Reference article
Fossil
A fossil showing the bones of a huge creature.
Finding it
This is story treasure 3. The cave checks for uncollected story entries and creates at most one story treasure room per run. Collect it, then return to town to see its cutscene.
Story reward
A fossil showing the bones of a huge creature. Returning to town unlocks Shirt ID 47. Later chapters also satisfy earlier chapter reward checks.
Version and distance
The source uses authored position gates, rather than a race win. Exact displayed distances and original Flash gates remain to confirm.