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

The earliest eutherian mammals were small-bodied locomotor generalists with a forelimb morphology that strongly resembles that of extant rats. Understanding the kinematics of the humerus, radius, and ulna of extant rats can inform and constrain hypotheses concerning typical posture and mobility in early eutherian forelimbs. The locomotion of Rattus norvegicus has been extensively studied, but the three-dimensional kinematics of the bones themselves remains under-explored. Here, for the first time, we use markerless XROMM (Scientific Rotoscoping) to explore the three-dimensional long bone movements in Rattus norvegicus during a normal, symmetrical gait (walking). Our data show a basic kinematic profile that agrees with previous studies on rats and other small therians: rats maintain a crouched forelimb posture throughout the step cycle, and the ulna is confined to flexion/extension in a parasagittal plane. However, our three-dimensional data illuminate long-axis rotation (LAR) movements for both the humerus and the radius for the first time. Medial LAR of the humerus throughout stance maintains an adducted elbow with a caudally-facing olecranon process, which in turn maintains a cranially-directed manus orientation (pronation). The radius also shows significant LAR correlated with manus pronation and supination. Moreover, we report that elbow flexion and manus orientation are correlated in R. norvegicus: as the elbow angle becomes more acute, manus supination increases. Our data also suggest that manus pronation and orientation in R. norvegicus rely on a divided system of labor between the ulna and radius. Given that the radius follows the flexion and extension trajectory of the ulna, it must rotate at the elbow (on the capitulum) so that during the stance phase its distal end lies medial to ulna, ensuring that the manus remains pronated while the forelimb is supporting the body. We suggest that forelimb posture and kinematics in Juramaia, Eomaia, and other basal eutherians were grossly similar to those of rats, and that humerus and radius LAR may have always played a significant role in forelimb and manus posture in small eutherian mammals

What lies beneath: sub-articular long bone shape scaling in eutherian mammals and saurischian dinosaurs suggests different locomotor adaptations for gigantism.

Eutherian mammals and saurischian dinosaurs both evolved lineages of huge terrestrial herbivores. Although significantly more saurischian dinosaurs were giants than eutherians, the long bones of both taxa scale similarly and suggest that locomotion was dynamically similar. However, articular cartilage is thin in eutherian mammals but thick in saurischian dinosaurs, differences that could have contributed to, or limited, how frequently gigantism evolved. Therefore, we tested the hypothesis that sub-articular bone, which supports the articular cartilage, changes shape in different ways between terrestrial mammals and dinosaurs with increasing size. Our sample consisted of giant mammal and reptile taxa (i.e., elephants, rhinos, sauropods) plus erect and non-erect outgroups with thin and thick articular cartilage. Our results show that eutherian mammal sub-articular shape becomes narrow with well-defined surface features as size increases. In contrast, this region in saurischian dinosaurs expands and remains gently convex with increasing size. Similar trends were observed in non-erect outgroup taxa (monotremes, alligators), showing that the trends we report are posture-independent. These differences support our hypothesis that sub-articular shape scales differently between eutherian mammals and saurischian dinosaurs. Our results show that articular cartilage thickness and sub-articular shape are correlated. In mammals, joints become ever more congruent and thinner with increasing size, whereas archosaur joints remained both congruent and thick, especially in sauropods. We suggest that gigantism occurs less frequently in mammals, in part, because joints composed of thin articular cartilage can only become so congruent before stress cannot be effectively alleviated. In contrast, frequent gigantism in saurischian dinosaurs may be explained, in part, by joints with thick articular cartilage that can deform across large areas with increasing load

Our XROMM setup for capturing skeletal movements of walking rats.

<p>Rats were trained to walk down a narrow trackway towards a dark hide box placed on the other side of the two C-arm videofluoroscopes.</p

Forelimb Kinematics of Rats Using XROMM, with Implications for Small Eutherians and Their Fossil Relatives - Fig 2

<p>The reference frame (zero position) of the left forelimb rig in (A) left lateral and (B) dorsal views. The bones have been made semi-translucent to enhance visualization of the three Joint Coordinate Systems (JCS) that comprised the virtual shoulder, elbow, and radioulnar joints. All angles and translations are recorded in relation to the reference frame. Note that the reference frame is not intended to be a natural orientation: in fact, in this orientation the radius is unnaturally rotated so that it is entirely supinated relative to the ulna, a posture impossible for a rat. However, this reference posture provides a point against which the degree of pronation can be quantified. To account for body movements, the shoulder JCS has the sternal manubrium centered on the caudal humeral head. Each JCS was based on an Euler angle ZYX rotation order that followed the right-hand rule. In this way, movement of the Z-axis also moved the Y- and X-axes as well as the bone model; movement of Y-axis also moved the X-axis and the bone model; and movement of the X-axis only moved the bone model.</p

Average motion in degrees at the shoulder joint coordinate system (JCS) for a <i>R</i>. <i>norvegicus</i> step cycle.

<p>Above the graph is a representation of the forelimb posture relative to the step cycle. Here, all ten trials from all three rats were binned for every 5% of the step cycle. Blue = Z-axis (protraction/retraction); Green = Y-axis (abduction/adduction); X-axis (long-axis rotation).</p

Changes in femur shape saurischian dinosaurs and alligators.

<p>Maximum femur shape change in the sample is shown on the Y-axis (PRIN 1), whereas femur shape changes associated with size are shown on the X-axis. As with the humerus, changes in femur shape along the PRIN 1 axis and X-axis are similar in that larger taxa have more proximally and distally expanded ends. Overall, the sub-articular region expands tremendously whereas its overall shape remains gently convex, although the distal condyles become somewhat more pronounced.</p

Joint excursions based on joint coordinate systems (JCS) for the shoulder, elbow, and radioulnar joint.

<p>Joint excursions based on joint coordinate systems (JCS) for the shoulder, elbow, and radioulnar joint.</p