Earlier, the Mesozoic, or Middle Life era, during which the reptiles and the great dinosaurs flourished, extended through the Cretaceous and preceding periods from about 225 million years ago. Before that the Palaeozoic, or Old Life era, went back another 375 million years to the very dawn of animal life, about 600 million years ago. The only fossil remains of the Palaeozoic era are those of the early invertebrates, the first clearly recognizable forms being shellfish. Undoubtedly even more primitive forms had evolved through the preceding ages when the earlier sedimentary rocks were being laid down, a period lasting for another 1,000 million years. Before that we travel back another 3,000 million years to the formation of the Earth's crust.