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Studies of soft tissue effects on joint mobility in extant animals can help to constrain hypotheses about joint mobility in extinct animals. However, joint mobility must be considered in three dimensions simultaneously, and applications of mobility data to extinct taxa require both a phylogenetically informed reconstruction of articular morphology and justifications for why specific structures' effects on mobility are inferred to be similar. We manipulated cadaveric hip joints of common quail and recorded biplanar fluoroscopic videos to measure a 'ligamentous' range of motion (ROM), which was then compared to an 'osteological' ROM on a ROM map. Nearly 95% of the joint poses predicted to be possible at the hip based on osteological manipulation were rendered impossible by ligamentous constraints. Because the hip joint capsule reliably includes a ventral ligamentous thickening in extant diapsids, the hip abduction of extinct ornithodirans with an offset femoral head and thin articular cartilage was probably similarly constrained by ligaments as that of birds. Consequently, in the absence of extraordinary evidence to the contrary, our analysis casts doubt on the 'batlike' hip pose traditionally inferred for pterosaurs and basal maniraptorans, and underscores that reconstructions of joint mobility based on manipulations of bones alone can be misleading.

Author(s): Bhart-Anjan S. Bhullar [sup.1] [sup.2] , Armita R. Manafzadeh [sup.3] , Juri A. Miyamae [sup.1] [sup.2] , Eva A. Hoffman [sup.4] , Elizabeth L. Brainerd [sup.3] , Catherine Musinsky [...]

Over the past two centuries, mammalian chewing and related anatomical features have been among the most discussed of all vertebrate evolutionary innovations.sup.1-3. Chief among these features are two characters: the dentary-only mandible, and the tribosphenic molar with its triangulated upper cusps and lower talonid basin.sup.3-5. The flexible mandibular joint and the unfused symphysis of ancestral mammals--in combination with transformations of the adductor musculature and palate--are thought to have permitted greater mobility of each lower jaw, or hemimandible.sup.6,7. Following the appearance of precise dental occlusion near the origin of the mammalian crown.sup.8,9, therians evolved a tribosphenic molar with a craggy topography that is presumed to have been used to catch, cut and crush food. Here we describe the ancestral tribosphenic therian chewing stroke, as conserved in the short-tailed opossum Monodelphis domestica: it is a simple symmetrical sequence of lower tooth-row eversion and inversion during jaw opening and closing, respectively, enacted by hemimandibular long-axis rotation. This sequence is coupled with an eversion-inversion rotational grinding stroke. We infer that the ancestral therian chewing stroke relied heavily on long-axis rotation, including symmetrical eversion and inversion (inherited from the first mammaliaforms) as well as a mortar-and-pestle rotational grinding stroke that was inherited from stem therians along with the tribosphenic molar. The yaw-dominated masticatory cycle of primates, ungulates and other bunodont therians is derived; it is necessitated by a secondarily fused jaw symphysis, and permitted by the reduction of high, interlocking cusps.sup.10-12. The development of an efficient masticatory system--culminating in the tribosphenic apparatus--allowed early mammals to begin the process of digestion by shearing and crushing food into small boli instead of swallowing larger pieces in the reptilian manner, which necessitates a long, slow and wholly chemical breakdown. The vast diversity of mammalian teeth has emerged from the basic tribosphenic groundplan.sup.13. The ancestral tribosphenic therian chewing stroke is conserved in Monodelphis domestica, and couples tooth-row eversion and inversion in jaw opening and closing, respectively, with a rotational grinding stroke., Author(s): Bhart-Anjan S. Bhullar [sup.1] [sup.2] , Armita R. Manafzadeh [sup.3] , Juri A. Miyamae [sup.1] [sup.2] , Eva A. Hoffman [sup.4] [sup.5] , Elizabeth L. Brainerd [sup.3] , Catherine [...]

Paleobiological reconstructions of joint mobility are an essential component of functional analyses of extinct animals. Over the past half-decade, the methods underlying mobility studies have advanced rapidly in three main areas: increasing complexity of virtual joint manipulation, formalizing pose viability criteria, and constructing more rigorous quantitative frameworks. Here we contextualize and review the recent history of this field, and call attention to remaining challenges and potential future directions. Additionally, we make available and describe a set of user-friendly scripts for the animation software Autodesk Maya. In doing so, we aim to make many of the latest approaches for virtual mobility reconstruction more easily accessible to other researchers, encouraging their broader adoption and collaborative improvement. [ABSTRACT FROM AUTHOR]

Extant archosaurs exhibit highly divergent articular soft tissue anatomies between avian and crocodilian lineages. However, the general lack of understanding of the dynamic interactions among archosaur joint soft tissues has hampered further inferences about the function and evolution of these joints. Here we use contrast‐enhanced computed tomography to generate 3D surface models of the pelvis, femora, and hip joint soft tissues in an extant archosaur, the American alligator. The hip joints were then animated using marker‐based X‐Ray Reconstruction of Moving Morphology (XROMM) to visualize soft tissue articulation during forward terrestrial locomotion. We found that the anatomical femoral head of the alligator travels beyond the cranial extent of the bony acetabulum and does not act as a central pivot, as has been suggested for some extinct archosaurs. Additionally, the fibrocartilaginous surfaces of the alligator's antitrochanter and femoral neck remain engaged during hip flexion and extension, similar to the articulation between homologous structures in birds. Moreover, the femoral insertion of the ligamentum capitis moves dorsoventrally against the membrane‐bound portion of the medial acetabular wall, suggesting that the inner acetabular foramen constrains the excursion of this ligament as it undergoes cyclical stretching during the step cycle. Finally, the articular surface of the femoral cartilage model interpenetrates with those of the acetabular labrum and antitrochanter menisci; we interpret such interpenetration as evidence of compressive deformation of the labrum and of sliding movement of the menisci. Our data illustrate the utility of XROMM for studying in vivo articular soft tissue interactions. These results also allow us to propose functional hypotheses for crocodilian hip joint soft tissues, expanding our knowledge of vertebrate connective tissue biology and the role of joint soft tissues in locomotor behavior. [ABSTRACT FROM AUTHOR]

Besides manatees, the suspensory extant 'tree sloths' are the only mammals that deviate from a cervical count (CC) of seven vertebrae. They do so in opposite directions in the two living genera (increased versus decreased CC). Aberrant CCs seemingly reflect neck mobility in both genera, suggesting adaptive significance for their head position during suspensory locomotion and especially increased ability for neck torsion in three-toed sloths. We test two hypotheses in a comparative evolutionary framework by assessing three-dimensional intervertebral range of motion (ROM) based on exhaustive automated detection of bone collisions and joint disarticulation while accounting for interacting rotations of roll, yaw and pitch. First, we hypothesize that the increase of CC also increases overall neck mobility compared with mammals with a regular CC, and vice versa. Second, we hypothesize that the anatomy of the intervertebral articulations determines mobility of the neck. The assessment revealed that CC plays only a secondary role in defining ROM since summed torsion (roll) capacity was primarily determined by vertebral anatomy. Our results thus suggest limited neck rotational adaptive significance of the CC aberration in sloths. Further, the study demonstrates the suitability of our automated approach for the comparative assessment of osteological ROM in vertebral series.