The Early Paleozoic Fauna: Ordovician radiation of the Cambrian survivors EPSC233 Earth & Life History (Fall 2002)

Recommended reading: STANLEY “Earth System History” Chapter 13, pp Keywords: phyla (arthropods, poriferans, brachiopods, echinoderms, mollusks, hemichordates), reef builders (archeocyathids), deposit feeders (trilobites, mollusks), filter feeders (eocrinoids, crinoids, brachiopods, mollusks), predators (cephalopods).

Apart from the occasional footprint, there is no evidence of terrestrial animal life in Cambrian and Ordovician sedimentary rocks. Oxygen levels had probably reached values close to the present atmospheric level.

Throughout the Paleozoic era, the rocks containing the most diverse fossils are typically ancient shallow- water marine limestones, and particularly fossilized reefs.

Reef: CaCO 3 framework of biogenic origin that rose over the sea floor. This framework is solid from the start and is not compacted or crushed during burial.

The earliest reef builders in shallow Paleozoic seas were not corals... Other organisms have the ability to build housings of CaCO 3 and thrive in shallow water. Stromatolitic reefs of the Precambrian were replaced by algal reefs (like those next to the Burgess Shale) and by new types of animals... the archeocyathids.

Archeocyathids were simple, filter-feeding, sponge-like animals (phylum Porifera). Their calcified skeletons built the earliest metazoan (i.e. animal) reefs. Calcified corals had not evolved yet.

These simple animals diversified and spread rapidly: over 200 genera are known worldwide from the middle Cambrian. But they went extinct before the end of the Cambrian period.

Poriferans (sponges) and cnidarians (jellyfish, sea pens and corals), the simplest type of animals found today, were probably among the earliest to appear. Several Ediacaran fossils are interpreted as poriferans or cnidarians because of their radial symmetry.

Sponges (poriferans) had been around since the latest Precambrian. Most of them leave little trace in the fossil record. In some cases, their soft body is supported by a flimsy skeleton of mm-size spicules which falls apart upon death. Usually, scattered spicules are the only remains found.

An early Cambrian reef was a busy place… good for shelter and to ambush prey.

During the Ordovician period new animals will evolve to build reefs. Cnidarians (sea anemones, jellyfish) give rise to calcified corals. Poriferans give rise to stromatoporoids. A type of filter-feeding worm gives rise to a new phylum, the bryozoans.

Primitive corals first appear in the Ordovician. Some were solitary, horn shape (rugose corals). Their growth bands have been used to calculate the number of days in Paleozoic years.

Other corals are colonial, like this tabulate coral Favosites.

stromatoporoids: sponge-related (not corals) Shapes like stromatolites, but the framework clearly includes mineralized pillars and layers that used to support a filter-feeding organism.

Life in sediments - advantage: offers shelter - disadvantage: - access to oxygen? - how not to choke in mud - Life on sediment: - but how not to sink in...

Brachiopods (a phylum of shelly invertebrates) will become the most abundant shelly fauna of the Paleozoic era. Most shells on today’s beaches are bivalve mollusks (clams, oysters, mussels). Until the end of the Paleozoic, brachiopods were the most successful group in the niche occupied today by mollusks.

Mollusks and brachiopods belong to two different phyla. They show similar use of hard parts even though they are different types of invertebrate animals. Their shells are -“anchors” for muscles - offer protection from predators - help keep out sediment from breathing and feeding organs.

Brachiopods are adapted to life on top of the sediment. Many have a pedicle (brown) to anchor themselves to a solid object (reef, or other shells or rock).