Six to ten thousand years is not enough time for what?
I know that a few thousand years is not enough time to explain the wide variety of species we see upon the Earth if you think they all came from those few species that could fit on a boat of known dimensions.
So what mechanism would you use to explain how we went from a few species a few thousand years ago to the innumerable species we have today? Do you think you don't need a physical mechanism to explain it? Either way - if this is your paradigm then you believe in evolution, speciation, and the (semi) common descent of species - and at a rate and with a power well beyond that ever proposed by any competent evolutionary biologist.
Why wouldn't the fossils of extinct species be “fully developed”?
An Australopithocine is not a undeveloped human being - it is a “fully developed” Australopithocine. A flying squirrel is not a undeveloped flying creature - it is a “fully developed” gliding arboreal mammal.
The ark had dimensions and shape similar to a modern day barge, would barely fit on a football field and be about 4 stories tall.
Here’s a description from Dr. Walt Brown PhD from his online website creationscience.com:
a. A boat large enough to hold representatives of every air-breathing land animal—perhaps 16,000 animals in all. (Of course, sea creatures did not need to be on the Ark. Nor did insects or amphibians. Only mammals, birds, reptiles, and humans. Much plant life survived the flood in a surprisingly simple way.)
b. The Ark, having at least 1,500,000 cubic feet of space, was adequate to hold these animals, their provisions, and all their other needs for one year.
c. Since the flood, many offspring of those on the Ark would have become reproductively isolated to some degree due to mutations, natural genetic variations, and geographic dispersion. Thus, variations within a kind have proliferated. Each variation or species we see today did not have to be on the Ark. For example, a pair of wolflike animals were probably ancestors of the coyotes, dingoes, jackals, and hundreds of varieties of domestic dogs. (This is microevolution, not macroevolution, because each member of the dog kind can interbreed and has the same organs and genetic structure.)