Irregularia

Irregularia Temporal range: Lower Jurassic–recent PreꞒ Ꞓ O S D C P T J K Pg N
Echinocardium cordatum
Scientific classification
Kingdom:	Animalia
Phylum:	Echinodermata
Class:	Echinoidea
Subclass:	Euechinoidea
Infraclass:	Irregularia Latreille, 1825
Superorders
Atelostomata Neognathostomata

Irregularia is an extant infraclass of sea urchins that first appeared in the Lower Jurassic.

Description and characteristics[]

These particular sea urchins are distinguished from other sea urchins by their irregular shape: the anus and often even the mouth are no more at the two poles of the test, creating a bilateral symmetry instead of the classical 5-fold symmetry of echinoderms. The group includes the well known heart urchins, as well as flattened sand dollars, sea biscuits and some other forms. Most of them live inside the sediment, moving in thanks to their particular spines, and feed on its organic fraction.

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  Test of a spatangoid (oral face) : the mouth is on the right and the anus on the left.
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  Echinocyamus pusillus.
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  Test of a Mellita quinquiesperforata.
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  A "sand dollar" in its environment.

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  Clypeaster reticulatus (family Clypeasteridae).
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  Encope emarginata (aboral and oral faces) drawn by Ernst Haeckel (1904).
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  Clypeaster rosaceus, ibid.
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  Many slices of a , showing buttresses and pillars.

Taxonomy[]

Fossil of an Hemipneustes pyrenaicus (Holasteroida).

Fossil of a (Echinoneoida).

Fossil of a (Clypeasteroida).

Echinolampas ovalis, Middle Eocene, Civrac-en-Médoc, France.

Echinolampas ovalis, Middle Eocene, Civrac-en-Médoc, France; oral surface.

• super-order Atelostomata
  • fossil family Mintz, 1968 †
  • fossil family d'Orbigny, 1853 †
  • fossil family Gras, 1848 †
  • order Holasteroida
  • order Spatangoida
  • fossil family Tithoniidae Mintz, 1968 †
• fossil genus Lambert, 1931 †
• fossil family Lambert, 1911a †
• order Echinoneoida
• fossil family Lambert, 1911a †
• fossil genus Michelin, 1854 †
• fossil order Holectypoida †
• fossil family Lambert, 1911 †
• fossil genus Neumayr, 1889 †
• super-order Neognathostomata
  • familia Apatopygidae Kier, 1962
  • fossil family Cotteau & Triger, 1869 †
  • order Cassiduloida
  • order Clypeasteroida
  • fossil family Clypeidae Lambert, 1898 †
  • fossil family Clypeolampadidae Kier, 1962 †
  • order
  • fossil family L. Agassiz & Desor, 1847 †
  • fossil family Lambert, 1905 †
  • fossil genus Saucède, Dudicourt & Courville, 2012 †
• fossil order Oligopygoida Kier, 1967 †
• fossil family Lambert, 1900 †
• fossil family Lambert, 1909b †

References[]

• Barnes, Robert D. (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. p. 981. ISBN 0-03-056747-5.
• Kroh A, Mooi R (eds.). "Irregularia". World Echinoidea Database. World Register of Marine Species.

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