ABSTRACT
Extensive spatangoids assigned to Eemiaster and Periaster species and collected in Cenomanian to Coniacian strata in Tunisian continental shelf ‚show numerous changes concerning test shape, tubercles density of aboral side, arrangement of fascioles and specialisation degree of the anterior ambulacrum. These changes seem to be related on the one hand to the adaptation of these spatangoids to burrow in the finegrained sediment and on the other hand to their burial depth. Thus, some taxa such as Hemiaster fourneli possess two fascioles: a peripetalous fasciole caracteristing the group and another latero-subanal one more or less developed. With this second fasciole Hemiaster tend to pass to the Periaster stage. At the same time, the tubercles density of aboral side becomes conspicously more important in species which have two fascioles than in species having diffuse peripetalous one. The sediment size analyse of the hosting rock reveals that all taxa assigned to Hemiaster or Periaster and having especialy two fascioles are very common in the sequences with extremely fine sediment (composed of about 50% - 60% grained-size less than 2 μm) deposed during decantation episodes in calm circalittoral environment. The acquisition of this laterosubanal fasciole in Hemiaster and Periaster, in addition to the peripetalous developed one is supposed to be an adptative answer of these echinoids to buried live in impervious and very fine and loose sediments limited to the continental shelf of the Cretaceous time. The increase of clavula bore by the tiny miliary tubercles of any fasciole is related to the increase of mucus output and current effect. All this will have benefited a spatangoid burrowing in very fine-grained sediments. Especialy the increase of mucus quantity is necessarily greater for the stabilization of the walls of the burrow (and its accessories) built into very fine sediments than that burrow made in coarser sediments of the saps shelf.
Moreover, among these spatangoids those which have a low test and equipped with an unspecialized ambulacrum III seem to live shallower buried in sediment (fine or coarse) than those with a high test and equipped with specialized ambulacrum III by having building-funnel tube feet.
