Page 273 - Introduction to Paleobiology and The Fossil Record

Chapter 11

The basal metazoans:

sponges and corals

Key points

• Parazoans are a grade of organization within the metazoans composed of multicellular
complexes with few cell types and lacking variation in tissue or organs; the sponges
(Phylum Porifera) are typical parazoans that lack a gut.
• Sponges are almost entirely fi lter-feeding members of the sessile benthos. The group
contains a variety of grades of functional organization that cut across the traditional
classification of the phylum.

• Sponge reefs were dominated, during most of the Phanerozoic, by calcareous grades
developed convergently across the phylum; siliceous sponges were important reef build-
ers mainly during the Mesozoic.
• Stromatoporoids are a grade of organization within the Porifera with a secondary cal-
careous skeleton, important in reefs during the mid-Paleozoic and mid-Mesozoic.
• Archaeocyaths are Cambrian organisms of sponge grade. They were mainly solitary but
developed a branching, modular growth mode and successfully built reefs in often
turbulent and unstable environments.
• Reef-type structures were already present in the Late Precambrian hosting large, robust,
colonial organisms.
• The cnidarians are the simplest of the higher metazoans with a radial diploblastic body
plan and stinging cells or cnidoblasts. The phylum includes sea anemones, jellyfi sh and
hydra together with the corals.
• The Paleozoic rugose and tabulate corals displayed a wide range of growth modes often
related to environments; neither group was a dominant reef builder.
• The scleractinians radiated during the Mesozoic with zooxanthellate forms dominating
biological reefs. Scleractinian-like morphs in Paleozoic faunas arose several times inde-
pendently from anemones with scleractinian-type polyps.
• Reef development through time has waxed and waned, dominated at different times by
different groups of reef-building organisms.
• Coloniality within the metazoans has evolved many times; one hypothesis suggests that
a Precambrian colonial organism may have been a source for the bilaterians.