Figure 6.1. A simple sponge, showing a cross-section of the body wall. Inset shows a magnified view of incurrent canals (ostia), collar cells and collar-cell chambers. Drawing by John Agnew.
Although sponges are regarded as the least specialized, hence most primitive of multicelled animals, they play an essential role as “sanitary engineers” in aquatic environments, living as active suspension feeders or filter feeders (Plate 3A). By removing minute organic particles from the water, sponges prevent decay products from poisoning the environment. This is a long-running role, as sponges first appear in the fossil record during the late Precambrian, over 540 million years ago.
Sponges
The body of a sponge lacks distinct cell layers, but is composed of different specialized types of cells that perform different life functions. The fundamental sponge cell is the collar cell, equipped with a waving flagellum that draws water into a cone formed of microvilli (Figure 6.1). The simplest sponge is a hollow tube, open at one end. Collar cells line the interior of the tube and create a feeding current that passes through the body wall via openings called ostia and tubular cells called porocytes. The collar cells remove food particles that are digested by amebocytes. The feeding current carries wastewater, depleted of nutrients, out of the sponge cavity through one or more chimney-like openings called oscula. Because sponges are fixed to the substratum and do not move about, they are often regarded as inert or nonliving. In fact they are actively circulating water and processing it for nutrients (Plate 3A).
The body of a sponge is mostly composed of a fibrous protein called spongin, which is also secreted by specialized cells. (This is what makes up a natural bath sponge.) After death, spongin readily decays, so that many sponges have little chance of becoming fossilized. Most sponges also secrete minute, mineralized spicules that are embedded within the spongin network. Spicules can be as simple in form as a single needle, but can also be very complex, burr-like, and even fused to form a basket-like lattice; their composition is either calcium carbonate or silicon dioxide. Once a sponge decays, the spicules are released into the sediment where they can be preserved as microfossils. Sponges that have dense or fused networks of spicules are more likely to be preserved intact, and it is these types that make up most of the fossil record of sponges.
In the late 1960s biologists diving on the coral reefs of Jamaica discovered a new group of living sponges that completely defied the concept of a typical sponge (Plate 3B). Moreover, these new sponges provided an important link to some fossils that had long been misclassified. Among these fossils are some found in the Cincinnatian. The new sponges are called sclerosponges or coralline sponges because they form a massive calcareous skeleton like that of coral. The sponge body is restricted to a thin surface layer in which typical sponge cells carry out filter feeding (Figure 6.2). Wastewater canals that converge on the oscula leave starburst patterns of grooves in the calcareous skeleton that match with structures called astrorhizae in the fossil group known as stromatoporoids (Figures 6.2, 6.3A, D). These and other similarities enabled the stromatoporoids to be recognized correctly as a new group of sponges, after they had been classified with cnidarians, bryozoans, and even protozoans. Stromatoporoids first appear in strata of Ordovician age, and were thought to have become extinct in the Cretaceous, until the living sclerosponges were found. During the Silurian and Devonian periods, stromatoporoids were major reef builders along with corals, but the modern sclerosponges (more distantly related to the Paleozoic forms) are restricted to deep reef environments and play a smaller role in reef building.
Figure 6.2. Reconstruction of a living stromatoporoid, modeled on a living sclerosponge. A section is removed to show internal laminae of the skeleton. Living tissue occupies only the uppermost layer, with excurrent canals radiating from oscula. Magnified inset shows surface tissue and microstructure of laminae. Compare to Figure 6.3D, below. Drawing by John Agnew.
Figure 6.3. Cincinnatian sponges and stromatoporoids. A. A cylindrical stromatoporoid, Aulacera undulata (Billings), MUGM 29618, Richmondian, horizon and locality unknown, × 0.3. B. A large sponge, Brachiospongia tuberculata James, holotype, CMC IP 209, Richmondian, Liberty Formation, Clinton County, Ohio, × 0.3. C. A sponge, Pattersonia tuberosa (Beecher), MUGM, Maysvillian, Fleming County, Kentucky, × 0.3. D. Polished cross-section of a stromatoporoid, Labechia huronensis (Billings), MUGM 634A2t, Richmondian, Montgomery County, Ohio, × 0.3.
Sponges are not common fossils in the Cincinnatian, although they often may be overlooked because they can resemble bryozoans or corals, and also have a rather nondescript appearance (Figure 6.3C). Five genera of sponges and three genera of stromatoporoids are recognized in the Cincinnatian (Dalvé 1948; Rigby 1996). Brachiospongia is the largest and most striking sponge (Figure 6.3B). This sponge has a hollow central cavity from which radiate 6–12 straight or curved, finger-like projections; its diameter can reach 28 cm (11 in). Other Cincinnatian sponges are described in detail and illustrated by Rigby (1996).
Stromatoporoids are not common in the Cincinnatian and may superficially resemble bryozoans in having a mound-like or encrusting form (Figure 6.3D). They differ from bryozoans in being densely covered with tubercles. Cross-sections of Cincinnatian stromatoporoids are composed of blister-like skeletal deposits called vesicles, whereas bryozoans have a tubular structure (Figure 6.2). Aulacera is a very distinctive, large Cincinnatian stromatoporoid with a cylindrical shape (Figure 6.3A). In life, Aulacera grew upright on the sea floor like a tree or a stalagmite, but they are preserved lying horizontally like fallen logs. Aulacera found in Ordovician strata slightly younger than the youngest Cincinnatian beds on Anticosti Island in the Gulf of St. Lawrence reached gigantic sizes, up to 28 cm in diameter and 1–2 m in length (Cameron and Copper 1994). In the Cincinnati region they are restricted to the Richmondian formations, the Elkhorn Formation in particular, and diameters up to 5–10 cm are known.
Cnidarians
The stony corals are the best known members of the phylum Cnidaria, which also includes jellyfish, sea anemones, and many groups like hydroids, sea fans, sea whips, and soft corals that are often mistakenly thought to be seaweeds. Despite such a bewildering array of forms, cnidarians share several features that indicate their common relationship as members of one of the simplest multi-celled animal (metazoan) phyla. Cnidarians stand apart from other animals in having two body forms, the polyp and the medusa. The polyp, as typified by a sea anemone or stony coral (Plate 3C; Figure 6.4), is cylindrical in shape and attached at the base to a hard substratum. The body wall consists of only two cell layers (unlike the three layers found in all other metazoans), separated by a non-cellular jelly layer called the mesoglea. There is a single opening (mouth) into the body cavity through which food is ingested and waste is expelled. A ring of tentacles surrounding the mouth serves for food capture and defense. The medusa or jellyfish has the same structure as the polyp (with thicker mesoglea) but is free-living, swimming by muscular pulsations with the mouth oriented downwards. Both polyp and medusa forms are present at different stages during the life cycle of some cnidarian species.
Figure 6.4. Reconstruction of the Cincinnatian solitary rugose coral, Grewingkia canadensis, showing the polyp with extended tentacles. Drawing by Kevina Vulinec.
The three major classes of cnidarians differ in their expression of the polyp and medusa stages. Hydrozoans (including hydroids, Portuguese Man-of-War, and fire corals) use both polyp and medusa. Scyphozoans (true jellyfish) restrict the polyp to a larval stage and live mostly as medusae. Anthozoans (anemones, corals, “soft corals”) live exclusively as polyps, and often as colonies of multiple polyps that are genetic clones of a single initial polyp. One other feature common to all cnidarians provides a clue to their mode of life. Microscopic stinging cells (cnidoblasts) are concentrated in the tentacles of all cnidarians. Upon contact with a foreign object the stinging cell releases a harpoon-like hollow thread that pierces soft tissue and injects a toxin. Small organisms are stunned or killed by the stinging cells, then grasped by the tentacles and stuffed into the mouth. Thus cnidarians live as predaceous carnivores, although polyps and weakly swimming jellyfish must rely on water motion to supply their prey. Because the prey of most cnidarians is mainly minute zooplankton, they are also considered to be passive suspension feeders.
In the Cincinnatian strata, two major groups of cnidarians are represented: the anthozoan corals and the scyphozoan conulariids. The corals include both solitary and colonial species.
Solitary Corals
Cincinnatian solitary corals, commonly called horn corals or cup corals, and one colonial coral belong to the order Rugosa. Rugosans first appeared in the Late Ordovician and became extinct by the end of the Permian. The common name “horn coral” refers to the conical or cylindrical shape of the calcitic skeleton (corallum); these corals are commonly mistaken for a fossilized cow’s horn (Figures 6.4, 6.5). The polyp occupied a depression (calice) at the wide end of the corallum. As the coral grew, the polyp deposited successive layers of skeletal material beneath it, thereby increasing the length of the corallum. Older parts of the corallum did not contain living tissue and thus were subject to physical abrasion or encrustation and boring by other organisms.
Figure 6.5. Cincinnatian rugose corals. A. Streptelasma divaricans (Nicholson), USNM 70211, several coralla attached to bryozoan, Richmondian, Waynesville Formation, Clinton County, Ohio, × 2.0 (from Elias [1982, plate 3, figure 5]). B. S. divaricans, USNM 135767, three coralla attached to margin of brachiopod Rafinesquina, Richmondian, Whitewater Formation, Wayne County, Indiana, × 1.3 (from Elias [1982, plate 3, figure 9]). C. S. divaricans, USNM 40086, two coralla attached to brachiopod Lepidocyclus, Richmondian, Whitewater Formation, Butler County, Ohio, × 2.2 (from Elias [1982, plate 3, figure 8]). D. Grewingkia canadensis (Billings), CMC IP 50667, Weaver Collection, well-preserved corallum, Richmondian, Adams County, Ohio, × 0.6, inset showing epithecal growth-lines, scale in mm. E. G. canadensis, CMC IP 45413, typical abraded corallum with broken rim, showing circular borings of Trypanites, Richmondian, Whitewater-Elkhorn Formations, Wayne County, Indiana, × 1.0 (from Elias [1982, plate 9, figure 10]). A–C reprinted by permission of the Paleontological Research Institution.
Figure 6.6. Cincinnatian colonial corals. A–E. Corallites as seen on external surface or in cross sections of corallum, at same scale × 3.7. A–E from Elias (1998). A. Cyathophylloides, a colonial rugosan, Richmondian. B. Foerstephyllum, a tabulate, Richmondian. C. Calapoecia, a tabulate, Richmondian. D. Nyctopora, a tabulate, Richmondian. E. Tetradium, a tabulate, Richmondian. F. Coral bed, Richmondian, Madison County, Kentucky, length of hammer 25 cm (from Elias [1998, figure 5]). G. Protaraea richmondensis Foerste, MUGM 5435, a tabulate encrusting brachiopod shell, Richmondian, Liberty Formation, Preble County, Ohio, × 1.7. H. Octagonal tool house built ca. 1900 in John Paul Park, Madison, Indiana, constructed entirely of colonial corals from the Richmondian coral beds exposed in the vicinity. A–F reprinted by permission of the Mid-America Paleontology Society.
Although the soft polyp of rugosans is never preserved, rugosans are known to be corals because the calice has multiple radiating partitions called septa that are found in living corals. Septa are secreted by soft tissue partitions of the internal body cavity called mesenteries. In living anthozoans, mesenteries serve important functions in digestion and reproduction. The number and arrangement of the septa are traits used in the classification of corals. In rugosans the septa have a roughly four-fold symmetry, compared to living corals that have six-fold symmetry.
Two species of solitary rugosans occur commonly in the Cincinnatian, both in the Richmondian strata (Figure 6.5; Elias 1982, 1998). Grewingkia canadensis (Billings) is the largest and most common rugosan (Figures 6.5D, E). Coralla reach lengths over 13 cm (5 in) but are generally in the range 10–60 mm (0.5–2 in); the diameter ranges from 22 to 40 mm (0.9–1.6 in). Specimens are almost always found lying on their sides and appear highly abraded, encrusted, and bored (Figure 6.5E). External concentric growth lines are rarely preserved (Figure 6.5D, inset). Like some living solitary corals, Grewingkia probably lived upright, partly buried in soft sediment with the polyp exposed. Some encrustation and boring took place during life but continued after the coral was exhumed by storm activity and deposited on its side. Bryozoans are the most common encrusters and a worm probably formed the borings (trace fossil name Trypanites, see Elias 1986). Field studies demonstrate that Grewingkia specimens on single bedding surfaces are oriented in preferred directions that probably resulted from alignment of corals during storms.
Streptelasma divaricans (Nicholson) is the other solitary rugose coral found in the Richmondian section (Figures 6.5A–C; Elias 1982, 1998). Unlike Grewingkia, Streptelasma is found in growth position, on the upper surfaces of limestones. The coralla are usually 6–13 mm (0.25–0.5 in) in length, rarely exceeding 25 mm (1 in), with a diameter of 13 mm (0.5 in). Coralla occur individually and in clusters, often attached to brachiopods, bryozoans, or even coralla of Grewingkia. In many cases the brachiopod or host was living at the time the corals attached. The outer layer of the corallum (epitheca) shows septal grooves and interseptal ridges in contrast to the smooth, worn epitheca of Grewingkia.
Colonial Corals
Most Cincinnatian colonial corals belong to another extinct coral group called the Tabulata. Tabulates are commonly called honey-comb corals because of their multiple, polygonal corallites (skeletal tubes secreted by individual polyps) (Figures 6.6, 6.7B). Tabulates originated in the Early Ordovician and became extinct by the end of the Permian. Coralla of tabulates vary in shape from sheet-like to hemispherical to spherical, reaching diameters of about 4 meters (13 ft). Individual tabulate polyps built tall, narrow corallites. Polyps periodically deposited a transverse basal plate (tabula) as they grew; thus, broken coralla or longitudinal polished sections have a characteristic ladder-like appearance (Figure 6.7B). In life, tabulate polyps were truly colonial because corallite walls are shared and have interconnecting pores. Septa are not well developed, leading some specialists to question whether tabulates were in fact corals. An extraordinary discovery of soft tissue polyps preserved in a Silurian tabulate (Copper 1985) settled the debate for most tabulates, although some, like the Cincinnatian Tetradium, are very similar to some living sponges that build a calcareous skeleton with a tabulate structure.
Figure 6.7. Cincinnatian colonial corals. A. Cyathophylloides stellata (Hall), MUGM 5285, a colonial rugosan, Richmondian, Liberty Formation, Nelson County, Kentucky, × 3. B. Foerstephyllum vacuum (Foerste), MUGM 5301, a tabulate in vertical section, showing tabulae, Richmondian, Liberty Formation, Nelson County, Kentucky, × 0.8.
Colonial corals occur exclusively in the Richmondian Waynesville, Liberty, Whitewater, Saluda, and Elkhorn Formations. Within these formations, there are as many as four distinct horizons where colonial corals are concentrated into “coral beds” up to about 4 m (12 ft) thick, traceable for great distances along the outcrop belt of the Richmondian around the Cincinnati Arch (Figure 6.6F; Browne 1964, 1965; Hatfield 1968; Elias 1998). Four genera of massive colonial tabulates (Foerstephyllum, Calapoecia, Nyctopora, and Tetradium) and one colonial rugosan (Cyathophylloides) are found in these beds (Figures 6.6A–E). Another tabulate, Protaraea, occurs in the Richmondian but did not form massive colonies. Instead, Protaraea exclusively encrusts the shells of brachiopods and other objects (Figure 6.6G). In John Paul Park, in Madison, Indiana, there is a unique, octagonal tool house built entirely of colonial corals gathered from the coral bed exposed north of the town (Figure 6.6H). Colonial corals are also incorporated into stone-walls beside some of the elegant houses in Madison.
Are Reefs Present in the Cincinnatian?
Although the coral beds of the Richmondian have some characteristics of reefs, they are not considered to be true reefs. Why is this so, and are there other reef-like concentrations of fossils elsewhere in the Cincinnatian? To answer the second question first, the only other reef-like structures reported in the Cincinnatian are small mounds, only 0.3 m high by 3 m across, which were composed of trepostome bryozoans (Cuffey 1998). These mounds occur in the Maysvillian Grant Lake Limestone near Maysville, Kentucky, but unfortunately the outcrop has been destroyed.
There are three reasons why true reefs might be expected in the Cincinnatian. First, organisms with reef building potential certainly existed in abundance throughout the Cincinnatian, including corals, sponges, and bryozoans. Second, these same organisms were constructing true reefs by Early Ordovician time in other regions worldwide (Plate 2; Copper 1997). By Middle Ordovician time, bryozoans assumed a major role in reef building in regions as close to the Cincinnati Arch as Tennessee (Alberstadt et al. 1974). By Cincinnatian time, a diverse cast of reef-building organisms had assembled that would dominate reef building worldwide during the ensuing Silurian and Devonian (Copper 1997). Third, the tropical to subtropical paleolatitude of the Cincinnati region during the Late Ordovician was well within the climatic range where reef building might be expected and indeed was occurring elsewhere worldwide (Plate 2; Copper 2001; Webby 2002).
The coral beds of the Richmondian, as well as the Maysvillian bryozoan mounds, have not been regarded as true reefs because they did not grow into an interconnected framework that developed significant relief in relation to the surrounding sea floor. Rather, they were restricted to low-relief colonies living very close to sea level (Hatfield 1968). Within the coral beds, concentrations of corals are no more than a few meters wide (Figure 6.6F). There is no differentiation of the coral cluster as a reef “core” from the beds lying adjacent to it. A reef core usually indicates the corals constructed a mound having relief greater than that of a single colony, less than one meter. Coral clusters probably existed as small patches on a level sea floor, similar to small living patch reefs. The size of some tabulate colonies is comparable to that of many living corals in patch reefs. Hatfield (1968) showed that the coral zone within the Saluda Formation acted as a low, coral barrier surrounding a central lagoon where fine-grained carbonate sediments accumulated. In this way the coral zone acted as do present-day reefs to influence water movement and sediment deposition. Application of the terms patch reefs and biostromes to the Richmondian coral beds is therefore quite reasonable.
Figure 6.8. Conulariid, Conularia formosa Miller and Dyer, University of Cincinnati collections, Maysvillian, Corryville Formation, Butler County, Ohio, × 1.2.
The inability of Cincinnatian corals and other potential reef builders to construct major reefs has several possible explanations. First, corals were present in the region only during Richmondian time (Webby 2002). The absence of corals during Edenian and Maysvillian time is puzzling, because of the similarity of the rest of the fauna throughout the Cincinnatian. Environmental conditions in the Cincinnati Arch region may have been unsuitable for solitary and colonial corals during the Edenian and Maysvillian, but it is difficult to identify the factors responsible. Abundance of fine-grained sediments and frequent disturbance of the sea floor by storms are two factors that might have restricted the presence of corals. However, both factors are pervasive throughout the Cincinnatian, and it is not certain that either decreased significantly during the Richmondian. According to Elias (1982) solitary corals were introduced during an early Richmondian invasion from sources to the west. Solitary corals are present in the Edenian-Maysvillian strata of the Maquoketa Group to the west.
The introduction of corals to the Cincinnati region during Richmondian time possibly was related to progressive shallowing of the region during the Cincinnatian that culminated in the Richmondian (Anstey and Fowler 1969; Hay 1998). Coral beds developed on a shallow platform that was flanked by deeper water toward the west and north (Elias 1982). Present-day coral reefs develop along similar platform margins where a break in slope separates shallow from deeper water. Prior to the Richmondian, the Cincinnatian platform may have been deeper and without a break in slope toward the west that have favored coral development.
Conulariids
The conulariids are a minor group yet are among the most problematic fossils to be found in the Cincinnatian. They are usually found in a compressed condition in shales and siltstones of Maysvillian and Richmondian formations. As compressed specimens, conulariids appear to have a high triangular shape, with finely striated markings in a chevron-like pattern on a very thin integument, brown or black in color (Figure 6.8). The integument is calcium-phosphatic in composition. Uncompressed specimens are found elsewhere that show the original shape to be pyramidal and four-sided, and some have triangular flaps extending from the sides at the wide end, suggesting a means of closure. Cross-sections of uncompressed specimens reveal a bifurcating septum originating from each of the four sides. Evidence for attachment at the apical end is occasionally found, but is often lacking. Possibly conulariids lived part of their lives attached and later became free-living.
The zoological affinities of conulariids have been debated for a long time. Chiefly on the basis of their four-part structure, conulariids have been classified with the scyphozoan cnidarians, which have a tetrameral (four-fold) body plan. There are living scyphozoans with a chitinous theca and some that live attached by means of a stalk. Some have argued that conulariids should be recognized as a distinct phylum (Babcock 1996b; Babcock and Feldmann 1986), but recent work by Van Iten and others (1996), and Hughes and others (2000), confirms that the similarities between conulariids and scyphozoans are indicative of a close evolutionary relationship. Conulariids are found in marine strata of Cambrian through Triassic age. A single species, Conularia formosa Miller and Dyer, is recorded from the Cincinnatian of the Cincinnati Arch region.
