Eighth Edition
by Harold L. Levin

Chapter 12 - page 1

Life of the Paleozoic

Introduction

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.

Paleozoic Life

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:

1. Unicellular eukaryotes (Phylum Sarcodina)
   1. Foraminifera (including fusulinids)
   2. Radiolaria

2. Animals
   1. Invertebrates with shells or other hard parts (both aquatic and terrestrial). Includes groups such as:
      1. Sponges (Phylum Porifera)
      2. Archaeocyathids (extinct Phylum Archaeocyatha)
      3. Corals (Phylum Cnidaria)

         
         Fossil of a rugose coral.

      4. Moss animals (Phylum Bryozoa)

         
         Fossil of a fenestrate bryozoan.

      5. Brachiopods (Phylum Brachiopoda)

         
         Fossil of an Ordovician strophomenid brachiopod.

      6. Clams and snails (Phylum Mollusca)
      7. Trilobites, eurypterids and insects (Phylum Arthropoda)

         
         Fossil of a Silurian trilobite.

      8. Starfish, crinoids, and blastoids (Phylum Echinodermata)

         
         Fragments of crinoid stems in Mississippian limestone.

         
         Reconstruction of crinoids on the Mississippian seafloor

   2. Vertebrates with cartilage and/or bone (both aquatic and terrestrial)
      1. Fish. The first fish were jawless. (Jawless fish appeared in the Cambrian, as seen in Cambrian rocks in China). Fish with jaws appeared later (Silurian). Major evolutionary expansion of fish in the Devonian.
      2. Amphibians (beginning in the Late Devonian). Four legs (tetrapod), walked on land; eggs survived only in water.
      3. Reptiles (beginning in the Pennsylvanian). Amniotic egg could survive on land (amniotes).
      4. Synapsids (sometimes called "mammal-like reptiles"), such as the therapsids, by Permian time. Mammal-like characteristics.
3. Plants (both aquatic and terrestrial)
   1. Seedless, spore-bearing plants such as those in Carbonferous coal swamps.
   2. Seed-bearing, non-flowering plants (gymnosperms) - like conifers.

   
   Fossil seed fern from the Pennsylvanian.