Youngest fossil occurrence of ichthyosaurs from the Southern Hemisphere

Ichthyosaurs maintained substantial species diversity throughout the Early Cretaceous, yet experienced a dramatic decline at the beginning of the Cenomanian. Reliable records of ichthyosaurs in the middle and upper Cenomanian are extremely scarce, with only one previous unequivocal record from the upper Cenomanian of Germany. Here, we describe an isolated ichthyosaur phalanx recovered from the ‘upper’ Gearle Siltstone in the lower Murchison River area of Western Australia. This fossil can be assigned to the terminal ichthyosaur clade Brachypterygiidae based on its distinctly rectangular shape. Stratigraphical bracketing using calcareous nannofossils delimits a Cenomanian age, which we further constrain as middle–late Cenomanian using elasmobranch teeth extracted from the same depositional horizon as the phalanx. The ‘upper’ Gearle Siltstone ichthyosaur occurrence thus represents the geologically youngest example of the group documented from the Southern Hemisphere, and implies a widespread distribution prior to their final extinction in the late Cenomanian.