Revision of the Cretaceous echinoids Heterodiadema and Trochodiadema, and description of two new genera from the Middle East and Arabian Peninsula

The Cretaceous hetero diadematid echinoids Heterodiadema Cotteau, 1862 and Trochodiadema de Loriol, 1900 are revised and provided with amended diagnoses. Of six species previously referred to Heterodiadema, only three are now retained in this genus, namely Heterodiadema libycum (Desor in Agassiz & Desor, 1846), Heterodiadema ouremense de Loriol, 1884 and Heterodiadema buhaysensis Smith, 1995. Similarly, seven species were previously assigned to Trochodiadema, but here only Trochodiadema abramense de Loriol, 1900 is left in this genus. Based on material previously referred to Trochodiadema, two new genera and one new species are established: Loriolidiadema gen. nov. from the Lower Cretaceous of the Middle East, and Arabicodiadema gen. nov. and Arabicodiadema alii sp. nov from the Albian and Cenomanian of the Arabian Peninsula. Trochodiadema isidis Fourtau, 1921 is transferred to Aegyptiaris Abdelhamid & Abdelghany, 2015. New morphological data on primary and secondary spines and perignathic girdle are presented; these improve our understanding of the evolution and relationships of the family Heterodiadematidae Smith & Wright, 1993.