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By Victor A. Ramos, J. Duncan Keppie

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The eastern belt consists mainly of carbonates and shallow marine clastics of shelfal and restricted environments, whereas the western tectofacies includes a thick wedge of slope to basin olistostromes, turbidites, and basinal black shales, the latter interbedded with mafic lava flows (some pillowed) and intruded by basic dikes. The overall western belt succession is affected by a very low-grade regional metamorphism. Figure 1. Location map showing basins and geologic regions mentioned in the text.

Among the latter, one of the most important is the type of larval development. At the end of the larval stage, the success of settling in a new area depends on the availability of a niche to colonize, the absence of competitors, and an adequate substrate-level environment. Influence of such factors depends, in turn, on the opportunistic or highly selective character of the species. Abiotic factors include direction and velocity of marine currents, water salinity, oxygenation, and temperature. Some planktotrophic larvae can survive several weeks before settlement.

Several additional factors could have been important to sponge dispersal, such as flotation of colonies and rafting. Biogeographic distribution of present-day sponges shows remarkable differences between interoceanic populations (Wilkinson, 1987). During the Ordovician, some genera were widespread while others are endemic or have more restricted distribution. Sponges have been considered as a very conservative group (long stratigraphic ranges of genera and families), which implies slow but persistent migration rates and, conversely, low rates of speciation.

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