Forearm Posture and Mobility in Quadrupedal Dinosaurs

Abstract

Quadrupedality evolved four independent times in dinosaurs; however, the constraints associated with these transitions in limb anatomy and function remain poorly understood, in particular the evolution of forearm posture and rotational ability (i.e., active pronation and supination). Results of previous qualitative studies are inconsistent, likely due to an inability to quantitatively assess the likelihood of their conclusions. We attempt to quantify antebrachial posture and mobility using the radius bone because its morphology is distinct between extant sprawled taxa with a limited active pronation ability and parasagittal taxa that have an enhanced ability to actively pronate the manus. We used a sliding semi-landmark, outline-based geometric morphometric approach of the proximal radial head and a measurement of the angle of curvature of the radius in a sample of 189 mammals, 49 dinosaurs, 35 squamates, 16 birds, and 5 crocodilians. Our results of radial head morphology showed that quadrupedal ceratopsians, bipedal non-hadrosaurid ornithopods, and theropods had limited pronation/supination ability, and sauropodomorphs have unique radial head morphology that likely allowed limited rotational ability. However, the curvature of the radius showed that no dinosaurian clade had the ability to cross the radius about the ulna, suggesting parallel antebrachial elements for all quadrupedal dinosaurs. We conclude that the bipedal origins of all quadrupedal dinosaur clades could have allowed for greater disparity in forelimb posture than previously appreciated, and future studies on dinosaur posture should not limit their classifications to the overly simplistic extant dichotomy.

Forelimb Kinematics of Rats Using XROMM, with Implications for Small Eutherians and Their Fossil Relatives

“…Although the broad aspects of small therian mammal forelimb kinematics are well delineated, other questions remain. Manus pronation is often assumed to play a significant role in parasagittal locomotion by ensuring that manus flexion and extension occurs in line with the direction of travel [ 23 , 24 ]. Manus pronation in a variety of therian mammals is often accomplished via long-axis rotation of the radius about the ulna [ 25 ].…”

“…Manus pronation in a variety of therian mammals is often accomplished via long-axis rotation of the radius about the ulna [ 25 ]. In fact, radial head shape and shaft curvature are often correlated with the ability to pronate the manus [ 24 ]. Understanding the extent and degree to which long-axis rotation of the radius could occur in early eutherians, for example, would further illuminate and constrain their potential locomotor envelopes.…”

“…Given the small size of the manus in rats, we were unable to directly track hand movements. However, long-axis rotation of the radius is correlated with manus pronation in eutherian mammals [ 24 , 25 ]. Therefore, movement of the radius serves as a proxy measure of manus pronation.…”

Articular soft tissue anatomy of the archosaur hip joint: Structural homology and functional implications

“…Because the function and homology of extant archosaur hip joints are not well understood, inferences of locomotor behavior and evolution in fossil archosaurs are hampered. The insufficiency of knowledge in archosaur joint anatomy limits the biological applicability of joint range of motion (Carpenter and Smith, ; Carpenter, ; Paul, ; Senter and Robins, ; Senter, ; Hutson and Hutson, ; Liparini, ; VanBuren and Bonnan, ) and inferred limb stress (Kubo and Benton, ; Bonnan et al, ) studies based on bony morphology alone.…”

Alligator (Alligator mississippiensis) sternal and shoulder girdle mobility increase stride length during high walks

“…Extant crocodilians have been central to interpreting locomotor evolution of Archosauria (Gatesy, 1991;Hutchinson, 2006;Reilly and Elias, 1998), because of their unique locomotor capacities and relatively basal phylogenetic position. Several major evolutionary changes in primary locomotor mode occur on the line to and within archosaurs: the origin of parasagittal gaits (Padian et al, 2010;Parrish, 1987), two of the three origins of vertebrate flight ( pterosaurs and birds), the origin of bipedalism in dinosaurs (Persons and Currie, 2017) and possibly pterosaurs (Padian, 2008), and multiple secondary returns to quadrupedalism (Maidment et al, 2014;VanBuren and Bonnan, 2013). All of these transitions involve changes in the types of external forces experienced by the forelimb which, in turn, affect the kinematics and kinetics of the limb joints.…”