Massetognathus Research Articles

Comparative bone microstructure of two non-mammaliaform cynodonts from the Late Triassic (Carnian) Chañares formation of Northwestern Argentina

ABSTRACT The aim of the present contribution is to examine the bone histology of two non-mammaliaform cynodonts recovered from the Late Triassic (Carnian) Chañares Formation, at Los Chañares locality, La Rioja province, Northwestern Argentina. The taxa analysed here are the Traversodontidae Massetognathus pascuali and the Probainognathidae Probainognathus jenseni, both forms known from multiple skeletons. Our osteohistological results reveal an uninterrupted growth pattern and high intraspecific histovariability in Massetognathus. On the other hand, Probainognathus presents cyclical growth, a cortex dominated by parallel-fibred bone tissue and scarce vascularisation. In spite of being derived non-mammaliaform eucynodonts, Massetognathus and Probainognathus show high growth plasticity, a feature usually regarded as plesiomorphic for cynodonts. Both taxa show a thick cortex in the long bones, a feature that could be linked to counteract the strong flexion forces during digging.

The first pectoral and forelimb material assigned to the lagerpetid Lagerpeton chanarensis (Archosauria: Dinosauromorpha) from the upper portion of the Chañares Formation, Late Triassic

The posture of the earliest dinosaurs is thought to be bipedal whereas their pseudosuchian relatives and stem archosaurs are thought to be typically quadrupedal. Therefore, the transition from quadrupedality to bipedality lies somewhere between the origin of Avemetatarsalia (bird-line archosaurs) and Dinosauria. However, studying this transition is hampered by the lack of forelimb fossils from many of the close relatives of dinosaurs and it is not clear if the morphology of the few dinosauromorphs that have forelimb material is unique or represents the plesiomorphic condition. New forelimb fossils of dinosaur relatives and careful assessments of their osteology are sorely needed to help address this knowledge gap. Here we present the first pectoral (left scapulocoracoid) and forelimb (right humerus) bones of an avemetatarsalian from the upper portion of the Chañares Formation of Argentina. We hypothesize that the bones pertain to the important early ornithodiran Lagerpeton chanarensis, but the scapulocoracoid and humerus does not precisely match any non-dinosaurian avemetatarsalian. The bones were prepared from a concretion that only consisted of Lagerpeton chanarensis and the cynodont Massetognathus pascuali. We identify the bones as belonging to Lagerpeton chanarensis because the distal end of the femur possesses an inflated crista tibiofibularis – a lagerpetid character state – and the newly recognized pectoral and forelimb bones are generally similar to those of the lagerpetid Dromomeron romeri and Ixalerpeton polesinensis, with tall and constricted anteroposteriorly narrow scapular blades and a humerus with a highly asymmetrical proximal end. The length of humerus and the proportions of the proximal and distal end in Lagerpeton chanarensis are also more similar to that of Dromomeron romeri. Overall, the scapulocoracoids and humeri of lagerpetids are similar in proportion across taxa, but comparing the length of the forelimbs to the hindlimbs is hampered by the lack of articulated or unambiguously associated individuals of any member of the group. Currently, it is still not clear if the anatomy of the pectoral girdle and forelimb of lagerpetids, and thus posture, is unique for lagerpetids or represents the ancestral condition for ornithodirans.

Three-dimensional mobility and muscle attachments in the pectoral limb of the Triassic cynodont Massetognathus pascuali (Romer, 1967).

The musculoskeletal configuration of the mammalian pectoral limb has been heralded as a key anatomical feature leading to the adaptive radiation of mammals, but limb function in the non-mammaliaform cynodont outgroup remains unresolved. Conflicting reconstructions of abducted and adducted posture are based on mutually incompatible interpretations of ambiguous osteology. We reconstruct the pectoral limb of the Triassic non-mammaliaform cynodont Massetognathus pascuali in three dimensions, by combining skeletal morphology from micro-computed tomography with muscle anatomy from an extended extant phylogenetic bracket. Conservative tests of maximum range of motion suggest a degree of girdle mobility, as well as substantial freedom at the shoulder and the elbow joints. The glenoid fossa supports a neutral pose in which the distal end of the humerus points 45° posterolaterally from the body wall, intermediate between classically 'sprawling' and 'parasagittal' limb postures. Massetognathus pascuali is reconstructed as having a near-mammalian complement of shoulder muscles, including an incipient rotator cuff (m.subscapularis, m.infraspinatus, m.supraspinatus, and m.teres minor). Based on close inspection of the morphology of the glenoid fossa, we hypothesize a posture-driven scenario for the evolution of the therian ball-and-socket shoulder joint. The musculoskeletal reconstruction presented here provides the anatomical scaffolding for more detailed examination of locomotor evolution in the precursors to mammals.

New information on the postcranial skeleton of Massetognathus ochagaviae Barberena, 1981 (Eucynodontia, Traversodontidae), from the Middle Triassic of Southern Brazil

New postcranial remains of Massetognathus ochagaviae are described based on a new specimen collected at the Dinodontosaurus Assemblage Zone from the Middle Triassic of Southern Brazil. Several isolated teeth collected together with the postcranial skeleton allowed a taxonomic assignation of the specimen to M. ochagaviae. Its postcranial morphology is quite similar to Massetognathus pascuali, especially regarding the morphology of the transverse process of presacral vertebrae (i.e. laminar in the anterior most and rounded in the posterior most presacral vertebrae); humerus (i.e. the head is dorsally deflected with a ‘fan-shaped’ distal end a deltopectoral crest abruptly ending at the middle of shaft); and femur (with a bulbous and dorsomedially inclined femoral head, lesser trochanter which abruptly begins near the intertrochanteric fossa and extending up to the middle of the femoral shaft and the medial condyle more pronounced and ventrally projected than the lateral one). On the other hand, the clavicle of M. ochagaviae is less lateromedially elongated than in M. pascuali.

Gomphodont cynodonts of the Chañares Formation: the analysis of an ontogenetic sequence

ABSTRACT A revision of gomphodont cynodonts from the Chañares Formation of Argentina, is presented. Four species in two genera: Massetognathus pascuali, M. teruggii, M. major, and Megagomphodon oligodens were originally described by Romer in 1967 and 1972. Qualitative characters proposed originally as diagnostic of the species were revised, and a bivariate analysis of allometry of 17 linear skull measurements was performed upon 31 specimens. Allometry relationships account for most of the differences among specimens. There are neither size gaps nor systematic trends across the sample. The temporal region scales with a ‘positive’ allometry, whereas components of the neuro-cranium and muzzle show ‘negative’ or isometric scaling. This results in a change from short, stocky skulls with large muzzles in small specimens, to slender skulls with small muzzles and large temporal fossae characteristic of large individuals. The reduction and ontogenetic loss of the parietal foramen in a cynodont is reported for the first time. Consideration of qualitative characters together with the allometric results lead us to conclude that all forms belong to a single species in a growth series. Consequently, Massetognathus pascuali is recognized as the only valid entity, an idea previously suggested but without a justification based upon extensive analysis.

Homologies of the Prootic Canal in Mammals and Non-mammalian Cynodonts

ABSTRACT The term prootic canal has been used to describe vascular canals in the adult petrosal bones of extant monotremes and marsupials and presumed vascular canals in the petrosals of extinct “non-tribosphenic” mammals (i.e., morganucodontids, docodontids, triconodontids, multitubercu- lates, Vincelestes). Study of the ontogeny of these canals in extant mammals and of their morphology in extinct mammals reveals that the term prootic canal has actually been applied to four different arrangements of canals. The prootic canal variously encloses the prootic sinus (most extinct “non- tribosphenic” mammals), the prootic sinus in company with the ramus superior of the stapedial artery (the platypus Ornithorhynchus and most multituberculates), the prootic sinus and the lateral head vein (the echidna Tachyglossus), and the lateral head vein alone (didelphid, caenolestid, and some dasyurid marsupials). We propose restricting the term prootic canal to the canal enclosing the prootic sinus (middle cerebral vein) at its exit from the cranial cavity. As so defined, the prootic canal is a canal that: (1) in extinct “non-tribosphenic” mammals passes through the lateral trough of the petrosal; (2) in the platypus forms between the petrosal and lamina obturans and also transmits a meningeal branch of the ramus superior; (3) in the echidna forms within the petrosal bone; and (4) in marsupials forms between the petrosal and squamosal. Several presumed vascular canals in non-mammalian cynodonts have been proposed as precursors of the canal enclosing the prootic sinus in mammals. We review the vascular structures of the ear region and lateral braincase in non-mammalian cynodonts and conclude that the most likely precursor is a canal through the lateral flange of the prootic similar to those interpreted as enclosing the prootic sinus in Probainognathus jenseni and Massetognathus pascuali. However, until a prootic canal is found in other non-mammalian cynodonts, those in Probainognathus and Massetognathus are most reasonably viewed, given current phylogenies, as acquisitions independent of the prootic canal in mammals.