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CHAPTER 1
INTRODUCTION TO ICHNOLOGY OF THE GEORGIA COAST
THE MYSTERY OF THE BROKEN BIVALVE
The large clam fell, however improbably, from a great height above the sandy tidal flat. Three fragments of its thick shell lay in front of me: an entire valve and a small part of the other were still connected by a hinge, and the other chunk was nearby. The main part of its fleshy body was now mostly gone, as were several more pieces in this once-living puzzle. Considering that bivalves are not often prone to aerial activities or spontaneously exploding, nor are they likely to will their soft parts to disappear, I wondered what else might have caused such a fatal mishap that fine spring day on Sapelo Island of coastal Georgia (Figure 1.1).
A glance at the sandy surface revealed a few clues: the impression where the shell bounced on the hard-packed sand surface; the dispersal of shell fragments from that impact site; the remaining soft parts of the bivalve, partially eaten; and the tracks around the semiconsumed remains. Whose tracks? It was two legged, and its tracks started with the feet nearly together, left in front of right, and none before these. After this, it alternated its feet as it walked, left–right and right–left in a tight, linear path to the aged about 10 cm (4 in) between each step, and decreased as it approached the bivalve. In these tracks, its digits pointed slightly toward the midline of the body, giving the trackway a pigeon-toed appearance. Its feet came together again and just to one side, followed by much shuffling in the same small area. The tracks were about 5 cm (2 in) long and almost as wide. These indicated feet with three narrow digits, widely spaced but all pointing forward, and a thin, curved line denoted webbing toward the ends of these forward-facing toes. A nub of a toe, its mark rarely seen, was at the rear of the tracks, and faint claw marks accented each digit.
I expanded the search for more clues. Spiraling outward from the shell remains, I looked for other indentations made by the clam bouncing on the sand flat, and found two more. One of these marks was near a shallow excavation, a vertically oriented hole in the sand that matched the external profile of the entire bivalve. This is where the clam had started its airborne exploits. Within a meter (3.3 ft) of the bivalve resting trace were the paired tracks of the same bipedal animal seen at the scene of the bivalve's demise. The same trackway pattern was there, starting with two tracks, changing to slow, pigeon-toed walking, and ending with side-by-side tracks directly in front of where the bivalve had its last moment of peace. Prod marks on one side of the bivalve resting trace showed where the trackmaker had, with much persistence, pried out the recalcitrant mollusk with a hard, sharp-edged digging tool (Figure 1.2).
The trackmaker was the laughing gull (Larus altricilla), the bivalve was the giant Atlantic cockle (Dinocardium robustum), and the traces there on the sand flat clearly told the story of how the cockle met its end through the actions of the gull. Well before my arrival on the scene, millions of years of evolutionary history contributed to a present-day behavior that had preserved the cockle's lineage. That past, though, was now meeting the outcome of natural selection manifested in the gull. The tide had dropped in the preceding few hours, leaving the cockle stranded on a tidal flat drained by nearby runnels and water trickling down between grains of sand. As the tide began to ebb, the cockle used its muscular foot to extend, anchor, and pull the shell into the still-saturated sand — a simple form of burrowing that exposed a bare minimum of its heart-shaped outline. As pore waters continued to drain from the sand flat, sand grains pulled together around the cockle, further securing its position as it waited for the next tidal cycle. Unfortunately for the prey, but fortunately for the predator, the partly buried clam was spotted from the air by the gull, which translated this search image to "food." The gull landed nearby, walked up to the cockle, and wedged it out of its formerly safe burrow with its beak. Having no hands, it used its beak again to grasp the cockle on the edge of its shell and took off with its payload. Perhaps as much as 10 m (33 ft) above the tidal flat, it let go of the bivalve. It did not break. A second time the gull landed, picked up, flew, and dropped its intended meal, but still with no satisfactory results. The third time was the proverbial charm, and the once-protective shell of the hapless clam fractured on the sand flat's surface. The gull's strategy was keen. Through a combination of interstitial moisture and gravity, the low tide had caused the fine-grained sand to pull together in such a way that the intertidal surface became nearly as solid as concrete. Ironically, the same sedimentary circumstances that initially helped to protect the cockle from predators later served as the instrument of its death.
But was this an isolated instance, a singular circumstance of a gull genius? (How difficult it is to say those last two words sequentially!) The shortest possible answer to that question — no — connected to other stories that confirmed this inventive, rapacious behavior in birds. For example, just two years before encountering this scene on the Georgia coast, I saw newly broken shells of freshwater gastropods on a paved road in Everglades National Park. At first I thought cars had crushed them, but I looked closer and noted that the fracture patterns were more from point impacts, and the remaining soft parts held distinctive beak marks. I modified my hypothesis: perhaps these snails were nicked by passing cars and scavenged by birds that saw or smelled the freshly killed food. The final modification of the hypothesis happened as soon as direct observation of a fish crow (Corvus ossifragus) provided enlightenment. The crow flew down into the freshwater marsh, emerged with something in its beak, flew up, and let go of the object about 10 m (33 ft) above the road. Crack! Gastropod sashimi was being served within minutes, and the idea that birds would use hard surfaces as tools for gaining food entered my consciousness as an idea to hold, remember, apply, and test elsewhere. Indeed, soon after seeing the results of this avian predation in both Florida and Georgia, I investigated the peer-reviewed literature and found that other scientists had observed and documented the same behavior in various species of gulls, crows, and ravens (Zach 1978; Beck 1980; Kent 1981; Cadée 1989; Gamble and Crisol 2002). In other words, I was not alone in my witnessing of such shell-breaking behaviors. In fact, some of these scientists even proposed how these techniques represented a sort of avian culture, wherein birds of the same species watched a few individuals successfully procure molluscan flesh by dropping shells, then imitated it enough that it spread from there, perhaps over generations (Zach 1978; Gamble and Cristol 2002).
Now let's put on our stylish paleontologist hats and project these modern observations into the future, perhaps long after our species, or that of the cockle and gull, have gone extinct. If just the pieces of the bivalve had been found in the fossil record, would we have known it was the victim of predation? Even if we noticed that its shell had unusual breaks imparted when the animal was still alive, would we have even guessed that its shell was broken by its hitting sediment — not rock — from high above a sand flat? How would we have inferred the identity of the predator, and its methods for acquiring and killing its prey? If only the bivalve's burrow were found, would we have been able to tell that a predator forcefully extracted its occupant? What about the bounce marks made by the bivalve — could we have recognized them for their true identity as traces of predation?
Welcome to the science of ichnology, the study of life traces, and one of the best places in the world to study it, the Georgia coast in the southeastern United States. The Georgia barrier islands collectively reflect a setting for many overlapping cycles of life and death, all leaving clues in sand, mud, plants, shells, and bones for us to read, understand, and project into the past and future — that is, if our senses are trained well enough to discern the many stories told by these traces, and if our imaginations can envision the intersecting lives and behaviors of plants and animals that make these vestiges.
WHAT IS ICHNOLOGY, AND WHAT IS A TRACE?
Ichnology is simply the study of traces (from ichnos, "trace," and logos, "study"), where a trace is any indirect evidence of an organism exclusive of body parts that also reflect the organism's behavior (Frey 1975; Ekdale et al. 1984; Bromley 1996; Seilacher 2007). Ichnology can be divided into neoichnology, the study of modern traces, and paleoichnology, the study of ancient traces; ancient traces are known more commonly as trace fossils or ichnofossils. The majority of this book is devoted to neoichnology, although toward its end, some of that focus is also applied to the study of trace fossils (Chapter 10). What are some examples of traces? Tracks, trails, burrows, nests, feces, borings, and tooth marks comprise a few types of traces, but virtually any mark left on a medium of some sort (sand, mud, shells, bone, flesh) caused by the behavior of a living organism can constitute a trace (Figure 1.3). In contrast, dead organisms, appropriately enough, do not behave; hence these do not make traces.
Although this might be the gentle reader's first encounter with the word, ichnology is probably one of the oldest sciences known to humankind. This science developed out of our evolutionary past through the recognition and classification of animal tracks and other signs, as well as interpreting animal behavior from these traces, otherwise known simply as tracking (Liebenberg 2001). In a recursive way, humans left their own traces, reflecting their awareness of animal traces, where petroglyphs and other markings on rock perfectly mimic the forms of animal tracks, trackway patterns, and nests. For example, rock art from Australia and South Africa from tens of thousands of years ago includes track iconography identifiable to local species, some of which are now extinct (Flood 1997; Eastwood and Eastwood 2006). The main purposes of such art were likely similar to those of this book: to increase consciousness of tracks and other traces in surrounding environments, as well as to teach future generations about the tracemakers and behaviors that led to the given traces. In other words, ichnology is a part of our human heritage, and learning about it ought to be as natural as following a series of tracks on a sandy beach.
Ichnologically speaking, though, traces are treated as separate from a few other types of indirect signs often associated with traditional tracking. For example, when encountering feathers, fur, skulls, and other body parts in the field, ichnologists do not consider these as traces per se, but as the actual remains of animals. Similarly, a leaf impression seen in an urban sidewalk, formed by a leaf falling onto wet cement, is not a trace, because it only represents a plant body part, rather than actual behavior of the plant. In short, anything constituting a body part and not reflecting behavior from a living organism is not a trace. Nonetheless, body parts can certainly contain traces of the behavior of another tracemaker. For example, the investigative story I used at the start of this chapter illustrates this distinction: the bivalve shell itself is a body part, but the broken shell reflects the predatory behavior of the gull. So it is both a body part (of the bivalve) and a trace (of the gull). Similarly, a scattered assemblage of songbird feathers encountered in the middle of a maritime forest is both a collection of body parts from that songbird and a trace of a raptor feeding on the songbird (Chapter 8). Raccoon feces packed with fiddler crab parts are a trace only of a raccoon, unless one wants to count fiddler crab gullibility and helplessness as traces (Chapters 6, 8). The same leaf impression in a sidewalk mentioned previously may also contain evidence of incisions along its margins and within the leaf, traces of bees, beetles, or other insects that fed on the leaf (Chapter 5). The plant also reacts to these attacks by toughening the tissue around the wounds; this wound response, then, is a trace of the plant's behavior (Chapter 4). In other words, traces are commonly preserved in or otherwise associated with the bodily remains of other organisms, especially traces that involve the consumption of another organism.
A lesson I often use in teaching my students the distinction between traces and other natural phenomena (what is a trace and what is not?) is told from a forensic perspective: When a person drags the dead body of a murder victim across a sandy area, what traces are left, and who is the tracemaker? After some brief debate and argument, the answers emerge. The dead body may have made the drag marks, but the body itself would not have formed the traces (zombie-movie scenarios aside) without the behavior of the person moving it. Hence the body was only a tool (leaving a tool mark), and the resultant drag marks coupled with footprints are traces of the person doing the moving, not of the victim. Furthermore, the tracemaker may not be the murderer; he or she is simply moving the body after someone or something else caused the death (Figure 1.4a). Another "trace or not a trace?" riddle is posed by a stalk of grass rooted in a sand dune that may have been pushed in different directions by the wind, then whipped back to its former position, leaving intricate impressions around the stalk (Figure 1.4b). This movement was not a result of plant behavior but was caused by the wind, so these marks are not traces either. Traces thus must involve behavior by a living tracemaker, and to emphasize this point, I will occasionally refer to them, somewhat redundantly, as life traces. With enough practice and application of healthy skepticism — that is, trying to prove yourself wrong, the trait of a real scientist — the distinction between traces and nontraces will become even more obvious.
Of course, what really makes a trace (or not) is a tracemaker, and a threefold line of integrated inquiry must be used to understand traces: (1) What is (or was) the medium (substrate) hosting the trace?; (2) What aspects of the tracemaker's anatomy were used to make the trace?; and (3) What behaviors by the tracemaker caused its anatomy to interact with materials in a way to leave a trace? In homage to my Catholic upbringing, I often call this mode of study the Holy Trinity of ichnology, but in a more ecumenical spirit, as well as to honor T. E. Lawrence, a fellow appreciator of sandy substrates, I also call it the Three Pillars of Ichnologic Wisdom. Regardless of how one remembers these tenets, this triad can be used a basic guide to understanding traces, from which novices and experts alike will benefit.
THE THREE PILLARS OF ICHNOLOGIC WISDOM: SUBSTRATE, ANATOMY, AND BEHAVIOR
As just mentioned, the gestalt of any trace is defined by the interaction of three factors: (1) the substrate preserving the trace; (2) the anatomy of the tracemaker; and (3) the behavior of the tracemaker. If any of these three are missing, a trace will not be made, nor will it be preserved. Accordingly, we will also not detect it with any of our senses, no matter how well honed our observation skills, catalog of search images, cognitive abilities, or other means of detection.
In sedimentary geology, a substrate is typically some sort of sediment, such as clay, silt, or sand. Likewise, these substrates are the ones we most commonly associate with ichnology. Other substrates include rock, shells, wood, or bones; these types of traces require scratching, breaking, drilling, or other such alterations of these consolidated media. Substrates, however, can be even more ephemeral. For example, I have seen wakes left by alligators in ponds that persisted for several hours after an alligator swam through algae floating on the pond surface. In such instances, the algae comprised the substrate (Figure 1.5). For many animals, pheromones and scent markings are traces used to communicate with other species, creating invisible olfactory landscapes (Chapters 5, 8). For example, owners of domestic dogs and cats are often conscious of how their pets always seem eager to detect scents. These animals also leave their own distinctive calling cards for others of their species through body rubbings, urine, or feces, announcing, "I was here!" Perhaps the most fleeting of traces, though, are sounds, in which compressional waves, imparted on air, received by ears, and translated by brains, vanish instantly after the vibrations alter their substrates, whether these are air, water, wood, or animal bodies. (Or is the brain a substrate changed by the sound, thus preserving it as a more lasting trace? Ponder this thought as it continues to alter your mind.) In ichnology, then, a substrate is any medium that holds the effects of an organism's behavior, however long that effect might last. The Georgia coast and some other coastal areas of the world thus can become ichnological playgrounds, filled with a surfeit of substrates that are constantly shifting with deposition, erosion, and other transformations inherent to coastal systems (Chapters 2, 3). Yet some are also preserved in just the right medium to become part of the geologic record, thereby becoming trace fossils (Chapter 10).
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Excerpted from "Life Traces of the Georgia Coast"
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Copyright © 2013 Anthony J. Martin.
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