I have yet to find a turtle skeleton that does not seem to have some degree of perpetual smile. Snapping turtles maybe, they scowl when alive so it is a good bet they do as skeletons, but certainly not Cretaceous turtles like Toxochelys. The Tylosaur chasing it would never be accused of smiling and the fact that it is being chased and still has that perpetual smile on its skeletal face almost makes this exciting chase scene seem exceedingly ludicrous. However, happy turtles, whether being chased or not, make me happy, and I therefore endorse the happy facade of the endangered turtle. We all know in reality that, with knowledge of the ambush from behind that the surge of water has certainly given the turtle, it would be wide-eyed and paddling as fast as its little paddling feet could go. The hard shell of an older adult would probably go a great way toward deflecting and deterring a mosasaur from trying to eat the turtle, but juvenile mosasaurs or desperate adults would probably still attempt to bite into this hard target.
Paintings and other illustrations of Toxochelys, while the most populous turtle fossil in Kansas, are rare even in Kansas. However, we do have some fun, and scary if the prior image is included, scenes acted out by skeletons in museum displays. The availability of the skeletal material for full recreation and casting allows for more interested museums to possess displays, though obviously Toxochelys is not the most often displayed fossil in the country. Thankfully those that do use it for display have done so in fun ways. The chase scene before between potential predator and prey is like a scarier version of this turtle chase scene. The number of reasons for solitary animals like sea turtles to chase one another is minimal, so, were this to be evidenced by fossils or witnessed in the wild the assumptions would probably pool down to 1) mating season or 2) territorial encounters. Considering most sea turtles of this time would probably have been wandering animals "territorial encounters" is probably less feasible, but considering we cannot watch these animals interact, anything could be true honestly!