Irregularia is an
extant infraclass of
sea urchins that first appeared in the Lower
Jurassic
The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...
.
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
echinoderm
An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the s ...
s. The group includes the well known
heart urchins, as well as flattened
sand dollar
Sand dollars (also known as a sea cookie or snapper biscuit in New Zealand, or pansy shell in South Africa) are species of flat, burrowing sea urchins belonging to the order Clypeasteroida. Some species within the order, not quite as flat, are k ...
s,
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.
Image:Heart urchin Spatangoida 1380049 Nevit.jpg,
Image:Echinocyamus pusillus.jpg,
Image:Keyhole sand dollar 01.jpg,
Image:Live Sand Dollar trying to bury itself in beach sand.jpg,
Image:Clypeaster reticulatus both sides.jpg,
Image:Encope emarginata (Leske, 1778) derivate 2013.jpg,
Image :Clypeaster rosaceus (Linnaeus, 1758) derivate 2013.jpg,
Image:Clypeaster aegypticus (inside).JPG,
Taxonomy
* 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
The Holectypoida are an order of sea urchins related to the sand dollars. The order consists of just two living genera, but was once more diverse.
Description and characteristics
In appearance, the living holectypoids are similar to sand dolla ...
†
* 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
Sand dollars (also known as a sea cookie or snapper biscuit in New Zealand, or pansy shell in South Africa) are species of flat, burrowing sea urchins belonging to the order Clypeasteroida. Some species within the order, not quite as flat, are k ...
** 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
*
*
Echinoidea
{{echinoidea-stub