Trace Fossils

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INTRODUCTION

Trace fossils (also called ichnofossils or lebenspuren) are the evidence of animal's activity. Unlike molds and casts which are evidence or replicas of skeletal remains or body impressions, trace fossils are sedimentologic or lithologic disturbance from an animal's (or plant's) activity such as resting, locomotion, or feeding.

The utility in trace fossils comes not from their biostratigraphic value, but instead their use in the interpretation of paleoenvironments. All too often, the organism that produced the trace is unknown. Given that trace fossils reflect activity, many different organisms doing the same thing can produce similar traces. Likewise, an organism engaged in different activity can leave more than one trace.


CLASSIFICATION

Ichnofossils

Resting & Hiding Traces

Dwelling Traces

Locomotive Traces

Feeding Traces

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Ichnofacies

Scoyenia Ichnofacies

Trypanites Ichnofacies

Skolithos Ichnofacies

Cruziana Ichnofacies

Zoophycos Ichnofacies

Nereites Ichnofacies

 

 


  

Bioturbation and Ichnofabric

Bioturbation can be defined as the disturbance of sediments due to biologic activity. An important component in understanding the activity of ancient organisms is an estimation of trace fossil abundance which in turn may offer clues as to the environment during deposition of the sediments (see Fig. 13.1). For example, in paleoenvironments that were unfavorable to trace-making organisms, the original sedimentary bedding or laminations will be left intact (see §13.1). As paleoenvironmental conditions improve, bioturbation activity increases leaving the original laminations disturbed (e.g. §13.2) to the point at which no primary bedding is observable (e.g. §13.3).


Morphology of Ichnofossils

The morphology of trace fossils can be described with respect to their position relative to their original depositional surface. Please see Figure 13.2 below and aquatint yourself with the various terms applied to trace fossil morphologies.


Activity Traces

Locomotive Traces (Repichnia): usually straight or slightly curved trails in addition to tracks and trackways. See the tracks (§13.4 and §13.5) and horizontal burrows called Planolites (§13.6).

Resting and Hiding Traces (Cubichnia): characterized by the fillings of shallow excavations that mimic the morphology of the trace maker. See Rusophycus (§13.7) for an example. Given the size and shape of the example (§13.7), what organism do you think may have made it?

Dwelling Traces (Domichnia): this group includes burrows, borings, or other excavations. They can be vertical unbranched cylinders or U-shaped. See Skolithos (§13.8) for a vertically oriented tubular burrow and the larger burrows (§13.9 and §13.10) made by shrimps and borings ( §13.11) made by Recent pholad bivalves.

Feeding Traces (Fodinichnia): Includes the burrows of sediment feeders usually with a distinct three-dimensional morphology. They may be dendritic or multibranched or dendritic and take on other geometric patterns. See Zoophycos or Taonorus §13.12 to §13.14. Note on these traces the U-shaped backfill is called sprieten. Other feeding traces include Thalassanoides (§13.17) and Ophiomorpha (§13.18) which consists of large vertical and horizontal branching tubes.

Additionally, trace fossils inductive of feeding behavior (and diet) include predatory borings (e.g. §13.17 and §13.18) and fossilized excrement (called coprolites). Smaller fecal pellets (microcoprolites) are a common constituent in many sediments. See example §13.19 for examples of Triassic microcorprolites that were probably deposited by anomurian crustaceans (shrimps and crabs).

Grazing Traces (Pascichnia): Generally grazing burrows are two-dimensional features which occur along bedding surfaces as a spiral, S-shaped series of curves, or other geometric pattern. Grazing organisms are efficient feeders. Rarely do grazing burrows cross paths. No examples, sorry.


Paleoecology and Ichnofacies

An ichnofacies is a recurring assemblage of one or more ichnofossils which are characteristic of a particular environment. Trace fossils within a given inchnofacies often have similar morphologies which is presumably due to a similar activities of the fossil organisms which made them. Ones you will be responsible include:

Scoyenia Ichnofacies: These are nonmarine trace fossils that mainly include foot prints, trails, and trackways which were originally made in moist sediments.

Trypanites Ichnofacies: The Trypanites ichnofacies is characterized by mostly dwelling borings into lithified sediments in the intertidal to shallow subtidal zone.

Skolithos Ichnofacies: Sometimes grouped together with the Glossifungites ichnofacies, the Skolithos ichnofacies occurs in soft or firm (but not lithified) within the intertidal or shallow subtidal zone. The Skolithos ichnofacies is characterized by vertical dwelling burrows and tubes (some which may be U-shaped) that can extend many centimeters into the sediment.

Cruziana Ichnofacies: The Cruziana ichnofacies are characterized by simple locomotion traces with some U-shaped dwelling and shallow resting and hiding traces. The Cruziana ichnofacies is common in middle to outer shelf clastic settings.

Zoophycos Ichnofacies: The Zoophycos ichnofacies commonly contain three-dimensional feeding traces which were constructed by a variety of organisms in often poorly sorted and unlithified sediments. The Zoophycos ichnofacies is common below wave base in clastic shelf environments.

Nereites Ichnofacies: The Nereites Ichnofacies is characterized by an abundance of grazing traces and occasional three-dimensional feeding traces. The Nerites ichnofacies is occurs in deep-water (typically bathyal or abyssal) environments often is sediments interpreted as low-oxygen.


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