But amphibians made their first appearance at the end of the Devonian period about 345 million years ago. Primitive amphibians were creatures called Labyrinthodonts (many specimens have been dug up in Sydney) and were long bodied, short limbed animals resembling crocodiles with fins on their tail. They were up to 4 m in length. The earliest known frog-like fossil is Triadobatrachus, from Madagascar. The skull is frog-like being broad with large eye sockets, but the fossil has a number of other features differing to modern amphibia. These include a different ilium, a longer body with more vertebrae, the lack of a urostyle and vertebrae in its tail. It is now viewed as pre-anuran, and the transitional stock from which modern frogs evolved.
Modern frogs first appear in the fossil record in the Jurassic period of North and South America and Europe, about 280 million years ago. Many of them are assigned to the same families as living genera of frogs. It thus seems likely that the evolution of modern anura was completed by the Jurassic period. The main evolutionary changes involved shortening of the body and loss of the tail.
Frog fossils have been found on all continents, including Antarctica, and fossil tadpoles have been found in Israel. Australian fossil frog sites include Lake Palankarinna in South Australia, Riversleigh in Queensland and King Island in Bass Strait. Fossil sites in Western Australia include the well known Devil's Lair in Margaret River. The oldest Australian frog fossil to date is 54.5 million years, from a clay deposit at Murgon in Queensland. Studies of frog fossils to date, indicate that only one genus of frog is extinct, this is Australobatrachus.
(After Tyler, 1994 and Barker, Grigg and Tyler, 1995)