Many paleontologists, ourselves included, became fascinated with fossils and embarked on scientific careers long before we ever encountered living marine animals. For many of us, the greatest thrill has been our first encounters with living representatives of the animal groups we knew first only as grey, lifeless forms encased in rock. Both of us have been privileged to examine firsthand living relatives of animals of our favorite groups of fossils—crinoids for Meyer and nautiloid cephalopods for Davis. Our experiences have fueled a curiosity that affects practically anyone who contemplates the fossil richness of the Cincinnatian or other comparable fossiliferous strata. Many times, in the field, we stand on a Cincinnatian outcrop where fossils are abundant in almost every rock, and we wonder: what did the Cincinnatian sea actually look like? How did these creatures behave when alive? If we could travel back in time to dive into the Cincinnatian sea, what would we see?
In his book The Crucible of Creation, the paleontologist Simon Conway Morris (1998) takes the reader on a journey through time in an imaginary time machine that lands on the shores of the Cambrian sea in western Canada of 520 million years ago. The time machine then descends into the sea and enables time traveling scientists to view the varied and bizarre animals found as fossils in the famous Burgess Shale. Conway Morris recreated the environment of the Cambrian sea and the life within it from the evidence of the fossils and rocks, but he embellished the scenario with a measure of speculation and fantasy.
Were we to travel back to the Late Ordovician, would we need a deep-diving submersible to explore the Cincinnatian world in a similar, imaginary journey? Based on the evidence from the rocks and fossils (see chapter 15), the time traveler to Cincinnati in the Late Ordovician would have to land on the sea surface, because no dry land would be found. With no familiar landmarks—the Ohio River valley, the Suspension Bridge, or Carew Tower—breaking the horizon, we would be attracted to the area only by vast shallows appearing as varying shades of aquamarine, with occasional shoals marked by breakers.
Our time machine transforms into a small boat as we land, and we check our position. There is no Global Positioning System of satellites, so we use a sensitive dip-needle that indicates that we are at 25° south latitude, but we have no reliable indicator of longitude. The water feels cool, about 65°F, so we need wet suits for the dive. Breakers off to the south look menacing, so we keep our distance and drop anchor where we can see the bottom. Let’s see what’s down there!
We don masks and snorkels for a quick reconnoiter, and slip over the side. We have anchored over very shallow water, and the bottom appears only a meter or so beneath us. As we take a closer look at the bottom, we notice that it is irregular, with low mounds separated by patches that are more level. The mounds are actually clumps of large, ribbed brachiopods, Platystrophia ponderosa. Living animals with articulated shells are intermingled with separated valves, some broken and worn smooth. It is the environment that, millions of years in the future, will be preserved as the Mt. Auburn Member of the Grant Lake Limestone. We easily scoop up a sample of specimens of Platystrophia because they have no pedicle attachments.
Clearly there is much of interest to see here, but we return to the boat because snorkel diving is not adequate for prolonged exploration. We have not been able to hold our breath during our dives as long as we normally do—we had to come up quickly, gasping for air. A check of our air quality monitor reveals the reason: the Late Ordovician atmosphere has only a fraction of the oxygen content of present-day air, perhaps as little as 1 percent. Fortunately we have brought along some sophisticated diving gear that will let us fill our diving cylinders with compressed air in which we have boosted the oxygen content to its present-day level, 21 percent. Because the rest of our compressed gas mixture is predominantly nitrogen, we still have to follow the dive tables that tell us how long we can dive at a given depth and resurface without suffering decompression sickness. We need to be really careful: we could not be farther away from a recompression chamber!
Equipped with collecting gear and cameras, we resume our dive using scuba gear. The water clarity actually is quite good: we can see perhaps 15 meters (50 feet) horizontally. However, there are patches of fine mud and silt covering wide areas of the bottom that could be easily stirred up if surface waves picked up. Visibility could drop sharply. As we settle to the bottom right beneath the boat, we can feel some wave motion. Although the sea floor appears very flat, we can see that there is a gentle slope falling off toward the north, so we swim slowly toward deeper water. We cover a lot of distance, but the depth changes very little. We see vast areas of the sea floor covered with bryozoans in dense thickets or folded sheets like a rumpled carpet. As we go deeper, more bush-like bryozoan colonies appear, some very delicate. It is amazing to see how similar the colony forms are to those of present-day bryozoans. But we know that, in the Ordovician, we are looking at trepostomes, not at animals of the groups that thrive in the seas of today. In places the sea floor is a hard, limestone pavement with a convoluted surface; depressions are filled with fine silt or shelly sand. Bryozoans and brachiopods are attached to the hard surfaces. The heart skips a beat as we spot for the first time a living edrioasteroid echinoderm, also attached like a present-day barnacle to the hardground. Because edrioasteroids became extinct in the Late Paleozoic, we have never been able to envision the living animal with confidence until now. A clump of tall, pillar-like specimens of Streptaster shows that Colin Sumrall had the right idea in suggesting that some edrioasteroids could inflate their thecae and telescope up or down from the substratum. Fine tube feet extending from the ambulacral grooves recall the reconstructions made by Bruce Bell. With a rock pick we easily break off a piece of the hardground with edrioasteroids attached, and we bag it for study in the lab. The hardground gives way to an area covered with thin-shelled Rafinesquina bachiopods, forming shell pavements like those we found in rock units with names like Corryville, Bellevue, and Fairview in the distant future from which we came. The concavo-convex brachiopods rest with the convex valve either down or up, and many are encrusted with small bryozoans or edrioasteroids. Brachiopods of taxa like Zygospira and pelecypods of genera like Caritodens are attached. Other clams, of taxa like Modiolopsis, poke up through the sediment between shells. Small Flexicalymene and Acidaspis trilobites glide over the surface, and here and there a crinoid has the stem coiled around a bryozoan. This is a diverse habitat.
But the water mass above the sea floor is surprisingly empty compared to the scene in present-day shallow seas. Today, fish are everywhere in the sea, filling a wide variety of ecological roles. Where are the fish in this Ordovician sea? Only small nautiloid cephalopods are jetting about, against a backdrop of pulsating jellyfish. At close range, we can pick out very small strings suspended in the water with clumps of minute tentacles arranged in vertical series. These are graptolites. Closer to the bottom some small, spiny trilobites, probably odontopleurids, swim above the bottom for short distances. Although early, jawless fish inhabit some Late Ordovician seas elsewhere, they had not yet spread to the Cincinnati region. It truly is a sea without fish! We do not have to worry about sharks either, because they will not evolve until the Devonian—millions of years in the future.
Our depth gauge shows that, as we swim farther away from the boat, the bottom gradually rises, and it becomes littered with brachiopods and bryozoans. Here and there mounds rise up as tall as a meter from the bottom. Although the mounds look somewhat like coral colonies, closer inspection reveals that they are heavily calcified colonies of bryozoans, whose fuzzy-looking surfaces are millions of oh-so-tiny tentacles extended into the water. They retract when we brush against the surface, and we can see the minute honeycomb-like skeleton beneath. Some colonies are rounded and boulder-like; others are branching or composed of complexly folded sheets. The patches between bryozoans are littered with broken fragments of bryozoans and brachiopods of genera like Hebertella and Platystrophia, including both living animals and dead, disarticulated shells. Here and there we see bryozoan colonies apparently turned upside down; we know that because the rounded side of each is on the bottom, or the branches do not radiate upward from a basal plate. These hefty, overturned colonies suggest that conditions can be far more turbulent than the placid calm in which, with great good luck, we landed. These waters sometimes must be raked by great storms. We feel stronger wave motion and soon we are breaking the surface on this shoal. Patches of dead, thin-shelled Rafinesquina brachiopods are turned on their edges and packed tightly together by wave action—what will be called “shingled Rafinesquina” beds in the far-distant future. Bryozoan branches and sheets are stacked into patches of coarse rubble. This is the environment of the Bellevue Limestone to be.
A gully leads us back into deeper water. Here we find vast areas covered with brachiopods of taxa like Rafinesquina and Strophomena or branching bryozoans. There are intervening patches of mud about equal in area to the shelly patches; this must be the environment of the Fairview, with continuous, even beds that will produce about 50 percent limestone and 50 percent shale.
In the distance we spot a ridge of bryozoans standing almost a meter above the surroundings, and we head toward it. Could we be approaching a sharp drop-off leading to deeper water? Approaching closer, we can see a richness of life around this ridge, and we sense a gentle current flowing along the bottom, parallel to the gradual slope, which intensifies as we reach the ridge. The depth gauge reads 15 meters (50 feet), and we have descended below the depth where the small surface waves stirred the bottom. Here, current flow takes over and follows the contours of the slope. The ridge is actually a mound built entirely of the branching bryozoan colonies of genus Parvohallopora; it projects outward from the slope. The current is diverted and gains velocity as it flows over the ridge. Crinoids forming a dense clump are of genus Glyptocrinus; their stalks are coiled around the bryozoan branches, and the crinoids stand above them like a forest canopy. Their crowns are splayed out in feathery filtration fans that are all aligned perpendicular to the current. Standing above them are a few gigantic crinoids of genus Anomalocrinus, with thick stems a meter or more in length and broad, dense filtration fans some 30 centimeters in diameter, also oriented by the current. Many other invertebrates make their homes in this intricate thicket. Cyclonema snails are attached to the calyces of practically every specimen of Glyptocrinus and clumps of small Zygospira brachiopods attach to the crinoid stems. Other brachiopods of genus Platystrophia and pelecypods of Ambonychia are nestled in among the bryozoan branches.
Suddenly we are startled by streaky shadows passing over us, and look up to see large, conical nautiloid cephalopods gliding above us, aligned like airplanes in formation. They behave much like schooling squid, but each has an outer shell, with a pattern of color banding. One is holding a large, wriggling trilobite in its tentacles. These cephalopods are the largest creatures we have seen, and some of these reach a meter or more in length, and their large eyes and numerous tentacles are more than a little menacing. We hunker down next to the ridge and point the camera upward as they shoot by overhead. If this picture does not make the cover of Science, it will at least be the hit of the next meeting of the Paleontological Society! In a flash they are gone, and our breathing rate settles down.
In a hollow of the ridge we find a large Isotelus trilobite, its pygidium (tail shield) folded neatly under its cephalon (head shield)—it must have seen us coming and rolled up for protection. It is a giant, about 25 centimeters between the sharp tips of the genal spines. The temptation to grasp a living trilobite is too great, and as we reach out for this prize, the animal suddenly unfolds with a snap and glides away, ventral side up, out of reach. It then flips back over and clings tightly to the bottom, pressing the dorsal carapace down into the sediment. Had this one not gotten away, it might have surpassed the world record specimen of Isotelus from the Ordovician of northern Canada! Nevertheless, we have managed to gather up several smaller trilobites, and maybe their DNA finally will resolve the question of how trilobites are related to other arthropods.
We venture out beyond the bryozoan ridge onto a seemingly level plain with patches of brachiopod pavement and bryozoans. The brachiopods are noticeably smaller forms like Dalmanella and Sowerbyella. Bryozoans are delicate, twig-like colonies with fewer sheet-like or massive forms. Flexicalymene trilobites are here, but are smaller than those we found in shallower water; some new trilobites of genera like Cryptolithus and Triarthrus cruise on the muddy patches, leaving grooved trails. Extending upward from some bryozoan thickets are very slender yet long-stemmed crinoids of the taxa Ectenocrinus and Cincinnaticrinus. Although they have fewer arms than the other crinoids we have seen today, they splay them into conical filtration fans in alignment with the gentle current (not open into the flow but with the concave side bearing the food grooves downcurrent). Some sediment patches are sands entirely composed of fragments of disarticulated crinoids, with odd bundles of long, still-articulated stems. The sands have broad, sinuous ripple marks like those along a beach. How could such ripples form at our present depth of 30 meters (100 feet)? We sense no wave motion and only slight current; however, as experienced divers, we know that severe storm conditions at the surface, including hurricanes, can produce occasional, strong wave oscillation or currents along the deeper sea floor where normal, fair weather wave motions do not penetrate. We have an uneasy feeling that we have ventured into an environment that can turn violent very quickly, as we see around us the remnants of shattered bryozoans and current-winnowed shells. Areas of pure mud have now become wider than areas of shelly sediment, and we recognize the characteristics of what will come to be known as the Kope Formation.
A glance at the dive computer shows we have almost exceeded our allowable bottom time at the maximum depth of 30 meters, so it is time to return to the shallows. We turn back on a reverse compass course, hoping we will surface close to the boat. As we again approach the bryozoan ridge, we notice an overhang we had overlooked. Some kind of movement is apparent in the shadows beneath this ledge. Whip-like, spiny appendages are waving at us; this is something new—we must investigate! Using a long, slender pole with a hook at one end, we reach back under the ledge. After a few unsuccessful tries, we have snagged a large and bizarre creature. We recognize the flailing appendages of a eurypterid of genus Megalograptus. It is the size of a Maine lobster but has very strange and spiny, non-lobster-like appendages. The elongate, segmented tail section is flexible like a lobster tail. We do not have time to observe the eurypterids now, so we pop a few into a mesh bag for later study. The mystery of how they use their appendages will remain until then. Perhaps we will see whether one tastes like lobster!
After more finning toward the shallows, our dive computer beeps, and we must ascend. We break the surface and switch to snorkel because we are low on air. We spot the boat in the distance, perhaps 200 meters away. It is a long swim on the surface. We are laden with specimen bags and gear, and, by the time we reach the boat, we are exhausted from gulping the oxygen-poor air. We haul ourselves over the gunwale, peel off our wetsuits, and just lie in the boat, catching our breath, our minds racing with the sights we have seen. What a dive!
Suddenly, we sense the warmth of the afternoon sun, amid the coolness of a crisp autumn day. We are staring at the fossil-covered surface of a bed of Ordovician limestone littered with rusty autumn leaves. We are sitting on the banks of Stonelick Creek, in Clermont County, Ohio, having lunch on a field trip with our students—and maybe you!