Why Are All the Dinos in Layers After the Mesozoic?

If selphices, alisorus and terexes were all living at same time before the flood, then they all would have died at the same time when the flood struck. And as a result, we should find the bones of these different species jumbled up together in the same rock layers. But that's not what we see - with the light weight celephices on the bottom, the heavier alisaurues above them, and the super heavy trexes on top. Wut. What if someone claimed that the shorter dinosaurs would have drowned first as the flood waters rose, and that's why they'd be in the lower rock layers? That won't work because for