REASSESSMENT OF THE MOSASAUR PECTORAL GIRDLE AND ITS ROLE IN AQUATIC LOCOMOTION

Mosasaurs are large marine squamates that were prominent global predators in the Late Cretaceous. Two clades, Mosasaurinae (Mosasaurus, Plotosaurus, Clidastes) and Russellosaurina (Tylosaurus, Platecarpus, Tethysaurus) consist of most derived mosasaur taxa. Derived mosasaurs exhibit convergent marine morphology including streamlined bodies, paddles, and caudal flukes. With regards to aquatic locomotion, recent functional analyses have likened mosasaurs to whales based on morphological similarities. However, both Mosasaurinae and Russellosaurina have distinct massive pectoral girdles which whales lack. We analyzed the morphology of the pectoral girdle across both mosasaur clades and suggest that they were actively utilizing their forelimbs for aquatic locomotion. Past studies did make note of large pectoral girdles in mosasaur genera, but pectoral limb propulsion models have typically been considered less robustly supported than caudal propulsion models. However, given the morphology of the mosasaur pectoral girdle, a dual-module propulsive system, in which both the caudal and pectoral modules contribute significantly to propulsion, remains plausible. Based on the large, nearly equal surface areas of the coracoid and scapula in LACM 397, a Plotosaurus (Niobrara Formation) we determined that the adduction power would be at least double that of the abductor power at the shoulder. The craniocaudally elongate sternum of LACM 397 (and other mosasaurs) is indicative of significant retraction capacity at the shoulder, as well. The shape of the glenoid is consistent with this interpretation. The pectoral limbs in mosasaurs could have been used in drag-based propulsion for fast starts. They may also have been used in a coordinated fashion with the caudal module, perhaps using a “feathered” stroke similar to modern sea lions and fur seals. This does not preclude the use of the pectoral limbs for control and stabilization during sustained swimming, particularly at high speeds. Given their large pectoral girdles, we suggest that mosasaurs may have utilized a dual caudal and forelimb propulsion system not seen in extant animals.