| |size. First appearance | 45
| | |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. |
---------+---------------+--------------+--