Ichnology: Study of trace fossils (biogenic sedimentary structures)

Ichnofabric & bioturbation: degree of overturning of sediment by organisms. Excessive biological activity obliterates individual trace fossils and sedimentary structures, mixing layers, etc.

Ichnotaxonomy: by tradition, individual trace fossil forms are given "ichnogenus" and "ichnospecies" (and "ichnofamily" and so on) names, even though they do NOT in reality represent a nested hiearchy based on patterns of descent!! Two trace fossils represent the same ichnospecies if they are identical in form regardless of whether they are made by the same species or different species; additionally, the same species can produce many different ichnospecies depending on its behavior, the substrate, etc.

Other Ichnonomenclature:

• By Behavior:
  • Cubichnia: resting
  • Domichnia: dwelling
  • Fugichia: escape traces
  • Repichnia: moving
  • Paschichnia: grazing
  • Agrichnia: farming
  • Fodinichnia: Deposit feeding (3-D)
• Ichnofacies: set of trace fossils associated with a particular set of environmental conditions. Form excellent record of paleoenvironment, esp. as many ichnogenera persist for hundreds of millions of years! Some major ichnofacies:
  • Skolithos: Rapidly moving sediment, made by filter feeders:
    • Skolithos: vertical tubes (Cam-K), unknown builder
    • Ophiomorpha: brick-built, (P-today), callianassid shrimp make these today
    • Diplocraterion
  • Cruziana: Shallow marine, between low tideline and storm wave base (middle-outer shelf):
    • Cruziana: trilobite walking traces (Cam-P)
    • Thalassinoides: complex mining (domichnia/fodinichnia) traces, probably by decapods
    • Asteriacites: starfish cubichinia
    • Rhizocorallium: horizonal version of Diplocraterion
  • Zoophycos: Low energy muds and muddy sands, dysaerobic and organic rich conditions:
    • Zoophycos: 3-D feeding trace
  • Nereites: Deep water (associated with turbidites and pelagic muds)
    • Nereites itself is one of many complex surface pascichnia or fodinichnia
    • Paleodictyon: a series of hexagonal structures. Builder unknown, but deep sea submersibles have found Paleodictyon is still being formed in the abyssal realm!
  • Other zones are less significant

Terrestrial trace fossils:
Both arthropod and vertebrate trackways are common. Unlike marine ichnofossils, repichnia dominate by orders of magnitude. However, some domichnia present:

• Termite burrows
• Rodent burrows (such as Daemonelix)

Also, feeding traces can be found as bite marks on bones or leaves, and there are the occasional resting tracks.

Additionally, considerable study of corpolites, and eggs and nests.

Some aspects of terrestrial (primarily vertebrate (primarily locomotory)) ichnology:

• Relatively good at identifying trackmakers: perhaps not to the genus/species level, but often easier to resolve than marine ichnites:
  • In part because vertebrates have individual footfalls
  • In part because vertebrate diversity is generally lower than invertebrates (bigger, rarer animals)
  • However, there have been some famous mistakes (such as identifying limulid (horseshoe crab) tracks as pterosaurs, or pterosaur tracks as crocodilians)
• Relatively good ability to reconstruct aspects of locomotion
  • Individual attributes, such as size and stance of the animal
  • Dynamic attributes, such as pace angulation and stride length
  • From some basic mechanics, and by comparison with modern animals, one can estimate speeds from trackways
• Can occasionally get estimates of other behaviors. For example, this one is REALLY cool: an apparent record of a large carnivorous dinosaur (Acrocanthosaurus or a close relative) attacking a large herbivorous dinosaur (Sauroposeidon or a close relative) in the Early Cretaceous of Texas. Note the "right-left-right-right-left-right" sequence in the carnivore's steps: one hypothesis is that the predator was grabbing onto the herbivore, and was dragged for a step before being dislodged.

Pliocene hominin tracks from Laetoli, Kenya.

Juassic pterosaur ptracks from Crayssac, France.

An important thing to remember is that an ichnite is the fossil of the animal while alive, not after it was dead!

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