Non-mammaliaform Cynodonts Research Articles

Sensory Anatomy and Sensory Ecology of Vintana Sertichi (Mammalia, Gondwanatheria) from the Late Cretaceous of Madagascar

ABSTRACT—Living mammals are distinguished from other extant tetrapods by adaptations for improved senses of hearing, touch, and smell. These adaptations, and concomitant reductions in visual anatomy, evolved during the Mesozoic in the mammalian and therian stem lineages. Here, we present a comparative study of the sensory anatomy of the Late Cretaceous gondwanatherian mammaliaform Vintana sertichi in order to draw inferences regarding its sensory abilities and sensory ecology. Our analyses demonstrate that Vintana has relatively large orbits that may have accommodated large eyes. Vintana also possessed cochlear primary and secondary osseous laminae and a cochlear canal that was relatively longer than in non-mammaliaform cynodonts but shorter than in extant therians. These features suggest that Vintana had some capacity for high-frequency hearing (i.e., >20 kHz), but that its cochlea may have encoded a more limited range of frequencies than the cochleas of most extant therians. The semicircular canals of Vintana have large radii of curvature and are nearly orthogonal, suggesting high sensitivity to angular head accelerations. These vestibular features may have evolved in order to stabilize large eyes during rapid and/or agile locomotion. Combined with evidence for large olfactory bulbs and a large trigeminal endocast, these data reveal that Vintana possessed a unique suite of sensory adaptations that distinguish it from other Mesozoic mammaliaforms. If these inferences are correct, then Vintana was probably a large-eyed and agile species, with a keen sense of smell and better high-frequency hearing than most other Mesozoic mammaliaforms.SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP

Endocranial and Inner Ear Morphology of Vintana Sertichi (Mammalia, Gondwanatheria) from the Late Cretaceous of Madagascar

ABSTRACTWe present the first digital reconstruction of the endocranial cavity and endosseous labyrinth of the Late Cretaceous gondwanatherian mammal Vintana sertichi from the Maevarano Formation of Madagascar. The Malagasy specimen is exceptionally well preserved and represents the only described cranium known for Gondwanatheria, an enigmatic clade from the Late Cretaceous and Paleogene of Gondwana. The endocranial cast of Vintana is relatively small for an animal of its estimated body mass. Its encephalization quotient is 0.28–0.56 for a range of body mass estimates, which is similar to that of basal mammaliaforms. The olfactory bulbs are very large, occupying over 14% of the endocranial volume. The cerebral hemispheres are only slightly expanded, more similar to the condition in Morganucodon than to that in multituberculates and monotremes. Unlike the condition in other Mesozoic mammaliaforms, the endocast is greatly flexed at the circular fissure. The osseous labyrinth displays a mixture of derived and primitive features. The cochlear canal is only slightly curved and short compared with that of extant therians. The ratio between total cochlear canal length and maximum cranial length is smaller than in basal mammaliaforms and approximates that of non-mammaliaform cynodonts. By contrast, the presence of both primary and secondary osseous laminae, the tractus foraminosus, and Rosenthal's canal represent derived characteristics of the mammalian inner ear typical of cladotherians. A modern innervation of the cochlea has either evolved independently in Vintana and cladotherians or was already present in the last common ancestor of both clades.SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP

Nasal anatomy of the non-mammaliaform cynodont Brasilitherium riograndensis (Eucynodontia, Therapsida) reveals new insight into mammalian evolution.

The mammalian nasal cavity is characterized by a unique anatomy with complex internal features. The evolution of turbinals was correlated with endothermic and macrosmatic adaptations in therapsids and in early mammals, which is still apparent in their twofold function (warming and moistening of air, olfaction). Fossil evidence for the transformation from the nonmammalian to the mammalian nasal cavity pattern has been poor and inadequate. Ossification of the cartilaginous nasal capsule and turbinals seems to be a feature that occurred only very late in synapsid evolution but delicate ethmoidal bones are rarely preserved. Here we provide the first µCT investigation of the nasal cavity of the advanced non-mammaliaform cynodont Brasilitherium riograndensis from the Late Triassic of Southern Brazil, a member of the sister-group of mammaliaforms, in order to elucidate a critical anatomical transition in early mammalian evolution. Brasilitherium riograndensis already had at least partially ossified turbinals as remnants of the nasoturbinal and the first ethmoturbinal are preserved. The posterior nasal septum is partly ossified and contributes to a mesethmoid. The nasal cavity is posteriorly expanded and forms a distinctive pars posterior (ethmoidal recess) that is ventrally separated from the nasopharyngeal duct by a distinct lamina terminalis. Thus, our observations clearly demonstrate that principal features of the mammalian nasal cavity were already present in the sister-group of mammaliaforms.

Study of a digital cranial endocast of the non-mammaliaform cynodont Brasilitherium riograndensis (Later Triassic, Brazil) and its relevance to the evolution of the mammalian brain

A digital cranial endocast of the specimen UFRGS-PV-1043-T, Brasilitherium riograndensis, was obtained from high-resolution computed tomography (μCT) scan images. This taxon is a small cynodont from the Late Triassic of Brazil, and has been used as the sister-group of the mammaliaforms in cladistic analyses. The digital endocast of UFRGS-PV-1043-T is mostly complete, allowing the description and collection of accurate linear and volumetric measurements, which were taken and compared with other non-mammaliaform cynodonts. Impressions of vessels were observed in the inner walls of the braincase. Despite the lack of a cribiform plate and the presence of a wide orbital vacuity, the endocast of Brasilitherium shows olfactory bulb casts that are relatively larger than in other non-mammaliaform cynodonts, suggesting a pattern of gradual increase in size and improvement of the olfactory sense for these structures toward the mammalian condition. The cerebral hemispheres are elongated and clearly divided by a median sulcus. The parafloccular casts are well defined, and their position corresponds to the maximum width of the endocast. In the ventral view, a large hypophyseal cast and a wide opening for the cavum epiptericum are evident. The encephalization quotient (EQ) calculated for Brasilitherium is greater than the range of EQs reported for most non-mammaliaform cynodonts (although it may be similar to that of some taxa, according to the equation used to estimate their body masses), but it is smaller than that of the mammaliaforms and mammals. A slighter increase in the brain size of Brasilitherium compared with other non-mammaliaform cynodonts was observed, along with a more significant increase in the size of the olfactory bulbs. This study supports the proposition of an early evolution of the mammalian brain associated with selective pressures for better sensorial acuity, especially regarding improved olfaction, which began with small Triassic mammaliamorphs.

Digital Reconstruction of the Otic Region and Inner Ear of the Non-Mammalian Cynodont Brasilitherium riograndensis (Late Triassic, Brazil) and Its Relevance to the Evolution of the Mammalian Ear

The external anatomy of the petrosal, the bony labyrinth of the inner ear, and the stapes of Brasilitherium riograndensis (specimen UFRGS-PV-1043-T) were investigated by digital 3D reconstructions based on μCT scan images. Brasilitherium is the most basal taxon bearing a distinct promontorium, although less inflated than that of Morganucodon and still lacking a flat medial facet. A bony wall formed by the petrosal separates the cochlear canal and the vestibule from the brain cavity, with an internal acoustic meatus bearing distinct foramina for the facial nerve (VII) and vestibulocochlear nerve (VIII). The semicircular canals are irregular in shape, the anterior canal being the largest and the lateral one the smallest. Brasilitherium has an elongated but straight cochlear canal. The stapes resembles the morphology of derived non-mammaliaform cynodonts, such as Probainognathus and Pachygenelus, and differs from Thrinaxodon. By the allometric relationship of the cochlear canal and the estimated body mass, Brasilitherium can be grouped with Yunnanodon and Morganucodon in a regression line, which is below the line of mammals and above the line of non-avian reptiles. Brasilitherium fits in a sequence of gradual elongation of the cochlear canal associated with the enhancement in the capacity to hear higher frequencies. Among the constraints that might have triggered these transformations in small, insectivorous, and possibly nocturnal Mesozoic cynodont taxa is the improvement of detecting acoustically active insects.

Forerunners of mammals: radiation, histology, biology

Preface Acknowledgments List of Contributors 1. The Origin and Radiation of Therapsids Tom S. Kemp 2. Therapsid Biodiversity Patterns and Paleoenvironments of the Karoo Basin, South Africa Roger Smith, Bruce Rubidge, and Merrill van der Walt 3. The Microstructure of Bones and Teeth of Nonmammalian Therapsids Anusuya Chinsamy-Turan 4. The Paleobiology and Bone Microstructure of Pelycosaurian-Grade Synapsids Adam K. Huttenlocker and Elizabeth Rega 5. Dicynodont Growth Dynamics and Lifestyle Adaptations Sanghamitra Ray, Jennifer Botha-Brink, and Anusuya Chinsamy-Turan 6. Biological Inferences of the Cranial Microstructure of the Dicynodonts Oudenodon and Lystrosaurus Sandra C. Jasinoski and Anusuya Chinsamy-Turan 7. Bone and Dental Histology of Late Triassic Dicynodonts from North America Jeremy L. Green 8. Bone Histology of Some Therocephalians and Gorgonopsians, and Evidence of Bone Degradation by Fungi Anusuya Chinsamy-Turan and Sanghamitra Ray 9. The Radiation and Osteohistology of Nonmammaliaform Cynodonts Jennifer Botha-Brink, Fernando Abdala, and Anusuya Chinsamy-Turan 10. The Radiation, Bone Histology, and Biology of Early Mammals Jorn H. Hurum and Anusuya Chinsamy-Turan 11. The Evolution of Mammalian Endothermy John A. Ruben, Willem J. Hillenius, Tom S. Kemp, and Devon E. Quick References Index

Mandibular histology and growth of the nonmammaliaform cynodont Tritylodon

AbstractAn investigation of bone microstructure of nonmammalian therapsids has revealed distinctive signals pertaining to their ontogenetic growth and biology. Until now, histological studies of the nonmammaliaform cynodonts have focused only on postcranial material. Through the examination of micro‐computed tomography (micro‐CT) scans and serial thin sections, the current study provides a novel perspective on the structure and growth of the mandible of Tritylodon, a derived herbivorous cynodont from southern Africa. By tracking histological features across the serial thin sections, trends in relocation and modelling are documented for the growing Tritylodon mandible. For example, during growth, localized changes in the cross‐sectional shape of the mandible occurred. Localized deposits of new lamellar and fibrolamellar bone on the lateral edge indicate widening of the mandible during different episodes of growth. The presence of radial channels indicates the deepening of the mandible at its anterior and posterior ends. The relocation of the paired mental foramina suggests that the mandibular body lengthened mainly in the posterior direction. The medial movement of a posterior postcanine tooth during growth and eruption is recorded in the histology. This histological assessment also documents the presence of Sharpey’s fibres in the cellular cementum of the first incisor, providing novel and unequivocal evidence that it was attached to the Tritylodon jaw by a periodontal ligament. This is the first comprehensive study that uses histological analysis to document the growth dynamics of the mandible of a nonmammalian therapsid, thus providing a unique perspective of localized mandibular growth in a fossil animal.

Trucidocynodon riograndensis gen. nov. et sp. nov. (Eucynodontia), a new cynodont from the Brazilian Upper Triassic (Santa Maria Formation)

An almost complete skeleton of a new carnivorous cynodont from the Upper Triassic of Southern Brazil, Trucidocynodon riograndensis gen. nov. et sp. nov., is described. The new taxon is very similar to Ecteninion lunensis Martinez et al. 1996 from Upper Triassic of Argentina (Ischigualasto Formation). Both have an elongated skull, large pterygoid flanges, a well developed orbitosphenoid, a relatively short osseous secondary palate, greatly developed canines and sectorial postcanines with posteriorly directed cusps. However, the new taxon shows some differences relative to E. lunensis such as an open pterygoparoccipital foramen, the posterior opening of the post-temporal foramen enclosed by tabular and squamosal, and upper incisors with serrated cutting edges. The more remarkable features of the postcranium of T. riograndensis are the presence of 32 presacral vertebrae, a greater number than in most of non-mammaliaform cynodonts, the accentuated lumbarisation of the posterior trunk vertebrae and a remarkable morphological gradient in the caudal vertebrae; the femur and humerus show some interesting adaptations suggesting a more upright limb posture. The overall morphology of the astragalocalcaneal complex is similar to that of Diademodon and of the therocephalian Bauria.

The oldest cynodont: new clues on the origin and early diversification of the Cynodontia

Early mammaliaforms and their extinct relatives, nonmammaliaform cynodonts, have long been the focus of intense research in attempting to unravel how and when major changes toward mammalness occurred. The earliest well-known representatives of cynodonts are latest Late Permian in age. Here, we describe Charassognathus gracilis gen. et sp. nov., from the early Late Permian of South Africa, representing the oldest cynodont yet found. This specimen displays a notch on the dentary in the same location as the base of the masseteric fossa in the basal cynodonts Procynosuchus and Dvinia, and represents the first indication in theriodonts of an invasion of occlusal musculature onto the dentary. A phylogenetic analysis of seven therocephalians and ten non-mammaliaform cynodonts and equally weighted characters resulted in nine most parsimonious trees, the strict consensus of which shows a basal polytomy in cynodonts, including Charassognathus, Dvinia, Procynosuchus and a clade including the remaining cynodonts. The basal polytomy in the majority rule consensus tree is reduced, as Procynosuchus and Dvinia form a clade. One most parsimonious tree, from an analysis using implied weights, positions Charassognathus as the most basal cynodont. This result implies that the Cynodontia initially diversified in Permian Gondwana, in what is now southern Africa. © 2007 The Linnean Society of London, Zoological Journal of the Linnean Society, 2007, 149, 477–492.

A new mammaliaform from the early Jurassic and evolution of mammalian characteristics.

A fossil from the Early Jurassic (Sinemurian, approximately 195 million years ago) represents a new lineage of mammaliaforms, the extinct groups more closely related to the living mammals than to nonmammaliaform cynodonts. It has an enlarged cranial cavity, but no postdentary trough on the mandible, indicating separation of the middle ear bones from the mandible. This extends the earliest record of these crucial mammalian features by some 45 million years and suggests that separation of the middle ear bones from the mandible and the expanded brain vault could be correlated. It shows that several key mammalian evolutionary innovations in the ear region, the temporomandibular joint, and the brain vault evolved incrementally through mammaliaform evolution and long before the differentiation of the living mammal groups. With an estimated body weight of only 2 grams, its coexistence with other larger mammaliaforms with similar "triconodont-like" teeth for insectivory within the same fauna suggests a great trophic diversity within the mammaliaform insectivore feeding guild, as inferred from the range of body sizes.