How did shallow-water marine communities change during the Ordovician Period?

During the Ordovician Period, shallow-water marine communities underwent significant ecological shifts as part of the Ordovician radiation.

Initially, these environments were dominated by Cambrian fauna, primarily trilobites and inarticulate brachiopods. During the Early Ordovician, these groups began to decline in shallow habitats, replaced by articulate brachiopods, gastropods, and cephalopods. As the period progressed, articulate brachiopods and gastropods expanded further offshore, while trilobites and inarticulate brachiopods became increasingly restricted to deepwater environments. By the Late Ordovician, bivalve communities emerged in shallow waters, displacing the existing brachiopod-gastropod communities to deeper zones.12

Beyond these taxonomic shifts, communities became more complex. Organisms began utilizing new habitats like reefs and crinoid gardens, and developed vertical "tiering," growing up to 50 cm above the seafloor. Additionally, invertebrates increased their intensity of burrowing, reaching depths of up to one meter into the sediment.12