Forelimbs of the theropod dinosaur dilophosaurus wetherilli: Range of motion, influence of paleopathology and soft tissues, and description of a distal carpal bone

Document Type

Article

Publication Date

1-1-2019

Abstract

Forelimb bones of the Early Jurassic theropod Dilophosaurus wetherilli were manually manipulated to determine the range of motion (ROM) of each forelimb joint and to test functional hypotheses of forelimb use. Using bony articular surface margins to delimit ROM, the humerus can be retracted to a position subparallel to the scapular blade, protracted no further than a subvertical position, and elevated only 65°. The elbow cannot be flexed beyond a right angle to the humerus. Supination and pronation are prevented by lack of rolling radio-ulnar joints, and the palms face medially. The distal carpal block of Dilophosaurus, described here for the first time, is relatively flat and lacks a proximal trochlea, suggesting limited wrist mobility. The fingers diverge during flexion and are extremely hyperextensible. Experiments on American alligator cadavers show that ROM exhibits limited intraspecific variation and is greater in the fully-fleshed elbow than skeletal manipulation suggests. Here, we apply these findings and those of previous studies on extant archosaurs to reconstruct ROM in the forelimbs of live Dilophosaurus by adjusting the results of the bare-bones manipulations to account for the known influence of soft tissues. We also discuss the effects of skeletal deformities on inferred ROM in Dilophosaurus. Forelimb morphology and ROM are consistent with two-handed prehension, clutching objects to the chest, manually hooking objects, and seizing prey beneath the predator’s chest or the base of its neck (but no further forward). Limited shoulder mobility suggests that the mouth, not the hands, made first contact with prey.

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