Ontogeny

Ontogeny is the biological life cycle of an animal; for the trilo-bite this would be from the presumed egg to the smallest larvae and the various intermediate stages, to the end of its life cycle. Considerably more is known about the adult phase of the trilo-bite life cycle because the vast preponderance of the fossil record is composed of the pieces of the exoskeleton representing late growth stages. Careful workers have found, however, a significant amount of information related to trilobites' earlier growth stages. Chatterton and Speyer (1997) provide an extensive and current review of the present state of knowledge of trilobite ontogeny.

Trilobites most likely began life as an egg, which was either laid outside the body or hatched within the animal, although there is no unequivocal evidence of trilobite eggs. C. D. Walcott (1877c), in a study of remarkably well-preserved trilobites in New York, found spherical objects within the cross sections of some trilo-bites, which he identified as eggs. This and earlier reports about trilobite eggs by Barrande in 1852 apparently have not been investigated completely.

The well-defined phases of trilobite growth are labeled the protaspid, meraspid, and holaspid phases. Fortey and Morris (1978) described some small exoskeletons from the Lower Ordovician as a pre-protaspid phase of trilobite called phaselus. Chatterton and Speyer (1997) also found phaseluses in beds with silicified trilobite remains. It is still not clear whether these are from trilobites or another fossil arthropod.

The first well-defined phases in growth of the trilobite, after the presumed egg and the phaselus, is the protaspid phase. (The name protaspis (plural, protaspides) was given to the individual silicified exoskeletons found in or on rocks by C. Beecher (1893a, 1893b).) Protaspides are difficult to find because of their small size. The larger ones are about the size of the "o" in the printed word "protaspis," and most are between that and half that size.

One needs very sharp eyes to spot them among the normal fossil debris, or to spend hours with a stereomicroscope scanning likely surfaces. Most protaspides that have been studied are those in which their exoskeletons have been replaced by silica, since silici-fied fossils survive acidic dissolution of a limestone or shaly matrix. One then uses the microscope to pick out the important material from the insoluble residue, including protaspides. The advantage of this procedure is that in silicified material, very fine details, including spines and in some cases hypostoma, are often preserved.

The shape and other physical features of the protaspis are unique for specific fossil families and genera, and for this reason the protaspides are used for taxonomic assignments and confirmation (Chatterton and Speyer 1997). Protaspides are assigned to specific trilobites and growth series through association with pieces of the more mature fossil in the same debris and through the prior knowledge of protaspis-adult relationships. Using the presence of mature trilobites, however, is not always a secure way to assign protaspis-adult relationships.

It is probable that the pre-protaspid and protaspid phases of the trilobite growth cycle served to disperse the trilobites in their environment, similar to the situation with many modern marine Crustacea. Some protaspides were benthic (bottom dwelling) and others planktic (in the plankton), drifting in the Paleozoic seas. Because of their differing life-mode and preservation, some pro-taspides may never be found or never be associated with their later growth and adult phases.

Parts B and C in Figure 2.8 show two protaspides of trilo-bites from the Lower Devonian of New York. The one in B is a lichid (family Lichidae) and the one in C is a phacopid (family Phacopidae). They are magnified to the same scale and each is about a millimeter in actual length. At this phase of growth, morphological features, which represent a future body part in the adult, are given the profo-prefix. Thus, the protaspis has a protocephalon, described similarly to the cephalon of the adults, and a protopygidium. Trilobites also had growth stages within the protaspid phase. This is observed by increases in the number of lateral lobes on the glabellar axis and the general size (Figure 2.8A). The protocephalon and protopygidium, however, are fused and remain together in the molted protaspid exoskele-ton, although the free cheeks and the hypostome may be detached. Each successive molt is called an instar (Figure 2.8A). Whether or not there is a direct correlation between the number of instars and the number of morphologically defined phases is not clear.

When the trilobite larvae clearly begin to display the separation of the protocephalon from the protopygidium, they are referred to as being in the meraspid phase (Figure 2.8E). The segments develop from the anterior of this transitory pygid-ium. (Once the segmentation is definite, the protopygidium is no longer "proto" but is still not the final pygidium, as the thoracic segments are being formed anterior to this new pygidial

Arthropod Ontogeny

FIGURE 2.8. Ontogeny of the trilobite. A. Flexicalymene senaria protaspides from the Ordovician of New York. These silicified specimens were prepared and reported on by Chatterton et al. (1990). Reproduced with permission. B. Possibly a lichid protaspid from the work of Beecher (1893a) and reproduced by Whittington (1957). C. A phacopid protaspid from the same source as B. D. The protaspid of Isotelus gigas reported on by Chatterton and Speyer (1990). Reproduced with permission. The shape precludes this protaspid from being a bottom dweller. It probably was planktic, living and drifting near the surface of the sea. E. Triarthrus meraspid instars of degree 1, 2, and 4 from Whittington (1957). This is part of a nearly complete growth series collected and reported on by Walcott (1918). The meraspides are reproduced at about eight times their life size. F. A growth series of Isotelus gigas holaspides from Raymond (1914). The holaspids are natural size. Young Isotelus holaspides have prominent genal spines, which are lost, in New York specimens, when they reach about 50mm long. G. A molting sequence for calymenid trilobites proposed by Mikulic and Kluessendorf (2001). The trilobite on the right first pushes its pygidium into the surface as an anchor, the cephalic sutures open, and the animal crawls forward and out. The cephalic parts held in position by the ventral integument fall back together, leaving a molt with the cephalon and pygidium curved downward and the thorax in a concave curve due to the pushing up of the cranidium during the process.

FIGURE 2.8. Ontogeny of the trilobite. A. Flexicalymene senaria protaspides from the Ordovician of New York. These silicified specimens were prepared and reported on by Chatterton et al. (1990). Reproduced with permission. B. Possibly a lichid protaspid from the work of Beecher (1893a) and reproduced by Whittington (1957). C. A phacopid protaspid from the same source as B. D. The protaspid of Isotelus gigas reported on by Chatterton and Speyer (1990). Reproduced with permission. The shape precludes this protaspid from being a bottom dweller. It probably was planktic, living and drifting near the surface of the sea. E. Triarthrus meraspid instars of degree 1, 2, and 4 from Whittington (1957). This is part of a nearly complete growth series collected and reported on by Walcott (1918). The meraspides are reproduced at about eight times their life size. F. A growth series of Isotelus gigas holaspides from Raymond (1914). The holaspids are natural size. Young Isotelus holaspides have prominent genal spines, which are lost, in New York specimens, when they reach about 50mm long. G. A molting sequence for calymenid trilobites proposed by Mikulic and Kluessendorf (2001). The trilobite on the right first pushes its pygidium into the surface as an anchor, the cephalic sutures open, and the animal crawls forward and out. The cephalic parts held in position by the ventral integument fall back together, leaving a molt with the cephalon and pygidium curved downward and the thorax in a concave curve due to the pushing up of the cranidium during the process.

structure — hence, the name transitory pygidium.) In trilobites the early meraspides show a line of separation from the cephalon on the anterior margin of the transitory pygidium but no thoracic segments. This is referred to as a "degree 0 meraspis." During successive molts, as the segments detach from the transitory pygidium and can be considered separate, the meraspis grows in size and degree number. Each detached thoracic segment is counted, and this number is the degree assigned to the meraspis. The fact that segments are added from the anterior side of the transitory pygidium is ascertained by the growth of trilobites with pleural or axial spines on specific thoracic segments. The spine first appears on a segment adjacent to the transitory pygidium, and each succeeding segment is added behind it until the adult number of segments is reached.

There are usually profound changes in the shape of the cephalon during the meraspid phase. When the meraspid trilo-bite molted, the cephalon and transitory pygidium separated, and thus they are found as distinct elements, unlike the one-part pro-taspis molts. The meraspis can grow until it gains the number of free thoracic segments found in the adult, at which time it is called a holaspis.

Figure 2.8E illustrates a limited growth series of Triarthrus eatoni, part of the only nearly complete series known from a New York trilobite. The meraspides appear to be missing their free cheeks and are probably molts. Meraspids are also known for Elliptoccphala asaphoides from the Lower Cambrian of New York (Ford 1877, 1878), but articulated holaspids are rare. Parts D and F of Figure 2.8 illustrate the morphological changes in Isotelus gigas from the protaspid (Figure 2.8D) through the early holaspid (Figure 2.8F). It is surprising, given the number of adult trilobite remains found in New York, that so few examples of growth series have been recorded. Either the exoskeleton on the juvenile forms of most trilobites is too fragile and not easily preserved in the fossil record, or the juveniles did not occupy areas where their remains could be readily preserved or found.

The trilobite exoskeleton did not necessarily become fixed in its physical structure at the early holaspid phase. The achievement of the holaspid phase generally only means that no more thoracic segments were added during continued growth. There are, however, variations in the number of thoracic segments in the holaspis of a very few species; in Aulacopleura koninki, from the Silurian of Bohemia, thoracic segments were added after the holaspid phase was reached (Hughes and Chapman 1995). For all other trilobites the segment number was stable. Trilobites did not reach maturity, or at least their final body proportions, until they grew substantially from the first holaspis. In some trilobites such as Eldredgeops species, the growing holaspis changed very little and the small ones looked essentially like the adults. Isotelus gigas, on the other hand, possessed long genal spines in meraspids and early holaspides (Figure 2.8F) and did not lose the spines until it reached about one-third the size of a full mature specimen. Isotelus tergites or molt remains from specimens longer than

125 mm (5 inches) are common in the Middle Ordovician Trenton age rocks of New York.

As in all arthropods, growth in trilobites required that the exoskeleton be shed or molted at regular intervals. In modern arthropods, molting (ecdysis) begins by the formation of a new cuticle or shell beneath the current one, separation of these shells by a space filled with molting fluid, and resorption of much of the old cuticle to provide the base chemicals to finish the new one. When this process is complete, the old shell splits and the animal emerges with a soft cuticle in place. By swelling this soft cuticle through the intake of liquids, the new body rapidly becomes larger. The new exoskeleton then hardens and the animal has grown one more increment. Because of the resorption of some of the old cuticle, the molted shell is significantly thinner and more fragile than it was on the animal. This process also lowers the energy requirements of growth because the resorbed chemicals are available for the new exoskeleton.

Trilobite molts, on the other hand, are robust and at least as thick as molts attributed to living animals that died and were preserved. This information indicates that trilobites did not resorb a significant amount of the minerals of the old exoskeleton and must have emerged from the molting process with a rather thin cuticle that was little more than a soft template within which the new mineralization took place. Very thin and compressed or wrinkled trilobite fossils are known and usually are attributed to the exoskeletal remains or impressions of recently molted or "soft-shelled" individuals. There is at least one example of a trilo-bite preserved in the Burgess Shale beds that is completely unmineralized and believed to be a very recently molted individual (Whittington 1985). This finding suggests that trilobites were vulnerable to predators and external trauma for an extended time and that building the new exoskeleton required significant energy from the animal. It also suggests that as trilobites gained size, they became increasingly vulnerable during molting and that the number of trilobites that might grow to an exceptional size for the species was severely limited. Early trilobite predators such as Anomalocaris species have been identified in the Middle Cambrian, and the number of potential predators such as cephalopods and fish increased throughout the Paleozoic.

There are many possible strategies for trilobite molting, and the cephalic sutures play a part in most of them. Henningsmoen (1975), McNamara and Rudkin (1984), Speyer (1985, 1990b, 1990c), and Whittington (1992, 1997) discussed specific molting strategies in detail. Disarticulated exoskeletons are common fossils in many New York rocks. Since most of these parts are attributable to molts, significant information is gained from their examination. For example, in Eldredgeops rana, the very common trilobite of the Middle Devonian of New York, complete cephala and pygidia are the parts most often found. This evidence shows that the facial sutures were fused and that the connections between the cephalon and thorax and between the thorax and pygidium were opened or weakened during the molting process.

The animal generally emerged through the split between the cephalon and thorax.

One often-illustrated set of molt remains is the upright thorax and pygidium, with the inverted cephalon just in front of it. In this case the molting animal must have pushed forward after the cephalon and thorax parted, with the forward margin of the cephalon pushed down in the sediment. This action would pivot the cephalon molt over upside down, and the animal could then emerge with the thorax and pygidium almost intact and right side up. That the pygidium is usually found separate suggests that the connection is weakened during the molting process and sometimes separates from the thorax during or shortly after ecdysis. There are other arrangements, besides the one just discussed, of the molt remains of E. rana found in western New York. S. E. Speyer (1990c), who has studied these molt remains, summed it up well: "Trilobites, like modern arthropods, displayed a variety of moult behaviors which vary according to ecological considerations (e.g., substrate consistency) and individual convenience."

Most other trilobites, however, lose their free cheeks during the molting process. The most common fossil remains of /. gigas, for example, are cranidia, free cheeks, hypostoma, and pygidia. Separated whole cephala of Isotelus, or articulated specimens without their free cheeks are very rarely found in the fossil record. /. gigas evidently molted by the facial sutures opening and the free cheeks separating, with the suture between the cranidium and the doublure opening, possibly along with a break in the connection between the thorax and the cephalon. These breaks permitted ecdysis by the trilobite moving straightforward. This molting strategy is important because /. gigas in the early phase had quite long genal spines, and it had to be able to free them to molt properly. The most efficient way to do this was to emerge in a forward direction through the opened sutures in the cephalon.

Spines on the trilobites were hollow, and the tissue inside had to be withdrawn during molting (Figure 2.7D). Any molting strategy of an individual trilobite must accommodate the physical shape of the animal. Since the newly molted animal was unmineralized and soft, there must have been some strategy to provide some protection while the new exoskeleton became suitably mineralized and hardened.

It is not always possible to distinguish the molted parts from the disarticulated remains of a dead trilobite. A complete exoskeleton, with free cheeks, is almost always the fossil of the carcass of a trilobite. As in any rule, however, there are exceptions. Trinucleids and harpids do not have dorsal facial sutures. The dorsal cheeks and preglabellar areas are separated from the ventral doublure by a suture that runs parallel to the horizontal plane. In other words, there is a suture separating the dorsal surface from the ventral surface, and the suture line is around the edge of the cephalon. Molting occurs by the opening of this suture and the trilobite emerging forward. In trinucleids the genal spine is on the lower lamella. An articulated specimen of Cryp-tolithus without the genal spines can be reasonably assumed to be a molt (Plates 163 and 165). In some extant arthropods like the horseshoe crab, however, which uses this same molting technique, the suture can reseal after molting and the molted exoskeleton remains whole. Thus, when one finds a whole, articulated Cryp-tolithus specimen with its genal spines, one cannot say with absolute certainty it is not a molt.

Most authors agree on the generalities of trilobite molting, but some unanswered questions are rarely discussed. The roles of the ventral, unscleritized integument and the scleritized appendages have been largely ignored (but see Whittington 1992). It is not surprising that there is little preserved evidence for the ventral membrane; there is so little soft tissue evidence from the fossil record, and only one of the known soft tissue preservation sites contains any significant ventral anatomical information beyond the appendages (Walcott 1881, 1918). In the trilobites in which the loss of the free cheeks is an important first step in ecdysis, the inversion of these parts helps demonstrate the actual process (McNamara and Rudkin 1984). Brief mention is made of the possibility that the free cheeks may still have been attached to the ventral membrane and would have been inverted as they came away from the animal. McNamara and Rudkin as do others, explain the inversion of the cephalon in molt remains as evidence that the trilobite pushed its cephalon down in front while arching its thorax to break away the cephalon at the cephalothorax suture. As the trilobite continued to move forward, the cephalon inverted. Many trilobites are found with the inverted cephalon under the thorax. Many of these also have long genal spines.

Some trilobite genal spines are totally enclosed except for the area at the genal angle. For this mechanism to take place, the soft spines must be dragged from their exoskeleton prior to total inversion of the free cheeks. Such a mechanism is illustrated by Whittington (1992, Figure 9). Not illustrated is the final struggle of the animal during the ecdysis process, which drags the dorsal cuticle forward over the now-inverted free cheeks.

Based on the observation that calymenid trilobites from the Silurian of Wisconsin and Illinois are often found with the cephalon and pygidium curled somewhat down, with a distinct concave sway to the thorax, Mikulic and Kluessendorf (2001) propose that the trilobite molted by pushing its pygidium into the substrate to anchor it, by the sutures between the free cheeks and cranidium as well as the anterior cephalic suture opening, and by the animal crawling forward, leaving the old exoskeleton behind. They further propose that since the upper and lower portions of the cephalon are held in place by the ventral integument, after the molting process the parts fall back into place, leaving a molt that may be indistinguishable from a carcass (Figure 2.8G). The same process is seen in extant horseshoe crabs, which leave behind an intact molted exoskeleton.

Growth was rapid during the early holaspid phase and could be expected to slow as the animal reached maturity. In a few rare cases, long periods between molts of larger individual species have been inferred. Tetreault (1992), Kloc (1993, 1997), and Brandt (1996) observed epizoans (encrusting animals) on whole, articulated exoskeletons of several species of trilobites. Such encrusters must have been on the living animal, as articulated trilobites would not remain whole unless buried a very short time after death and epizoans would have little opportunity to become attached to the buried carcass. These observations indicate either a terminal molt, after which there is little growth in the animal with no further molting, or long intervals between molts of the mature species. Tetreault further observed that brachiopods on the exoskeleton of Arctinurus boJtoni were in four distinct size classes. From this he deduced, assuming these brachiopods spawned once a year, that the largest brachiopods were 4 years old and that in mature Arctinurus animals molting was terminal or occurred at as much as 4-year intervals.

Many collectors find populations of trilobites that are significantly larger than the norm. Excellent specimens of E. rana 5 to 6.4cm (2 to 2.5 inches) long and /. gigas 13 to 15cm (5 to 6 inches) long are not uncommon. Specimens of E. rana of 10 to 13 cm (4 to 5 inches) and 7. gigas of 30.5 cm (12 inches) and larger are found, but rarely. The largest reported articulated trilobite is an Ordovician asaphid from the Arctic. It is 72 cm (28 inches) long. This indicates that some trilobites continued growth throughout life, as do extant lobsters, and could achieve an exceptional size in favorable environments. Raymond (1931), describing an unusually large hypostome of an IsoteJus species from the Ordovician Chazy limestones, estimated the length of the trilo-bite at 61 to 64cm (24 to 26 inches). Estimates of sizes of other New York trilobites, from molt remains or partial specimens, listed by Raymond are as follows:

Basilicas

whittingtoni

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