Ammonite is a fossil shell of a large extinct group of mollusks related to the living chambered Nautilus. They are among a suite collected by the National Museum staff in April 1948 at Tignoan creek, Mansalay, a town in Mindoro Island. They are included as one of the oldest rock-dated fossils in the Philippine archipelago. It ranges according to the result obtain from both field and laboratory work to Middle Jurassic age, 160-175 million years old.
The first evidence of Mesozoic Era in the Philippines was the discovery of Ammonites in 1940 by Hollister of the National Development Company Petroleum Survey (Corby et.al.) from exposures south of Mansalay bay near Colasi Pt. in southeastern Mindoro. The ammonite-bearing formation is designated as the Mansalay Formation, name after a district of Mansalay where it was first encountered.
Ammonite is the most important and interesting fossil not only because it is an index fossils recognized in the Philippines but it could be use to elucidate past geographic relationship between the Philippines and other area. Whether the Philippines was part of the mainland Asia or not, one thing is obvious, that part of the archipelago was certainly under the sea (Pacific Ocean) at the time of formation.
The occurrence of Jurassic fossil around Mansalay, Oriental Mindoro, has been reported by various authors (Hayasaka, 1943; De Villa, 1944; Rivera, 1954; Kobayashi, 1957; Teves, 1957; and Sato, 1961).