Sponges, Cnidarians, and Ctenophores
Françoise Debrenne and Joachim Reitner
Sponges and coralomorphs were sessile epibenthic suspension feeders living in normal marine environments. Sponges with calcified skeletons, including archaeocyaths, mainly inhabited shallow to subtidal and intertidal domains, while other sponges occupied a variety of depths, including slopes. The high diversity of sponges in many Cambrian Lagerstätten suggests that complex tiering and niche partitioning were established early in the Cambrian. Hexactinellida were widespread in shallow-water conditions from the Tommotian; some of them may have been restricted to deepwater environments later in the Cambrian. Calcareans (pharetronids), together with solitary coralomorphs, thrived in reef environments, mostly in cryptic niches protected from very agitated waters. Rigid demosponges (anthaspidellids and possible axinellids) appeared by the end of the Early Cambrian and inhabited hardgrounds and reefs from the Middle Cambrian. The overall diversity of sponge and coralomorph types indicates that during the Cambrian these groups, like other metazoans, evolved a variety of architectural forms not observed in subsequent periods.
RAPID DIVERSIFICATION near the Proterozoic-Phanerozoic boundary implies the mutual interactions of ecosystems and biotas. One of the most striking features in the distribution of Early Paleozoic sessile benthos is the poor Middle—Late Cambrian record (Webby 1984).
The present contribution deals with the ecologic radiation of sponges and cnidarians.
Sponges are a monophyletic metazoan group characterized by choanoflagellate cells (choanocytes). Based on studies made by Mehl and Reiswig (1991), Reitner (1992),