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The functional anatomies of avian finger joints have never been compared with those of the basal avian Archaeopteryx lithographica or maniraptoran theropod dinosaurs. These startling oversights are due to unfamiliarity of the joints outside of highly specialised studies of bird-wing biomechanics. Here fleshed to skeletonised repeated-measures analyses of finger-joint ranges of motion (ROMs) in a basal palaeognath, the Ostrich Struthio camelus, were utilised as a model to begin filling in these knowledge gaps. Results verified that all of the flight-adapted, avian-specific finger joints are retained in the flightless, yet large and well-differentiated fingers of the Ostrich. Moreover, Ostrich finger joints still possess flight-restricted ROMs within the semi-pronated plane of the manus. The osteological markers of these highly conserved flight-adapted characters in Ostrich show that prior descriptions have unknowingly demonstrated that the main avian finger-joint complex began evolving at the base of Penna...

Although investigations of forelimb characteristics are central to therian evolutionary studies, the functional origins of forearm pronation are neglected. However, recent research based on bipedal manipulations strongly suggests that proximal radioulnar joint mobility is highly conserved in tetrapods. This new information calls for a replication of previously published physical simulations of forearm bone movements, to investigate whether active therian pronation/supination evolved from the plesiomorphic mechanism via which locomotor-induced torsion is passively alleviated during forelimb retraction. Preliminary results using representative extant and extinct tetrapod forelimb elements are supportive, and also offer insight into why another overlooked forearm trait, osteological full pronation (mechanically aligned elbow and wrist/finger joints), evolved only in therians and chameleons. During forelimb retraction in tetrapods with unfused radii/ulnae, the radius unexpectedly remains fixed in place as a functional complex with the firmly planted manus/carpus, which the ulnar complex (ulna/humerus) displaces relative to. Therefore, the highly conserved functional morphology of the tetrapod forearm indicates that enhanced therian manual dexterity, which emphasizes isolated radial movements bipedally, was preceded by the locomotor evolution of ulnar supination relative to the radius quadrupedally. This counterintuitive information indicates that the traditional hypothesis, that therian pronation/supination evolved arboreally to amplify radial mobility, requires modification. The authors propose that proximal long-axis rotations of the therian ulnar complex co-evolved with osteological full pronation during a period of arboreal, chameleon-like locomotion, to continue allowing torsion at a reinforced proximal radioulnar joint. These adaptations were later or simultaneously co-opted for object manipulation using active radioulnar pronation/supination.

In response to increased limb bone loads many tetrapod clades have converged upon similar adaptations to reinforce the elbow joint by reducing independent movements of the forearm bones. However prior studies have not examined how these changes occurred phylogenetically or functionally, such as during the transition from prehensile forelimbs in dinosaurs to gliding/flapping flight in bird wings. Here, a functional analysis of forearm bone mobility in extant archosaurs shows that crossing and uncrossing of the radius and ulna can be forced in alligators via a passive gliding mechanism recently described in lacertilians, while birds are adapted to inhibit this motion. A comparison of these findings with a sample of extinct quadrupedal archosaur forearms strongly suggests that, due to the highly conserved morphology of tetrapod forearms in general, the lacertilian mechanism broadly describes the plesiomorphic mechanism via which tetrapod forearm bones passively cross in response to locomotor-induced...

Quadrupedal dinosaurs and other archosaurs are hypothesized to alleviate differences in height between shorter forelimbs and longer hindlimbs via digitigrady (standing on fingers). An alternative hypothesis suggests that they trend toward metacarpogrady (vertical, columnar palms with finger reduction and loss) to abandon semi-pronated (mechanically misaligned) wrist and finger joints. Range of motion (ROM) studies provide support for the latter hypothesis by showing that some dinosaurian fingers were held hyperextended away from the palm at metacarpophalangeal (MCP) joints. However, recent studies dispute traditional MCP hyperextension characters. Moreover, in vivo ROM is assumed to be less than fossil ROM because of the constraining influence of lost soft tissues, further implying that reports of MCP hyperextension in fossil archosaurs may be overstated. Because crocodilian fingers reportedly hyperextend during the high walk, a ROM study of their MCP joints could clarify these issues. Here a repeated-measures analysis was used to gather ex vivo finger joint ROM data in degrees from fully fleshed to skeletonized conditions in digit III of the American alligator Alligator mississippiensis. Results verify that MCP joints hyperextend, and show that soft tissues significantly affect ex vivo ROM. Skeletal ROMs were greater than fleshed ROMs in the three interphalangeal joints; however the MCP joint had the opposite result after losing curved articular cartilages. These results suggest that studies of flattened metacarpo- and metatarsophalangeal joints in fossil archosaurs may underestimate in vivo hyperextension. A comparison of characters in Alligator mississippiensis and the rauisuchid Postosuchus suggests that Postosuchus metacarpal adaptations represent a transitional stage toward vertical quadrupedal support with hyperextended MCP joints. This information provides support for the preexisting hypothesis that quadrupedal archosaurs tend to transform their wrist + palm into a rigid stilt-like extension of the forearm, which implies that locomotor abandonment of semi-pronated wrist/finger joints and alleviation of limb disparity evolve synchronously.

SUMMARY Recent studies have presented range of motion (ROM) data in degrees for dinosaur forelimb joints, usually via physical manipulation of one individual. Using these data, researchers have inferred limb orientations, postures, gaits, ecological functions and even phylogenetic trends within clades. However, important areas of concern remain unaddressed; for example, how does ROM at a forelimb joint change after soft tissues are lost in archosaurs? And are fossil ROM methodologies amenable to reproducibility and statistical analysis? Here, we investigated these questions using the extant phylogenetic bracket of dinosaurs. Repeated measures of elbow joint ROM from Struthio camelus and Alligator mississippiensis forelimbs were statistically analyzed as they were sequentially dissected through five levels of tissue removal treatment. Our data indicate that there are no statistically significant differences in repeated measures of ROM between observers who use the same techniques. Extrinsic soft tissues, such as integument, muscles and ligaments were found to impede ROM at the elbow joint. Intrinsic soft tissues, such as articular cartilage, may increase ROM. The hypothesis that the articular surfaces of the bones within the elbow joints of archosaurs provide a general approximation of mobility is supported. Final ROMs were less than the initial ROMs in both taxa, which suggests that prior reports of elbow joint ROMs in degrees for nonavian dinosaurs may represent conservative estimates. We conclude that if observer bias and other variables are controlled for, ROM studies of fossil archosaur limbs can obtain useful degree data for inferring joint mobility in vivo.

Therians (marsupials and placentals), archosaurs, and chameleons are remarkable in that they evolved postures and gaits with inturned forelimbs. However, recent studies have indirectly recognized that, unlike fully pronated therian and chameleon forearms, dinosaur forearms were mechanically constrained by semi-pronated (misaligned) joints. This has led to the hypothesis that quadrupedal dinosaurs mitigated this constraint via proximal migration of the radius, indirectly forming a more pronated, tubular manus distally. To test this hypothesis, a standardized pose was used to examine the forearm pronation of ornithischian dinosaurs that were obligatory quadrupeds and facultative bipeds. Results show that only restructuring of the distal, not the proximal radius, causes additional pronation of the pre-axial edge of the carpus, but also unexpectedly reveal that the ulna may help form a tubular manus by supinating the post-axial edge. Thus, relative to the plane of the elbow joint the wrist and finger joints r...

A recent study hypothesized that avian-like wrist folding in quadrupedal dinosaurs could have aided their distinctive style of locomotion with semi-pronated and therefore medially facing palms. However, soft tissues that automatically guide avian wrist folding rarely fossilize, and automatic wrist folding of unknown function in extant crocodilians has not been used to test this hypothesis. Therefore, an investigation of the relative contributions of soft tissues to wrist range of motion (ROM) in the extant phylogenetic bracket of dinosaurs, and the quadrupedal function of crocodilian wrist folding, could inform these questions. Here, we repeatedly measured wrist ROM in degrees through fully fleshed, skinned, minus muscles/tendons, minus ligaments, and skeletonized stages in the American alligator Alligator mississippiensis and the ostrich Struthio camelus. The effects of dissection treatment and observer were statistically significant for alligator wrist folding and ostrich wrist flexion, but not ostrich wrist folding. Final skeletonized wrist folding ROM was higher than (ostrich) or equivalent to (alligator) initial fully fleshed ROM, while final ROM was lower than initial ROM for ostrich wrist flexion. These findings suggest that, unlike the hinge/ball and socket-type elbow and shoulder joints in these archosaurs, ROM within gliding/planar diarthrotic joints is more restricted to the extent of articular surfaces. The alligator data indicate that the crocodilian wrist mechanism functions to automatically lock their semi-pronated palms into a rigid column, which supports the hypothesis that this palmar orientation necessitated soft tissue stiffening mechanisms in certain dinosaurs, although ROM-restricted articulations argue against the presence of an extensive automatic mechanism. Anat Rec, 297:1228–1249, 2014. © 2014 Wiley Periodicals, Inc.

Summary Using the extant phylogenetic bracket of dinosaurs (crocodylians and birds), recent work has reported that elbow joint range of motion (ROM) studies of fossil dinosaur forearms may be providing conservative underestimates of fully fleshed in vivo ROM. Since humeral ROM occupies a more central role in forelimb movements, the placement of quantitative constraints on shoulder joint ROM could improve fossil reconstructions. Here, we investigated whether soft tissues affect the more mobile shoulder joint in the same manner in which they affect elbow joint ROM in an extant archosaur. This test involved separately and repeatedly measuring humeral ROM in Alligator mississippiensis as soft tissues were dissected away in stages to bare bone. Our data show that the ROMs of humeral flexion and extension, as well as abduction and adduction, both show a statistically significant increase as flesh is removed, but then decrease when the bones must be physically articulated and moved until they separate from one another and/or visible joint surfaces. A similar ROM pattern is inferred for humeral pronation and supination. All final skeletonized ROMs were less than initial fully fleshed ROMs. These results are consistent with previously reported elbow joint ROM patterns from the extant phylogenetic bracket of dinosaurs. Thus, studies that avoid separation of complementary articular surfaces may be providing fossil shoulder joint ROMs that underestimate in vivo ROM in dinosaurs, as well as other fossil archosaurs.