Hypothesis On The Influence Of Hardground Feedback On The Benthic Fauna Evolution

The extremely fast growth in the number of echinoderms should have resulted in high rates of evolutionary innovation in this phylum. From the conventional classic point of view, the self-reproducing hardgrounds that appeared for the first time in the

Ordovician represented a new system of ecologic niches facilitating the existence of many benthic groups, particularly and primarily echinoderms. Colonization of these niches should have been accompanied by specialization and ubiquitous morphogeneses of the pioneer fauna. Hardgrounds were likely to be disjunct and patchy and to have appeared suddenly as a result of storm erosion, thereby favoring r-selection strategies, at least among the pioneers. Frequent burial and overturning of cobbles in high-energy seas would certainly select for progenetic lineages. Therefore, r-selection promoted the early sexual maturity of individuals and the subsequent shift of ancestral juvenile features to the mature stages in descendants (paedomorphosis or progenesis). This mode of natural selection might have resulted in the appearance of many new higher taxa, especially among the Echinodermata.

In essence, classes of animals, including those well documented in the paleonto-logic record, are distinct groups, between which obvious morphologic hiatuses exist, whereas the intermediate forms are absent. Roots of many classes cannot be traced beyond the Ordovician or the Cambrian. It is possible that the classes—for example, echinoderm classes—that are known since the Ordovician had only latent ancestors among Cambrian skeletal echinoderms and did not arise from soft-bodied forms. Therefore, since we do not see their direct ancestors in the paleontologic record of the forms with mineralized skeletons, we may assume that they originated from certain Cambrian forms as a result of changes in ontogeny and that these changes were rapid enough to escape the fossil record. The ontogenetic changes that generated evolutionary transformations are based on heterochronic shifts of the relative rates of different processes in individual development. At present, one kind of heterochrony, paedomorphosis, is recognized as one of the most probable mechanisms for the acceleration of macroevolutionary rate and saltatory speciation, because it provides a large evolutionary potential as the mechanism that permits rapid and profound coordinated changes in morphology, physiology, biochemistry, and behavior through an insignificant initial somatic disturbance (McKinney and McNamara 1991; Smir-nov 1991). Thus, paedomorphosis not only could play a role in the main morpho-genetic mechanism during the Ordovician radiation of marine biota but also could be the most important mechanism in the origin of higher taxa, especially of echinoderm classes.

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