Figure 8.1. Comparison of a brachiopod with a pelecypod. A and B, pelecypod. C and D, Platystrophia ponderosa showing sulcus and fold. Drawings from Meek (1873), courtesy of the Ohio Department of Natural Resources Division of Geological Survey.
Brachiopods are among the most common fossils in the Ordovician rocks of the Cincinnati area. Only fossils of bryozoans are more numerous to the naked eye. In a study of type-Cincinnatian limestones, Martin (1975) reported that brachiopods and bryozoans together constitute about 60 percent of the fossil fragments comprising the limestones. There even are some layers, for example, in the Bellevue Limestone, in which the rock is a veritable coquina, in this case consisting of complete and nearly complete shells of large, flat brachiopods of a single genus. These aptly named “shingled Rafinesquina beds” commonly are thought of as remains of very shallow water deposits reminiscent of the shingled beaches of today. Although they have been living on Earth since the Cambrian Period, brachiopods are not well-known animals to most of us. In fact, many folks confuse them with that group of molluscs that includes the clams. Members of the phylum Brachiopoda and those of the molluscan class Pelecypoda are bivalved animals, that is, each has a shell that consists of two valves. But there the resemblance ends. The brachiopods and pelecypods are otherwise strikingly different animals.
First, the orientations of pelecypods and brachiopods are different (Figure 8.1). The two valves of a clam are anatomically left and right in position, with the hinge connecting the valves located at the top of the animal (technically called dorsal). However, the two valves of a brachiopod are dorsal and ventral, respectively, and the hinge is at the rear of the animal (posterior). Thus, although both pelecypods and brachiopods are bilaterally symmetrical animals, the planes of symmetry of the two are at a right angle to one another (Figure 8.1). We can conclude from this that, although animals of both groups each have two valves, “bivalvedness” in each group evolved independently; the two groups are not at all closely related, and neither evolved from the other.
From a practical point of view, however, it happens that the difference in orientation generally provides a convenient way to tell fossil brachiopods from fossil pelecypods. Each valve of most brachiopods can be divided into two halves that are mirror images of one another (Figure 8.1C). On the other hand, it is the two valves of most fossil pelecypods that are the mirror images of one another (Figure 8.1B). In other words, the plane of symmetry of an ordinary pelecypod is between the valves; in an ordinary brachiopod, it is down the middle of each valve.
Moreover, in general, the two valves of a brachiopod shell are NOT mirror images of one another (Figure 8.1D). For example, one valve may be decidedly deeper than its opposite. In addition (or instead), one valve may have a portion along the midline that is distinctly convex toward the outside of the animal (this structure is called a fold), and the other valve may have a distinct concavity in the same position (called a sulcus).
Figure 8.2. A. Cross-section of an articulate brachiopod. B. Interior views of pedicle valve (left) and brachial valve (right) of the Cincinnatian orthid brachiopod Hebertella. Drawings by Kevina Vulinec.
As the overall symmetry of pelecypods and brachiopods differs, so too does the operation of the shells. In a pelecypod, the two valves are joined at the hinge by an elastic pad or ligament. When the shell is held closed, the ligament is stretched, so that when the animal relaxes the two valves gape apart from one another. To close the shell, the pelecypod animal must contract one or two adductor muscles (the number depends on the type of clam).
In brachiopods, however, there is no ligament. The animal must contract what are called diductor muscles to open the shell (Figure 8.2A). It uses adductor muscles to close the shell; these adductors are stretched when the shell is open.
Thus, when a pelecypod relaxes, its shell opens. By way of contrast, when a brachiopod relaxes, its shell tends to close. This has important implications for how one finds pelecypods and brachiopods as fossils. Upon death (the ultimate relaxation), the individual valves of a pelecypod tend to get separated from one another, because they gape apart, allowing currents, for example, to tear them asunder. On the other hand, the two valves of a brachiopod shell more commonly remain together, and are found that way by the intrepid fossil collector.
Brachiopod shells tend to be better preserved than are pelecypod shells for another reason, too. Most brachiopod shells consist of calcium carbonate, but so do pelecypod shells. However, calcium carbonate exists in more than one form. Most brachiopod shells are of the mineral calcite, whereas pelecypods consist of or include aragonite. The atoms of calcium, carbon, and oxygen are arranged differently in aragonite and calcite, and the two substances have different properties. Because of this, pelecypods tend not to be preserved as well as brachiopods. The practical result is that you may find many well-preserved brachiopods in the rocks of the Cincinnati area, but pelecypods, with few exceptions, are preserved as internal molds.
Brachiopods are filter feeders. They extract small particles of organic matter from the sea water. These particles are captured by a ciliated structure called the lophophore (Plate 3F; Figure 8.2A). The lophophore occupies much of the space between the valves, forming a pair of tubular “arms” that extend on each side of the plane of symmetry. A gutter-like food groove runs along the arms from which ciliated tentacles extend to form a filter (Plate 3F; Figure 8.2A). The beating of the cilia causes water to flow into the shell, along or through the tentacles, and then out of the shell again. The food particles stick to the cilia and are transported by them to the food groove and mouth, which is located on the centerline of the animal. As with all animals, food is metabolized, and waste is expelled.
In at least some brachiopods, the lophophores are supported by projections from the interior surface of one valve. This so-called brachial valve is the one that is anatomically dorsal in position. In some instances, each branch of the lophophore is complexly coiled; in such cases, the lophophore support may be coiled, too. It is the two “arms” of the lophophore that give the brachiopods their name; the ancient Greek word “brachion” means “arm.” The “pod” part of the name comes from “podos,” one of the parts of speech of the ancient Greek word “pous,” which means “foot”; it recalls a time when brachiopods were thought to be close relatives of the molluscs, which include the gastropods, pelecypods, and cephalopods, among others.
Inarticulates vs. Articulates
Before proceeding further, it must be admitted that the picture of brachiopods painted above is a bit over-simplified. The brachiopods do not comprise a single, homogeneous lineage of animals. The brachiopods portrayed above mostly fall into a group called articulate brachiopods. They are called articulates, because the two valves of the shell are articulated—they are connected together by way of a well-developed hinge (Figure 8.2B). Along the hinge of one valve are projections, called teeth, that fit into sockets in the hinge area of the opposing valve. Because of the interlocking teeth and sockets, the so-called dentition, it would be difficult for a would-be devourer of brachiopod flesh to twist the valves apart to get at supper.
Animals of the other major group of brachiopods, not surprisingly, are termed inarticulates. In these animals, there are neither teeth nor sockets. Not having a real hinge, the task of keeping the valves together is a greater challenge for an inarticulate. The musculature is a good deal more complicated in inarticulates than in articulates—to keep the two valves from being twisted apart from one another.
In an articulate, there is a hinge, which serves as a fulcrum. The diductor muscles and the adductor muscles, in order to open and close the shell, operate against one another about the fulcrum (Figure 8.2A). But an inarticulate has no such fulcrum. The animal opens its shell, not by contracting diductors, but by pulling the body back toward the rear of the shell, thereby causing the valves to gape sufficiently for the animal to feed, respire, and perform other necessary activities.
Another common difference between articulates and inarticulates involves the composition of the shell. In most inarticulates the shell consists, not of calcium carbonate, but, rather, of calcium phosphate. (Note, however, that this is not a universal rule, for the shells of some of the animals traditionally called inarticulates are calcium carbonate, like those of the articulates.)
Studies of present-day forms have been taken to suggest that the articulate and inarticulate brachiopods may, in fact, not be particularly closely related. An inarticulate brachiopod has both mouth and anus, whereas an articulate has no anus. The early life histories of the animals of the two groups are different, too; for example, the pedicle of an inarticulate has a different origin than does the pedicle of an articulate. Detailed studies of the genetics of present-day animals are beginning to throw light on the issue of the relationships of the various groups of brachiopods (Cohen and Gawthrop 1997). However, pending the amassing of further information, we will follow the usual tradition of considering the Articulata and the Inarticulata to be two subphyla of the phylum Brachiopoda. One alternative scheme would be to recognize those two groups as separate phyla. On the third hand, some experts on brachiopods prefer to do away with the formal taxa Articulata and Inarticulata altogether and recognize instead three subphyla (Williams et al. 2000); followers of that scheme retain the concepts of articulated brachiopods and inarticulated brachiopods, but only as descriptive terms.
Brachiopod Life Habits
Regardless of such taxonomic issues, the brachiopods as a whole are benthic creatures. Some of them simply lie on the sea floor, whereas others each are physically attached to the sea floor by means of a fleshy stalk called a pedicle. In 1972 Peter Richards published a study that explored the relationship between shell form and life habits of many characteristic Cincinnatian brachiopods. His research demonstrated that Cincinnatian brachiopods have a variety of life habits and that careful attention to morphological features of the shell as well as preservational evidence helps to explain the diversity seen in this important group. His work has guided and inspired much subsequent research and the discussion that follows.
Some present-day inarticulates (genus Lingula and its relatives) spend much of their time in a burrow. When it comes time to feed, the pedicle extends so that at least the anterior part of the shell projects up into the water. In some brachiopods, there is an opening in the shell through which the pedicle extends. In general, this pedicle foramen is in the anatomically lower valve, which, hence, is called the pedicle valve. The pedicle of some brachiopods may be attached to another shell on the sea floor; this may result in pedicle attachment scars which consist of tiny, characteristic pits in the other shell. In an inarticulate brachiopod, as in you, there is a mouth, esophagus, stomach, intestine, and anus. In an articulate brachiopod, however, there is no anus (Figure 8.2A). What might have been a complete digestive tract is, in fact, a cul de sac. Thus, the only egress for waste material is back out the mouth. In present-day animals that have been studied, solid waste is regurgitated as small pellets; these are then expelled from the shell by rapid snapping of the valves.
Although the articulates are more readily noticed, inarticulates are not rare. Most of them, however, grew on the shells of other animals and, hence, are relatively small and easily overlooked (Figure 8.3). Presumably, the other shells provided a solid substrate to which to attach; it could well be that the little “hangers-on” (technically called epizoa) made their living by consuming the waste matter expelled by their “hosts.” In any case, the shells of these inarticulates commonly look a bit like scabs, blisters, or little volcanoes on the shells of articulate brachiopods. Some of them grew so tightly affixed to their larger cousins that the ribbing of the shells of the latter deform or show through the shells of the inarticulates.
Another kind of inarticulate may be found in the rocks of the Cincinnati region by the sharp-eyed collector. Given the moniker “inarticulate,” it is ironic that this other group comprises animals that have tongue-shaped shells. Luckily for paleontologists, these so-called lingulides are represented in present-day oceans, so that we readily can see them. An individual member of genus Lingula (Latin for “tongue”), from which the group gets its name, has a way of life reminiscent of that of some burrowing worms. The animal anchors the distal end of its long pedicle to the sea floor and uses its shell to dig vertically down into the sediment front end foremost. In due course, the burrower veers to the horizontal and then back up to the sea floor. Most of the U-shaped burrow collapses from behind, so that only the one vertical tube remains, with the opening of the shell at or near the sea floor and the pedicle pointing down into the sediment. When necessary (or desired?), the gape between the valves can be raised above the sea floor by extension of the pedicle, and the animal can feed, or whatever. In times of danger, the animal can retreat into its burrow by contracting the pedicle.
Lingulides are not particularly common in the type-Cincinnatian. Very occasionally a specimen is found with its shell oriented perpendicular to the stratification, in what appears to be its life position, that is, the animal’s orientation in space during its life. The individual in Figure 8.3A is a case in point.
Figure 8.3. A. Pseudolingula sp., CMC IP 51994, Cincinnatian, horizon and locality unknown, × 1.9. Specimen oriented perpendicular to bedding, presumably in life position with beak downward. B. Trematis millepunctata Hall, CMC PT 585, brachial valve interior, Cincinnatian, horizon and locality unknown, × 3. C. Petrocrania scabiosa (Hall) encrusting on Hebertella sp., MUGM 29461, Arnheim, Oxford, Ohio, × 1.3. D. Petrocrania scabiosa (Hall) encrusting on brachial valve of Rafinesquina sp., Bruce and Charlotte Gibson Collection, no. 1042, Richmondian, Franklin Co., Indiana, approx. × 1.5. E. Philhedra laelia (Hall), MUGM 26219, two specimens encrusted on Rafinesquina brachial valve exterior, Liberty Formation, Preble Co., Ohio, × 3. F. Schizocrania filosa Hall, CMC IP 36, encrusted on Rafinesquina pedicle valve exterior, Corryville Formation, Warren Co., Ohio, × 1.8.
It is only by great good fortune that that particular individual made it into a museum. Ralph Dury was a Cincinnati entomologist of some considerable repute. However, his actual livelihood was in real estate. According to Charles Dury, Charles’s son and director of the Cincinnati Museum of Natural History for more than five decades, the specimen in Figure 8.3A started its museum career in a real estate project. Charles Dury was having a stone wall built. Being a meticulous fellow, he visited his sites on a regular and frequent basis. He happened to make one of these visits to the building site shortly after a load of stone had been delivered. In the course of examining the stone to be certain that it was up to his standards, he saw the chunk with the lingulide in its life position. So, instead of ending up as part of a wall, the piece of rock ended up as part of the collections of the Cincinnati Museum of Natural History, along with Charles Dury’s insect collection—but that is another story (Vulinec and Davis 1984).
The vast majority of the brachiopods that one sees in the Ordovician rocks of the Cincinnati area are articulates. There is a tremendous variety of them (Figures 8.4–8.9). Some have an external shape that makes them easy to identify to genus or, even, species. On the other hand, there are some that are beastly difficult to tell apart. The problem is that brachiopods that are only distantly related may look alike externally (Figure 8.6). It is only when one carefully studies the various features on the inner surface of each valve that their true relationships may become apparent. Useful features include the scars where muscles attached to the valves, the structures that supported the lophophores, and so on. One even may need to examine the internal structure of the shell material that makes up the valves. It seems that the environment in which the brachiopods lived led different lineages to evolve similar external shapes. The phenomenon is called convergent evolution, and the result is homeomorphy—the existence of two or more kinds of animals that are not closely related but that nonetheless look alike. This is a common enough occurrence that it is an old saying: “Homeomorphy is rife amongst the brachiopods.” This, of course, presents a problem to the collector of fossil brachiopods. It means that he or she must make a real effort to obtain loose valves that have been naturally cleaned over time so as to reveal their internal features. Failing that, the collector must break, cut, or grind specimens open and clean them meticulously to reveal the inner truth.
Some of the articulate brachiopods must have spent much of their lives anchored by their pedicles to shells, hardgrounds, or other solid objects on the sea floor (Figure 8.9E). They were not frozen in position, though, because they apparently had adjustor muscles that allowed the individual animal to move its shell with respect to the pedicle. We know this because present-day articulates have such adjustor muscles, and where these adjustors attach to the insides of the valves, there are muscle scars that occur in particular positions. Many fossil brachiopods have comparable muscle scars.
Figure 8.4. A, B. Plectorthis neglecta (James), MUGM uncatalogued. Fairview Formation, Hamilton Co., Ohio. A. Brachial valve exterior. B. Pedicle valve exterior, both × 1.8. C, D. Dalmanella emacerata (Hall), MUGM 24362, Kope Formation, Hamilton Co., Ohio. C. Pedicle valve exterior. D. Brachial valve exterior, both × 2.4. E, F. Sowerbyella rugosa (Meek), MUGM 24564, Kope Formation, Cincinnati, Ohio. E. Pedicle valve exterior. F. Brachial valve exterior, both × 2.2. G. Slab covered with Sowerbyella rugosa, Kope Formation. Courtesy of Paul E. Potter.
Figure 8.5. A–D. Hebertella occidentalis (Hall), MUGM 11205, Waynesville Formation, Franklin Co., Indiana. A. Brachial valve exterior. B. Pedicle valve exterior. C. Pedicle valve interior, showing muscle scars and triangular pedicle opening. D. Posterior view of articulated valves and triangular pedicle opening, all × 1.2. E–H. Glyptorthis insculpta (Hall); E–F, MUGM 22450, Liberty Formation, Preble Co., Ohio. E. Brachial valve exterior. F. Pedicle valve exterior, × 2.8. G–H, MUGM 29458, Arnheim Formation, Butler Co., Ohio. G. Pedicle valve interior, showing muscle scars, triangular pedicle opening, and encrusting cyclostome bryozoans, × 2.2. H. Brachial valve interior, showing muscle scars and cardinal processes, × 1.8.
Figure 8.6. A–C. Plaesiomys subquadrata (Hall), MUGM 22933 Liberty Formation, Preble Co., Ohio. A. Brachial valve exterior, inarticulate brachiopod Philhedra laelia on beak, × 2.1. B. Pedicle valve exterior, × 2.1. C. Pedicle valve interior, showing muscle scars, × 2.4. D–F. Retrorsirostra carleyi (Hall), MUGM 23037, Arnheim Formation, Butler Co., Ohio. D.
Brachial valve exterior. E. Pedicle valve exterior, note triangular pedicle opening and flanking inter-area. F. Pedicle valve interior, showing muscle scars, × 2.
Figure 8.7. Three of the many described species of Platystrophia. A–D. Platystrophia ponderosa Foerste, MUGM 24060, Maysvillian, Campbell Co., Kentucky. A. Pedicle valve exterior, × 1.5. B. Brachial valve exterior, × 1.5. C. Pedicle valve interior, showing pedicle opening and deep muscle scar, × 1.0. D. Brachial valve interior, showing muscle scars, × 1.3. Early collectors of Cincinnatian fossils referred to this large, robust brachiopod as the “double-headed Dutchman.” E, F. Platystrophia laticosta (Meek), MUGM 11315, Maysvillian, Cincinnati, Ohio. E. Brachial valve exterior. F. Pedicle valve exterior, both × 1.5. G, H. Platystrophia acutilirata (Conrad), MUGM 23360. G. Pedicle valve exterior., H. Brachial valve exterior, Whitewater Formation, Preble Co., Ohio, both × 1.4.
Figure 8.8. The many faces of Rafinesquina: Rafinesquina alternata (Conrad). A. University of Cincinnati collections, left, pedicle valve up, with encrusting edriasteroids and cyclostome bryozoans, right, brachial valve up, with encrusting edrioasteroid Streptaster vorticellatus and crinoid Iocrinus subcrassus wedged beneath lower left edge, Corryville Formation, Boone Co., Kentucky, scale in mm. B. Pedicle valve interior, CMC IP 51111, showing muscle scars, scale in mm. C. Brachial valve interior, University of Cincinnati collections, showing muscle scars and pair of cardinal processes along hinge at top, Corryville Formation, Boone Co., Kentucky, size about same as in B. D. Rafinesquina pavement, with pedicle valves up, Fairview Formation, Kenton Co., Kentucky. E. Shingled Rafinesquina bed, with valves perpendicular to bedding, Fairview Formation, Kenton Co., Kentucky.
Figure 8.9. A, B. Hiscobeccus capax (Conrad), MUGM 25490, Liberty Formation, Preble Co., Ohio. A. Brachial valve exterior. B. Posterior view of articulated valves showing small pedicle opening, × 2.2. C, D. Rhynchotrema dentatum (Hall), MUGM 25933, Whitewater Formation, Preble Co., Ohio. C. Pedicle valve exterior. D. Anterior view of articulated valves showing pronounced sulcus, × 3.4. E. Zygospira modesta (Say), CMC IP 51112, Corryville Formation, Boone Co., Kentucky, attached in life position to bryozoan Parvohallopora sp., × 1.9. F. Catazyga schuchertana (Ulrich), MUGM 7614, Waynesville Formation, Jefferson Co., Indiana, brachial valve exterior, × 3.4.
Figure 8.10. Environmental distribution of brachiopods in the Cincinnatian Series. Shoreface environments are equivalent to the shallow subtidal (1–2 m or 3–6 ft); transition zone environments are deeper subtidal (3–6 m or 10–20 ft), and offshore environments are deeper water, with a maximum depth of about 30 m (100 ft). The heavy lines indicate the environments where each genus is most abundant and thin lines indicate environments where a genus is present at lower abundance. From Holland (1997), in Paleontological Events. Copyright 1997 Columbia University Press. Reprinted with permission of the publisher.
On the other hand, some kinds of fossil articulates have no pedicle foramen, the opening through which the pedicle extends beyond the hinge area of the shell. There has been a great deal of discussion as to how these creatures survived in areas where the sea floor was soft mud.
Brachiopods of genus Rafinesquina (Figure 8.8) are, perhaps, the most common of the larger articulates in the rocks of the Cincinnati area. The overall shape of the shell is described as “concavo-convex”; the brachial valve (anatomically dorsal) is concave to the exterior, and the other valve (anatomically ventral) is convex to the outside. The whole shell, then, is saucer- or bowl-shaped. The adult animal does not seem to have had a pedicle—no pedicle foramen. Hence, it must have been free on the sea floor. But imagine a saucer-shaped shell in a current; all too easily, it would have been flipped so that the convex side was uppermost. The result would have been that the commissure, the opening between the valves, would have been against the sea floor. If that sea floor were soft mud, the animal would have had considerable difficulty generating sufficient currents with the cilia of its lophophore so as to bring life-giving nutrients and oxygen between the valves. Thus, there seems to be a conflict between hydrodynamics and biology.
The bleak picture of the brachiopod, upside down with its opening in the mud, may be misleading. It portrays the animal as an immobile lump unable to right itself. True, there was no pedicle on which the animal, using adjustor muscles, could twist and turn itself back into a viable position. But, what if the animal were less like an inanimate saucer and more like a living scallop of today? A scallop is a kind of bivalved mollusc. The “scallop” you enjoy in your favorite seafood restaurant is an adductor muscle of one of those pelecypods. The adductor of a scallop is powerful enough, in life, to snap the animal’s valves together so swiftly that the creature can be lifted above the sea floor. Some scallops can even swim for some distance, although rather jerkily and in decidedly irregular trajectories.
Dattilo (2004) has found evidence that one Cincinnatian brachiopod with a concavo-convex shell, Sowerbyella, was capable of escaping from burial beneath sediments stirred up by storms, presumably by snapping its valves. Individuals of Rafinesquina may have had similar capabilities, because convex-up specimens are found with a moat-like furrow around the commissure that formed while the brachiopod was alive (Meyer 2006). These recent findings suggest that these brachiopods without pedicle attachment might have led much more active lives than previously realized.
Distribution of Type-Cincinnatian Brachiopods in Time and Space
Despite the high diversity of type-Cincinnatian brachiopods, the distribution of species is not uniform throughout the stratigraphic succession. There are distinct associations of species and shell types that characterize different stratigraphic intervals and even individual beds. Recent research by Steven Holland, in collaboration with Arnold Miller, David Meyer, and Benjamin Dattilo (Holland et al. 2001) showed that the relative abundance of brachiopods and other fossils changes within the Kope Formation, a unit generally regarded as having a uniform shaley lithology. In the lower Kope, fossil assemblages as found on characteristic limestone bedding surfaces are dominated by the small, thin-shelled Sowerbyella, along with branching bryozoans, small, slender crinoids like Cincinnaticrinus and Ectenocrinus, and the trilobite Cryptolithus. Higher in the Kope, another thin-shelled but larger brachiopod, Dalmanella, becomes more abundant. In the highest sections of the Kope, the large thin-shelled brachiopods Rafinesquina and Strophomena and the large, rather thick-shelled Platystrophia become the dominant brachiopods. In conjunction with changes in the lithology, bedding, and other characteristic fossils, Holland, Miller, Meyer, and Dattilo (2001) interpreted the changes in the Kope Formation as a paleobathymetric gradient reflecting transition from deeper to shallower water.
Throughout all type-Cincinnatian depositional sequences the nature of the brachiopod assemblages provides one of the most reliable and abundant indicators of paleoenvironmental conditions such as depth and level of water movement energy (see chapter 15). The relationship between brachiopod morphology and depth reflects both the type of substratum and the level of water movement energy. In the deeper water, muddy environments, brachiopods with small, thin, flat shells acted like snowshoes on the soft sediment. In shallower water, larger, concavo-convex brachiopods like Rafinesquina and Hebertella were better adapted to stronger wave energy. Other larger brachiopods like Platystrophia, with thicker shells and well-developed plications (ribs radiating from the beak), characterize some of the shallowest, highest wave-disturbed environments. Figure 8.10 shows the environmental distribution of other brachiopods within the type-Cincinnatian.
Even on a smaller scale, brachiopods reveal some very basic aspects of life on the Late Ordovician sea floor. Very densely populated limestone beds, featuring brachiopods like Rafinesquina, Strophomena, and Dalmanella, are very common throughout the type-Cincinnatian and are called shell pavements (also called shingled beds as noted above). In such shell pavements, the brachiopods are usually preserved with convex valves upward, sometimes covering the entire bed surface (Figures 8.4G, 8.8D, 8.8E). Shell pavements can be as thin as a single layer of shells, or thicker, with the entire thickness up to a few tens of centimeters consisting of stacked brachiopods. In some cases, the valves are vertical or tilted at various angles and packed closely together in an edgewise shell bed. The edgewise shell pavements are good evidence of water movement in the form of wave oscillations because there are present-day examples of edgewise shell beds and shale fragments formed in very shallow water by wave oscillations. If the hingelines or beaks of the brachiopods were always directed downward in an edgewise bed, it might be possible that the densely packed shells actually had lived in a manner similar to an oyster bed. However, analysis of valve orientation within edgewise shell beds shows that valves do not show such a pattern and even can be predominantly hingeline-upward (Seilacher 1973, pers. comm.).
There has been considerable debate as to how shell pavements could have formed. Many workers felt that the thin, concavo-convex shells of the characteristic brachiopods like Rafinesquina and Strophomena initially lived on a soft, muddy sea floor, with the convex valve downward. During a storm, the fine-grained muds could have been swept away, leaving a concentration of shells (called a lag deposit) to form the shell pavement. Possibly the shells were even carried some distance by storm currents to be deposited later. In cases where pavements form edgewise beds, storms could very well have been involved, but not all pavements are edgewise.
It is also possible that shell pavements accumulated by abundant production of shells of a single species over some time span in one place. In one shell pavement from the Corryville Member of the Grant Lake Formation in northern Kentucky, mostly convex-upward shells of Rafinesquina form a kind of imbricated or shingled bed, but the spaces between the shells are mud-filled, forming a type of limestone known as packstone (Meyer 1990). If the mud had been removed by a storm, the remaining shell bed could have formed a grainstone. Shells in the upper surface of the bed are a mixture of articulated shells with good preservation of fine surface features and disarticulated shells that are abraded and broken. Both abraded and unabraded shells are encrusted with bryozoans and edrioasteroid echinoderms. Some articulated brachiopods have the moat-like feature mentioned above that suggests activity of the living brachiopod. All these features are evidence that the shell bed accumulated gradually without significant transportation. Ultimately the entire bed was smothered by an influx of mud, probably produced by a storm.
Characteristically, a stratigraphic section with the type of shell pavement described above will contain repetitions of thin shell-pavements smothered by shales. Harris and Martin (1979) described this pattern in the Waynesville Formation as a form of paleoecologic succession (see Figure 4.8). In present-day settings, ecologic succession occurs when one assemblage of animals or plants alters the habitat in such a way that other species can replace the so-called pioneer species. Harris and Martin (1979) suggested that thin-shelled brachiopods were pioneer species that first colonized soft muddy patches of the sea floor and provided a pavement on which encrusting animals like bryozoans and inarticulate brachiopods could settle. Eventually other species could take advantage of the shell pavement and thickets of bryozoans, so that the diversity of the assemblage increased upward from the bottom of a pavement bed. Storms frequently smothered the shelly patches with mud, thus interrupting the succession until brachiopod larvae once again could colonize the barren muds. Some paleoecologists have questioned whether paleoecologic succession comparable to present-day succession can be detected in the fossil record because most stratigraphic changes in fossil assemblages represent a much longer time scale than the scale of years to decades over which present-day succession occurs. Although we still do not know how much time was required for the formation of characteristic, thin type-Cincinnatian shell pavements, it is possible that they formed over a short time scale. It also seems correct to view the brachiopods as having a pivotal role in providing a hard substratum onto which encrusting animals could settle, thus altering the habitat in the manner of successional pioneers. Clearly, succession at the scale of individual shell pavements was an important act on the stage of the Cincinnatian sea floor, and brachiopods played a major role in that evolutionary play.
