Palaeo - Ichnology

Question 1

Types of trace

Answer 1

• Tracks
o ‘Footprints’

• Trackways
o Series of footprints

• Resting traces
o e.g. Rusophycus
 Appreared before trilobite fossils which shows there was something with the gills of a trilobite before something with the hard skeleton, which would mineralise

• Burrows
o e.g. Thalassinoides
 “dichotomously or T-branched boxworks, mazes and shafts, unlined and unornamented”. Facies of Thalassinoides increased suddenly in abundance at the beginning of the Mesozoic

• Borings

Question 2

Martinsson’s scheme

Answer 2

Preservation
Full relief - burrow erodes out sediment (mud)
Epirelief - Mud collapsed by lower burrow
Full relief - burrow erodes out sediment (sand)
Hyporelief - Burrow eroded and sand deposited on top

Location
Exichnia - Trace out of sediment 
Epichnia - Trace on sediment surface
Endichnia - Trace in sediment
Hypichnia - Trace on sediment bottom

Question 3

Challenges in naming ichnofacies

Answer 3

• Identifying the trace maker
o Don’t know how organism interacts with surface
o Trace morphology differs by surface substrate
• ‘unsquashing’ sedimentary layers
o Depth of sediment changes impression morphology by compression
Trace fossils therefore have their own genus-and-species naming scheme that runs in parallel to standard Linnean taxonomy.

Question 4

Behaviour

Answer 4

• 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.
Dwelling traces – Domichnia – Domestic burrow
Feeding burrows – Fodichnia – Feeding on sediment
Grazing trails – Pascichnia – Feeding on a surface
Movement traces – Repichnia – Repositioning
Resting traces – Cubichnia

Escape burrows = high energy instable env
Farming = low energy stable env

Question 5

Ichnofacies

Answer 5

Distinctive environments characterized by recurrent and particular assemblages of ichnofossils

Question 6

Skolithos ichnofacies

Answer 6

• 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 facies is Ophiomorpha
o Ophiomorpha comprises networks of burrows; in order to stop them collapsing, burrow walls are lined with a stucco of faecal pellets.
• Conditions:
o Hostile
o Instability
o Dynamic
o High energy
o Quick movement needed
 Burrowing bivalves

Question 7

Cruziana ichnofacies

Answer 7

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 deposit- feeding worms, showing spreiten superficially similar to those of Diplocraterion.
• Palaeophycus is another ichnogenus of unbranching, horizontal burrow.
• Conditions
o Shallow
o Photic zone
o Oxygenated + nutrient rich water
o High diversity
o Stable

Question 8

Zoophycos ichnofacies

Answer 8

• 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.
• Conditions:
o Shelf margin
o Below photic zone
o Slow to no currents
o Little oxygen
o Low diversity
o Harsh environment

Question 9

Nereites ichnofacies

Answer 9

• 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.
• Conditions:
o Abyssal depth
o Stable
o Little food