TRILOBITE TALES -Jack R

TRILOBITE TALES -Jack R. Holt TREASURE At last I found a trilobite. The rock simply parted around the animal, like some sort of revelation. -Richard Fortey I was a college student at a school in Arkansas during the first years of the 1970’s. Then, I majored in Biology but I had become completely absorbed in collecting fossils which were plentiful in the Ozark Plateau. These were mainly marine deposits from the mid Paleozoic. Some of the locations were very rich with abundant and diverse forms of extinct animal remains. Some, like clams and snails, were familiar. Others like crinoids, nautiloids, and brachiopods, were even more common and became familiar. I came to know them well and could recognize them from tiny fragments or exposed parts. However, trilobites (see Figure 1), more than all of the other organisms, fired my imagination and kept me looking. FIGURE 1. Phacops from Devonian beds of central Oklahoma. The scale is in centimeters. They attracted me in part because they were relatively rare and because they were so exotic in appearance. They occurred almost always as dissociated parts, particularly the tail or pygidium. However, parts of the head also survived, but more rarely. These so occupied my attention that I ordered books on fossils and then the great volumes O and P on Trilobites and related animals from the Treatise of Invertebrate Paleontology (1959). I struggled through the two volumes and attempted to master the terminology as well as I 1 could. My struggle was compounded by the frustration that there was no one at my college who could help me with the technical terms. The foundation of my frustration was that a particular fundamentalist perspective determined the college curricular philosophy. Still, I searched for fossils and attempted to explain their occurrences and meanings. By the end of my fourth year there, I had amassed quite a nice collection. Then, I searched for fossils as one might search for treasure. I did not really study them. I possessed them. Then they began to possess me. My fossil hunting continued through my graduate school days in Oklahoma where I collected some beautiful specimens from Ordovician and Pennsylvanian fossil beds. Apart from their exotic beauty (Yes, I find them beautiful), what is the value of a trilobite? Why would anyone spend time studying them? What secrets can they reveal? TRILOBITES AS ANIMALS Aquatic Arthropoda with preoral antennae and remaining appendages of typical or modified trilobite type, biramous appendages characterized by the presence of lateral gill branch attached to very base of walking leg. Introductory description of Trilobites by Leif Stormer in the Treatise of Invertebrate Paleontology (1959). Although trilobites have been gone for hundreds of millions of years, we can know much about them. First, we know that they existed. They were animals, in particular Arthropods which include insects, crustaceans, spiders, etc. As such they had a hard outer or exoskeleton, jointed appendages, and a segmented body. The body plan of the typical trilobite included three general regions (cephalon or head, thorax, and pygidium; see Figure 2). Also, the body of the animal had three longitudinal lobes (a central axial lobe flanked by two pleural lobes) that traversed the three body regions (thus the name tri-lobite, three-lobed animal). The cephalon was a shield-like head to which a pair of antennae attached. Also, trilobites had complex compound eyes, prominent features on most whole specimens. The pygidium was made of a fusion of the terminal segments into a terminal shield-like structure. Both the head and tail were inflexible, but the thorax was made of multiple articulated segments that allowed the animal to bend. In fact many of them could roll up in what can only be interpreted as a defensive posture (see Figure 3). The exoskeleton was made of chitin impregnated with calcium carbonate. This biomineralized skeleton made its survival as a fossil much more likely than if it had been made of chitin alone. Also, like other arthropods, the animals had to molt or cast off the exoskeleton and grow a new one at successive stages in their growth because the exoskeleton did not stretch with them as they grew. Thus, any one animal left several cast molts throughout its life. The calcium carbonate hardened skeleton and the numerous molts (5 or more) of any one individual, increased the likelihood of preservation as a fossil, a remarkably unlikely event. Because the molts from the youngest and smallest to the adult form could be preserved, we now have been able to reconstruct their somewhat complex development. The smallest instar (larval stage) had a single segment. This form was very small (around a millimeter long) and likely lived as a member of the free-floating plankton, as many marine arthropods do today. In successive molts, the animal separated the head from the 2 pygidium and then added thoracic segments, one per molt. Curiously, they seem to have added the segments from the top of the pygidium until they attained their full adult size. FIGURE 2. Elrathia, a small (about 2cm long) but very abundant animal that shows the dorsal anatomy of a typical trilobite. I have labeled the cephalon (C), thorax (T), and the pygidium (P). Note the eyes and the three longitudinal lobes. FIGURE 3. Bollandia from the Carboniferous of Oklahoma. These small animals were preserved in an enrolled position. The cephalon was clamped tightly to the pygidium in a position that protected their legs, their antennae, and the non-mineralized ventral skeleton. Some exceptional fossil beds preserved the most delicate structures of trilobites. The most famous site is a mid Cambrian outcrop in British Columbia called the Burgess Shale, which was discovered by Charles Doolittle Walcott (1850-1927; see Figure 4), 3 former head of the Smithsonian Institution in Washington, D.C. The trilobites contained in those shales had their legs, antennae, and internal organs preserved as thin carbonized films. They showed that the legs resembled those of crustaceans (shrimps and their relatives). That is, each walking leg had an additional leg-like lobe that must have functioned as a gill (the combined walking leg and gill is called a biramous leg; see Figure 5). They had a pair of biramous legs on each thoracic segment. We can infer that they used their legs to walk over the bottom mud or to burrow in the sediment. The numerous tracks and burrows that match the coexisting trilobites in width support such an inference. The legs also functioned to gather food, tear it into small pieces and transfer the pieces to the mouth, which was located on the underside of the cephalon. Again, this is similar to the feeding mode of the legs of many aquatic arthropods today. FGURE 4. Charles Doolittle Walcott posing with the Burgess Shale in 1913. FGURE 5. A rendering of a trilobite biramous leg by Sam Gon. The walking portion of the leg is jointed and has a feathery gill branch near the body. Perhaps, the strongest evidence that they were bottom-dwellers is that they had flattened bodies with eyes on the upper surface of the cephalon. The eyes were compound, some of which had thousands of tiny calcium carbonate crystalline lenses. 4 Many species lost their eyes presumably because they lived in environments where eyesight was not necessary (burrows or deep ocean sediments). Some abandoned the typical trilobite scavenger lifestyle and took to the open ocean. Their bodies were bullet- like and their eyes were huge. The phacopid trilobites (see Figures 1 and 6) had eyes with larger lenses, but fewer of them. The physics of the lensing system suggests that they had quite good visual acuity. FGURE 6. A large Phacops from Devonian deposits in Morocco. Note the relatively large lenses (see the enlarged eye in the inset). Also, the eyes were as large as the whole animals in Figure 3. LESSONS IN EVOLUTION The most erroneous stories are those we think we know best – and therefore never scrutinize or question. -Stephen Jay Gould It was while studying the Devonian phacopids, especially their eyes that Niles Eldridge (see Figure 7) documented the stability of trilobite species over periods of millions of years and then a sudden (sudden in Geological time is on the order of hundreds of thousands of years) change followed by stability, etc. In particular, he documented changes in the number of rows of the crystalline lenses of various Phacops species (see the insert in Figure 6). A species would show stability with regard to the number of rows followed by a change in number. These changes seemed to correlate with the changes in species. He realized that many other fossil groups also document stasis followed by rapid change followed by stasis. Together with Stephen Jay Gould (1941-2002; see Figure 10), he called this mode of evolutionary changed punctuated equilibrium. This met stiff resistance from the strict Darwinians. They subscribed to a view that said change occurs gradually, and, given enough time, new species appear. They argued that the appearance of stasis and rapid change was just an artifact of the fossil record. Really, gradual change had occurred. This debate has been quite acrimonious over the past 25 years, but the stasis appears to be real. 5 FIGURE 7. Niles Eldredge More recently, Ken McNamara studied the Cambrian genus Olenellus. Here, he documented true gradual change in the number of body segments in mature adults. Thus, evolutionary change might be gradual or sudden. The solution to this dilemma can be found in the realization that living things are not putty to be formed gradually or suddenly into something new.

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