GEOL2301: Palaeoecosystems

With few exceptions, it is difficult to determine the exact biological identity of a trace maker – very different taxa can produce very similar impressions on the sediment, particularly if they share similar behavioural modes. Conversely, behaviours that only occur in specific settings can give rise to trace fossils that are distinctive to a very specific depositional environment. As such there are a variety of different classification schemes based around different properties of trace fossils.

Define and distinguish the characteristics of the major types of trace:

• Tracks
• Isolated 'footprints'
• Trackways
• Series of 'footprints' (or trails, etc.) from which movement patterns can be reconstructed
• Resting traces, e.g. Rusophycus
• Interactions with the sediment surface from an organism whilst 'at rest', i.e. not relocating itself, but possibly relocating the sediment – feeding on it, or moving in situ to retain a flow of fresh water over respiratory surfaces.
• Burrows, e.g. Thalassinoides
• Traces made within the sediment, by burrowing
• Borings
• "Burrows" in a hard substrate, such as wood or lithified rock.

Classifying by preservation

An additional consideration is the fashion in which the fossils are preserved, which may affect their observed morphology. Martinsson’s scheme has the widest currency in the modern literature.

Rindsberg 2012

Naming ichnofossils

Can we not just name trace fossils after the organism that created them? This can pose some challenges, both in terms of identifying the trace maker, and ‘unsquashing’ sedimentary layers.

Trace fossils therefore have their own genus-and-species naming scheme that runs in parallel to standard Linnean taxonomy.

Behavioural classification

Another way to classify trace fossils is according to the type of behaviour that generated them – common categories include dwelling traces, grazing traces and resting traces. As different behaviours suit different sedimentary environments, the mode of behaviour expressed can help to resolve certain characteristics of the likely depositional environment.

Gingras et al. 2009

Characterize the main behavioural categories of trace fossil, noting where appropriate the environments with which they may be associated:

• Dwelling traces – Domichnia

• Domestic bliss? Domichnia are traces in which an organism resided.

  Why live within sediment? Protection from a stressful environment, or from predators?

• Feeding burrows – Fodichnia
• Obtaining Fo[o]d: the sediment must be food-rich, suggesting that the sort of environment where deposit feeders outnumber suspension feeders. What sort of environment would that be? Look back on your notes from previous sessions to jog your memory.
• Grazing trails – Pascichnia
• Feeding on organisms that grow on the sediment surface, rather than within the sediment – I think of cows as familiar terrestrial grazers. Most such foodstuffs will be photosynthetic, though exceptions exist – how deep is the photic zone again?
• Movement traces – Repichnia
• The need to reposition indicates some degree of instability in an environment – even if this is just the presence of predators (and thus an ample supply of nutrients). Escape-type burrows in particular hint at rapid changes in sediment position or water chemistry.
• Resting traces – Cubichnia
• It's nice to have time to chill… could this reflect the abundance of nutrients one might associate with the Cruziana ichnofacies (more on that story later)? Other explanations are available, but this is often a useful hypothesis to have in mind.

Inferring environmental conditions from trace fossils

Around a dozen separate ichnofacies – that is, distinctive environments characterized by recurrent and particular assemblages of ichnofossils – have been proposed. We won’t meet them all: we’ll focus on the four marine softground ichnofacies, as these occur consistently through space and time (since the Cambrian, at least!).

Ichnofacies are typically named after an ichnogenus that characteristically occurs within them, so trace fossil assemblages deposited in the Skolithos ichnofacies often (but not always) include the ichnogenus Skolithos, but may alternatively contain some or all of Arenicolites, Diplocraterion and/or Ophiomorpha.

• Summarize the environmental conditions and ichnological composition of the Skolithos ichnofacies
• This ichnofacies corresponds to shallow water (high energy, possibly intertidal) settings with relatively clean and unconsolidated sandy sediment. Most of its inhabitants are suspension feeders that sit in the sand as they feed; such creatures might produce simple vertical or U-shaped dwelling burrows (Skolithos, Arenicolites). To keep pace with influxes of sediment or periods of erosion, the burrows may need to move up or down, leaving evidence of vertical movement (spreiten) as they migrate (Diplocraterion). Another characteristic ichnogenus of the Skolithos ichnofacies is Ophiomorpha. Ophiomorpha comprises networks of burrows; in order to stop them collapsing, burrow walls are lined with a stucco of faecal pellets.

Cruziana ichnofacies

• Summarize the environmental conditions and ichnological composition of the Cruziana ichnofacies
• This ichnofacies typifies shallow marine settings with poorly sorted, mud-rich (and therefore cohesive) sediments set in areas of low to medium energy. Such settings are compatible with both deposit and suspension feeding, giving rise to reasonable biodiversity and plenty of opportunity for predation. Cruziana itself represents the locomotion of (predatory?) trilobite-like arthropods, whereas Rusophycus is a resting trace of the same organisms. The more placid and stable environment is more compatible with horizontal burrowing than the Skolithos ichnofacies. Rhizocorallium are sub-horizontal U-shaped burrows thought to be made by burrowing depositfeeding worms, showing spreiten superficially similar to those of Diplocraterion. Palaeophycus is another ichnogenus of unbranching, horizontal burrow.

• Summarize the environmental conditions and ichnological composition of the Zoophycos ichnofacies
• This ichnofacies is associated with sediments that are fine grained (silty clays or muds) and organic rich, with slow deposition rates. As such the sediments are often prone to periods of low oxygen conditions and intensely bioturbated. Such conditions are common in distal settings and deeper waters (below storm wave base) but can occur elsewhere. In such low-energy settings, deposit feeding is of course dominant. Zoophycos itself looks something like a broad spiral staircase, with lots of lateral burrows corresponding to the steps as it screws down into deeper sediments. The trace fossil assemblage is typically low diversity (perhaps owing to periodic anoxia); another taxon that might be encountered is the branching, root-like Chondrites, itself thought to prosper in low oxygen conditions; it has been suggested that the burrow system serves to culture methanogenic bacteria.

• Summarize the environmental conditions and ichnological composition of the Nerites ichnofacies
• The Nerites ichnofacies occurs in deep marine settings; sediment typically rains down from the overlying water at incredibly slow rates, with occasional interruptions from sandy turbidite packages. With food scarce and sometimes occurring in thin horizontal layers (perhaps corresponding to the fallout of an algal bloom, for example), deposit feeders are careful to cover ground as efficiently as possible, perhaps by spiralling out from a central point (Spirorhaphe), or perhaps by mapping out a labyrinth-like pattern by sticking as close as possible to previous burrows (Helminthoida). Nereites itself is a more meandering trace, again horizontal, with lateral lobes. Palaeodictyon is a more unusual trace in the form of a horizontal hexagonal grid; the trace maker would have cultivated bacteria on the walls of the grid, left them to grow using nutrients in the sediment, then fed on the bacteria once they had flourished.

Bryozoans are both a substrate that can contain impressions of other organisms, and a permanent impression of competitive interactions – offering a complementary glance into ancient behaviour.

Cheilostomes win two-thirds of their competitive interactions with members of the other bryozoan group, the cyclostomes; this can be attributed to their competitive advantages: larger zooid size; outward-directed exhalent flow; often more sophisticated colony structure. (Some colonies even have a specialist class of frontier zooids that do not feed or reproduce, but are basically giant pincers that nip any competitor that gets too close.)

• If cheilostomes tend to outcompete cyclostomes whenever the two meet, why are cyclostomes not on a trajectory to extinction?

• One explanation is that cyclostomes are much better at dispersal and colonisation: they can rapidly colonise a new substrate before cheilostomes can get established, and have already spread their genes to a new site before they are smothered by a cyclostome.

  Is this explanation consistent with the differences in characteristics between cheilostomes and cyclostomes?