jewess: i’m sorry i don’t know where this is from, but i think there is one explanation that each day of creation could have been a thousand years for all we know, since a thousand years is like a day to G-d, who is not bound by time. and during this period when the world was in existence but before it was complete, animals could have lived and died out, leaving their bones as fossils.
another possibility i’ve heard is that fossils are the bones of the first generations of people (between adam and noach), who may have been much taller/bigger than people today. and the waters of the mabul may have warped their bones so they don’t really fit together as human skeletons anymore.
i don’t know if i got everything exactly right since it’s been a while since i heard this, and i can’t remember where. but whether one or neither explanation actually happened, it does show that there are possible explanations for what happened, even if we don’t know exactly what it was.