When did the apatosaurus went extinct?

Truth be told, there is ONLY one known specimen of Elasmosaurus platyurus.... the type specimen found by Turner in 1867 and described by Cope (see Carpenter, 1999), so it is difficult to estimate when the species died out. It seems likely,however, that E. platyurus would have been around for several million years about 80 million years ago and then been replaced by later occurring species of elasmosaurs....

If the question is when did elasmosaurs go extinct, then the answer is about the same time as the dinosaurs, about 65 million years ago.