An ichthyosaurian forefin from the Lower Cretaceous Zapata Formation of southern Chile: implications for morphological variability within Platypterygius

Abstract

Expeditions to the Tyndall Glacier (Torres del Paine National Park, South Chile) in 2009 and 2010 recovered an articulated ichthyosaurian forefin that displayed traits consistent with Platypterygius. The combination of features observed in this forefin has not been observed in other Platypterygius species, i.e.: (1) one preaxial row and three or even four postaxial rows; (2) the distal articulation of ulna with metacarpal five; (3) the shape and the articulation context of distal carpal three, metacarpal three and four; (4) the hexagonal shape of the intermedium; and (5) the presence of only two distal articular facets on the humerus. Nevertheless, the forefin is not referred to a new species, because it was found isolated and no other diagnostic skeletal elements are present. The forefin presented here is the first complete and articulated forefin of a Platypterygius from the South Pacific margin, and together with other ichthyosaurs from the Tyndall area, represents the southernmost occurrence of Early Cretaceous ichthyosaurs in the Americas.