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Phylum Echinodermata

  • Subphylum Echinozoa

    • Class Echinoidea — Late Ordovician-Recent

Echinoids (sea urchins) live in normal marine environments because they with a very limited range of salinity tolerance (generally only a few ppm).

They occur mainly as grazers or burrowers in sandy shelf areas or as grazers and bioeroders along rocky shorelines. They occur in deeper waters as well, extending to abyssal depths. Fossil forms are most common in normal marine, open shelf or platform deposits.

Echinoids are common in both warm- and cold-water settings, although they rarely are major rock-forming organisms (i.e., they rarely exceed 10-15% of the total sediment).

Modern and ancient echinoids are/were composed of moderate- to high-Mg calcite. Modern forms contain between 2 and 17 mole% Mg; the Mg content varies with generic group and increases with increasing water temperature (see Milliman, 1974, p. 130-134, for details and citations).

Echinoids, like all echinoderms at some stage in their life cycle, show pentameral (five-fold) symmetry. They have heavily calcified, globular to discoidal, hollow, endoskeletal tests (coronas) that are composed of individual sutured, interlocking or imbricated calcite plates. The calcitic coronal plates are porous and sponge-like; echinoids with rapid growth rates have spongier plates (with more holes and less calcification) than slowgrowing counterparts. Thus, slow growing, cold-water forms can be more heavily calcified than those from warmer waters (Raup. 1958)

In life, echinoid tests are covered with elongate, moveable spines (in some species extremely short, but in others longer than 8 cm). The spines normally detach after death and can themselves be significant sediment contributors.

Generally, each plate of an echinoid behaves optically as a single, extensively perforated, calcite crystal (see comments below). Echinoid teeth, however, are polycrystalline.

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