Abstract
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.
Highlights
The evolutionary ecology of Mesozoic mammals is often inferred from skeletal traits associated with locomotion
We confirmed that the humero-ulnar joint operates essentially as a single-axis hinge joint in rats [12,19]
We demonstrated that long-axis rotation (LAR) of the radius occurs during a normal, overground step cycle in Rattus norvegicus
Summary
The evolutionary ecology of Mesozoic mammals is often inferred from skeletal traits associated with locomotion. Following a long tradition of form-function studies, the skeletal morphology of Mesozoic mammal forelimbs has been used to support various ecomorphological hypotheses of locomotion [12,13] In this context, small-bodied therian mammals have often been utilized to decipher the locomotor habits of their ancestors given the similarities in their appendicular morphologies. Classic studies by Jenkins and colleagues [14,15,16,17,18,19,20] utilized a combination of radiographic stills, cineradiography, and the comparative method to deduce the probable locomotor habits of early therians and theriiomorphs This body of research suggested that many early therians were scansorial to arboreal in their habits. It is likely that a crouched but parasagittal forelimb posture was primitive for basal therians regardless of their locomotor habits [21,22]
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