Life Mode

The following discussion of life-mode is based almost exclusively on circumstantial evidence. As such, it is highly interpretive. Fortey (1985) pointed out that using the same body of knowledge, trilobites in the family Agnostidae have been hypothesized to be pelagic, benthic, parasitic, and epifaunal, possibly attached to algal strands. The fossil record does not often permit clear, unambiguous conclusions. However, one might assume that "form follows function" and that a trilobite's morphology is often a good indicator of its life habits.

Most paleontologists contend, by analogy with crustaceans, that for many trilobites the protaspid phase was planktic. That is to say that the larvae floated and drifted in the sea, ensuring a wide dispersal of the species.

Trilobites that were primarily benthic as adults probably settled to the sea bottom sometime during the meraspid phase. Pelagic, or free-swimming trilobites, may never have left the open seas during their transformation to adults. All through these changes, the immature trilobite was very vulnerable to predators and environmental stress, as the molting process left the animal

FIGURE 2.12. Internal anatomy of the trilobite. A. Internal organs of the cephalon of Triarthrus determined by Cisne (1975, p. 55, Fig. 9): m, muscles; c, crop or stomach; g, gut; h, hepaticopancreatic organ. Reproduced from Fossils and Strata, www.tandf.no/fossils, by J. L. Cisne, 1975, vol. 4, 45-63, by permission of Taylor and Francis AS. B. Cross section of the thorax with the internal organs (Cisne 1975, p. 53, Fig. 7): d, dorsal vessel or "heart"; m, muscles; g, gut. Reproduced from Fossil and Strata, www.tandf.no/fossils, by J. L. Cisne, 1975, vol. 4, 45-63, by permission of Taylor and Francis AS. C. Reconstruction of some internal anatomy of Cer-aurus pleurexanthemus by Raymond (1920a): d, dorsal vessel or heart; g, gut; m, muscles. D. Specimen of C. pleurexanthemus that cleaved to show the gut (g) as a dark ferruginous stain (MCZ 111716).

Dorsal Vessel Dinosaurs Heart

FIGURE 2.12. Internal anatomy of the trilobite. A. Internal organs of the cephalon of Triarthrus determined by Cisne (1975, p. 55, Fig. 9): m, muscles; c, crop or stomach; g, gut; h, hepaticopancreatic organ. Reproduced from Fossils and Strata, www.tandf.no/fossils, by J. L. Cisne, 1975, vol. 4, 45-63, by permission of Taylor and Francis AS. B. Cross section of the thorax with the internal organs (Cisne 1975, p. 53, Fig. 7): d, dorsal vessel or "heart"; m, muscles; g, gut. Reproduced from Fossil and Strata, www.tandf.no/fossils, by J. L. Cisne, 1975, vol. 4, 45-63, by permission of Taylor and Francis AS. C. Reconstruction of some internal anatomy of Cer-aurus pleurexanthemus by Raymond (1920a): d, dorsal vessel or heart; g, gut; m, muscles. D. Specimen of C. pleurexanthemus that cleaved to show the gut (g) as a dark ferruginous stain (MCZ 111716).

with little in the way of defense for a period of time and the energy burden of continually building new exoskeletons was considerable. In extant arthropods Clarkson (1979) noted that 80% to 9 0% of mortality comes during the molting process. The more common trilobites had to produce large numbers of larvae in order for significant numbers to reach maturity.

The metamorphic change in an asaphid trilobite, Isotelus, from the planktic protaspid (Figure 2.8D) to the benthic adult

(Figure 2.8F) is well documented. Both forms are well suited to their respective modes of life. The change to a bottom-dwelling (benthic) form comes at the metamorphosis from protaspis to the meraspis, which is shaped like the adult but with long genal spines. As mentioned earlier, the long genal spines last well into the early juvenile holaspis.

Some trilobite species are widely dispersed throughout the world, suggesting that they were planktic or pelagic as adults and

Hypodicranotus

FIGURE 2.13. Trilobite shapes and functions. A. Isotelus gigas (MCZ 311). This trilobite has a smooth shape suitable for shallow plowing of the surface muds for feeding. B. Hypodicranotus stnatulus (MCZ 100986). The streamlined shape and the 180-degree visual capability suggest a trilobite that might have been a good swimmer and was perhaps pelagic. C. Achatella achates (PRI 49659). This trilobite has a fairly flat body with eyes raised well above the rest of the cephalon. In analogy with bottom dwellers with raised eyes, one might expect this trilobite to rest on the bottom, with the body just under the surface of the substrate and the eyes above it. This position is a defense against predators and possibly a means of lying in wait for prey. D. Cryptolithus bellulus (PRI 49654). This trilobite has a prominent, robust cephalon compared to the light exoskeletal material on the thorax and pygidium. This feature and other evidence (Campbell 1975) suggest a sedentary lifestyle and filter feeding habit. E. Triarthrus eatoni (TEW collection). The appendage, including the exite or brachial branch, extends well beyond the edge of the thoracic shield. This configuration aids the trilobite to survive in the dysoxic, deep-water conditions suggested by the dark shales in which they are found.

FIGURE 2.13. Trilobite shapes and functions. A. Isotelus gigas (MCZ 311). This trilobite has a smooth shape suitable for shallow plowing of the surface muds for feeding. B. Hypodicranotus stnatulus (MCZ 100986). The streamlined shape and the 180-degree visual capability suggest a trilobite that might have been a good swimmer and was perhaps pelagic. C. Achatella achates (PRI 49659). This trilobite has a fairly flat body with eyes raised well above the rest of the cephalon. In analogy with bottom dwellers with raised eyes, one might expect this trilobite to rest on the bottom, with the body just under the surface of the substrate and the eyes above it. This position is a defense against predators and possibly a means of lying in wait for prey. D. Cryptolithus bellulus (PRI 49654). This trilobite has a prominent, robust cephalon compared to the light exoskeletal material on the thorax and pygidium. This feature and other evidence (Campbell 1975) suggest a sedentary lifestyle and filter feeding habit. E. Triarthrus eatoni (TEW collection). The appendage, including the exite or brachial branch, extends well beyond the edge of the thoracic shield. This configuration aids the trilobite to survive in the dysoxic, deep-water conditions suggested by the dark shales in which they are found.

moved freely throughout the seas. Planktic larval forms would also serve to disperse species but in a more limited manner. Dispersal into new areas by the normally benthic trilobites is expected to be followed by speciation, so one would not expect the species identity to be maintained if dispersal was into areas not previously occupied by the species.

There are wide variations in the size of trilobite eyes, the number of lenses, and the angle of vision. These variations are certainly some indication of the life-mode, but modern analogy is often necessary to come up with suggestions. Animals that bury themselves shallowly in the bottom sediment have eyes that are raised above the plane of the head, enabling them to see when they are slightly buried. Some trilobites have such raised eyes (e.g., Achatella achates (Figure 2.13C) and DaJmanites species), and it is reasonable to suggest that they too buried themselves under a thin layer of sediment.

Body shape suggests the life-mode of many trilobites. Fortey (1985) defined three morphologies of pelagic species: (1) large-eyed, epipelagic, slow-swimming trilobites; (2) pelagic, streamlined, faster swimmers; and (3) possible swimming IrvingeUa types, remopleuridids, and progenetic types. The fast-swimming trilobites have rounded streamlined shapes, which promoted buoyancy and low drag while swimming. These shapes are not often found among New York trilobites, but the Middle Ordovi-cian remopleurid Hypodicranotus striatulus has the right shape and is considered pelagic (Figure 3.13B, Plate 160).

Vaulted, smooth exoskeletons such as that on /. gigas (Figure 2.13A, Plate 150), DipJeura dekayi (Plate 97), and Trimerus del-

phinocephalus (Plate 99) were well designed to plow through the upper sediment layers in search of food. T. eatoni, with its thin exoskeleton and outer branches that extend beyond the pleurae, was unsuited for shallow, turbulent water and for plowing in sediment and was better designed for surface scavenging in deeper, less oxygenated environments (Figure 2.13E, Plate 172).

It has been proposed that to maximize visual effectiveness, the plane of the upper and lower edges of the eyes should be parallel to the substrate (Plates 5 and 6). In many outstretched trilobites it is apparent that the eyes are parallel to the substrate. However, many species of illaenids, when outstretched, have eyes that are angled upward and posteriorly. Westrop (1983) and Bergstrom (1973), among others, argued that these trilobites were infaunal, burying themselves backward into a soft bottom with their cephalon on the surface at an angle to the thoracopygidium.

In this attitude the eye base is parallel to the surface and gives the maximum all-around vision. These infaunal trilobites fed and breathed by the exchange of water on the buried ventral anatomy. This exchange of water was the result of the movement of the appendages and an upstream orientation of the burrow. There are other trilobites with this life-mode, which Westrop termed "illaenimorphs." Whittington (1997b) rebutted this view on the basis of the high flexibility of the thorax in illaenids and that they are more suited to crawling around the bottom and over irregular objects than living or resting primarily in burrows.

Westrop also pointed out that on some of the illaenimorphs there is a median tubercle on the glabella, midway between the palpebral lobes on the sagittal line. This tubercle is also a thin spot and is characterized as having possible light-sensing properties. It is the highest point when the body is in a normal life position, thus covering any blind spots of the conventional eyes. Ruedemann (1916b) found a significant number trilobites with such median tubercles, even nominally blind trilobites such as in the genus Cryptolitluis. It is reasonable to conjecture that the tubercles had a light-sensing utility and played a role in the trilo-bite's life-mode. Such light sensing tubercles probably could sense movement but not resolve objects.

In modern arthropods larger eyes are found on nocturnal species or those adapted to low daylight levels. Animals with a wide visual angle need it to watch for predators. Very large eyes and a wide visual angle are seen on some pelagic trilobites (Figure 2.13B). Fortey (1985) considered the large-eyed trilobites as epipelagic and slow swimmers.

Compound eyes permit insects to be highly aware of movement but do not necessarily provide high visual acuity. Reduction in eye size has been noted for trilobite genera that moved to deeper water through time. Blind trilobites, such as in the genus Cryptolithus (Figure 2.13D), may have burrowed into sediment where sight would have been less important than other sensory capabilities.

Most trilobites were benthic. They passed most of their life on or in the uppermost part of the sediment layer. Some trace fossils such as tracks, burrows, and distinctive pits preserved in the fossil record are attributed to trilobites, as supported by the rare find of a trilobite at the end of a trackway or in one of the burrows (Figure 2.14A). Two common traces — Cruziana and Rusophycus — are generally preserved as molds (fillings) on the basal contact of sandstones or carbonate beds in shales. One type of deep, inscribed, horizontal track or furrow, Cruziana, shows "V-like" scratch patterns made by the dactyls (claws) of the trilo-bite and a central groove corresponding to an axial ridge of debris pushed up by the trilobite (Figure 2.14B). The bilobed burrows, or resting pits also showing V-shaped scratches, are called Ruso-phycus, and the trackways consisting of small "footprints" on the substrate are sometimes known as Diplichnites (see Bromley 1990, p. 161), although Osgood (1970) did not hold this term in high regard.

Trace fossils attributed to various burrowing animals (worms?) have been found ending in Rusophycus, suggesting the trilobite had attacked another burrower. These fossils indicate that many trilobites walked around on the bottom and dug into the sediment for both food and resting places (Hall 1852, Plate 9, Figure 1).

In the case of Cruziana the direction of travel for the trilobite is toward the open end of the V-shaped appendage traces. Most Rusophycus traces form a V, with the gape end or anterior being wider than the posterior, suggesting that the trilobite rested (or hunted) with the cephalon toward the "gape" end. There is little argument that Rusophycus traces are most readily explained as trilobite hunting or resting pits, but the same is not true for Cruziana. Whittington (1980), based on his in-depth studies of Olenoides serratus, believed that trilobite appendages do not readily allow for the type of traces represented by Cruziana and that some other animal, perhaps not even an arthropod, may be responsible. However, many Cruziana traces end in Rusophycus, and since the latter are unambiguously trilobite, this reference is questionable.

In New York, traces attributed to trilobites are common in the Silurian Clinton Group, particularly on the base of sandstone layers near the village of Clinton, Oneida County. Osgood and Drennen (1975) provided a good description of these traces and their literature. In other strata they are far less well known, either because conditions were not right for their preservation or because little effort has been made to find and identify them in appropriate strata.

Fortey and Owen (1999) proposed that trilobite feeding habits can be related to the position and attachment of the hypostome and other physical characteristics. Trilobites that are considered predatory (i.e., they fed off macrofauna such as worms) had a conterminent hypostome fixed or strongly supported at the anteroventral cephalon. The hypostome provided a strong base for the appendages so the trilobite could manipulate and masticate the prey. Food was passed forward along the ventral median by the bases to the mouth at the posterior of the hypostome. Examples of this type of hypostome attachment in an Isotelus species may be seen in Figure 2.7E and Plates 153, 155, and 157.

FIGURE 2.14. Trilobite traces. A. Rusophycus pudicum (UCM 37574). Sandstone deposits over trilo-bite-rich bottom muds often have convex, slightly V-shaped traces on their lower surface. These traces are known as Rusophycus and have been long regarded as trilobite "resting traces." Osgood (1970) reported on a remarkable Rusophycus that had the trilobite responsible for it still in place. B. Flexica-lymene meeki. This trilobite was found on the Rusophycus in A. The trilobite is about 46 mm long and is from the Upper Ordovician of Ohio. C. Trachomatichnus numerosum (UCM 37695). A trilobite walking trace attributed to Cryptolithus (Osgood 1970). The illustration is life size. All figures reproduced with permission.

FIGURE 2.14. Trilobite traces. A. Rusophycus pudicum (UCM 37574). Sandstone deposits over trilo-bite-rich bottom muds often have convex, slightly V-shaped traces on their lower surface. These traces are known as Rusophycus and have been long regarded as trilobite "resting traces." Osgood (1970) reported on a remarkable Rusophycus that had the trilobite responsible for it still in place. B. Flexica-lymene meeki. This trilobite was found on the Rusophycus in A. The trilobite is about 46 mm long and is from the Upper Ordovician of Ohio. C. Trachomatichnus numerosum (UCM 37695). A trilobite walking trace attributed to Cryptolithus (Osgood 1970). The illustration is life size. All figures reproduced with permission.

Impendent hypostoma that are also attached to the doublure also suggest a predatory habit. Bellacartwrightia species (Plate 47) and Calyptaulax callicephalus (Plate 117) show this mode of hypo-stome attachment.

Trilobites whose hypostoma were not strongly attached to the cephalon (i.e., natent) are considered to be particle feeders. They relied on much smaller food particles swept up from the bottom, and the rigid hypostome was unnecessary. These trilobites tend to be smaller than the predatory ones because their food sources were not as rich and concentrated. The genera Harpidella (Plate 128) and Triarthrus (Plates 170 to 174) represent this group of trilobites.

The last feeding mode to consider here is filter-chamber feeding. This type of feeding is typical of trilobites with reduced mobility and relatively large cephala. These trilobites settled in a position and stirred up the sediment immediately under them, and then filtered out the minute food particles contained in the top layer of sediment. Movement occurred only after the food supply was exhausted. The genus Cryptolithus (Plates 163 to 167) is the example used, and the evidence is the relatively small

(weak) thorax and thoracic appendages compared to the cephalon, as well as trace fossils clearly attributable to Cryp-tolithus resting (and feeding) sites.

Trilobites are found in a variety of environments, from fairly shallow waters near shore, to reefs, continental shelves and slopes, and moderately deep basins. The observation has been made that trilobites from the shallower areas with more wave turbulence have thicker exoskeletons (Fortey and Wilmot 1991). These thicker exoskeletons are possibly an evolutionary response to the greater environmental energy. As mentioned earlier, exoskeletons are generally thicker in post-Cambrian trilobites, which may also signal the rise of better-developed predation, another form of environmental stress.

Many trilobites were gregarious, at least at some point during their life cycles. The large numbers of death and molt assemblages, well illustrated by E. rana in New York, are no statistical accidents. It is not uncommon, within a number of different trilobite species, for a large number of individuals to be found in local "pockets" on the same bedding plane or horizon, indicating some form of group behavior (Plates 59, 89, 102, 128, 146, 147, and 152). All this suggests that it was common for many trilo-bites to congregate for breeding or molting, or just because it was their normal life-mode to be together in "schools" (Speyer and Brett 1985, Speyer 1990a).

Trilobites such as phacopids and calymenids were capable of very tight enrollment, literally into a ball. This position was undoubtedly a defense mechanism resorted to in times of stress. If the stress was an undersea sediment flow too large to get out of, the trilobite would be entombed in the tightly enrolled position. Most of the post-Cambrian trilobites in New York could enroll, and the frequency of this position versus the open position is perhaps an indication that it was a common reaction to stress for the species.

Some benthic trilobites undoubtedly could swim. Modern horseshoe crabs, a normally benthic species, are good swimmers. They also swim upside down, and it has been shown that the hydrodynamics of their swimming works best in this attitude (Fisher 1975). It is reasonable to assume that some normally benthic trilobites could swim also and possibly quite well. Some likely swam upside down. (H. Burmeister first proposed this in 1843.) This mode of swimming may not have been involved in their food gathering, but they could move from place to place and evade danger by swimming. In one trilobite bed, the result of a burial event, at least 98% of the trilobites with a wide variety of sizes are found buried upside down (Whiteley et al. 1993; Brett et al. 1999). This observation applies to both the lower surface and internal to the limestone. For more on this, see Chapter 3.

Spininess in trilobites has invoked a number of explanations, most of them probably correct for one species or another. There seems to be little need to invoke just a single explanation, any more than there is one explanation for spininess in modern species of arthropods. Spines can be a defense mechanism against predators, provide support on a soft substrate, assist the animal in burying itself when necessary, and sensory devices. The Lower Devonian stratum in New York (and the Devonian strata of Oklahoma and Morocco) has trilobites with elaborate spines, some curling up and over their thorax. These spines of the genus Dicranurus commonly carried algae and other encrusting organisms on them, possibly as a defense mechanism (Kloc 1993, 1997). The spines provided good attachments and presumably helped break up the visual body lines to provide camouflage. Some modern arthropods do exactly this.

One can assume, based on modern analogy, that trilobites were subject to injury, parasitism, and predation, as reflected in their preserved parts. Owen (1985) extensively reviewed trilobite abnormalities. He listed three general types of trilobite abnormalities: injury, genetic or embryological malfunction, and pathological abnormalities.

Not uncommonly, trilobites are found with malformations that are ascribed to healed injuries (Figure 2.9B). Most of these injuries are seen on the pleura either by asymmetry or by healed damage. These malformations can come about in several ways, but damage due to problems in molting and damage caused by actual attack by a predator were probably the most common. The mechanism of a defect is not always clear, but healed punctures and crescent-shaped malformations are readily attributed to predation. Figure 2.15 illustrates four examples of exoskeletons with clear indication of predation. Panels A and B show "bite" marks out of trilobites from the Rochester Shale that have been through at least one molt, as shown by the broken edges of the thoracic segments being rounded to a new termination. Panel C shows the unusual situation of Dalmanites limulurus with the pygidial spine missing (compare to Figure 2.15A) and the damage healed over. Panels D through F show a calymenid from the Rochester Shale with evidence of boring on its exoskeleton. This finding is particularly interesting because crinoids from the Rochester Shale have been described with similar boring marks (Brett 1978, 1985). Signor and Brett (1984) and Pratt (1998) summarized the possible predators in the Paleozoic. Anomalocaris is a well-documented predator of the Cambrian, and many of the circular scars on trilobites are attributed to the circular mouth of Anom-alocaris. Cephalopods became a predation factor in the Ordovi-cian, and fishes became prominent in the Devonian.

Babcock (1993) has collected information related to the concept of behavioral asymmetry. This means that when a trilo-bite was attacked by a predator, its response was not random, but there was a preference to move in a manner that resulted in most damage to the right side of the animal. Conversely, preferred damage to the right side of the trilobite could suggest the attack strategy of the predator.

Genetic or embryological abnormalities cannot be easily diagnosed in fossils. Pathological abnormalities are due to disease and parasites. Disease is impossible to document in fossils, but parasitic attack is seen by the presence of worm-shaped borings and gall-like swellings. It is more difficult to determine if borings were made on the living animal (versus on the exuviae or postmortem) than to recognize a healed injury. Swellings are known from a number of different trilobite families, and some can be diagnosed as the result of parasites based on the direct presence of wormlike structures.

McNamara and Rudkin (1984) documented the death of a partially molted Pseudogygites latimarginatus. It seems likely that the animal was overcome and buried while molting.

Often trilobites are found in association with other fossils in rapid-burial deposits. If there is little indication of their being transported any distance, then this can be taken as evidence of their ecology. An example is the odontopleurid Meadowtownella trentonensis from the Trenton Group. This small, spiny trilobite is often found whole on fossil hash layers, particularly with branched bryozoans, and also in burial event deposits with the cystoid Cheirocrinus and fenestrate bryozoans. M. trentonensis was probably a scavenger on bottoms with animal debris accumulations and also in bryozoan thickets with their attached

FIGURE 2.15. Trilobite injuries. A. Dalmanites limulurus with a semicircular portion of the thorax missing. The damage has "healed" in that the edges of the damage are rounded, indicating the trilobite has molted at least once since the injury (K. Smith collection). B. Dicranopeltis nereus with an injury to the thorax. The trilobite has molted at least once since the injury (K. Smith collection). C. Dalmanites limulurus with the pygidial spine missing. The end of the pygidium is rounded, indicating one molt since the loss of the spine. (K. Smith collection). D-F. Calymene species, showing circular "borings" (arrows) similar to those of Tremichnus on several different species of crinoids from the Rochester Shale reported by Brett (1985) (NYSM 16796).

FIGURE 2.15. Trilobite injuries. A. Dalmanites limulurus with a semicircular portion of the thorax missing. The damage has "healed" in that the edges of the damage are rounded, indicating the trilobite has molted at least once since the injury (K. Smith collection). B. Dicranopeltis nereus with an injury to the thorax. The trilobite has molted at least once since the injury (K. Smith collection). C. Dalmanites limulurus with the pygidial spine missing. The end of the pygidium is rounded, indicating one molt since the loss of the spine. (K. Smith collection). D-F. Calymene species, showing circular "borings" (arrows) similar to those of Tremichnus on several different species of crinoids from the Rochester Shale reported by Brett (1985) (NYSM 16796).

FIGURE 2.16. Cryptolithus, the most-studied trilobite genus found in New York. A. Cryptolithus tessellatus (TEW collection, whitened). This trilobite, in New York, is found only in the Middle Ordovician Sugar River Formation. It is recognized by the three rows of pits anterior to the cheek area (arrow). B. Cryptolithus lorettensis (PRI 49657, whitened). In New York this trilobite is found only in the upper Sugar River Formation. It differs from C. tessellatus in that there are four rows of pits anterior to the cheek (arrow). The first approximately nine radial abaxial pit rows are well aligned. C. Cryptolithus bellulus (PRI 49654, latex pull, whitened) from the Upper Ordovician Lorraine Group in New York. This trilobite differs from C. lorettensis primarily in the poor radial alignment of the first four abaxial pit rows. D. A drawing of a silicified specimen of C. tessellatus by Campbell (1975). Reproduced with permission. E. A drawing of the same specimen by Bergstrom (1972) with the appendages drawn in. The appendage information is from Beecher specimens of C. bellulus. Reproduced with permission. F. Raymond's (1920a) reconstruction of the ventral anatomy of C. bellulus using specimens prepared by Beecher. G. Bergstrom's (1972) reconstruction of the same specimens. Reproduced with permission.

Cryptolithus Tessellatus Appendage

cystoids. A further possibility is that this trilobite fed off living bryozoa.

Trilobites of the genus Cryptolithus are among the best understood of the New York trilobite genera (Figure 2.16). There are several reasons for this. The genus is widely distributed geographically and geologically. Wide distribution means that it is preserved in a wide variety of environments and the possibility of new information on the lifestyle and biology is enhanced. Cn/ptolithus specimens with appendages are preserved in Beecher's Trilobite Bed, which enables detailed observations of the ventral anatomy (Beecher 1895a, Raymond 1920a (Figure 2.16F), Bergstrom 1972 (Figure 2.16G)). At least one whole, three-dimensional specimen, preserved in silica, is known. Figure 2.16D is a drawing of the specimen by Campbell (1975), and Figure 2.16E is a drawing of the same specimen by Bergstrom with the legs included. Walking and digging trace fossils unequivocally assigned to Cryptolithus are known from Kentucky. The Cryptolithus protaspid is sometimes found silicified and is easily characterized. The unique cephalon with its heavily pitted brim is readily recognized in stratigraphic samples, and it is impossible to mistake it for other genera. A number of authors have studied the distribution of Cryptolithus so there is a large database of information.

In New York there are three distinct species or "morphs" of Cryptolithus (Whittington 1968, Shaw and Lesperance 1994): C. tessellatus (Figure 2.16A), C. lorettensis (Figure 2.16B), both from the Middle Ordovician Sugar River Formation, and C. bellulus (Figure 2.16C) from the Upper Ordovician Lorraine Group, all in New York. They are distinguished primarily by the rows of pits or the pit arrangement, or both, so that it is justified to assume that any biological information from one species is essentially the same for all of them.

Trinucleids arose as a family in Europe and migrated to North America during the Middle Ordovician (Whittington 1968). The pelagic protaspis (Chatterton et al. 1994) ensured that once in the North American epicontinental sea, the trilobite spread rapidly wherever currents took the protaspis. Molts from protaspides and cephala of what is probably the meraspid phase are commonly found in the deep-water Frankfort Shales including the Beecher's Trilobite Beds. The holaspid is blind except for the possible eye spot or visual receptor centered on the median line of the glabella. There is a wide brim on all but the posterior of the cephalon. This brim is bilaminar, separating into an upper and lower lamella through a plane parallel to the ventral plane of the trilobite. The brim is perforated with holes, commonly referred to as pits, that pass through both lamellae. The pits are in circumradially arranged rows for the first three or four circumferal rows. There are a number of speculations as to the function of the pits. The most current thinking is that they were sensory devices, perhaps to determine current direction (Campbell 1975).

The long genal spines are on the lower lamella. Cephala are found with and without genal spines, as are near-whole articulated specimens. The trilobite molted by moving forward through a gap between the lamella, and this gap possibly closed after molting (a mechanism observed in extant horseshoe crabs). Alternatively, the lower lamella became detached during the molting process. Consequently, it is difficult to know if a completely whole articulated specimen with the genal spines intact is a molt or an animal killed and buried. The absence of genal spines and lower lamella, however, ensures it is a molt.

The thorax and pygidium are much reduced compared to the cephalon. There are six thoracic segments and a small triangular pygidium. A nearly whole silicified Cryptolithus species was dis covered by Whittington (1959). Photographs of this specimen reproduced by Campbell (1975) reveal that the ventral plane of the thoracopygidium was well above the apparent plane of the ventral surface of the cephalon. In order to move about on the bottom and to burrow, the trilobite must have had long, strong, ventral walking legs or have developed other modes of movement. Campbell (1975) explored this in detail, but in summary Cryptolithus species could not have been very active crawlers or deep burrowers (see Figure 2.16G). Osgood (1970, Plate 58, Figure 1 and 2) figured the resting trace fossil Rusophycus cryp-tolithi, which because of size and the impressions of genal spines could only have been made by Cryptolithus animals. These traces indicate that the trilobite sat in a shallow depression, facing into the current, and swept particles of detritus from the bottom into its mouth.

The mouth of Cryptolithus species is at the rear of the small hypostome in the ventral part of the cephalon. The stomach occupies the glabella, and digestive capability extends out into the genal area. The alimentary track lay along the axis of the thora-copygidium and occupied about 20% of the width of the axial rings. The anus is at the posterior point of the pygidium.

Fortey and Owens (1999) described Cryptolithus species as filter-chamber feeders, meaning that the resting trilobites stirred the sediment directly under them, using the cavity between the cephalon and the substrate, and filtered out the food particles from the rest of the suspended sediment. Fortey and Owens also suggested that the cephalic perforations provided channels for water to flow out of this cavity while it was swept forward by the appendages. This flow brought the food particles forward to the mouth and kept oxygenated water flowing over the exopods.

As in all specimens of trilobites with preserved ventral appendages, the biramous limbs of Cryptolithus included walking legs, endopods, and outer branches, exopods, which may have served as a breathing organ as well as provided some ability to sweep the surface under the trilobite. The exopods do not extend beyond the border of the thoracic segments. There are three pairs of appendages under the cephalon and one pair under each thoracic segment. Under the pygidium there are a large number of much-reduced appendages, reflecting the number of fused segments in the pygidium.

Assuming the relationship of the appendages to the exoskele-ton is correct, it is hard to see members of the genus Cryptolithus as active surface crawlers.

On the whole, one can assume that trilobites initially occupied many of the environmental niches that are occupied today by modern marine arthropods. The ecological niche for the individual species enabled trilobites to become part of a paleocom-munity. Within this community the trilobites shared the local environment with a variety of other animals and plant life. Wherever one finds the necessary associations within the rocks, one can expect to find the other members of the community. This is no different than what is seen in extant, and undisturbed, communities today. Some trilobites, such as the illaenids, are found in a fairly narrow range of environmental conditions, and some like in the genus Isotelus are found in a wide variety of communities from shallow nearshore areas to deep ramp-basin transition areas. Fortey (1975) defined communities in the northern Europe Ordovician. Two of his communities, the cheirurid-illaenid and the olenid, fit much of the New York Middle Ordovician very well.

The geographic restrictions of specific trilobite families and genera also are used to identify paleobiogeographic provinces. Whittington (1961b), Whittington and Hughes (1972), and Ross (1975), for example, used this approach to define the early positions of the paleocontinents.

The early evolutionary success of trilobites was probably due to their development of a mineralized exoskeleton and their wide geographic dispersal. The number of trilobite genera increased from their first appearance until the Late Cambrian and then decreased continuously through the Paleozoic. This is graphically shown in Ludvigsen (1979b, Figure 9). A number of explanations for this have been proposed. The number of potential predators continually increased during these same times. It is likely that the increased predation, increased number of competitors for environmental niches, and the burden of molting to a completely defenseless individual all played a part in the reduced success of trilobites through time. Trilobites disappeared completely at the end of the Permian.

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Responses

  • geronima
    What defense mechanism did trilobites develope against predators?
    6 years ago

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