Biota - Lifeforms

The Ordovician radiation took place over tens of millions of years and produced organisms that dominated the remaining marine ecosystems of the Paleozoic. The Ordovician radiation was an extension of the Cambrian explosion, from which all modern marine life (with the exception of the bryozoans) emerged.

As a result, almost all modern invertebrate phyla, as well as fish, originated in the Ordovician sea. The Ordovician sea was filled with a diverse community of invertebrates:

• brachiopods
  
• bryozoans
  
• trilobites
  
• mollusks
  
• echinoderms
  
• graptolities
  

On land the first plants appeared, as well as terrestrial arthropods.

Ordovician seas were characterized by a rich and diverse assemblage of species.

• stromatolites, are found in Ordovician rocks
  
• Chitinozoans
  
• microfossils with a hollow cavity and organic walls- phytoplankton
  
• photosynthetic organisms
  

Invertebrates

• stromatoporoids first appeared in the Ordovician.
  
• Tabulata
  
• rugosa corals
  
• Bryozoans
  
• brachiopods

Fish

Fish remains have been reported from the Late Early-Middle Ordovician of Australia and the Early Late Ordovician of Bolivia. Unidentified fish remains have also been reported from the Late Cambrian and Early Ordovician.

• The exact morphology of these fish is unknown, only fragments and individual scales have been found. Most specimens consist of individual bone plates covered with ridges and knots. The head appears to be covered with a dense row of plates, while the body is covered with a row of thin plates. The structure of the tail is unknown.

All of these fossils are assumed to be agnathans/jawless fishes.

Reefs

Reefs have been identified throughout the Ordovician, but the types of organisms that make up the reefs have changed over time. Most Ordovician reefs are small, but some can be up to 20-30 m thick.

Land plants

The Ordovician period contains the oldest generally recognized remains of land plants in the form of cuticle fragments and spores. Cuticle fragments lack pores or other structures, making identification difficult.