Trace Fossils & the Evolution of Behavior

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Remember our earlier intro to trace fossils:

  • Trace fossil: Evidence of the organism's activity as a living creature. These include:

    This lecture introduces ichnology, the study of fossil traces. Ichnology is fundamentally unlike the study of body fossils in three significant ways:

    Obviously, traces themselves don't evolve and speciate. What does the hierarchy of ichnotaxomony actually represent? There really are two hierarchies and signals:

    What does the trace fossil record tell us?

    Even though the trace-makers are enigmatic, the record of marine trace fossils documents the appearance and proliferation of burrowing animals during the Phanerozoic.

    Trace fossils on land

    Both arthropod and vertebrate trackways are common. Unlike marine ichnofossils, locomotion traces (trackways) dominate by orders of magnitude. However, some dwelling and resting traces are present, including:
    • Termite burrows
    • Rodent burrows (such as Daemonelix)

    Additionally, considerable study of corpolites and eggs and nests.

    Some aspects of terrestrial vertebrate ichnology:

    • It's easier to approximate the identity of the track-maker because:
      • vertebrates have individual footfalls
      • the foot, itself often has distinctive anatomy
      • vertebrate diversity is generally lower than invertebrates (bigger, rarer animals) with enough niche partitioning that you don't typically see many similar forms in the same environment (although there have been some famous mistakes, such as identifying horseshoe crab tracks as pterosaurs').
      Even so, we don't definitively know the identity of the track-maker of Cheirotherium (right).

    Our knowledge of living vertebrate locomotion enables us to extract much information about the locomotion of ancient track-makers from their trackways, including:
    • 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. (Reconstruction
  • Group trackways help distinguish between environments where many animals cross individually, and where they move as herds, interacting with and avoiding one another.
  • Sometimes, the movement of herds can be traced for tens of miles across megatrackways.
  • But caution! Trackways are, by definition, preserved when the animal walked across wet sediment. Thus, they were not moving normally, as they would on dry ground.
  • Pliocene hominin tracks from Laetoli, Kenya.

    Juassic pterosaur stracks from Crayssac, France.

    Nests, eggs, and babies:
    Nests are known from several land vertebrates, notably dinosaurs. These are bowl shaped depressions with eggs or egg shells in them. Dinosaurs particularly amenable to preservation because.: