The Echinoid Directory

Isomicraster Lambert, 1901, p. 959

Diagnostic Features	Test cordiform with distinct anterior sulcus and short, truncate posterior face. Test subconical in profile. Apical disc ethmophract with four gonopores; central. All five ambulacra petaloid aborally and weakly sunken. Pore-pairs in frontal ambulacrum a little narrower than those in other ambulacra. Petals long, parallel and with perradial groove. Peristome facing forward into frontal groove. Labrum projecting to cover peristome in oral view. Plastron plating with symmetical sternal plates. Labral plate longitudinally elongate. Periproct on posterior truncate face; below mid-height in posterior view. No fascioles. Aboral tubercles set in a groundmass of fine granules.
Distribution	Upper Cretaceous (Turonian to Campanian), Europe, former Soviet Union, North Africa.
Name gender	masculine
Type	Isomicraster stolleyi Lambert 1901, p. 959, by original designation.
Species Included	I. senonensis (Lambert, 1895); Santonian-Campanian, Europe. I. stolleyi Lambert, 1901; Campanian, North Africa, Europe and Ukraine. I. sp. (Ernst, 1970); Turonian, Germany.
Classification and/or Status	Spatangoida, Micrasterina, Micrasteridae. Paraphyletic or polyphyletic grouping of Gibbaster species lacking fascioles. Subjective junior synonym of Gibbaster Gauthier, 1889.
Remarks	Differs from Micraster in having all five ambulacra petaloid aborally, and in being more conical in profile, with the periproct positioned below mid-height. Traditionally such forms with a subanal fasciole have been placed in the genus Gibbaster Gauthier, 1887, while forms lacking a subanal fasciole as adults have been placed in the genus Isomicraster Lambert, 1901. According to Ernst (1970) these all form a single lineage within which the subanal fasciole is variably developed. There remain significant taxonomic problems within this genus (see Stokes, 1976). Ernst, G. 1970. Zur Stammesgeschichte und stratigraphischen Bedeutung der Echiniden-Gattung Micraster in der nordwestdeutschen Oberkreide. Mitteilungen aus dem Geologisch-Palaontologischen Institut der Universitat Hamburg 39, 117-135. Lambert, J. 1901. Essai d’une monographie du genre Micraster et notes sur quelques échinides. Errata et addenda. In Grossouvre, A. de, Recherches sur la Craie supérieur. Mémoires du Service de la Carte géologique de France, (3) 23, 957-971. Stokes, R. B. 1976. Distinction between sympatric species of Micraster (Echinoidea) from the English Chalk. Palaeontology 19, 689-697.