A Beginner's Guide to Collecting Coral

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Having established my goals, let me now diverge briefly to give a personal background. This should give the reader a sense of where I am coming from in terms of perspective. I was born in Montgomery, Alabama. My older sister was primarily responsible for my interest in collecting. When she was in elementary school, she would collect and show off shark's teeth she found in the school playground while the other kids swang and played kickball. At the age of 5, we moved to the Ordovician hills of the central basin of middle Tennessee. The rolling hills from Nashville down to the Alabama line are chock full of road cuts and outcroppings literally dripping with marine invertebrate fossils. Therefore, my experience and collection have been primarily concentrated upon the Paleozoic tableland of the east central United States. Much of our young adulthood was expended seeking that complete trilobite (which I will confess I never found in middle Tennessee, but I will one day!)

I realize this is not following the topic, but I really must mention an interesting story here. One day while passing through Cincinnati for the first time, I was impressed by the huge limestone cliffs along the highways.

Years later, I got back for that second visit, but just in for one day on business. On my morning walk, I began my trek from the hotel on the north side of town frantically searching for a road cut. No such luck. I only found open fields with construction going on. However, I desperately grasped a few local chunks of limestone strewn into the wet clay. "AH HA, brachiopods, just as I suspected, and Ordovician at that!" Well, I purposed my third visit would be different. Still just a callow youth now living in Knoxville, Tennessee (where I did find my first complete trilobite, a pair of doozies. (That's technical talk for pliomerops), I planned a visit to a friend for one night while passing through. I dropped the newlywed wife off at the mall and promised I would pick her up in a couple of hours. Like a madman, I recklessly careened toward Hamilton, where I was told by reputable sources that a nest of angry flexicalymene was carelessly overturned by innocent construction workers building a new Kroger. (Needless to say, it took quite a crew to exterminate the beasts and rid the world of the carnage.) Oh, too bad. Nothing but manicured mulch and boxwood at that site. However, on the way back, driving down US 23 I think it was, there they were big as life: cliffs by the road, in the southbound lane. Quick as a flash, I pulled over in the emergency lane (hey, it was an emergency), bounded out of the car, grabbed the pick hammer and immersed myself in the warm embrace of weathering bedrock. No it can't be, only 45 minutes left to pick up my wife. Oh, the fickleness of life. I ended up staying there only 1 hour. But I found my third complete trilobite! A small, unimpressive flexicalymene, but it's mine all mine.

WO! Corals, Albert corals. I'm terribly sorry. Flashbacks can be so vivid. I really will try to keep on track. I was trying to say that I grew up in middle Tennessee, but the rocks of Cincinnati are incredibly similar, the same age, fossil distribution and preservation. More trilobites in Cincinnati, but more coral in Tennessee. Coral? Did someone say coral? Yes, that reminds me of another story! Our grandparents lived in our hometown of Columbia, Tennessee. They had property at the bottom of a tall, round hill with exposed cliffs from which would weather out wonderful corals, nautiloids, bryozoan, etc. Anyway, while playing in the woods nearby, we discovered that someone had used the area to dump their garbage. So there were 50 year-old bottles and other rubbish laying just beneath the surface - exactly the kind of thing kids love. We were kicking through the leaves and what did my sister find? It was a perfectly preserved spherical coral colony 10 inches in diameter (see it here!). Apparently, someone's kid had found it weathered out and taken it home to mom and dad. "Oh, that's nice dear." Out with the garbage it went as soon as they weren't looking! So this twice found fossil now adorns my cluttered basement. Well, my sister's rock appeal has long since waned. Hence I have appropriated it, with the story, into my personal belongings.

See, that wasn't so bad, was it? It even had to do with coral. By the way, did you know that there are three types of coral? They are called rugose, scleractinian, and tabulate. I apologize for the long weird names, especially "scleractinian", but it's not my fault. I promise. The differences can be very hard to appreciate for the novice (like me!) and are not always obvious. To make matters worse, there is another critter that can look exactly like a coral and yet is not. That is a bryozoan, which are often found in great abundance along with corals. Without delving into the briny depths of zoology, I will try to give the casual and intermediate fossil collector the necessary information to distinguish these without the use either of stereo microscopes or advanced degrees.

Referring to figure 1, a typical cross section is drawn of a hypothetical rugose coral. As a very general rule with many exceptions, rugose coral have stronger radial septa than they do transverse platforms. If I may relate an individual coral tube to an orange, the septa may be thought of as the clear membrane separating the slices. They converge in the center and radiate outward from there. Baby corallites are born (OK, they aren't born, they bud asexually) with four cute little septa (see arrows in figure). As baby corallite grows, his calice becomes larger and the septa begin to spread further apart. So, two new septa are first added (kind of like baby teeth). As he continues to grow, four septa at a time are added to maintain a rigid structure. For this reason, rugose coral are also called tetracoral, since tetra means four.

Figure 1. Illustration of a generic Rugose corallite.

Long, long ago little rugose corallite babies played together in the deep blue sea (OK, they didn't often run around, but they did build reefs). The very first rugose coral was created in the Ordovician. They enjoyed a long and happy life, building reefs all over the world. Then, a very terrible thing happened. It was at the end of the Permian, I believe it was. Someone outlawed all rugose coral from ever building another reef. And being very humble animals, the rugose coral, sadly, never came back to play.

Many legends remain of the rugose coral. Stories passed down for generations. Everyone knows of the famous prismatophyllum, which are now loving referred to as "Petosky stones" because of their abundance in Petosky, Michigan. Michigan even bestowed upon them the honor of "State Stone". Then there are the grewinkia and their kinfolk, common in the upper Ordovician limestone of middle Tennessee and Cincinnati, as well as in the Silurian and Devonian deposits across New York, Ontario, and Michigan. Acrocyathus forms huge masses in the Mississippian highlands of the Cumberland plateau and is found both weathering out of the escarpment and as rounded creek rock. Rugose coral may grow either individually (horn coral) or together in colonies, as with the Petosky stone.

Then there are the tabulate corals. Tabulate coral have (again with exceptions) stronger transverse plates than septa. By transverse plate, I mean a membrane which cuts across the longitudinal axis, see figure 2. The soft body parts of the coral were supported by this platform. As the rim of the corallite grew taller, the coral would secrete a new higher platform in order to remain sitting at the top, where food was available. In the figure, I have drawn an individual corallite to help visualize. However, tabulate corals rarely occur individually.

Figure 2. Illustration of a generic tabulate corallite.

Tabulates were really quite home bodies. They almost never played by themselves, but grew up in neighborhoods or colonies, side by side with their tabulate buddies. And rightly so. Without strong radial septa, their structural rigidity was quite limited. The margin of figure 2 shows a typical corallite arrangement in tabulate coral. The section is parallel to the longitudinal axis. The calices at the top often form polygonal (honeycomb) patterns at the surface (hence the popular name of honeycomb coral). Rugose corals also got together sometimes in colonies, but often stood alone.

Being the socially dependent individuals that they were, tabulates often poked holes (OK, they grew holes) in their corallite walls to talk with, steal food from and spy upon their corallite neighbors. Although leaving little privacy, this behavior seemed to work well. Tabulates are abundant in Paleozoic marine sediment worldwide. Unfortunately, the same fate which eradicated the rugose coral from the face of the earth also affected the tabulates. They coexisted with their rugose cousins from the Ordovician to the Permian extinction.

The most commonly found tabulate is the favosites. Some of the many favosites localities I know of are: the Ordovician, Silurian, and Devonian limestone of Tennessee, Kentucky and Ohio, the Silurian / Devonian belt from New York to Michigan, and Silurian Iowa (like Coralville). Favosites is, in my unqualified opinion, the archetypal tabulate coral. They commonly form spheroidal colonies that range from very small to immense.

Now, if you think the differences in tabulate and rugose corals are exciting, just wait until hear about scleractinian coral! (You may want to be sitting down for this.) Once again, refer to the diagram in figure 3. Like rugose coral, platforms are weak or absent. But baby scleractinians are born with (and here's the thriller) six septa. Septa are then added between existing adjacent pair, hence 6,12, 24, 48 ... at a time.

Figure 3. Illustration of a generic scleractinian corallite.

Scleractinian corals first appeared in the Triassic, but were fairly gun shy after the Permian holocaust. They did not become abundant until the Jurassic. Now, you may wonder how, if every coral on earth was suddenly killed, they could have reappeared at a later date with almost identical structure. Well, most people admit that no one really knows. Some think a sea anemone converted to coralism and started a whole new sect. Or perhaps there may have been a mutant, renegade rugose coral hiding out at a deserted south pacific island until after the dawn of the Mesozoic. Whatever happened, scleractinia is now the only remaining coral subclass. It forms reefs in tropical climates around the world, especially off the eastern coast of Australia.

Scleractinia is hard to distinguish visually from rugosa (the officially sanctioned subclass name), but is simple to determine if you know the locality. If the locality is Mesozoic or Cenozoic, it is scleractinian. If it is Paleozoic, it is either rugose or tabulate. If you aren't sure the age of the host rock, get a good geologic map if possible, or ask local knowledgeable sources.

Corals have soft body parts that are not usually well preserved in the fossil record. Their bodies were somewhat similar (biologically that is) to jellyfish, sea fans, and sea anemones. Most varieties of colonial coral form roughly spherical colonies at first. However as time progressed for the colony, preferential growth would lend itself to many forms: branchlike, sheetlike, or often similar to a lava flow, slowly engulfing everything in its path such as other rocks or animal skeletons. Solitary corallites are normally conical, either straight or curved. There may be either circumferential or longitudinal ridges, indicative of the septa and platforms present on the inside.

Commonly, corals may be misidentified by beginners because of these distinct shapes. Horn coral are often thought to be teeth, since teeth and claws are the most common naturally occurring conical objects in our life. Many a branching colonial coral, especially if the corallites are small, has been proudly brought home (or to museums!) as a "dinosaur bone." But even if they are not as dramatic as a tyrannosaurus rex, coral fossils can be quite impressive and attractive. And because of their relative abundance and durability, can be collected by amateurs easily and in many localities.

I had promised earlier to help distinguish between corals and bryozoa. This is not always an easy task. Without careful microscopic analysis, coral with very small corallites can have the same external shape and fibrous appearance. Normally, however, anything with "holes" (corallites for coral, zooecium for bryozoa) 1mm or larger will be a coral. Also, corallites typically open to the surface of the colony with a "cheese grater" roughness, whereas a bryozoan will normally have a smoother external surface. These two guidelines work in about 99% of the cases. The other 1% is best taken to a university geology department or good fossil museum curator.

Well, enough technical content, let's have some more stories! I moved up to the frozen north (west Michigan that is) about seven years ago. I had never lived in a place without abundant bedrock outcroppings before. It has been a shock to be certain. Gone are the days when afternoons were spent scaling road cuts, enduring chiggers, broken bottles, ticks, curious policemen, copperheads, and the insults of passing motorists all for the glory of bringing home a neater rock than the day before.

My first trip north of Cincinnati brought me along highway I-75 en route to Detroit to meet my future in-laws. Well, I figured I would stop at famous Silica, Ohio (famous to fossil collectors, anyway). I drove around and was surprised to find that the landscape was flat as a pancake and the only possible collecting would be in the quarries. So I pulled into the main quarry office, walked in, introduced myself and being the southern bumpkin I am, asked if I could collect and that I would be willing to pay, if they wished. One of the men (whom I have seen later in fossil shows) kept silent. The other fellow said there might be a quarry in Lucky (an hour back south) that would permit collecting. But that if he caught me on his property again, he would call the police! Welcome to northern hospitality!

Well, once again, I am straying from my topic. And since you now know at least as much as I do about coral, the time has come to bring this article to a close. I must admit, I rarely go collecting for the specific purpose of looking for coral. There are more kudos from fellow fossil hunters for finding a perfect nautiloid, trilobite, or even gastropod than a "mere" coral. Yet because it forms huge durable reefs, coral represents a major and important portion of the fossil record. Here's wishing you many enjoyable hours of hunting that may bring you coral specimens worthy to show off to friends and relatives.

Illustrations by the author.