ABSTRACT

In 2008, an articulated distal forelimb of the type specimen of the large pliosauromorph Megalneusaurus rex (UW 4602) was discovered adjacent to the original excavation pit from which two hindlimbs had been collected in 1895.

The new material includes six complete or partial carpals, four metacarpals, and nearly all of the phalanges. Although the new bones were damaged by weathering and gypsum crystal growth, the articulated arrangement of the bones is preserved. Important features include (1) broadly flared metacarpals that articulate proximally and distally with the adjacent metacarpals; (2) curved facets on phalanges that are concave proximally and convex distally; (3) laterally interlocking phalanges between digits I and II and between digits IV and V for the entire length of the outer digits; and (4) tightly articulated, interlocking phalanges among all digits distal to the 3th phalanx. Examination of the material collected in 1895 indicates that a similar structure occurred on the hindlimb as well. The results of this arrangement are rigid, reinforced leading and trailing edges of the flipper, as well as a stiff distal end. During swimming, the limb moved as a rigid unit, with no flexibility at any articulation distal to the head of the propodial. The stiff flipper generated thrust by pushing backward and downward against the water during power stroke; and generated lift when the limb was rotated and moved forward and upward during the recovery stroke.