Fossil Corals

"Hundreds of thousands of individual animals -- Each with just one opening for living."

In the Earth's geological past, corals were abundant, and just like modern-day corals are found in the clear, warm and tropical waters of some particular part of the world, fossil corals made reefs deep under the ancient seas.

There are some coral reefs that lie in the form of great structures amidst sedimentary rocks.

The ancient reefs are not made completely of corals. Here's an illustration of an Ordovician Reef (488 - 443 MYA), where life continued to increase in complexity from that of the Cambrian, and where the extensive coral reefs were created.

Figure 1.

Ordovician life included a diversity of predators, including straight-shelled "nautiloid-like" or "squid-like" creatures.

The algae and sponges and fossilized remains of echinoids, brachiopods, bivalves, gastropods, and trilobites contributed to making the fossil reefs. In fact, fossil coral reefs are the prime locations to see different types of fossils.

Here's another illustration of a coral-based Devonian reef (416 - 359.2 MYA).

Figure 2.

Can you pick out the corals?

Corals were not limited to reefs only. There are several corals that are found in rocks where reefs were not present. Geologists have dated the age of the rocks in which they are found. This is particularly the case of corals found in Carboniferous limestones (359 - 2.5 MYA).

Some Types of Fossil Corals

Here's a few of the corals in our collection, and some are still being classified.

Rugose Corals

The Rugosa or "rugose corals" (referring to their wrinkled appearance) are also known as "horn corals." And they were abundant in Middle Ordovician to Late Permian seas.

You can find these corals in both solitary and colonial forms, but the former are more common.

Here's an end view:

Figure 3. Heliophyllum Halli

Here's a side view:

Figure 4. Heliophyllum Halli

Here's an Ohio horn coral.

Figure 5. Aulpcophyllum sp. in silica shale

Here's one from the Middle Devonian, found in New York:

Figure 6.

Amplexiphyllum hamiltoniae Coral in Windom shale.

Here's a Devonian “Sun Coral.”

Figure 7. Middle Devonian (375 Million Years Old)

Coral Geodes

We have two coral geodes in our collection.

Here's one, an example of agatized coral, first found in the Tampa Bay area around 1825. This is considered by some to be the only gemstone of note from the State of Florida.

These corals are found in two forms, as geodes, which represent partial replacement of coral, and as solid pieces which represent total replacement.

The coral is replaced by a waxy, translucent, varicolored chalcedony (See below).

Here's what they look on the outside:

Figure 8. Miocene/Pliocene (a few million years old to tens of millions).

However, on the inside they are quite beautiful:

Figure 9.

Petoskey Stones

These are coral fossils, and some have been prepared using cutting and polishing techniques.

Petoskey stones are fragments of a coral reef that was originally deposited during the Devonian period, about 350 million years ago. They have a distinctive mottled pattern of a six-sided colony of coral.

Figure 10.

The scientific name is hexagonaria Percarinata (six sided).

Calceola

Calceola sandalina, both by its generic and specific name, means a little shoe or slipper. It is a common fossil in the Devonian rocks.

Figure 11. Middle Devonian

This is a lid coral, and as such it had a lid, the operculae (A horny or shelly plate), and a hinge between its Calice (opening) and lid.

This lid proved to be a survival adaptation for special conditions, such as high sedimentation rates.

Tabulate Corals

Tabulate corals were common from the Ordovician to the Permian. And recently, a Lower Cambrian coral, Moorowipora chamberensis, has been found in south Australia. It appears to be a tabulate coral, although this is not absolutely certain.

However, if it is a true tabulate, it extends the history of tabulate corals considerably. (Sorauf and Savarese, 1995)

Tabulate corals receive their name from horizontal internal partitions known as tabulae, as seen in the following specimen. Most tabulates were colonial, with some forming substantial reefs.

Tabulate corals, as well as Rugose corals (See above), went extinct at the end of the Permian, about 245 million years ago, victims of the heaviest mass extinction ever.

Figure 12.

Terms

As with other earth events and features, the unique composition of geodes can teach us about minerals, how they interact to produce structure, and ultimately the world we live on.

Definition of Chalcedony (kāl-sěd'n-ē)

n. A cryptocrystalline, translucent variety of quartz, having usually a whitish color, and a luster nearly like wax.

www.hpwt.de cal·ced·o·ny (kāl-sěd'n-ē)

Definition of Quartz

n. A form of silica, or silicon dioxide (SiO2), occurring in hexagonal crystals, which are commonly colorless and transparent, but sometimes also yellow, brown, purple, green, and of other colors; also in cryptocrystalline massive forms varying in color and degree of transparency, being sometimes opaque.

    
www.charmsoflight.com

The crystalline varieties include:

Amethyst, violet;
Citrine and false topaz, pale yellow;
Rock crystal, transparent and colorless or nearly so;
Rose quartz, rose colored;
Smoky quartz, smoky brown.

The chief cryptocrystalline varieties are:

Agate, a chalcedony in layers or clouded with different colors, including the onyx and sardonyx;
Carnelian and sard, red or flesh-colored chalcedony;
Chalcedony, nearly white, and waxy in luster;
Chrysoprase, an apple-green chalcedony;
Flint, hornstone, basanite, or touchstone, brown to black in color and compact in texture;
Heliotrope, green dotted with red;
Jasper, opaque, red yellow, or brown, colored by iron or ferruginous clay;
Prase, translucent and dull leek-green.

Quartz is an essential constituent of granite, and abounds in rocks of all ages. It forms the rocks quartzite (quartz rock) and sandstone, and makes most of the sand of the seashore. Origin: G. Quarz. Source: Websters Dictionary (01 Mar 1998)

Definition of Cryptocrystalline

a. Indistinctly crystalline; -- applied to rocks and minerals, whose state of aggregation is so fine that no distinct particles are visible, even under the microscope.

Links

Here's a good site that uses images to give a quick sketch of the History of Life:

www.geology.wisc.edu

The Ordovician period.

en.wikipedia.org/wiki/Ordovician

The Devonian period:

en.wikipedia.org/wiki/Devonian

Here's “The Five Worst Extinctions in Earth's History:”

www.space.com

Here's a good reference dictionary:

www.lexic.us/

Figures & Acknowledgments

Figures

Figure 1. www.geology.wisc.edu
Figure 2. www.geology.wisc.edu
Figure 3. www.indiana9fossils.com

Figure 4. www.indiana9fossils.com

Figure 5. www.indiana9fossils.com

Figure 6. www.indiana9fossils.com

Figure 7. www.indiana9fossils.com

Figure 8. www.indiana9fossils.com

Figure 9. www.indiana9fossils.com

Figure 10. naturestreasures.art.officelive.com

Figure 11. upload.wikimedia.org/wikipedia/commons

Figure 12. www.humboldt.edu