The fossil record is progressive and stratified. The global flood as described by YEC reshaped the planet. A flood of that power would jumbo everything together.
The identifiable geological eras from the Cambrian forward are identifiable by geological characteristics but also by their fossil inclusions. The fossil record displays a general progression from more primitive organisms, extinction boundaries, and sorting of creatures into their associated epochs. This is precisely what would be expected given life evolving over a span of hundreds of millions of years.
Given that YEC holds that all the animals that ever breathed coexisted together before the deluge, this presents a challenge - if they lived together why did they not die together? The entire geologic column constituting most of the fossil bearing sedimentary rock found on the planet, would have been laid down roughly over the course of a year.
No humans and dinosaur fossils have ever been found together, but that is only the thinnest sliver of the problem. Bear in mind that YEC believes that pretty much every creature that ever lived was present in the days leading to the flood. So we should find not just people with dinosaurs, but trilobites with crabs, dimetrodons with velociraptors, plesiosaurs with whales, triceratops with elephants, pterosaurs with buzzards - one could go on all day.
If the animals lived together, perhaps they segregated as the flood waters arose. Could it have been by the size of the animal? Only a minority of dinosaurs grew to the size of large sauropods or T. Rex, most were comparable to elephants and buffalo, and many down to the size of badgers. There is also no reason to believe that dinosaurs moved particularly faster or slower than an assortment of other animals. There are over a thousand identified species of dinosaur of many shapes and sizes; if they lived at the same time there is no reason to expect there would be no inter-sorting between Permian amphibians, cretaceous reptiles, and Holocene mammals. Finally, could they have sunk at different rates? That makes no sense for sorting either, some animals would be tangled up or suffer trauma and not even float to begin, and again, many amphibians, dinosaurs, and mammals are about the same size and would bloat and sink randomly. These things are not like clockwork.
So it makes little sense that if animals from the various epochs were all alive at the time of the flood, that there would be any effective mechanism that would result in any sorting whatsoever. But the fossil record is screaming clear. There has never been a solitary dinosaur found above the KT boundary, and never a modern mammal from below.
Flood geology holds that the deluge was an extraordinarily violent affair, spreading the continents apart, grinding primal rock to massive sedimentary formations, and jutting mountain ranges skywards. It is inconsistent that all this fury would reshape the planet but placidly lay each animal to rest in peace in some well ordered sequence followed the world over. That would be like, …what would be a good analogy? Well, it would be like a tornado in a junkyard assembling a 737 jetliner without anything out of place or in the wrong order. No Cambrian pieces after Jurassic, no Paleocene pieces before Triassic; nothing at all out of place.
The fossil record confirms a succession of ecologies, and contradicts a global flood.