, the vermis of the cerebellum, the superior sagittal sinus, a junction of several blood vessels, the pineal gland or other midbrain structures were considered. Neutron tomography of a skull of Diictodon feliceps (Therapsida, Anomodontia) revealed some clear impressions of canals in this region of the brain cavity. Furthermore, the prootic sinus probably ran on the internal surface of the pila antotica and had a similar course in anomodonts as it has been proposed for cynodonts and Mesozoic mammals. Comparisons with the vascular systems of nonmammalian synapsids and mammals suggest that the unossified zone is best interpreted as a terminal chamber of the anterior segment of the medial head vein, which housed the junction of the superior sagittal sinus and the transverse sinuses. Consequently, the system of cranial vessels in Diictodon reveals a partial division of the medial head vein system into an anterior and a posterior segment at an early stage of synapsid evolution, which is consistent with the well-known common pattern of early ontogenetic development in amniotes. J. Morphol., 2017. © 2017 Wiley Periodicals, Inc.

Correlation between Hox code and vertebral morphology in the mouse: towards a universal model for Synapsida

BackgroundThe importance of the cervical vertebrae as part of the skull–neck system in facilitating the success and diversity of tetrapods is clear. The reconstruction of its evolution, however, is problematic because of the variation in the number of vertebrae, making it difficult to identify homologous elements. Quantification of the morphological differentiation in the neck of diverse archosaurs established homologous units of vertebrae (i.e. modules) resulting from Hox gene expression patterns within the cervical vertebral column. The present study aims to investigate the modularity of the cervical vertebral column in the mouse and to reveal the genetic patterns and changes underlying the evolution of the neck of modern mammals and their extinct relatives. In contrast to modern mammals, non-mammalian synapsids are characterized by a variable cervical count, the presence of free cervical ribs and the presence of a separate CV1 centrum. How might these evolutionary modifications be associated with changes in the Hox code?ResultsIn combination with up-to-date information on cervical Hox gene expression including description of the vertebral phenotype of Hox knock-out mutants, the 3D landmark-based geometric morphometric approach demonstrates a correlation between Hox code and vertebral morphology in the mouse. There is evidence that the modularity of the neck of the mouse had already been established in the last common ancestor of mammals, but differed from that of non-mammalian synapsids. The differences that likely occurred during the evolution of synapsids include an anterior shift in HoxA-5 expression in relation to the reduction of cervical ribs and an anterior shift in HoxD-4 expression linked to the development of the highly differentiated atlas-axis complex, whereas the remaining Hox genes may have displayed a pattern similar to that in mammals on the basis of the high level of conservatism in the axial skeleton of this lineage.ConclusionThus, the mouse Hox code provides a model for understanding the evolutionary mechanisms responsible for the great morphological adaptability of the cervical vertebral column in Synapsida. However, more studies in non-model organisms are required to further elucidate the evolutionary role of Hox genes in axial patterning of the unique mammalian body plan.

Neutron Tomography and X-ray Tomography as Tools for the Morphological Investigation of Non-mammalian Synapsids

As having evolved on the stem line of mammals, the taxonomy and phylogeny of therapsids (Synapsida) are of special interest with respect to early mammalian evolution. Due to the fact that in most cases soft tissue of fossil vertebrates is not preserved, species can only be distinguished by diagnosis of morphological features of the skeleton. Moreover, investigations of vertebrate fossils are often obstructed, because internal cranial anatomy is usually hidden and parts of the fossils may be embedded in stone matrix. As a consequence, most species of non-mammalian synapsids were only defined on the basis of external skeletal features. Our investigations on Diictodon skulls (Therapsida, Anomodontia) show that non-destructive methods are very useful to clearly distinguish fossil species. We, therefore, propose using modern non-destructive techniques such as neutron tomography, synchrotron tomography, and micro-computed tomography (μCT) as standard tools for the investigation and virtual reconstruction of fossils and to include features of the internal cranial anatomy into morphological descriptions and phylogenetic analyses of fossil vertebrates.

Morphological evolution of the mammalian jaw adductor complex.

ABSTRACTThe evolution of the mammalian jaw during the transition from non‐mammalian synapsids to crown mammals is a key event in vertebrate history and characterised by the gradual reduction of its individual bones into a single element and the concomitant transformation of the jaw joint and its incorporation into the middle ear complex. This osteological transformation is accompanied by a rearrangement and modification of the jaw adductor musculature, which is thought to have allowed the evolution of a more‐efficient masticatory system in comparison to the plesiomorphic synapsid condition. While osteological characters relating to this transition are well documented in the fossil record, the exact arrangement and modifications of the individual adductor muscles during the cynodont–mammaliaform transition have been debated for nearly a century.We review the existing knowledge about the musculoskeletal evolution of the mammalian jaw adductor complex and evaluate previous hypotheses in the light of recently documented fossils that represent new specimens of existing species, which are of central importance to the mammalian origins debate. By employing computed tomography (CT) and digital reconstruction techniques to create three‐dimensional models of the jaw adductor musculature in a number of representative non‐mammalian cynodonts and mammaliaforms, we provide an updated perspective on mammalian jaw muscle evolution.As an emerging consensus, current evidence suggests that the mammal‐like division of the jaw adductor musculature (into deep and superficial components of the m. masseter, the m. temporalis and the m. pterygoideus) was completed in Eucynodontia. The arrangement of the jaw adductor musculature in a mammalian fashion, with the m. pterygoideus group inserting on the dentary was completed in basal Mammaliaformes as suggested by the muscle reconstruction of Morganucodon oehleri. Consequently, transformation of the jaw adductor musculature from the ancestral (‘reptilian’) to the mammalian condition must have preceded the emergence of Mammalia and the full formation of the mammalian jaw joint. This suggests that the modification of the jaw adductor system played a pivotal role in the functional morphology and biomechanical stability of the jaw joint.

Dental peculiarities in the silvery mole-rat: an original model for studying the evolutionary and biological origins of continuous dental generation in mammals.

Unravelling the evolutionary and developmental mechanisms that have impacted the mammalian dentition, since more than 200 Ma, is an intricate issue. Interestingly, a few mammal species, including the silvery mole-rat Heliophobius argenteocinereus, are able to replace their dentition by the addition of supernumerary molars at the back of jaw migrating then toward the front. The aim here was to demonstrate the potential interest of further studying this rodent in order to better understand the origins of continuous dental replacement in mammals, which could also provide interesting data concerning the evolution of limited dental generation occurring in first mammals. In the present study, we described the main stages of the dental eruptive sequence in the silvery mole-rat and the associated characteristics of horizontal replacement using X-ray microtomography. This was coupled to the investigation of other African mole-rats which have no dental replacement. This method permitted to establish evidence that the initial development of the dentition in Heliophobius is comparable to what it is observed in most of African mole-rats. This rodent first has premolars, but then identical additional molars, a mechanism convergent to manatees and the pygmy rock-wallaby. Evidence of continuous replacement and strong dental dynamics were also illustrated in Heliophobius, and stressed the need to deeply investigate these aspects for evolutionary, functional and developmental purposes. We also noticed that two groups of extinct non-mammalian synapsids convergently acquired this dental mechanism, but in a way differing from extant mammals. The discussion on the diverse evolutionary origins of horizontal dental replacement put emphasis on the necessity of focusing on biological parameters potentially involved in both continuous and limited developments of teeth in mammals. In that context, the silvery mole-rat could appear as the most appropriate candidate to do so.

Virtual reconstruction and description of the cranial endocast of Pristerodon mackayi (Therapsida, Anomodontia).

The cranial endocast of Pristerodon mackayi is described, which has been virtually reconstructed on the basis of neutron tomographic data. The gross anatomy of the cranial endocast of Pristerodon resembles other nonmammalian synapsids such as Thrinaxodon liorhinus in having a narrow, tubular forebrain, well developed olfactory bulbs, a large parietal foramen and unossified zone. As it is the case in cynodonts the hindbrain of Pristerodon is broader than the mid- and forebrain. Large paraflocculi are developed. The medulla oblongata can be well distinguished from the pons. The pons is divided by a median ridge into two portions. There is no evidence for a neocortex, which seems to be also reflected in the low encephalization quotient of 0.18 estimated according to the method of Eisenberg.