The Echinoid Directory

Contributed by Andrew Smith, October 2006

Heterodiadema Cotteau, 1864, p. 521

Diagnostic Features	Test circular, depressed in profile with ambitus about mid-height. Apical disc large, key-hole shaped and projecting into the posterior interambulacrum, this projection getting deeper in larger individuals. Plating not preserved, presumed monocyclic; interradial notches distinct. Ambulacra straight and moderately broad, with trigeminate ambulacral plating; all elements reaching the perradial suture, but with central element the largest. Pore-pairs uniserially arranged throughout. A large primary tubercle overlaps all three elements of a compound plate on ambital and adoral plates. Above the ambitus tubercles decrease in size markedly and take up an adradial poisiton, but still straddle all three elements; A broad granular zone is present perradially above the ambitus. Interambulacral plates wider than tall, each with a large subcentral primary tubercle, with a slightly incised areole. Primary tubercles decrease in size markedly on the upper surface. Well developed adradial and interradial zones of granulation are developed, especially aborally. Tubercles perforate and crenulate up to the ambitus; smaller primary tubercles on apical surface usually imperforate and non-crenulate. Peristome slightly sunken, with deep buccal notches that have a small tooth-like partition. Long, naked and slightly sunken zones extend along the adradial margin of interambulacral zones behind each buccal notch. Spines and lantern unknown.
Distribution	Upper Cretaceous (Cenomanian) of southern Europe; North Africa (including Niger) and the Middle East as far as Karakorum.
Name gender	neuter
Type	Hemicidaris libyca Desor, in Agassiz & Desor, 1846, p. 338, by monotypy.
Species Included	H. libycum Desor, in Agassiz & Desor, 1846; Cenomanian, North Africa, Middle East H. ourmemense de Loriol, 1890; Cenomanian, Portugal.
Classification and/or Status	Euechinoidea, Acroechinoidea, Diadematoida, Heterodiadematidae
Remarks	Although no lantern has yet been described from this taxon, it seems likely that Heterodiadema is a diadematoid, based on its tuberculation and deep buccal notches. It is very close to Trochodiadema in test morphology, but that genus has a regular apical disc that is not extended posteriorly. Juvenile Heterodiadema and Trochodiadema may be very difficult to differentiate. Cotteau, G. H. 1862-1867. Paléontologie Française. Terrain Crétace, Tome Septième. Échinides. Masson et fils, Paris. 892 pp., pls 1007-1204. Geys (1980) reported the presence of Heterodiadema lybica from the Campanian of Belgium, but this was based on an old museum collections and likely represents a mislabelled specimen. Geys, J. F. 1980. Heterodiadema libycum (Agassiz & Desor, 1846) . a hemicidaroid echinoid from the Campanian of Belgium. Anales de la Societe geologique du Nord 99, 449-451. Smith, A. B., Simmons, M. D. & Racey, A. 1990. Cenomanian echinoids, larger foraminifera and calcareous algae from the Natih Formation, central Oman Mountains. Cretaceous Research 11, 29-69.