Echinoderms and Fossil Corals

Echinoderms and corals built much of the marine fossil record's biomass and reef framework, recording deep-time ecology through their distinctive skeletons.

Definition

Echinoderms are pentaradial marine invertebrates with a calcite endoskeleton of porous plates; fossil corals are cnidarians that secrete calcareous skeletons and, in colonial forms, construct reefs.

Scope

This topic covers echinoderms (crinoids, echinoids, blastoids, and others) with their unique calcite stereom skeletons and water vascular system, and the major reef-building corals including extinct rugose and tabulate corals and the modern scleractinians.

Core questions

• What is the stereom skeleton and why is it diagnostic of echinoderms?
• How do crinoid and echinoid forms reflect their modes of life?
• How do rugose, tabulate, and scleractinian corals differ?
• How do fossil corals record ancient reefs and climate?

Key concepts

• Pentaradial symmetry and water vascular system
• Stereom calcite skeleton
• Rugose, tabulate, and scleractinian corals
• Crinoidal limestones and reef framework

Key theories

Stereom microstructure

The single-crystal, porous calcite stereom of echinoderm plates is a unique tissue that survives as recognizable ossicles even when skeletons disarticulate.

Coral reef succession through time

Reef-building passed from Paleozoic rugose and tabulate corals to Mesozoic and Cenozoic scleractinians, with reef collapses tracking major extinctions and climate events.

Clinical relevance

Crinoidal and reefal limestones are major rock-forming deposits and hydrocarbon reservoirs, and fossil coral skeletons preserve growth-band and isotope archives used to reconstruct past sea temperatures and reef health.

History

Crinoids and echinoids have been collected since antiquity, and nineteenth-century work established echinoderm classification and the recognition of pentaradial body plans. The history of coral reefs became a major theme after Darwin's theory of reef formation and later studies of reef gaps following mass extinctions.

Debates

Origins of Mesozoic scleractinian corals

Whether scleractinians descend from skeletonless soft-bodied anemones or from Paleozoic coral lineages remains debated.

Key figures

• George D. Stanley
• David Nichols
• Euan Clarkson

Seminal works

• clarkson1998
• stanley2003

Frequently asked questions

What are crinoids?

Crinoids, or sea lilies and feather stars, are stalked or free-living echinoderms whose stem and arm ossicles form thick crinoidal limestones in the fossil record.

Did all fossil corals build reefs?

No, many corals were solitary, but colonial rugose, tabulate, and scleractinian corals built the framework of reefs through different geological eras.