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, regardless of whether you keep it or trade it for Fame/Funds, it unlocks an extra Prehistoric world option on the World Map when choosing your next destination (this does not apply to Expedition 6).