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

Area of Science:

• Paleontology
• Biomechanics
• Mammalian Evolution

Background:

• Earliest eutherian mammals were small-bodied, with forelimbs resembling those of modern rats.
• Understanding rat forelimb kinematics can inform hypotheses about early eutherian posture and mobility.
• Previous studies on *Rattus norvegicus* locomotion lack detailed three-dimensional bone movement analysis.

Purpose of the Study:

• To explore the three-dimensional kinematics of the humerus, radius, and ulna in *Rattus norvegicus* during walking.
• To investigate long-axis rotation (LAR) of forelimb bones and its role in manus orientation.
• To infer potential forelimb posture and mobility in early eutherian mammals based on rat kinematics.

Main Methods:

• Utilized markerless X-ray Rotational Angiography (XROMM) for three-dimensional analysis of bone movements.
• Focused on normal, symmetrical gait (walking) in *Rattus norvegicus*.
• Quantified movements of the humerus, radius, and ulna throughout the step cycle.

Main Results:

• Confirmed crouched forelimb posture and parasagittal ulna movement in rats.
• First-time discovery of significant long-axis rotation (LAR) in both the humerus and radius.
• Identified correlations between elbow flexion, manus pronation/supination, and LAR, suggesting a divided role between ulna and radius for manus orientation.

Conclusions:

• Humerus and radius LAR are key to maintaining manus pronation during weight-bearing in rats.
• Rat forelimb kinematics, including LAR, likely reflect ancestral eutherian traits.
• These findings provide a model for understanding forelimb function in basal eutherians like *Juramaia* and *Eomaia*.