Thanks, gentle readers, for indulging my ichno-fetish.
Trace fossils represent the intersection of critterology with sedimentology, preserving a wide variety of information valuable to both the sedimentologist/stratigrapher and the paleontologist. Importantly, trace fossils are essentially un-reworkable, and therefore reflect biological activity preserved in situ within the host sediment. The morphology of an individual trace is determined by three things: the Morphology of the animal, the Behavior of the animal, and the Substrate characteristics in which the trace is being made. It’s the intersection of these three attributes that define the resultant trace. For instance, if a trilobite is hanging around the sea-floor, resting up for a late night foraging party, it might produce Rusophycus; if that trilobite gets scared by some predatory critter swimming overhead and decides to make a run for it, it might produce a Cruziana. So the behavior of the little guy decides the morphology of the trace (similarly, think about sediment bulldozers scooping up food versus sediment probers that search out organic matter systematically). Substrate can have a very strong influence on traces; when a shrimp is burrowing in a firm substrate, it might produce a Thalassinoides, whereas in goopy or shifting sediment conditions, it will make an Ophiomorpha.
The recognition of various repetitive behaviors has lead to a categorization of traces based on classes of activity. This ethological nomenclature has produced seven categories:
CUBICHNIA: resting traces
DOMICHNIA: dwelling traces
FODICHNIA: food mining traces
AGRICHNIA: farming traces
PASCICHNIA: grazing traces
REPICHNIA: crawling traces
FUGICHNIA: escape traces
Importantly, some of these behaviors may overlap. For instance, some critters may live in a burrow, and probe the surrounding sediments for food, producing a combination feeding trace/dwelling trace.
Of course, though trace fossils may represent these above behaviors, we all know that the nomenclature of traces is considerably more complicated. In reality, ichnologists classify individual traces on the basis of their morphological characteristics, separating individual morphs into ichnogenera and ichnospecies based on specific characteristics. The interpretation of the trace maker’s biology is secondary to the classification of traces; indeed, traces can be produced by a variety of disparate critters which have evolved to exploit the same niche (Trilobites and Horseshoe crabs, for instance) and therefore perform the same behaviors.
However, the ethological concept does have important implications for the recognition of patterns within the trace fossil record. Importantly, the behaviors and morphology of the critters in question are the result of evolutionary processes; natural selection driving species to exploit specific niches in order to survive. Important selective pressures in the ocean include food and light availability, salinity, substrate consistency, and wave or current energy, which are themselves often contingent on geologically important processes. So suites of organisms have evolved to live in the upper shoreface (for instance), where energy is high (and substrates are therefore sandy and often mobilized) and food is scare. Similarly, some organisms have evolved to live in deep water conditions, with low energy but lots of organic matter available. The strategies that these organisms employ for their survival determine the traces they leave behind. This is the foundation for the concept of ichnofacies.
This image is a picture of a pyritized Ophiomorpha burrow from the Eagle Sandstone in Montana (Upper K, and equivalent to the Milk River in Canada).
This image is a heavily bioturbated sandstone from the Thermopolis shale (Albian) near Bozeman, MT. This is a view of the sole of the bed. Faint HCS could be seen in the bed, and the sands exhibit a sharp contact with the underlying offshore muds. Above, the beds grade into Swaley X-strata and low-angle and planer cross-beds, so it’s interpreted as a shallowing upward shoreface succession.
This medium- to coarse-sandstone bed shows basal scours, rip-ups, and HCS, and has a pretty heavily churned upper surface. Handlens and cathead for scale.
I think next time we’ll dive into the nitty gritty of ichnofacies.