30.3: Marine cohorts - the evolutionary faunas

When Sepkoski was gathering his data, he recognized another fascinating set of patterns in his Phanerozoic diversity curve. Doing some basic analyses, he was able to identify groups of marine taxa with hard skeletal material that tend to originate at the same time, co-exist for a few geologic periods, and then go extinct at relatively the same time. He called these groups evolutionary faunas. The Phanerozoic diversity curve is divided into three of these evolutionary faunas: the Cambrian fauna, the Paleozoic fauna, and the Modern fauna. It is important to recognize that these groups of organisms are not mutually exclusive, and indeed, throughout much of the Phanerozoic, they coexist. The idea is that, typically, one of the groups is most dominant in ecosystems at particular points of time. Therefore, if you have a general sense of which fossil groups “hang out together” you will be able to make quick deductions about the age of the rocks containing the specimens.

The Cambrian fauna

Figure \(\PageIndex{1}\): Asaphiscus wheeleri, Cambrian trilobite from Utah. (Public domain image.)

The Cambrian fauna is responsible for the incredible increase in diversity seen in the fossil record, known as the Cambrian explosion. Some major taxa included in the Cambrian fauna are trilobites, inarticulate brachiopods, and early representatives of echinoderms. Trilobites are a superphylum (Trilobitomorpha) of arthropods that are a hallmark fossil of not only the Cambrian, but also much of the Paleozoic. Many of the taxa of the Cambrian fauna, particularly the trilobites, were hit hard by the end-Ordovician mass extinction, and trilobites are one of many taxa that go extinct during the end-Permian mass extinction.

Figure \(\PageIndex{2}\): Inarticulate brachiopod Lingula sp. from the Cambrian Conasauga Formation of Alabama. (Photo courtesy of Encyclopedia of Alabama)

Inarticulate brachiopods are rather distinctive and typically fossilize with flattened, dark-colored shell with distinctive curved lines on the shell surface. Some genera, like Lingula sp. possessed a long, fleshy stalk to burrow into shallow marine sediments. While still alive today, inarticulate brachiopods are much less common than they were in the early Paleozoic.

The Paleozoic fauna

Figure \(\PageIndex{3}\): Diorama of a Late Ordovician seafloor with many representatives of the Paleozoic fauna. (Creative Commons Attribution 2.0 Generic license)

The Paleozoic fauna includes many of the taxa that are responsible for creating and maintaining the Paleozoic plateau of diversity from the Ordovician to the end of the Permian. These groups developed during the Great Ordovician Biodiversification Event (GOBE) as noted from the steep incline on the graph at the onset of the Ordovician. The GOBE is a summation of many increases in diversity among planktonic, benthic, and reef communities during the Ordovician. The Paleozoic benthic seas were dominated by a wide array of relatively abundant invertebrates, including trilobites, articulate brachiopods, bryozoans, reef building tabulate and rugose corals, crinoids, and blastoids. Interestingly, after initial bursts of species diversity of benthic organisms, there are also increases in the use of ecospace through the Paleozoic. Many of the taxa that evolved during the GOBE were filter feeding organisms, and despite diversification and abundance of phytoplankton and other prokaryotes and unicellular eukaryotes at the base of the food web, there was an increase in competition for resources. Therefore, as the Paleozoic continues, there is an increase in the number of organisms burrowing deeper under or growing taller (e.g., stalked crinoids and blastoids) above the seafloor. These adaptations also aid in maintaining diversity as sediments eroding into epicontinental seas from tectonic activity increase turbidity. Fish also became dominant during this time, including sharks and other groups of armored fish. These armored fish were likely diversifying in part due to evolutionary selective pressures from other fish and large arthropods of the Paleozoic oceans, such as eurypterids.

The Modern fauna

Figure \(\PageIndex{4}\): “Duria Antiquior,” a famous watercolor by the geologist Henry de la Beche depicting life in ancient Dorset based on fossils found by Mary Anning. (Public domain image.)

The Modern fauna follows the ecological reset that occurs after the end-Permian mass extinction. Because so many taxa from the Cambrian and Paleozoic faunas were exterminated during the end-Permian event, many ecological niches were open and available for evolutionary innovations. A number of planktonic clades that are found in modern oceans evolved at this time, including silica-based diatoms and radiolarians, and calcite-based coccolithophores and foraminifera (note forams first evolved in the Cambrian). Modern (scleractinian) corals arose as the primary reef building organisms, along with echinoids (sea urchins and sand dollars) as the main benthic community echinoderms. Most importantly, however, was the rise of molluscs—bivalves, snails, and cephalopods diversified significantly and dominated benthic communities. Bivalves include a wide array of common shells you mind find on the beach or at the seafood market, such as clams, scallops, mussels, and oysters. Bivalves and snails also eventually worked their way into terrestrial ecosystems. Shelled (nautiloids, ammonoids, belemnoids) and unshelled (e.g., octopus, squid) swimming cephalopods competed for food with marine vertebrate groups of the Modern fauna in the pelagic realm for fish and other swimming invertebrates. These vertebrate groups included a wide diversity of fish, swimming reptiles (e.g., icthyosaurs, plesiosaurs, sea turtles), and swimming mammals (e.g., whales and their ancestors, dolphins, sea lions).

To learn more about important fossil groups through time, see the Digital Atlas of Ancient Life from Paleontological Research Institute.

---

Further reading:

Sepkoski Jr., J.J. 2002. A Compendium of Fossil Marine Animal Genera. Bulletins of American Paleontology (363), 16 Sep 2002: 1-560. [Online]

Servais, T., Harper, D.A.T., Munnecke, A., Owen, A.W. and Sheehan, P.M. 2009. Understanding the Great Ordovician Biodiversification Event (GOBE): Influences of paleogeography, paleoclimate, or paleoecology? GSA Today 19(4): 4-10. doi: 10.1130/GSATG37A.1