Irregularia is an

extant Extant is the opposite of the word extinct. It may refer to: * Extant hereditary titles * Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English * Extant taxon, a taxon which is not extinct, ...

infraclass of

sea urchin Sea urchins () are spiny, globular echinoderms in the class Echinoidea. About 950 species of sea urchin live on the seabed of every ocean and inhabit every depth zone from the intertidal seashore down to . The spherical, hard shells (tests) ...

s that first appeared in the Lower

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), 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 J ...

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 urchin The heart urchins or Spatangoida are an order of sea urchins. Their body is a somewhat elongated oval in form, and is distinguished by the mouth being placed towards one end of the animal, and the anus towards the other. As a result, heart urchi ...

s, 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 The Atelostomata are a type of sea urchins. They are distinguished from other sea urchins by their irregular shape and the absence of a feeding lantern. The group includes the well known heart urchins, as well as some less familiar and extinct fo ...

** fossil family Acrolusiidae Mintz, 1968 † ** fossil family Collyritidae d'Orbigny, 1853 † ** fossil family Disasteridae Gras, 1848 † ** order

Holasteroida Holasteroida is an order of irregular sea urchins. Characteristics These irregular sea urchins are characterized by a particularly marked bilateral symmetry, including for the apical system, which is highly elongated. In some contemporary aby ...

** order

Spatangoida The heart urchins or Spatangoida are an order of sea urchins. Their body is a somewhat elongated oval in form, and is distinguished by the mouth being placed towards one end of the animal, and the anus towards the other. As a result, heart urchi ...

** fossil family

Tithoniidae Tithoniidae is an extinct family of sea urchin Sea urchins () are spiny, globular echinoderms in the class Echinoidea. About 950 species of sea urchin live on the seabed of every ocean and inhabit every depth zone from the intertidal seasho ...

Mintz, 1968 † * fossil genus '' Atlasaster'' Lambert, 1931 † * fossil family Desorellidae Lambert, 1911a † * order

Echinoneoida Echinoneoida is an order of echinoderms belonging to the class Echinoidea Sea urchins () are spiny, globular echinoderms in the class Echinoidea. About 950 species of sea urchin live on the seabed of every ocean and inhabit every depth zon ...

* 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 dollar ...

† * fossil family Menopygidae Lambert, 1911 † * fossil genus '' Mesodiadema'' Neumayr, 1889 † * super-order

Neognathostomata The Neognathostomata are a superorder of sea urchins. Description and characteristics They are distinguished from other sea urchins by their irregular shape and a highly modified feeding lantern. The group includes the well known sand dollars, ...

** familia

Apatopygidae Apatopygidae is a family of echinoderms belonging to the order Cassiduloida Cassiduloida is an order (biology), order of sea urchins. The group was extremely diverse with many families and species during the Mesozoic, but today, only seven exta ...

Kier, 1962 ** fossil family Archiaciidae Cotteau & Triger, 1869 † ** order

Cassiduloida Cassiduloida is an order of sea urchins. The group was extremely diverse with many families and species during the Mesozoic, but today, only seven extant species remain. A 2019 phylogenetic systematics study by Souto et al. presented a revised c ...

** 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 ''Clypeidae'' is a family of sea urchins belonging to the order Clypeasteroida. Fossil record This family is known in the fossil record from the Jurassic ( Bajocian age) to Cretaceous (Santonian age) (age range: from about 164.7 to 94.3 mill ...

Lambert, 1898 † ** fossil family

Clypeolampadidae ''Clypeolampadidae'' is a family of sea urchins belonging to the order Clypeasteroida. Fossil record This family is known in Cretaceous the fossil record of France, Oman, Spain and Turkey. Genera Genera within this family include: * ''Clypeola ...

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 Oligopygoida is an extinct order of irregular Echinoidea, echinoids. The group is closely related to the Clypeasteroida, and may be representative of its ancestral clypeasteroid. The order was once grouped together with the cassiduloids. Howev ...

Kier, 1967 † * fossil family Pygasteridae Lambert, 1900 † * fossil family Pygorhytidae Lambert, 1909b †

References

* * Echinoidea {{echinoidea-stub