euechinoids

Euechinoids, or true echinoids, are a clade of echinoderms within the class Echinoidea that includes most living sea urchins and their fossil relatives, while excluding the more primitive cidaroids. The group encompasses a wide range of body forms, from the spherical regular echinoids to the laterally flattened irregular echinoids such as heart urchins and sand dollars.

Morphology and anatomy: Like other echinoderms, euechinoids have a rigid calcareous test (shell) made of interlocking plates and a water vascular system with tube feet. Regular euechinoids typically display pentaradial symmetry with a more or less spherical or discoidal test and often possess Aristotle’s lantern, a complex jaw apparatus used for grazing. Irregular euechinoids have undergone morphological shifts that produce bilateral or semi-bilateral shapes, with adaptations for burrowing or living on soft substrates; their mouth and periproct positions are often relocated compared to regular forms.

Evolution and ecology: The earliest euechinoids appear in the Paleozoic and underwent major diversification in the Mesozoic era. Today they occupy a broad array of marine environments, from shallow rocky reefs to deep-sea floors. Regular euechinoids commonly inhabit hard substrates and are mobile grazers, while irregular euechinoids are frequently detritivores or deposit feeders adapted to burrowing or living on soft sediments.

Taxonomy and classification: In modern classifications, euechinoids are the sister group to cidaroids. Within Euechinoidea, two major lineages are recognized: Regularia (regular echinoids with radial symmetry) and Irregularia (irregular echinoids with bilateral tendencies). The term “euechinoids” thus denotes the clade containing the bulk of living echinoids and their close fossil relatives.