The pectoral girdle and forelimb of Baryonyx exhibit a suite of adaptations that reflect its semi‑aquatic hunting strategy. The shoulder region, composed of a scapula and coracoid, is unusually robust for a large theropod, supporting an enlarged musculature that powered a massive, curved claw on the thumb. In addition to the prominent ungual, the architecture of the pectoral girdle reveals a series of modifications that enhanced stability, increased leverage, and facilitated a broader range of motion in an environment where prey items were often elusive, partially submerged, or fast‑moving. Compared with typical allosauroids, the forelimb of Baryonyx shows a distinct shift toward a more flexible, yet powerful, design that may have aided in grappling fish, small crocodiliforms, or other aquatic vertebrates.
Skeletal architecture of the pectoral girdle
The scapula is a narrow, elongated blade that measures approximately 55–60 cm in the type specimen (NHMUK R9952). Its dorsal margin is straight, while the ventral edge is slightly curved to accommodate the glenoid fossa. The curvature of the ventral margin not only forms a shallow, saddle‑shaped glenoid surface but also provides a recessed area for the origin of the Mm. supraspinatus and infraspinatus, allowing a large cross‑section of muscle fibres to converge on the forelimb. The scapular blade extends posterodorsally, terminating in a pointed end that would have been positioned just above the ribcage when the animal was in a standing posture. This elongated profile suggests that the scapula acted as a lever arm, increasing the mechanical advantage of the shoulder musculature during limb protraction and retraction.
The coracoid, which is fused to the scapula in most spinosaurids, forms a ventrolateral process that expands the glenoid surface. In Baryonyx, the coracoid is notably hypertrophied, projecting anteriorly and laterally to broaden the glenoid fossa. This expansion creates a shallow, cup‑like articulation that enables the humeral head to move through a wider arc than in many other large theropods. Moreover, the coracoid’s lateral process bears a distinct ridge that probably served as the insertion point for the M. biceps brachii, a muscle that would have contributed to rapid flexion of the elbow during a strike. The combined scapulocoracoid unit therefore presents a robust, yet highly mobile, platform for the forelimb.
- Scapular blade morphology: The elongated blade measures roughly 55–60 cm and exhibits a subtle, caudodorsal curvature that aligns the glenoid fossa slightly posteroventrally, allowing the humerus to be held in an abducted position while still maintaining a secure articulation. Surface texture indicates the presence of ligamentous scars that would have anchored the scapular part of the Mm. trapezius and levator scapulae.
- Coracoid expansion and ventrolateral process: The coracoid’s ventrolateral process lengthens the glenoid surface, increasing its contact area with the humeral head. This expanded socket facilitated a greater range of forearm rotation, a critical adaptation for sweeping motions used to capture slippery prey.
- Glenoid orientation and functional implications: The glenoid fossa is directed caudodorsally, a geometry that encourages an outward‑swinging movement of the forelimb. Such orientation is reminiscent of semi‑aquatic mammals and indicates that Baryonyx could swing its arm in a quasi‑lateral plane to strike water surfaces or to grasp objects underwater.
- Muscle attachment sites: Prominent rugosities on the dorsal border of the scapula mark the origins of the Mm. deltoideus and supraspinatus, muscles that would have provided substantial power for elevation of the forelimb. The coracoid bears a pronounced tuberculum for the insertion of the M. biceps brachii, emphasizing the ability to generate rapid elbow flexion.
- Humerus‑glenoid interaction: The humeral head is stout and slightly flattened, fitting snugly into the expanded glenoid socket. This arrangement distributes stress evenly across the joint during forceful claw strikes and helps prevent dislocation during high‑impact prey capture.
- Acromial process and possible furcula remnant: A small, ridge‑like acromial process extends from the anterior edge of the scapula, possibly representing a vestigial acromion that, in life, may have supported a cartilaginous arch akin to a reduced wishbone. Though no ossified furcula has been reported in Baryonyx, the presence of this process suggests an evolutionary tendency toward a more flexible shoulder girdle.
- Biomechanical inferences: The combination of a long scapular lever, expanded coracoid socket, and robust musculature indicates that the forelimb of Baryonyx was adapted for producing high torque at low angular velocities. Such a system would have been effective for delivering deep, sweeping motions that could scoop water and seize prey, reinforcing the hypothesis of a semi‑aquatic predatory lifestyle.
Collectively, the skeletal architecture of the pectoral girdle in Baryonyx underscores a suite of functional adaptations that bridge the divide between terrestrial hunting and aquatic predation. The enlarged glenoid surface, reinforced scapulocoracoid union, and extensive muscle scarring all point to a forelimb capable of both precision and power—a morphological signature that likely facilitated the animal’s ability to capture and manipulate slippery aquatic prey while still retaining the capacity for forceful strikes on land. These insights not only refine our understanding of spinosaurid ecology but also illustrate how selective pressures can remodel the shoulder apparatus of large theropods to meet the demands of a semi‑aquatic existence.