| | |of amphibians and land | millions | | |plants. | of +---------------+--------------+-----------------------+ years. PALEOZOIC| | |Shellfish develop | Era of | _Silurian_ | |fully. Appearance and | Old Life | | |culmination of crinoids| | | |or sea-lilies, and | | | |large scorpion-like | | | |crustaceans. First | | | |appearance of | | | |reef-building corals. | | | |Development of fishes. | +---------------+--------------+-----------------------+ | | |Sea animals develop | | _Ordovician_ | |shells, especially | | | |cephalopods and | | | |mollusk-like | | | |brachiopods. Trilobites| | | |at their height. First | | | |appearance of insects. | | | |First appearance of | | | |fishes. | +---------------+--------------+-----------------------+ | | |More highly developed | | _Cambrian_ | |forms of water life. | | | |Trilobites and | | | |brachiopods most | | | |abundant. Algae. | ---------+---------------+--------------+-----------------------+---------- | _Algonkian_ | |The first life which | | | |left a distinct record.| 33 | | |Very primitive forms | millions PROTEROZOIC | |of water life, | of | | |crustaceans, | years. | | |brachiopods and algae. | ---------+---------------+--------------+-----------------------+---------- | _Archean_ | |No fossils found, but | | | |life inferred from the | | | |existence of iron ores | ARCHEOZOIC | |and limestones, which | | | |are generally formed in| | | |the presence of | | | |organisms. | ---------+---------------+--------------+-----------------------+----------