Ichthyosaurs had been dwindling in numbers for some time; they were no longer the force they once were in the Upper Triassic and Lower Jurassic. By the middle Jurassic, it was thought thought they all belonged to the single clade, the Ophthalmosauridae. By the Cretaceous it was thought that only three genera survived. For the last 50+ years it has been thought that only one genus, "Platypterygius", was known at the time of the anoxic event in the Upper Cretaceous.