The Dinosaur Skull is a rare item that can be found very rarely at Trading Caravans, as buried treasure, and possibly in cave crevices. It can appear only in Expeditions 3-6.
It is considered a treasure and can be traded in-expedition or donated/sold upon return to London.
When brought to London, if you keep it, it unlocks an extra Prehistoric world option, on the World Map when choosing your next destination (this does not apply to Expedition 6).
Note: in earlier versions extra Prehistoric world option was unlocked regardless of whether you keep it or trade it for Fame/Funds.