Irregularia

Group of sea urchins

Irregularia Temporal range: Lower Jurassic–recent PreꞒ Ꞓ O S D C P T J K Pg N
Echinocardium cordatum
Scientific classification
Domain:	Eukaryota
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 at the two poles of the test, creating a bilateral symmetry instead of the classical 5-fold symmetry of

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.

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

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

Taxonomy

Hemipneustes pyrenaicus (Holasteroida

).

Clypeasteroida

).

• super-order Atelostomata
  • fossil family Acrolusiidae Mintz, 1968 †
  • fossil family Collyritidae d'Orbigny, 1853 †
  • fossil family Disasteridae Gras, 1848 †
  • order Holasteroida
  • order Spatangoida
  • fossil family Tithoniidae Mintz, 1968 †
• fossil genus Atlasaster Lambert, 1931 †
• fossil family Desorellidae Lambert, 1911a †
• order Echinoneoida
• fossil family Galeropygidae Lambert, 1911a †
• fossil genus Grasia Michelin, 1854 †
• fossil order Holectypoida †
• fossil family Menopygidae Lambert, 1911 †
• fossil genus Mesodiadema Neumayr, 1889 †
• super-order Neognathostomata
  • familia Apatopygidae Kier, 1962
  • fossil family Archiaciidae Cotteau & Triger, 1869 †
  • order Cassiduloida
  • order
    Clypeasteroida
  • fossil family Clypeidae Lambert, 1898 †
  • fossil family Clypeolampadidae Kier, 1962 †
  • order Echinolampadoida
  • fossil family Nucleolitidae L. Agassiz & Desor, 1847 †
  • fossil family Pygaulidae Lambert, 1905 †
  • fossil genus Pygolampas Saucède, Dudicourt & Courville, 2012 †
• fossil order Oligopygoida Kier, 1967 †
• fossil family Pygasteridae Lambert, 1900 †
• fossil family Pygorhytidae 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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