The Paleozoic was a time with abundant fossils of multicellular organisms with shells. As a result, the fossil record improves dramatically at the beginning of the Paleozoic Era.
Representatives of most major invertebrate phyla were present during the Paleozoic, including sponges, corals, bryozoans, brachiopods, molluscs, arthropods, and echinoderms. Almost all of the common invertebrate phyla in existence today had appeared by the Ordovician.
Vertebrates evolved during the Paleozoic, including fishes, amphibians, reptiles,
and synapsids (so-called mammal-like reptiles).
The first vertebrates were jawless fishes, which are found in rocks as old as Cambrian in China.
An advanced lineage of fishes with primitive lungs and stout fins gave rise to the four-legged animals or tetrapods.
The transition from water-dwelling vertebrates to land-dwelling vertebrates depended on the evolution of the amniotic egg.
The first primitive land plants appeared near the end of the Ordovician. Vascular plants expanded across the face of the land, eventually forming great forests. The plants progressed from seedless, spore-bearing plants to plants with seeds but no flowers (the gymnosperms). (Flowering plants did not appear until near the end of the Mesozoic Era.)
Several mass extinctions occurred during the Paleozoic, including the largest extinction of all at the end of the Paleozoic Era (Permian period). Other mass extinctions occurred at the end of the Ordovician and Devonian periods.
Summary of invertebrate phyla with a good fossil record.
Paleozoic life includes some of the Precambrian forms, which survived into the Paleozoic, as well as more advanced forms:
Fossil of a rugose coral.
Fossil of a fenestrate bryozoan.
Fossil of an Ordovician strophomenid brachiopod.
Fossil of a Silurian trilobite.
Fragments of crinoid stems in Mississippian limestone.
Reconstruction of crinoids on the Mississippian seafloor
Fossil seed fern from the Pennsylvanian.
December 21, 2005