Forearm Posture and Mobility in Quadrupedal Dinosaurs

<div><p>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.</p></div

Radii of sprawling and parasagittal extant taxa.

<p>The radial head (A) and long axis (B) of <i>Caiman crocodylus</i> (ROM R7719) shows the flattened ulnar articular surface and relatively straight long axis typical of sprawling taxa. The radial head (C) and long axis (D) of <i>Ursus americanas</i> (USNM 49664) shows the rounded ulnar articular surface and curved long axis typical of parasagittal mammals and chameleons. Scale bar = 5 cm. Radial heads not to scale. <b>rh</b>, radial head; <b>sp</b>, styloid process; <b>us</b>, ulnar articular surface.</p

Results from the Kruskal-Wallis test on extant taxa.

<p>Results from the Kruskal-Wallis test on extant taxa.</p

Terrestrial radial head PCA.

<p>Principal component scores from the analysis of terrestrial taxa. PC1 vs. PC2 (A) and PC1 vs. PC3 (B) have 95% convex hulls representing the sprawled and parasagittal taxa. The origin of each axis is represented by the shape of the consensus (C). Shape change along the principal component axes is shown with the location of the consensus shown at the origin (C). Landmarks 1 (middle of the ulnar articular surface) and 10 are labeled.</p

Extant radial head PCA.

<p>A graphical representation of the first two principal components from the analysis containing extant taxa (A). The shape at the origin is represented by the consensus (B). Shape change along the principal component axes is shown with the location of the consensus shown at the origin (B). Landmarks 1 (middle of the ulnar articular surface) and 10 are labeled.</p

Angle of curvature measurement.

<p>The angle of curvature was calculated using ImageJ. The vectors for the angle are from the distal medial end up to the radial head and across the radial head. <b>an</b>, angle measurement; <b>p</b>, proximal.</p

Results from the Kruskal-Wallis test with non-avian dinosaurs.

<p>Results from the Kruskal-Wallis test with non-avian dinosaurs.</p

Antebrachia of sprawling and parasagittal taxa.

<p>The parallel radius of a sprawling alligator compared to a radius that crosses the ulna in a parasagittal cat. <b>r</b> = radius; <b>u</b> = ulna.</p