Trace Fossils

Introduction

Trace fossils are the geological record of life’s behaviour - burrows dug, surfaces crawled across, sediment searched for food. They are not body fossils: no skeleton was converted to mineral, no shell was preserved. What survives is only the shape that an organism’s activity left in the sediment. Yet that shape is often more informative than a body fossil, because it records the conditions of the environment at the time of life rather than at the time of death. Trace fossil assemblages can reconstruct water depth, substrate type, oxygen levels, energy, and sedimentation rate with a resolution that few other sedimentary tools can match.

What Trace Fossils Are

The burrowing, boring, feeding, and locomotion activities of organisms can produce a variety of trails, depressions, and open burrows and borings in mud or semiconsolidated sediment bottoms. Filling of those depressions and burrows with sediment of a different type or packing creates structures that may appear as positive-relief features - such as trails on the base of overlying beds - or as burrow or bore fillings on the tops of the underlying mud bed. Burrows and borings commonly extend down into beds, so these structures are not exclusively bedding-plane structures. ^[1]

Tracks, trails, burrows, borings, and other structures made by organisms on bedding surfaces or within beds are known collectively as trace fossils, also called ichnofossils or lebensspuren. The study of trace fossils constitutes the discipline of ichnology. ^[1]

Four Categories of Biogenic Structures

Trace fossils are not true bodily preserved fossils - they do not form by conversion of a skeleton into a body fossil. They are simply structures that originated through the activities of organisms. Interpreted broadly, biogenic structures fall into four categories: (1) bioturbation structures (burrows, tracks, trails, root penetration structures); (2) biostratification structures (algal stromatolites, graded bedding of biogenic origin); (3) bioerosion structures (borings, scrapings, bitings); and (4) excrement (coprolites, such as fecal pellets or fecal castings). Not all geologists regard biostratification structures as trace fossils, and these are not commonly included in published discussions of trace fossils. ^[1]

Classification into Ichnogenera and Ichnospecies

Trace fossils are classified into ichnogenera based on characteristics that relate to major behavioural traits of organisms, and are given generic names such as Ophiomorpha. Distinctive but less important characteristics are used to identify ichnospecies, for example Ophiomorpha nodosa. ^[1]

Trace fossils are produced by marine organisms such as crabs, flatfish, clams, molluscs, worms, shrimp, and eel. In nonmarine environments, insects, spiders, worms, millipedes, snails, and lizards produce burrows and tunnels; vertebrates leave tracks; and plants leave root traces. The trace-making organism is rarely preserved alongside its trace, so the names applied to ichnogenera and ichnospecies generally do not refer to the trace makers themselves. ^[1]

The separation of taxonomy from trace-maker identity is an important peculiarity of ichnology. Because the organism and its trace are almost never found together, ichnotaxonomy is based entirely on trace morphology - not on biology. This means that very different organisms can produce morphologically similar traces and receive the same ichnogenus name, while a single species may produce different traces in different substrates and receive different names for each.

Significance Beyond Environment

Although trace fossils are important palaeoenvironmental indicators, no single biogenic structure is an infallible indicator of depth and environment. The basic controls on trace fossil formation are not simply depth - they include the nature of the substrate, water energy, rates of deposition, water turbidity, oxygen and salinity levels, toxic substances, and quantity of available food. Individual trace fossils can overlap the depth zones they are associated with, and ichnofacies boundaries are gradational, not sharp. ^[1]

Trace fossils are also useful for estimating relative sedimentation rates: rapidly deposited sediments contain relatively fewer trace fossils than slowly deposited sediments. They can indicate whether sedimentation was continuous or punctuated by erosional breaks; provide a behavioural record of extinct organisms; assist paleocurrent analysis when organisms preferentially oriented themselves facing the current during resting; and allow beds to be identified as right-way up or inverted where U-shaped burrows that opened upward are preserved. They also have biostratigraphic and chronostratigraphic significance for zoning, correlation, and recognition of bounding discontinuities between stratigraphic successions. ^[1]

Master UPSC Geology Optional

Ex-ONGC Geologist & Rank Holder

Learn the exact analytical answer-writing patterns needed for UPSC Optional from an AIR 2 & AIR 25 holder.

1-on-1 Personalized Mentorship

Elite Batch (Strictly 10 Seats)

Targeted Strategy for AIR 1-100

Bilingual Conceptual Lectures

Join Us

Offline in Delhi