How to Dig a Hole
How animals dig holes has long been a matter of keen interest to paleontologists. Often, the only evidence of an animal's existence is a hole, burrow, or track it has left, a so-called trace fossil. Paleontologists are very skilled at squeezing the maximum amount of information from the slightest bits of evidence. By understanding how living animals dig holes and by correlating what they know about today's diggers with the holes that extinct animals have left behind, paleontologists are able to make remarkably detailed inferences about the lives of long lost animals.
Paleontologists specializing in trace fossils (ichno-paleontologists, as they call themselves, from the Greek, ichnos, for "footstep") have identified at least five common methods animals use for digging holes. These are (1) intrusion, (2) compression, (3) excavation, (4) backfilling, and (5) fluidizing.
Intrusion is probably the simplest and most primitive of all hole-building techniques, because it involves no development of new behaviors or specialized devices for digging. The animal simply intrudes itself into a substratum, usually a watery mud or clay that offers little resistance to progress of the animal's body. Once in the sediment, the animal literally swims through the substratum, using the same behaviors and devices that enable it to swim through water. Usually, intrusion does not result in a permanent burrow being left behind: the tunnel in the substratum made by a swimming animal simply collapses behind the animal as it progresses. Many nematode and polychaete worms and molluscs use intrusive digging methods to burrow into soft aquatic muds.
Compression burrowing is like driving a blunt dowel through mud. The animal forces its head (or at least the part of its body that is in front) into the substratum, simultaneously pushing sediment to the side and compressing sediment in front. As a result, the substratum at the leading edge and at the sides of the burrow is more compact and harder than the undisturbed sediment. Unlike intrusion, compression requires a specialized skill: the animal must be able to anchor at least one point of its body so that the forward compressive force will not simply push the body away in a "bounce back" effect. Consequently, compressive burrowers must be able to execute a complex series of movements. A typical compression bur-rower, for example, progresses through the substratum by executing a repeating sequence of push-pull movements. First, the rear of the body is formed into a penetration anchor, which holds it in place while the forward part is thrust forcefully into the sediment. Then, the forward part expands into a retraction anchor that holds the front part in place while the rear is pulled up. This is probably the most common method of burrowing among marine and aquatic invertebrates.
Excavation burrowing is still more complicated. It is common among animals that have articulated joints (like arthropods and vertebrates) and some kind of a hard digging edge, like a nail, tooth, or claw. Excavation is a method favored in soils relatively more compact than the soils that favor intrusion or compression. An excavator must detach a bolus of material from the substrate and then transport the spoil elsewhere. Excavation opens the possibility of creating burrows of complex shapes. For intrusion and compression burrowers, the burrow is typically a hollow tube. An excavation burrower can open up more complicated underground shapes, like branching burrows, chambers for turning around and other maneuvering, and galleries for storage of spoil or food or for living room for offspring or mates.
Backfill burrowing, like intrusion burrowing, typically does not leave behind an open burrow. It differs from intrusion burrowing in that it requires the development of new structures and behaviors. A backfill burrower picks up material ahead of it and transports it, as if by a conveyor belt, to its trailing edge, where the spoil is deposited. The animal's forward progress, therefore, is something like moving on a caterpillar tread. The transport mechanisms may be peristaltic waves on a soft body or cilia, but most commonly soil is transported by the tube feet characteristic of the echinoderms (starfish, urchins, sea cucumbers). Backfill burrowing is unusual in another respect. Intrusion, compression, and excavation burrowing are all most efficiently done when the burrow created is as narrow as possible. Consequently, these types of burrowing favor body shapes that are thin cylinders.
Backfill burrowing, on the other hand, is most efficiently done when the surface area of the "conveyer belt" is greatest. Thus, backfill burrowers tend to be broad and flat.
Finally, fluidizing burrowing involves elements of both intrusion and backfilling. It works by exploiting a phenomenon that is familiar (or at least should be) to most Californians, or to anyone who lives along an active earthquake fault. Soils and other substrata can, if they are shaken hard enough, become "fluidized." Put simply, a soil is fluidized when the forces that normally make soil grains stick together are overcome by some other input of energy. When this happens, the soil now flows as if it were a fluid, like air or water. In an earthquake, the source of energy is the kinetic energy released by the slippage of an earthquake fault. A burrower may fluidize soil by releasing kinetic energy by violent contractions of muscles. For example, some worms will, when disturbed, engage in violent peristaltic movements of the body. This sets water in motion, which helps fluidize the substratum, allowing the worm to sink rapidly into the mud. Similarly, many clams that live in otherwise fairly hard sediments, like mud flats, will, when disturbed, eject jets of water from their siphons. The mud surrounding it is then fluidized, and the clam literally swims away through the mud. If you have ever done any clamming, you will have seen this remarkable technique in action.
of trace fossils first appearing during the period (Table 6.1). There are two major bursts of diversification of burrows and burrow types, one during the Ediacaran period, roughly 650 million years ago, and one at the beginning of the Cambrian, roughly 570 million years ago. These parallel contemporaneous increases in the diversity of body types among animals (Box 6A). That these changes were probably steps in an evolutionary arms race is suggested by contemporaneous increases in the rates of extinction of new burrow types at this time: there was more testing of "novel designs" for burrows (Table 6.1).
The increasing complexity of Precambrian burrows also shows what some of these measures and counter-measures were, and how effective they were. Obviously, if a particular type of burrow "worked," the creatures that built it survived and left descendants. Trace fossils left by them would appear over a long period of the fossil record. Conversely, a "failed" burrow type would not persist in the fossil record for very long. The difference between success and failure seemed to hinge on three factors. First, burrows began to turn vertically and deepen during the Ediacaran period, and they persisted in this trend until the beginning of the Cambrian. Vertical burrows presumably put the animals building them out of harm's way more effectively than the shallow, horizontal burrows that preceded them. The second was the development of hard body parts made from calcite (in the case of the molluscs), chitin (in the case of the arthropods), and cartilage and bone (in the case of the chordates and vertebrates). These could serve as digging tools, enabling the animals possessing them to burrow more effectively into deeper and harder sediments. This trend began just before the dawn of the Cambrian period and peaked during the early Cambrian years. Following it was the third factor in success, namely the development of more complex burrow types: simple vertical shafts evolved into branched and ramifying networks of tunnels, some with complicated systems of conduits, false entrances, and cul-de-sacs.
The Greatest Ecological Catastrophe of All Time Burrows in muds do physiological work for the animals building them because of a legacy of events that occurred roughly two billion years ago. Let us now turn our attention back to that time, long before there were any animals.
I sometimes ask my students to identify the greatest ecological catastrophe of all time. Usually, they point to some recent newsworthy event, like the wreck of the Exxon Valdez in Prince William Sound or the nuclear disaster at Chernobyl in Ukraine. Some point to the extraordinarily high current rates of species extinction, presumably driven by human encroachment on
Table 6.1 Patterns of appearance and extinction of trace fossils during the transition between the Precambrian and Cambrian periods (Crimes 1994). Time frame is in units of millions of years before present (Haq and van Eysinga 1998). Percent extinctions is calculated with respect to the total number of genera present. The two bursts of diversification that are supposedly the result of an arms race are indicated by bold type.
(million years First Total Percent
Period before present) appearance Extinctions genera extinctions
Upper Cambrian Middle Cambrian Trilobite-bearing lower
Cambrian Pre-trilobite lower Cambrian Post-Ediacaran Precambrian Ediacaran Varangerian Riphaean
4 36 11
10 35 29
tropical forest ecosystems. Sometimes they refer to the asteroid that collided with the Earth about 65 million years ago and exterminated the dinosaurs (and a whole lot more). But they're all wrong, in my view. I would put my money on events much earlier, about two and a half billion years ago, when certain bacteria, blue-green algae specifically, learned how to use light to strip hydrogen off water and combine it with carbon dioxide to make sugars: in short, the origin of photosynthesis (Fig. 6.1).
Pointing to photosynthesis as an agent of catastrophe seems rather strange, since green plants are presently at the foundation of nearly all life on Earth. It has not always been so, however, and may be only partially true even today. For more than a billion years following life's origin, living things (all bacteria, then) relied on a diverse array of energy sources—such as atmospheric lightning, ultraviolet radiation, and complex molecules like ammonia, methane, and carbon dioxide—but left one unexploited, the virtually unlimited light streaming in from the sun. The origin of photosynthesis changed all that, because it gave the bacteria that were capable of it an immediate and insurmountable energetic advantage over their rela tively more plodding contemporaries. Consequently, photosynthetic bacteria soon became the dominant life form on Earth, and their abundance led directly to the second source of catastrophe. In stripping hydrogen atoms off water, photosynthesis produces oxygen gas (O2) as a by-product. As photosynthesizers became abundant, they churned out ever increasing quantities of this waste gas. At first, the impact was not great, because the oxygen could be absorbed into solution in the oceans and bound to oxygen-hungry minerals, like iron and silicon. Once these reservoirs were full, however, oxygen began to accumulate in the atmosphere, fundamentally changing the chemistry of the planet. This was very bad news for most of the organisms then living.
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