The Early Paleozoic Fauna:

earliest animal reef formers

and other benthos

186-233A Earth & Life History (Fall 2001)

Recommended reading:

STANLEY “Earth System History”Chapter 13, pp. 345-354.

Keywords:phyla (arthropods, brachiopods, echinoderms, mollusks), reef formers (archeocyathids), deposit feeders (trilobites, mollusks), filter feeder (eocrinoids, crinoids, brachiopods, mollusks), predators (cephalopods).

Wednesday October 3

- lecture cancelled

- students who gave their names should meet at my office (Frank Dawson Adams Bldg, Room 214, at 1h20).

- we leave from the FDA Building at 1h30.

Drift of continents during the Cambrian: 600, 540 and 525 million years ago.

A “proto-Atlantic” called Iapetus is created along east coast of N. America.

Most Cambrian sandstones are poor in fossils.

As sea level rose worldwide, continental shelves were flooded. Vast areas on the continental margins became hospitable to shelly marine faunas.

Cambrian sandstones grade upward to shallow-water limestones.

These limestones rarely contain stromatolites, unlike Precambrian limestones. They contain the remains of a diverse shallow-water community.

Burgess Shale fauna is a diverse assemblage of soft-bodied organisms. Some display body plans that have no counterparts to this day.

The finding of the Chengjiang fauna, 30 million years younger, confirmed the diversity of the early Cambrian fauna.

Ceraurus(Neuville, Quebec)

ElrathiaMiddle Cambrianwestern Canada

Several waves of extinction during Cambrian...

Archeocyathids were simple, filter-feeding, sponge-like animals. Their calcified skeletons built the earliest reefs of animal origin. Corals had not evolved yet.

Reef: framework of biogenic origin that rose over the sea floor. Provides shelter to a diverse community of organisms.

Primitive corals appear in the Ordovician. Some are solitary, horn shape (rugose corals). Their growth bands have been used to calculate the no of days in Paleozoic years.

Other corals are colonial, like this tabulate coral Favosites.

Sponges 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.

Only scattered spicules are found in sedimentary rocks.

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.

Brachiopods (a phylum of its own) will become the most abundant shelly fauna of the Paleozoic era.

Most shells on today’s beaches are bivalve molluscs (clams, oysters, mussels).

Until the end of the Paleozoic, brachiopods were the most successful group in the niche occupied today by molluscs.

Lingulepsis, an inarticulate brachiopod.

Inarticulate brachiopods survive today with shells very similar to those of their early Ordovician relatives. Most use their long pedicle to anchor themselves 10s of cm deep in the sediment.

Billingsella, a Cambrian orthid brachiopod

Brachiopods are different from mollusks. They anchor their shell to a firm ground using a muscular stem (pedicle).

This shell shows an opening for its pedicle.