I’m glad you asked, @ask217771.
Assuming you are totally bored with geology and physics, which demonstrate by multiple different methods that the Earth is 5 billion years old (not 5,000 years) … the unique pattern of fossils we find on Earth are completely impossible to produce from a world flood scenario 5,000 years ago.
I would reconsider the issue of geology and physics… it really does simplify matters. But let’s continue on to review your questions about fossils and extinct species:
If we look at the overall systematic pattern of fossils, we discover certain grand truths:
Fossils of Large fossils and large mammals are never found together. The Evolutionary hypothesis is that the larger a mammal becomes, the easier it is for carnivore dinosaurs to find them and eat them. So during the age of dinosaurs, only the smallest of mammals could survive extinction.
Fossils of aquatic mammals (any size) and aquatic dinosaurs (any size) are never found together. The Evolutionary hypothesis is that before aquatic mammals could evolve, we had to first have mammals, and especially the larger ones that could survive in an aquatic environment filled with fish. So invariably, when we find a fossil of an aquatic mammal, it is from millions of years after the extinction of all dinosaurs, let alone aquatic dinosaurs.
Using the same logic as (2), the existence of large terrestrial mammals (Elephants, Giraffes, Rhino’s etc) was not possible until the large dinosaur predators were gone. So whenever we find a large mammal, invariably it is from millions of years after the extinction of dinosaurs.
Then we have the Australian populations of mammals which until relatively recently, were all marsupials (the placental mammals that once lived there went extinct long ago). Apparently the marsupials in Australia, being isolated from placental evolution going on in the rest of the world, was protected from competition from the more robust placental mammals. And in the rest of the planet, only a handful of marsupials were able to compete against the other mammals (like the possum).
The YEC explanation for this pattern (Australian Marsupials vs. Placentals beyond Australia) does not exist. YEC’s would essentially have to propose the following sequence:
a) The Great Flood kills all animals in Australia and elsewhere.
b) When the animals are released from the Ark, the marsupial mammals (even the slow ones) manage to travel directly to Australia before it separates from the main continent…
c) … while the placental mammals completely ignore Australia for generations.
This, of course, is an impossible scenario.