ABSTRACT
Hemiasterid echinoids are part of the oldest and most varied spatangoids, gathering more than two hundred fossil species and a few Recent ones. Mecaster and Hemiaster differentiated from Toxasterids during Apto-Albian periods in the western Mediterranean Tethys, with the subgenera Mecaster (Palhemiaster) and Hemiaster (Hemiaster) respectively. The parallel diversification of Mecaster and Hemiaster groups during Upper Cretaceous, and then the development of both hemiasterids and schizasterids, show several contemporary and convergent morphological changes. From Paleogene to Neogene, Ditremaster thrived in the Mediterranean basin until it was sealed off from the Indo-Pacific in the Middle Miocene. It appears necessary, now, to examine schizasterids and, may be, spatangoids as the whole with the same critical eye.
