THE ADAPTATION IN THE SUPERORDER SPATANGACEA (ECHINOIDEA) AND THEIR EVOLUTIONARY ROLE

Some quantitative aspects of the evolution of the superoder Spatangacea (orders Spatangoida and Holasteroida) are related to the development of certain morphological adaptations. The generic diversity dynamics,rate of extinction at the Cretaceous/Tertiary boundary, average size of area inhabited by a genus and average generic life span have been compared in various morphological groups of the Spatangoids and Holasteroids. These quantitative parameters allow to distinguish the morphological characters which help to expand the adaptive zone and to occupy new habitats from those which result in specialization. Thus it ts possible to distinguish aromorphosis from allomorphosis or specialization. The progressive evolution of the Spatangacea was accompanied by gradual occupation of the new regions of the adaptive space. Echinoids with the most primitive morphological characters were able to survive only in a very narrow range of environmental conditions. Genera and species with more advanced morphology were usually more tolerant There were several significant aromorphoses in the evolution of the Spatangacea: the development of the frontal groove, petals, subanal and channel-building tubefeet, fascioles, etc. Each of these innovations significantly improved the ecological capabilities of the taxon. Only in a few cases the excessive development of certain characters resulted in specialization. The evolutionary pattern of this kind is characteristic of the groups whose origin was accompanied by the occupation of a vast vacant region of the ecological space. The original methods described here can be applied to other groups of animals.