Figure 11.1. The Ordovician trilobite Triarthrus. Left, dorsal view, right, ventral view. Drawings by Kevina Vulinec.
In terms of sheer abundance, species diversity, and exploitation of habitats, arthropods rank as the most successful of all living animals. More than 750,000 species (mostly insects) inhabit a vast range of environments on land, in the sea, and in fresh water. Living arthropods include the insects, crustaceans, horseshoe crabs, arachnids, centipedes, and millipedes. During the Ordovician, arthropods had not yet invaded the land, but trilobites were abundant and diverse in the sea, along with the eurypterids, ostracodes, and a few other minor groups.
Despite their bewildering variety of form, all arthropods share certain basic features. Like their close relatives, the annelid worms, arthropods have a segmented body. Unlike the annelids, the body and its appendages are encased in an exoskeleton composed of the protein chitin. The exoskeleton is much like a suit of armor in having rigid components articulated by flexible joints. (The name arthropod means “jointed legs.”) Not only does the exoskeleton shield the internal organs from predation and some environmental hazards, but it also provides rigid points for muscle attachment. Consequently, arthropods are capable of rapid locomotion by walking, swimming, or flying. The nature of the exoskeleton has two important implications for the fossilization potential of arthropods. First, because the chitinous exoskeleton decomposes after death, many arthropods are poor candidates for fossil preservation. However, arthropods that have thicker exoskeletons or incorporation of calcium carbonate into their skeletons (such as some crustaceans and trilobites) will have enhanced potential for preservation. Second, all arthropods grow by periodically shedding the exoskeleton and forming a new skin that accommodates growth. Each individual arthropod can contribute numerous shed exoskeletons (molts) as potential fossils during its lifetime. Molting may thus explain in part the abundance of some arthropod fossils.
Trilobites
Although trilobites achieved their maximum diversity during the Late Cambrian, they remained diverse and abundant during the Ordovician. Among the plethora of Cincinnatian fossils, trilobites are arguably the best known for their preservation and abundance. Trilobites are unique among the arthropods in having a characteristic lengthwise subdivision of the dorsal exoskeleton into three lobes, an axial lobe flanked by two pleural lobes (Figure 11.1). There is a distinct head shield or cephalon, a flexibly segmented thorax, and a tail shield or pygidium. The cephalon usually has a central, swollen region called the glabella. Although it resembles a nose or forehead, the glabella actually protected the trilobite’s stomach. Transverse glabellar lobes and furrows indicate fused segmentation of the head region. A prominent pair of compound eyes usually flanks the glabella. A sinuous facial suture crosses the cephalon alongside the eyes, separating the lateral free cheeks from the fixed cheeks. The facial suture provided a line of breakage across the cephalon during molting. For this reason, isolated free cheeks are commonly found as well as the cranidium, a single unit comprising the glabella and fixed cheeks. A pair of genal spines is often developed at the posterior corners of the cephalon, as seen in Flexicalymene meeki, Isotelus maximus, and Cryptolithus tessellatus from the Cincinnatian.
The segments of the thorax were connected by a thin integument that allowed the trilobite to flex its body and in many cases to achieve complete enrollment like a modern pillbug (an isopod crustacean). After death, decay of the articulating integument often released individual thoracic segments that resemble brackets ({) when preserved. The pygidium is commonly preserved as a single unit because its segments were fused. As a consequence of molting and post-mortem decay, trilobite fragments are abundant, but complete, articulated specimens are uncommon. There is considerable debate about whether complete specimens represent trilobites buried intact, because some may have molted without the exoskeleton breaking apart. Usually, however, articulated specimens, particularly enrolled ones, represent trilobites buried alive or very soon after death.
On the ventral side of the cephalon, a plate called the labrum or hypostome was positioned beneath the mouth and connected to the anterior margin of the cephalon (Figures 11.1, 11.2, 11.4B). The labrum is often found as an isolated fossil, but it rarely will be found in place. Unlike that of a crab or lobster, the ventral exoskeleton of trilobites was a thin, chitinous membrane to which the appendages were attached. Trilobite appendages were also weakly constructed and thus were preserved only under exceptional conditions (Figure 11.4D). A pair of jointed antennae was attached to the ventral side of the cephalon, followed by a series of paired, jointed appendages underlying each segment of the cephalon, thorax and pygidium. Although appendages are known from very few species of trilobites, the appendages are similar in having two branches: the walking leg or endopodite and a branch called the exopodite extending from the basal segment of the endopodite (Figures 11.1, 11.2). The exopodite carried numerous filaments giving it a comb-like appearance.
Although at least sixteen genera of trilobites are known from the Cincinnatian, only a few are common. Flexicalymene and Isotelus are the most common and are distributed throughout the Cincinnatian Series. The wide-spread distribution of these two signature trilobites, in shales and limestones representing the full range of Cincinnatian depositional environments, clearly suggests that both had very generalized habitat preferences. In contrast, most other Cincinnatian trilobites have much more restricted stratigraphic distributions, suggesting more limited environmental tolerances.
Flexicalymene is one of the world’s best-known trilobites, in large measure due to its abundance in Cincinnatian strata (Figure 11.3). Although a modern systematic review has not been done, as many as three species may be present: F. meeki (Foerste), by far the most abundant and widely distributed species, F. granulosa (Foerste), a small form restricted to the lower Cincinnatian Kope Formation, and F. retrorsa (Foerste), found only in the upper Cincinnatian Waynesville Formation. In the first volume of the Geological Survey of Ohio (1873), F. B. Meek included the Cincinnatian calymenid trilobites with Calymene senaria, originally described from New York by Conrad in 1841. Foerste (1910) proposed the name Calymene meeki for the species so well described by Meek from the Cincinnatian rocks of Ohio, but provided only a five-line description and a single illustration of an enrolled specimen. In the same paper Foerste named C. meeki-retrorsa, a form of Calymene meeki from the Waynesville, which differs chiefly in the narrower posterior width of the cephalon, resulting in more obtuse genal angles. The anterior border of the cephalon is more strongly reflexed, bringing it closer to the anterior margin of the glabella. The British worker Shirley (1936) placed the Cincinnatian species in Flexicalymene. Ross (1967) provided a more complete description of F. meeki and also reviewed the status of Foerste’s C. meeki-retrorsa. Ross considered F. retrorsa to be a valid species although it has little to distinguish it from F. meeki except the size, shape, and inclination of the anterior cranidial border. Ross was unable to verify the other differences asserted by Foerste. A morphometric analysis conducted by Danita Brandt (1980) in her unpublished master’s thesis led her to conclude that only a single species, F. meeki, is valid, and that both F. granulosa and F. retrorsa should be synonymized with F. meeki as intraspecific variants. More recent work by Brenda Hunda (pers. comm.) supports the recognition of F. meeki, F. retrorsa, and F. granulosa as valid species.
Figure 11.2. Ventral views of three Ordovician trilobites, showing reconstructions of the appendages. Anterior at the top in each. A. Flexicalymene senaria (Conrad). B. Composite of Isotelus maximus and I. latus, (exopodites omitted because they are unknown). C. Cryptolithus tessellatus Green. From Raymond (1920, figures 9, 16, 20) and reprinted by permission of the Connecticut Academy of Arts and Sciences.
Figure 11.3. A, B. Flexicalymene meeki (Foerste), University of Cincinnati collections, Maysvillian, Corryville Formation, Hamilton Co., Ohio, enrolled specimen, cephalic width 28 mm. C. Flexicalymene retrorsa (Foerste), CMC IP, Ferree Collection, Richmondian, Arnheim Formation, Highland Co., Ohio, × 1.4. D. Flexicalymene granulosa (Foerste), Mark Peter collection, Edenian, Kope Formation, Brown Co., Ohio, × 2.5.
Flexicalymene is commonly found as isolated partial exoskeletons (cephala, cranidia, free cheeks, thoracic segments, pygidia) in Cincinnatian limestones; complete specimens are less common and are usually found in shales as either enrolled or extended individuals. On rare occasions, these trilobites can be found in great numbers in yellowish shales known as butter shales. One of the most prolific trilobite discoveries ever made in the Cincinnatian was in such a shale within the lower Richmondian Waynesville Formation during construction of an apartment complex at Boudinot Avenue and Westwood Northern Boulevard in northwest Cincinnati in the 1950s. Literally thousands of Flexicalymene were collected from two shale beds 2.5 and 3 feet thick, and as the exposure weathered, trilobites became perched on pedestals of clay for easy picking (Caster, pers. comm.; Schweinfurth 1958). Taphonomic studies of occurrences of abundant, complete trilobites in Cincinnatian shales indicate that these are the result of mass mortalities of living trilobite populations smothered during storms or mass movements of fine-grained sediments (Brandt 1980, 1985; Schumacher and Shrake 1997; Hughes and Cooper 1999).
Our understanding of the life habits of trilobites has been hampered by the fact that trilobites are extinct and have no close living relatives, although horseshoe crabs and some crustaceans are often regarded as possible models (Plate 3G). The form of the appendages is closely related to life habits in living arthropods, but in trilobites the appendages are so rarely preserved that little information can be gained from them. A recent review by Fortey and Owens (1999) demonstrated that other preserved morphological features of trilobites can be used to determine their feeding habits. Fortey and Owens regard the calymenid trilobites like Flexicalymene to have been predators or scavengers because the hypostome is rigidly attached to the underturned edge of the cephalon (doublure). The hypostome may have acted as a grinding or manipulating surface for small prey items held between the basal segments of the appendages.
Further evidence for the predatory behavior of Flexicalymene comes from characteristic burrows (trace fossils named Rusophycus) formed by this trilobite. Rusophycus trace fossils are well known in strata of late Precambrian through Devonian ages, and most are thought to have been produced by trilobites digging into the sediment using the paired appendages (Häntzschel 1975). Although the digging activity could reflect different possible behaviors including sheltering, resting, egg laying, or feeding, recent discoveries suggest that trilobites were actively hunting prey buried within the sediment. Although by no means common, several remarkable cases of Rusophycus intersecting worm burrows are known from the Cambrian (Jensen 1990) and Cincinnatian (Brandt et al. 1995). In the Cincinnatian, Flexicalymene is unequivocally identified as the producer of one type of Rusophycus (R. pudicum) on the basis of a few exceedingly rare specimens that have the characteristic bilobate burrow preserved in place beneath the complete carapace of the trilobite (see Figure 14.2B; Osgood 1970). Although no specimens of Rusophycus pudicum from the Cincinnatian have been found intersecting worm burrows, the digging activity is consistent with predation on small, infaunal organisms (Fortey and Owens 1999).
Figure 11.4. Isotelus maximus Locke. A. Enrolled specimen, CMC IP 2250, Richmondian, Arnheim Formation, Highland Co., Ohio, × 1.3. B. Large hypostome, CMC IP 33067, Maysvillian, Clermont Co., Ohio, × 1. C. CMC IP 51, Robert Nestor collection, Maysvillian, Corryville Formation, Clermont Co., Ohio, × 1.8. D. Appendages on ventral side of a complete specimen, USNM 33458, Richmondian, Oxford, Butler Co., Ohio, × 0.75. This exceptional specimen was originally illustrated by Mickleborough (1883). Photo courtesy of Loren Babcock.
Despite the excellent state of preservation found in Cincinnatian Flexicalymene, remnants of the appendages have never been found. Stürmer and Bergstrom (1973) carried out x-radiographic studies that revealed preserved appendages in some trilobites, but similar studies by Brandt (1980) and by Hughes and Cooper (1999) detected no evidence of appendages in Flexicalymene. The study by Hughes and Cooper revealed pyritized material concentrated within the body cavity of Flexicalymene that may have originated as decaying soft parts. An approximate idea of the nature of the appendages in Cincinnatian Flexicalymene species can be gained from the restoration of the closely related F. senaria (Figure 11.2A; Raymond 1920).
Isotelus is the other highly characteristic and widely distributed trilobite of the Cincinnatian (Plate 7; Figure 11.4). Fragments of this large trilobite are found in every Cincinnatian formation, in both limestones and shales. Complete specimens are quite rare, but in certain shale horizons, particularly in the Waynesville Formation, numerous complete specimens have been found (Schumacher and Shrake 1997). Specimens of Isotelus from the Cincinnatian are among the largest-known trilobites. A specimen of Isotelus on exhibit at the Smithsonian Institution collected in 1919 during construction of the Huffman Dam near Dayton measures 37 cm long (14.5 in) by 26 cm wide (10.25 in). The complete specimen at Cincinnati Museum Center measures 37.5 cm in length (Plate 7). On the basis of partial specimens, Isotelus probably reached lengths of 80–90 cm (Babcock, pers. comm.). Recently, a specimen of Isotelus was found in Upper Ordovician strata in northern Manitoba that holds the current world record as the largest trilobite, at a length of over 70 cm (Rudkin et al. 2003).
Currently, two species of Isotelus are recognized in the Cincinnatian: I. maximus and I. gigas. I. maximus has well-developed genal spines. I. gigas has genal spines that are either shorter than those of I. maximus or lacking. Without a modern, critical analysis of the range of variation within these species, it is not clear how to separate I. maximus from I. gigas on the basis of genal spine length. Cincinnatian Isotelus having a very broad, flattened carapace were given the name I. brachycephalus by Foerste (1919), but this species was regarded as a variant of I. maximus by Babcock (1996), and hence a junior synonym of I. maximus. (If two different names have been given to the same species by different workers, one name can be subordinated as a junior synonym if it was described after the original name and if it is determined to be equivalent to the originally described taxon.) Because I. maximus and his I. brachycephalus occur together in the Richmondian strata of the Cincinnatian, Foerste suggested the possibility that the broader I. brachycephalus might represent females and the narrower I. maximus the males of a single species. This remains an intriguing possibility that has never been fully explored.
Figure 11.5. A. Decoroproetus parviusculus (Hall), CMC IP 46429, Edenian (figured in Davis [1992, plate 2, figure 23] as Proetus parviusculus), × 7.7. B. Triarthrus eatoni (Hall), Steve Brown collection, J. Rush collector, Edenian, Kope Formation, Hamilton Co., Ohio, × 4.6. C. Cryptolithus tessellatus Green, University of Cincinnati collections, Edenian, Kope Formation, Hamilton Co., Ohio, × 3.7.
Isotelus was one of the largest-known animals in the Cincinnatian sea, rivaled only by the less common eurypterid Megalograptus and endocerid cephalopods. By virtue of its large size alone, Isotelus might be suspected to have been a predator, but additional evidence also points clearly toward this interpretation of its ecological role. Like Flexicalymene, Isotelus has a hypostome rigidly attached to the cephalic doublure (Figures 11.2B, 11.4B); in addition the anterior cephalic margin is strengthened (Fortey and Owens 1999). Fortey and Owens mentioned several other unique features of the Isotelus hypostome that suggest its function as a rigid platform like an anvil for the manipulation of bulky food: its forked shape, and development of anterior wings provide a larger surface area, and the fine raised ridges on the inner surfaces of the fork could make it easier to hold the prey rigidly using the appendages. This hypostome is the most heavily calcified part of the Isotelus exoskeleton and is often found as an isolated component (Figure 11.4B).
In the Cincinnatian, large Rusophycus burrows have been attributed to Isotelus on the basis of their size (R. carleyi, see Osgood 1970). Specimens often show not only furrows created by the appendages, but also impressions of the cephalic and pygidial margins and pleurae. A remarkable specimen shows a horizontal worm burrow apparently truncated in the approximate location of trilobite’s mouth (see Figure 14.2A; Brandt et al. 1995)—a trace fossil recording the very act of predation. The trilobite evidently dug and drew itself down into a semi-cohesive mud substratum so as to impress the margins of its carapace like a cookie cutter. The trace shows impressions of the basal segments (coxae) of the appendages that probably seized the prey along the ventral midline and worked it toward the mouth. A single exceptional specimen preserving the appendages of Isotelus was found in the Cincinnatian near Oxford, Ohio, and was first reported by Mickleborough (1883) (Figure 11.4D). Only the walking legs are poorly preserved, but the large size of the coxae is evident.
Cryptolithus, the lace-collared trilobite, is another common Cincinnatian trilobite in the Edenian and Maysvillian formations; a single species, C. tessellatus, is present (Figure 11.5C). Its common name refers to the broad, perforated cephalon that bears genal spines. The thorax and pygidium form a small, short unit that is rarely found attached to the cephalon. Exceptionally well-preserved specimens from the Ordovician of New York revealed the appendages of Cryptolithus. Reconstruction of the appendages shows that each walking leg was accompanied by a branch (exopodite) that carried long, comb-like filaments (Figure 11.2C). The exopodites probably waved in unison to stir up the sediment and create a respiratory and feeding current. Trace fossils attributable to Cryptolithus (Rusophycus cryptolithi) suggest that this trilobite excavated a pit that created a filter-feeding chamber beneath the broad cephalon (Osgood 1970; Fortey and Owens 1999). Different workers agree that Cryptolithus used the appendages to extract food particles from the sediment stirred up in digging the pit, but there are varying interpretations of the function of the perforations of the cephalic fringe. The funnel-like perforations could have acted as a sieve to separate fine particles from a feeding current passing from the exterior of the cephalon to the interior (Cisne 1970; Seilacher 1970). According to Fortey and Owens (1999), it is more reasonable to suppose that a feeding current created by the exopodites (gills) stirred up food particles beneath the trilobite, then exited through the perforations from the interior to the exterior. Campbell (1975), however, pointed out that very little flow could have passed through the minute pores and thus it is more likely that the pores had a sensory function to orient the animal into the current. If this is true, the pores may have been the sites of sensory hairs that were not preserved.
Figure 11.6. A. Acidaspis cincinnatiensis Meek, Steve Brown collection, J. Rush collector, × 2.6. B, D. Odontopleurid, gen. and sp. Undetermined. B. MUGM 29056, Richmondian, Oxford, Butler Co., Ohio, × 4.0. D, Steve Brown collection, J. Rush collector, × 2.6. C. Primaspis crosotus (Locke), University of Cincinnati collections, Kope Formation, Hamilton Co., Ohio, × 6. E. Primaspis crosotus (Locke) on bryozoan Peronopora sp., MUGM 4001-A, × 2.8, from Shrake (1989, plate 3C). Note Catellocaula vallata bioclaustrations in upper right part of bryozoan. F. Primaspis crosotus (Locke) on bryozoan, MUGM 4010, ventral side of trilobite showing hypostome in place, × 5.2, from Shrake (1989, plate 4B).
Figure 11.7. Cincinnatian phacopid trilobites. A. Platycoryphe christyi (Hall), Mark Peter collection, Richmondian, Waynesville Formation, Montgomery Co., Ohio, × 2.6. B. Tricopelta breviceps (Hall), MUGM 29057, Richmondian, Waynesville Formation, Franklin Co., Indiana, × 5.5. C. Ceraurus milleranus Miller and Gurley, CMC IP 5199, enrolled, no unit or locality (figured in Davis [1992, plate 3, figure 28]), × 2.5. D. Ceraurinus icarus (Billings), Steve Brown collection, J. Rush collector, Richmondian, × 2.
Figure 11.8. A, C from Caster and Kjellesvig-Waering (1964, plate 45, figure 1, plate 46, figure 4) and reprinted by permission of the Paleontological Research Institution. A. Eurypterid Megalograptus ohioensis Caster and Kjellesvig-Waering, Richmondian, Elkhorn Formation, Adams Co., Ohio; ventral side of postabdomen and last (sixth) pair of legs (darker segments), dorsal imprint of prosoma (grey segments turned to right near top), holotype, CMC IP 24119A, × 0.14. B. Professor Kenneth E. Caster with type specimens of Megalograptus ohioensis. C. First walking leg, paratype, CMC IP 24117A, × 0.9. D. Aglaspid Neostrabops martini Caster and Macke, holotype, CMC IP 25569, Maysvillian, Corryville Formation, Clermont Co., Ohio, × 1.4. From Caster and Macke (1952, plate 109, figure 2), and reprinted by permission of the Society for Sedimentary Geology.
Figure 11.9. Eurypterid Megalograptus ohioensis Caster and Kjellesvig-Waering. C, D. From Caster and Kjellesvig-Waering (1964, plate 49, figure 2, plate 48, figure 1). A. Reconstruction of dorsal surface of adult female (from Caster and Kjellesvig-Waering [1964, plate 43]), × 0.14. B. Reconstruction of ventral surface of adult female (from Caster and Kjellesvig-Waering [1964, plate 44]), × 0.14. C. Distal end of second walking leg, note bulbous expansion (sensory?) at end of longest spine, paratype, CMC IP 24115, × 0.7. D. Third paired appendage, showing large coxa and part of first four joints, paratype, CMC IP 24103A, × 0.4. A–D reprinted by permission of the Paleontological Research Institution.
Triarthrus is restricted to the lowermost Kope Formation but is significant for several reasons (Figure 11.5B). Triarthrus is the last of the olenid trilobites that were prominent during the Cambrian. Pyritized specimens from the Upper Ordovician Utica Shale of New York are among the most well-preserved trilobites, from which detailed reconstructions of the appendages and internal soft anatomy have been made (Cisne 1970; Whittington and Almond 1987). In the Cincinnatian, two species, T. eatoni and T. spinosus, are recognized (Babcock 1996a) but preserved appendages have not been found. The structure of the appendages in Triarthrus suggests that it was a particle feeder, sorting food particles with the thoracic appendages, which then passed them toward the mouth along the ventral axis (Fortey and Owens 1999). Enlarged, spine-bearing basal limb segments (gnathobases) beneath the cephalon acted like jaws to process the food and transfer it to the mouth. The restriction of Triarthrus to the dark shales of the lower Kope Formation, and its dominance in some thin beds, both suggest that this trilobite was uniquely adapted to deeper water environments low in oxygen, as were other olenid trilobites (Fortey and Owens 1999).
Among Cincinnatian trilobites, two genera belonging to the Order Odontopleurida are unique in being festooned with spines and tubercles; these are Acidaspis and Primaspis (Figure 11.6). Acidaspis is distinguished by having a long spine originating on the occipital lobe of the cephalon and extending over the axial lobe of the thorax, as well as long genal spines and a fringe of short spines along the margin of the free cheeks (Figure 11.6A). A complete description of A. cincinnatiensis Meek is given by Whittington (1956). Primaspis lacks the occipital spine and has the cephalic spines lying in a curve (Figures 11.6C, E, F). Both genera have spines developed from the pleural lobes of the thorax and the pygidium. Ross (1979) provided excellent illustrations of these very small trilobites that are usually less than 1 cm in length and rarely found complete. Acidaspis occurs from the basal Kope Formation through the top of the Maysvillian Mt. Auburn Formation, and reappears in the Richmondian Waynesville Formation. Whittington (1956) suggested that a single species, A. cincinnatiensis, is present. Primaspis crosotus (Locke) occurs in the Kope Formation where it was found in association with a cluster of Flexicalymene (Hughes and Cooper 1999).
The extreme spinosity of odontopleurids has prompted much debate about their mode of life. The fringe of vertical cephalic spines in Acidaspis enabled the cephalon to be rested against the substratum with the thorax and pygidium outstretched and elevated slightly, in a probable feeding position (Whittington 1956). In this position the hypostome and mouth were situated close to the substratum. Clarkson (1969) pointed out that whereas Acidaspis had a single, fixed life position, Primaspis could assume the same feeding position and also a resting attitude by tilting the cephalon backwards so that the entire body was supported against the substratum. Odontopleurids are considered to have been potential predators by Fortey and Owens (1999), but their prey must have been very small.
Figure 11.10. Cincinnatian ostracodes. A. Ceratopsis chambersi (Miller), CMC IP 40171, Kope Formation, Cincinnati, Ohio. From Warshauer (1972, plate 5), × 30. B. Ctenobolbina alata Ulrich, CMC IP 33085, Kope Formation, Cincinnati, Ohio, × 33. C. Quadrijugator regularis (Emmons), CMC IP 28245, Waynesville Formation, Butler Co., Ohio, × 90.
Plate 6 and Figure 11.7 illustrate exceptionally well-preserved specimens of some of the rarest of Cincinnatian trilobite species.
Eurypterids
One of the most intriguing fossils known from the Cincinnatian is the eurypterid Megalograptus (Figures 11.8, 11.9). Eurypterids are chelicerate arthropods, distinguished by having the first pair of appendages (chelicerae) equipped with small pincers. Modern chelicerates are the horseshoe crabs (Plate 3G), and arachnids (scorpions, spiders, mites, and ticks). The body of the eurypterid is unique, with a distinct head (prosoma) bearing compound eyes followed by an elongated, segmented section called the opisthosoma, divided into a wider preabdomen and a narrower, tail-like postabdomen (Figures 11.9A, B). Six pairs of appendages were attached to the underside of the head and served functions of feeding and locomotion. Because the exoskeleton was not calcified, preservation of the chitinous remains of eurypterids was unlikely, and therefore they are usually very rare fossils.
Eurypterids first appeared in the Early Ordovician and attained their maximum diversity in the Silurian, but Megalograptus is significant for being one of the oldest and most unusual. When fragments of the animal were first described in 1874 by S. A. Miller, they were thought to be parts of a graptolite, hence the name. Later workers corrected the error on the basis of additional, albeit fragmentary discoveries. A truly phenomenal discovery made in 1938 of exceptionally well-preserved and nearly complete specimens from a single bed in the uppermost Cincinnatian Elkhorn Formation of the Richmond Group in Adams County, Ohio, led to a better understanding of the animal (Figure 11.8B). This material, which included male and female specimens, became the basis for a new species, M. ohioensis, described by Kenneth E. Caster and Erik Kjellesvig-Waering in 1964. One additional eurypterid species, Eocarcinosoma batrachophthalmus, was described from this bed on the basis of an isolated prosoma.
Megalograptus ohioensis was one of the largest creatures in the Cincinnatian sea floor community, reaching a length of over 50 cm. The first pair of appendages, the chelicerae, is small and located beneath the head. The next three pairs of appendages bear well-developed spines (Figure 11.9). The third appendages are most striking for their length and long spines directed toward the midline (Figures 11.9C, D). Exactly how the eurypterid used these spiny appendages is uncertain. Caster and Kjellesvig-Waering considered Megalograptus to have been a predator, and thus the appendages likely had some function in grasping prey. The basket-like structure of the long spines of the third appendages suggests that the animal might have raked them through the sediment in order to extract prey in a sieving fashion. Only a few other eurypterids have similar long spiny appendages. Tubular castings filled with fragments of eurypterid integument associated with the eurypterid material could represent feces of the animal, indicating cannibalistic behavior like that found in living eurypterid relatives among the scorpions and spiders. The fourth pair of appendages lacks spines, and the fifth pair has expanded, flattened segments giving them a paddle-like appearance (Figures 11.9A, B). These were most likely employed in swimming.
One of the most peculiar features of Megalograptus is the development at the end of the postabdomen of a pair of expanded, hook-like cercal blades flanking a spine-like telson (Figures 11.9A, B). Caster and Kjellesvig-Waering thought that the cercal blades could move laterally in a scissor-like motion, possibly serving to grasp either in defense or copulation. The paired, incurved posterior spines of earwigs are similar, but nothing like this structure appears in any other eurypterid.
In addition to the Adams County occurrence of Megalograptus ohioensis in the Elkhorn beds, Caster and Kjellesvig-Waering described two other species from the Cincinnatian. M. shideleri is known from fragments occurring in the Saluda Formation of the Richmond Group and M. williamsae from the Waynesville Formation of the Richmond Group. Fragments of M. shideleri suggest that it may have reached two meters in length. The type species, M. welchi, was originally described from the Liberty Formation, thereby indicating that the genus ranges through virtually the entire upper Cincinnatian. Megalograptus was not restricted to the Cincinnati Arch region, as M. alveolatus is known from the Upper Ordovician of Virginia.
In the Liberty occurrence, described by Foerste (1912), Megalograptus was found in a pocket together with crinoids. In the Elkhorn, Megalograptus was associated with a diverse marine fauna including trilobites, brachiopods, bryozoans, and molluscs. Despite the common notion that eurypterids lived in somewhat restricted or atypical marine environments, the Cincinnatian occurrences argue strongly for association with the normal marine biota. Their chitinous integument may account largely for their rarity in the Cincinnatian. The extraordinary quality and quantity of eurypterid preservation at the Adams County site may have resulted from smothering of the marine fauna by volcanic ashfall, because the 15 cm-thick shale within which the fossils were concentrated was found to contain bentonitic clays (Caster and Kjellesvig-Waering 1964).
In 1952 Caster and Macke described what they termed a maverick merostome, Neostrabops martini, from a single specimen found in the Maysvillian Corryville Formation in Clermont County, Ohio (Figure 11.8D). This fossil could be taken to be a trilobite, but lacks the characteristic lengthwise division into three distinct lobes. It also resembles the aforementioned eurypterids in having numerous narrow segments, although it lacks any demarcation of pre- and postabdomen. Neostrabops does resemble other arthropods known as aglaspids from the Cambrian. Aglaspids are regarded as early offshoots of the evolutionary lineage of modern horseshoe crabs, the well-known Limulus.
Ostracodes
Crustaceans are one of the most abundant and diverse groups of living arthropods, yet they are represented in the Cincinnatian by only one group, the ostracodes (major living crustaceans such as shrimps, lobsters, and crabs evolved much later than the Ordovician, and thus are not found in the Cincinnatian). Ostracodes are generally very small, less than 1 mm in length, but can exceed 1 cm in forms like the Ordovician Eoleperditia. They are distinguished by a calcitic, bivalved shell or carapace that can be smooth or have various surface features such as knobs, ridges, or spines (Figure 11.10). When the hinged valves of the carapace open, the appendages can be extended for feeding and locomotion. Modern ostracodes have a wide range of feeding habits, but many live as benthic suspension feeders and deposit feeders. It is very difficult to determine the specific habits of Cincinnatian species, because the appendages and other internal anatomy are not preserved.
Ostracodes are diverse and abundant throughout the Cincinnatian Series, although this is generally unappreciated because few collectors or researchers encounter these microfossils. Two major studies on Cincinnatian ostracodes provide a modern analysis of their diversity and classification, but there is no single comprehensive study that gives an overview of total Cincinnatian ostracode diversity. Warshauer and Berdan (1982) reported fifty-three species and thirty-nine genera of ostracodes belonging to two major orders, the Palaeocopida and the Podocopida, from the Middle Ordovician Lexington Limestone and the Upper Ordovician (basal Cincinnatian) Clays Ferry Formation of Kentucky. Species in these groups are all very small, and were extracted from disaggregated shales or by acid dissolution of the Lexington Limestone in which the fossils are silicified. Fourteen genera occur in the Clays Ferry Formation, but distribution within the rest of the Cincinnatian was not studied. Berdan (1984) reported on ostracodes of the order Leperditicopida from the Middle and Upper Ordovician of Kentucky and vicinity. These ostracodes are noteworthy for their size (sometimes > 1 cm long) and high abundance in single beds of fine-grained limestone. The occurrence of leperditicopids is restricted to fine-grained limestone facies deposited in extremely shallow subtidal to intertidal environments particularly well known from the Middle Ordovician High Bridge Group of Kentucky (Cressman and Noger 1976). This unique facies is absent from the deeper water facies of the lower and middle Cincinnatian but recurs in the Richmondian Sunset Member of the Arnheim Formation, in which four species are found.
