Trace Fossils

Biology, Taxonomy and Applications

Richard G. Bromley

• Publisher: Routledge
• Year: 2012
• ISBN:
  9781135076078
• Edition: 1
• Topic: Biological Sciences
• Subtopic: Biology
• Index: Biological Sciences

The new edition of this work includes an appendix listing criteria for the identification of ichnotaxa. It covers all aspects of tiering trace fossil diversity and ichnoguilds, and is aimed at advanced undergraduates and postgraduates in palaeoecology, paleobiology and sedimentology.

PART ONE

Neoichnology

1	Animal-sediment relationships

1.1 Why do animals burrow?

Animals disturb sediments for many reasons. Some do it accidentally, as the crab or quadruped leaves surficial footprints. At the other end of the scale are animals that spend their entire life constructing intricate structures as part of their feeding processes. The specialist endobionts have learnt to employ various properties of their substrate to serve a wide spectrum of the essential requirements of life: respiration, feeding, reproduction and protection.

1.1.1 Protection and concealment

Perhaps the most basic advantage for an animal that can establish itself beneath the depositional interface is the protection this confers from the above-sediment environment. In turbulent water conditions, and in environments that are susceptible to periodic sub-aerial exposure, the endo-benthic environment is far less rigorous than the epibenthic. A sun-baked beach exposed at low tide may seem a barren place, but a little spade-work may reveal it to be richly populated beneath the surface.

In addition to physical protection, endobenthic animals are concealed from potential predators, in particular those that hunt by sight. The so-called ‘resting traces’ so common in the photic sea floor largely represent such hiding traces; it is safer to be just covered with sediment than exposed on the sea floor. Deeply or rapidly burrowing animals find refuge from shallowly or slowly burrowing predators. However, the tables may be turned. Many predators (fish, crustaceans, polychaetes) put concealment within a burrow to their advantage and wait in ambush for passing epibenthic prey.

1.1.2 Respiration

Another advantage of residence within a cylindrical burrow is the relative ease with which water may be circulated through it. Different species bring different pre-adaptational mechanisms to work for this purpose: worms send peristaltic contractions along their elongate bodies and thus squeeze rings of water past them; crustaceans and fish modify swimming organs to waft a current.

In either case, a strong current can be produced using little energy, bringing quantities of water past the respiratory organs. Outside the burrow, the same activity would pass hardly any water over the gills. Thus, although penetration into the sediment brings the burrower into regions of lowered oxygen tension, irrigation using bottom water removes this problem. Indeed, the hydraulic efficiency of canalization allows bur-rowers to live at lower oxygen levels than most epibenthic animals (Sas-saman and Mangum 1972).

1.1.3 Suspension feeding

The current produced so easily within a burrow can also be used for gathering suspended food particles. Many different devices have been developed by suspension-feeding burrowers to intercept the seston suspended in the water current, including tentacles, sieves created by hairs on limbs, and nets of slime (section 3.3). Many epibenthic suspension feeders depend on natural currents to bring them food, and spread delicate catchment devices that are exposed and vulnerable. Burrow dwellers control their own current and have protection for their filtering equipment. They may thus colonize environments that are unsuitable for epibenthic suspension feeders.

1.1.4 Deposit feeding

By far the most endobenthos are deposit feeders, working the sediment for its contained nutrients. A host of diverse niches is available to multifarious specializations, and strategies are numerous within this way of life (sections 3.5, 4.1.1, 4.2.2 and 4.3.2). Few organisms engulf the sediment whole and uncritically, and digest what they can of it. Almost all are selective, extracting those grains that carry special organic components. Many animals digest microalgae and the bacteria that are taking part in decomposing the organic material (Taghon et al. 1978; Newell 1979; Hauksson 1979). Together with organic exopolymeric substances, these organisms form a biofilm covering grain surfaces (Westall and Rince 1994). Several of these deposit feeders find such material well below the redox level (RPD), deep beneath the depositional interface. Unlike seston and surface detritus, supplies of which fluctuate temporally, the organic content of the sediment constitutes a relatively predictable food source (Levinton 1972).

The enzymes available to deposit feeders are limited, and the animals may depend to some extent on gut and sediment microbes for the conversion of particulate organic matter into simpler, assimilable units. In a sense, the substrate can function as an ‘external rumen’ for deposit feeders (Rice and Rhoads 1989; Levinton 1989).

1.1.5 Surface detritus feeding

In aquatic environments, the richest supply of organic matter is that at the depositional interface – immature, copious and continually renewed. Many vagile epibenthic animals exploit this food resource. If supplies are renewed rapidly, however, they can also support stationary detritus feeders, and these generally avail themselves of the advantages of an endo-benthic domicile. Many species of sipunculan, echiuran and annelid worms, bivalves and crustaceans have adopted this mode of life, protected beneath the sediment surface and feeding radially around the burrow aperture or trapping detritus in a funnel (section 3.4.3).

Surface detritus feeding is not a universally recognized category. Ecolo-gists tend to view detritus feeders as either collecting their food in situ as deposit feeders, or resuspending it and catching it as suspension feeders. Moreover, many ecologists use detritus as a synonym for particulate organic matter, whether at the surface or buried (de Wilde 1976). Most phytodetritus arrives on the ocean floor pelagically as a surface layer (Billett et al. 1983; Suchanek et al. 1985; Rice et al. 1986; Carney 1989). Much less commonly it arrives in a turbidity current and is immediately buried (Reichardt 1987).

From a sedimentological point of view, feeders on the organic accumulations at the depositional interface represent a specialist group that is important in community and trace fossil analysis. Thus, the biologically impure group is separated here as a trophic unit. In this book, ‘detritus feeders’ are ‘surface detritus feeders’.

1.1.6 Gardening

The search for suitable bacteria may be alleviated by actually culturing them (or other food organisms) on burrow walls or on deliberately accumulated organic matter in chambers within the burrow. The activity of burrowers within the sea floor in any case increases the microbial activity in the sediment (e.g. Andersen and Kristensen 1991). Gardening is said to take place if the activities of a burrowing animal promote the production of microbes in such a way that this positively contributes to the food resources of the animal (de Wilde 1991).

As leaf-cutter ants culture fungi on leaf clippings in the terrestrial realm, so do some shrimps apparently cultivate bacteria on sea-grass cuttings (section 4.3.4) and some annelids and echiurans may culture organisms on their burrow walls (section 3.3.3).

1.1.7 and the opposite

A thin scattering of endobenthic animals, distributed over many phyla, produce highly toxic halogenated compounds (Woodin et al. 1987). These compounds occur in the body and are also commonly concentrated in the burrow wall. The toxins reduce or prevent the development of microbes and other fauna in the burrow walls, as well as acting as a deterrent to predators. One group, the enteropneusts, have taken this poisoning seriously (section 3.4.5).

1.1.8 Chemosymbiosis

Chemosymbiosis was first detected in deep-sea vent habitats, where communities of pogonophore worms and large bivalves have formed an association with autolithotrophic bacteria. These bacteria can oxidize HS^- from the vents, thereby allowing the fixation of carbon and production of carbohydrates and enzymes for the growth and metabolism of the host (e.g. Hand and Somero 1983; Felbeck et al. 1984; Jannasch 1984). Bacterial chemosymbiosis has since been recognized in other habitats where HS~ or methane occurs in close proximity with oxygenated water, as it is in the soft sea floor generally, and particularly around natural gas and petroleum seeps (Paull et al. 1984; Hovland and Thomsen 1989).

Some host animals carry the bacteria within their bodies, such as the lucinacean bivalves (Fisher and Hand 1984; Reid and Brand 1986). Others use a burrow to house the bacteria, usually in the wall, or they dig a deep ‘sulphide well’ into the anoxic sediment. These burrow modifications are exercising the imagination of ichnologists, and the trace fossil Chondrites has been convincingly reinterpreted as the work of a chemosymbiont (Seilacher 1990; Fu 1991).

Oxidation of such sulphide exploits energy that would otherwise be lost from the system since it occurs beneath the RPD. The symbiosis with invertebrates extends the habitat available for bacterial oxidation of sulphide, thus benefiting the bacterial side of the association.

1.1.9 Predation

Endobionts themselves constitute a food source. Some fossorial predators trap vagile endobenthos as the mole does earthworms in an extensive open-burrow system; others actively burrow and seek less mobile prey, as do priapulid worms and some carnivorous snails. Many predators find it more effective to search epibenthically, and then burrow down to their hidden prey as they find them, as do some rays and starfish (Smith 1961; Mauzey et al. 1968; Veldhuizen and Phillips 1978). Such epibenthic predators may cause considerably more disturbance of the substrate than do their truly endobenthic prey. As mentioned before, yet other predators produce permanent burrows in which to lie in ambush (section 4.4).

Finally, many sedentary burrowing animals are subject to browsing predation, whereby upper ends are browsed off or harvested by epibenthic predators without killing the prey animal. The lost parts are later regenerated (de Wilde 1976; Woodin 1982; section 4.1.2).

1.1.10 Reproduction and trauma

Special chambers for brooding young have been described, especially in the case of insects (e.g. Wohlenberg 1939, fig. 34; Frey and Pemberton 1985, fig. 10); their morphology is extremely variable. For example, the nests of termites make fine trace fossils (section 9.2.10).

Panic structures may be produced by animals escaping from predators or from sudden burial (section 9.2.9). In other cases, an animal may quietly shift its burrow upwards, adjusting its position in response to a gradually accreting sea floor (section 9.2.8).

1.1.11 Other behaviour

Endobenthic animals create sediment structures through still other behavioural traits than these, and the picture is further complicated by the fact that many broadly specialized burrowers exhibit several different feeding styles, adapting to changing environmental conditions (Cadée 1984). Indeed, individual burrow systems may comprise different parts that relate to different life activities. Thus, although it is not always possible to relate trace fossil morphology directly to animal behaviour, it is nevertheless possible to classify trace fossils in broad behavioural categories (section 9.2).

1.2 How do animals burrow?

As state...

Table of contents

• Cover Page
• Title Page
• Copyright Page
• Contents
• Preface to the second edition
• Preface to the first edition
• Introduction
• Part One Neoichnology
• Part Two Palaeoichnology
• References
• Glossary
• Index