''Micraster'' is an

extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

echinoid Sea urchins () are spine (zoology), 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 s ...

s from the

Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', the ...

to the early

Eocene The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene' ...

. Its remains have been found in

Africa Africa is the world's second-largest and second-most populous continent, after Asia in both cases. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 6% of Earth's total surface area ...

Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest contine ...

Europe Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a Continent#Subcontinents, subcontinent of Eurasia ...

, and

North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...

. Micraster was an

infauna Fauna is all of the animal life present in a particular region or time. The corresponding term for plants is ''flora'', and for fungi, it is '' funga''. Flora, fauna, funga and other forms of life are collectively referred to as '' biota''. Zoo ...

l echinoid living in a burrow below the sediment surface. The

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is clearly bilateral and there is a deep anterior groove to take in water containing organic particles to the mouth. The

tube feet Tube feet (technically podia) are small active tubular projections on the oral face of an echinoderm, whether the arms of a starfish, or the undersides of sea urchins, sand dollars and sea cucumbers; they are more discreet though present on britt ...

keep a supply of nutrient-laden water moving into the burrow. The anus has a waste tube behind it.

Continuous evolution

In the classic text ''

The Science of Life ''The Science of Life'' is a book written by H. G. Wells, Julian Huxley and G. P. Wells, published in three volumes by The Waverley Publishing Company Ltd in 1929–30, giving a popular account of all major aspects of biology as known in the 1 ...

'' (1931),

H. G. Wells Herbert George Wells"Wells, H. G."
Revised 18 May 2015. ''

Julian Huxley Sir Julian Sorell Huxley (22 June 1887 – 14 February 1975) was an English evolutionary biologist, eugenicist, and internationalist. He was a proponent of natural selection, and a leading figure in the mid-twentieth century modern synthesis. ...

and G. P. Wells use Micraster as an example of a fossil whose continuous evolution can be traced over some 10 million years through 450–500 feet of chalk beds of the Late Cretaceous. They point out: :"During this long period the fossil Micrasters are so abundant that hundreds of thousands can be collected and a gradual evolution can be traced as we pass upward. The changes are apparently trivial. There is a slow alteration of shape from rather flattened to rather arched, and from rather elongated to about as broads as long. The mouth creeps steadily forward, its distance from the front border of the lower surface decreasing from about a third of the body-length in the early types to a sixth in the latest, on a total length of fifty to sixty millimetres ... though the changes involved are small, they are absolutely continuous, the urchins found at one level grading quite imperceptibly into those of the rest; a single specimen, indeed, may show characters of one "species" in some of its tube-feet, characters of another in the rest." Micraster bajo

Species

* ''

Micraster norfolkensis ''Micraster'' is an extinct genus of echinoids from the Late Cretaceous to the early Eocene. Its remains have been found in Africa, Antarctica, Europe, and North America. Micraster was an infaunal echinoid living in a burrow below the sediment s ...

'' , 2012 † * '' Micraster burgiensis'' , 1935 † * '' Micraster coravium'' , 1959 † * '' Micraster decipiens 1878 † * ''

Micraster depressus ''Micraster'' is an extinct genus of echinoids from the Late Cretaceous to the early Eocene. Its remains have been found in Africa, Antarctica, Europe, and North America. Micraster was an infaunal echinoid living in a burrow below the sediment s ...

'' , 1937 † * '' Micraster desori'' , 1926 † * '' Micraster elevatus'' , 1949 † * '' Micraster gappi'' , 1925 † * ''

Micraster leskei ''Micraster'' is an extinct genus of echinoids from the Late Cretaceous to the early Eocene. Its remains have been found in Africa, Antarctica, Europe, and North America. Micraster was an infaunal echinoid living in a burrow below the sediment s ...

'' , 1855 † * '' Micraster piriformis'' , 1927 † * '' Micraster subglobosus'' , 1959 † * '' Micraster trangahyensis'' , 1936 † * '' Micraster turonensis'' 1878 † * '' Micraster uddeni'' , 1953 † * '' Micraster vistulensis'' , 1950 †

Sources

* ''Fossils'' (Smithsonian Handbooks) by David Ward (Page 185)

External links

''Micraster''
in the

Paleobiology Database The Paleobiology Database is an online resource for information on the distribution and classification of fossil animals, plants, and microorganisms. History The Paleobiology Database (PBDB) originated in the NCEAS-funded Phanerozoic Marine Pale ...

Notes

Spatangoida Prehistoric echinoid genera Cretaceous echinoderms Prehistoric echinoderms of Africa Extinct animals of Antarctica Prehistoric echinoderms of Europe Prehistoric echinoderms of North America {{paleo-echinoidea-stub