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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.

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.