Fossil Corals

drawing of horn coral

Description: Corals are simple animals that secrete skeletons made of calcium carbonate. They are close relatives of sea anemones and jellyfish and are the main reef builders in modern oceans. Corals can be either colonial or solitary.

As fossils, corals are found worldwide in sedimentary rocks. Based on these fossils, we know that the corals began their long evolutionary history in the Middle Cambrian, over 510 million years ago. In Kansas, they are fairly common in Pennsylvanian and Permian rocks, deposited from about 315 to 250 million years ago.

color photo of coral colony covering rock; fossils somewhat resemble links of chain

This sample of the colonial coral Cladochonus is from the Beil Limestone Member, Lecompton Limestone, Greenwood County. This limestone was deposited during the Pennsylvanian Period, about 300 million years ago.

Corals are simple animals, characterized by their radial symmetry and lack of well-developed organs. The polyp, the soft part of the coral, is essentially a small digestive sack made up of an inner and outer wall, separated by a gelatinous layer (see diagram). The mouth, surrounded by stinging tentacles, forms an opening through which food enters and waste products are expelled. The hard external skeleton is secreted by the polyp's outer wall. These calcium carbonate structures are the part of the animal most likely to be preserved as a fossil.

view A shows generalized coral, a stalk topped with a ring of tentacles; view B is a cross section, identifying the mouth, tentacle, gelatinous layer, outer wall, and inner wall

A, Generalized drawing of a coral polyp; B, cross section of simple polyp.

drawing of Syringopora, showing branching network of tubes, emerging as pores at top color photo of Syringopora fossil showing vertically aligned series of tubes

The tabulate coral Syringopora shows the structure of the hard parts that protected the individual polyps and formed the framework of the colony. The drawing shows the pores on the surface, from which the polyps extended their tentacles to feed; the photographed specimen is from the Plattsmouth Limestone Member, Oread Limestone, Douglas County (drawing by Al Kamb, KU Natural History Museum, Invertebrate Paleontology).

Corals live attached to the seafloor and feed by trapping small animals with their tentacles. They reproduce both sexually and asexually. Budding, a kind of asexual reproduction, occurs when the parent polyp splits off new polyps. Evidence of budding can be seen in fossil corals.

Modern corals inhabit deep-water environments as well as shallow reefs. Based on evidence from the rocks, scientists have determined that the Pennsylvanian and Permian corals of Kansas lived in warm, shallow, sunlit waters where the bottom was firm enough to offer a secure point of attachment.

Although corals are the main reef builders in modern oceans, not all corals build reefs. In addition to the corals, which are called framework organisms, other organisms contribute to the formation of reefs. For example, modern reefs are inhabited by binding organisms (such as encrusting algae) and filler organisms (such as snails, bivalves, and sponges), whose skeletons fill in the spaces in the reef after death.

Two groups of corals were important inhabitants of the Pennsylvanian and Permian seas--tabulate and rugose corals. Tabulate corals were exclusively colonial and produced calcium carbonate skeletons in a variety of shapes: moundlike, sheetlike, chainlike, or branching. Tabulate corals get their name from horizontal internal partitions known as tabulae. Some tabulate corals were probably reef builders (but not in Kansas).

color photo of branching coral covered with pores

The Pennsylvanian tabulate coral Thamnoporella illustrates the branching structure of some colonies. These corals were collected from the Bethany Falls Limestone Member, Swope Limestone in Labette County, Kansas.

color photo of moundlike coral covered with pores

Syringopora, another tabulate coral, illustrates the moundlike structure. This sample is from the Spring Branch Limestone Member, Lecompton Limestone, Woodson County, Kansas.

A common characteristic of rugose corals, from which they get their name, is the wrinkled appearance of their outer surface. (Rugose comes from the Latin word for wrinkled.) Rugose corals may be either solitary or colonial. Because solitary rugose corals are commonly shaped like a horn, these fossils are sometimes called horn corals.

color photo of three cornucopia shaped rugose corals

These Pennsylvanian rugose corals belong to the genus Caninia torquia, from the Beil Limestone Member, Lecompton Limestone, Douglas County, Kansas.

Both tabulate and rugose corals died out in the major extinction that occurred at the end of the Permian Period, roughly 250 million years ago. This extinction marked the end of the Paleozoic Era. The corals that inhabited the post-Paleozoic seas differ significantly from the earlier corals. Because of this, many specialists argue that these later corals may not be closely related to the Paleozoic corals.

Tabulate and rugose corals are common in eastern Kansas. Rugose corals are especially common in the Beil Limestone Member of the Lecompton Limestone in the vicinity of Sedan, Kansas.

Stratigraphic Range: Middle Cambrian to Holocene.

Taxonomic Classification: Corals belong to the Kingdom Animalia, Phylum Cnidaria, Class Anthozoa, Subclass Zoantharia. The subclass is divided into six orders, two of which--the Rugosa and Tabulata--are common Kansas fossils. Most living and post-Paleozoic fossil corals belong to a third order, the Scleractinia, the earliest fossils of which are from the Triassic Period, 14 million years after the end-Permian mass extinction.

Sources

Boardman, Richard S., Cheetham, Alan H., and Rowell, Albert J., 1987, Fossil Invertebrates: Boston, Blackwell Scientific Publications, 713 p.

Clarkson, E. N. K., 1979, Invertebrate Palaeontology and Evolution, 3rd Edition: London, Chapman and Hall, 434 p.

Fortey, Richard, 1999, Life--A Natural History of the First Four Billion Years of Life on Earth: New York, Knopf, 346 p.

Hill, Dorothy, 1981, Coelenterata, Anthozoa, Subclasses Rugosa and Tabulata; in, Treatise on Invertebrate Paleontology, Coelenterata, Supplement 1 (Rugosa and Tabulata), Part F, v. 1-2, Teichert, C., ed.: Boulder, Colorado and Lawrence, Kansas, Geological Society of America and The University of Kansas, 762 p.

Johnson, Kirk B., and Stuckey, Richard K., 1995, Prehistoric Journey--A History of Life on Earth: Boulder, Colorado, Denver Museum of Natural History and Roberts Rinehart Publishers, 144 p.

Moore, Raymond C., Lalicker, Cecil G., and Fischer, Alfred G., 1952, Invertebrate Fossils: New York, McGraw-Hill Book Co., 766 p.

Stanley, George D., Jr., and Fautin, Daphne G., 2001, The Origins of Modern Corals: Science, v. 291, p. 1913-14.

University of California Museum of Paleontology, 1994, Introduction to the Tabulata: www.ucmp.berkeley.edu/cnidaria/tabulata.html (4/12/00).

University of California Museum of Paleontology, 1994, Introduction to the Scleractinia: http://www.ucmp.berkeley.edu/cnidaria/scleractinia.html (4/12/00).

University of Newcastle, Department of Geology, 1998, Rugose and Tabulate Corals: http://www.newcastle.edu.au/discipline/geology/research/corals/corals.htm (Dec/24/03).

Text by Liz Brosius, Kansas Geological Survey. Drawing of rugose coral at top of page by Alan Kamb, KU Natural History Museum, Invertebrate Paleontology. Other illustrations by Jennifer Sims, Kansas Geological Survey; photographs by John Charlton, Kansas Geological Survey.

Ammonoids | Bivalves | Brachiopods | Bryozoans | Corals | Crinoids | Fusulinids
Gastropods | Insects | Sponges | Trilobites