Trace Fossils & the Evolution of Behavior
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:
- Only very rarely do we know the actual identity of the trace-maker
- It is possible to have too many specimens. With body fossils, generally more fossils give us larger samples and better statistical control. Trace fossils, particularly burrows, in contrast, bioturbate the sediment and destroy previous traces or render them unintelligible.
- In imitation of actual biological taxonomists, ichnologists use a system of Ichnotaxonomy that follows the pattern of Linnean taxonomy. 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!! Indeed, two distinct biological species can produce the same ichnospecies. Conversely, an individual critter can produce various ichnospecies in its life, as its activity and the substrate it moves across change.
Obviously, traces themselves don't evolve and speciate. What does the hierarchy of ichnotaxomony actually represent? There really are two hierarchies and signals:
- Groups by behavior:
- Groups by environment. Characteristic assemblages of distinct ichnotaxa are often found together in particular environments and are given technical "ichnofacies" names (remember "facies fossils?") The chart above shows the sequence near a rocky shore. Near a sandy shore, we see:
- Psilonichnus Ichnofacies: Backshore, dune, and supratidal transitional environments.
- Skolithos Ichnofacies: Transitional intertidal zone to shallow marine. Typical of environments with rapidly changing conditions (tides, storm-surges, etc.) in which critters may need rapidly to take shelter.
- Cruziana Ichnofacies: Shallow marine environments, but deep enough that they are not typically touched by tide or wave action.
- Zoophycos Ichnofacies: Continental slope and deep marine environments.
- Nereites Ichnofacies: Deep marine environments.
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.
- The earliest unambiguous trace fossil is from the Ediacaran Period, a horizontal surface trace made by a worm-like organism.
- Cambrian Substrate Revolution: During the Early Cambrian, surface locomotion traces like Climactichnites reflected the activity of surface grazers that left the sub-surface sediments essentially unidsturbed. As the Middle Cambrian approached, a fundamental change occurred as proper burrows appeared, and it became common for sediments to become significantly bioturbated. (Compare unbioturbated and bioturbated sediment in otherwise similar depositional settings.)
- Complexity: Over the course of the Early Paleozoic, simple horizontal burrows give way to coimplex deposit feeding and "farming" traces.
- Invasion of new environments: Over the Paleozoic, ichnofacies typically appear in shallow marine environments (hospitable to life) and spread into more challenging environments (transitional and continental environments, and the deep oceans).
- The post-Paleozoic world: From the Permian onward, the distribution of ichnofacies has held constant with one exception: the proliferation of the Nereites facies in the deep oceans. Maybe the post-Paleozoic oceans have more nutrients, or the deep oceans receive more sediments, or organisms have simply become more competent at living down there. One deep ocean enigma: Paleodictyon hexagonal network of mucus-lined burrows with numerous vertical shafts. The last known fossil occurance is about 50 My, however non-fossil examples have been recently spotted by submersibles!
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:
Even so, we don't definitively know the identity of the track-maker of Cheirotherium (right).
- 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').
Our knowledge of living vertebrate locomotion enables us to extract much information about the locomotion of ancient track-makers from their trackways, including:
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.
- 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
hominin tracks from Laetoli, Kenya.
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.:
- Eggs with a hard calcium carbonate shell (as opposed to the leathery shells of
lepidosaurs, turtles, egg-laying mammals)
- ALL non-avian dinosaur eggs are basketball-sized or smaller: Apparently all dinosaurs came from small babies! (Differs from the mammalian condition, where baby elephants etc. are BIG animals!)
- Some dinosaur nests associated with covered mats of vegetation: probably helped to keep
warm (as in croc nests).
"brooding position" over nests.
- Dinosaurs tend to have nests of about a dozen or so eggs each: more than found in modern
birds, less than in (for example) turtles. This is regardless of size: troodontids to
- Implies that unlike placental mammals, dinosaurs could produce a dozen or so offspring a
year regardless of size; among placental mammals, larger body size means LONGER gestation
- Two main potential life habits upon hatching:
- Precocial: able to move around easily shortly after birth.
- Altricial: nest-bound, wholly dependant on parents for food.
Some evidence of these habits in hatchling dinosaurs:
- Some baby species have poorly developed joint surfaces in the legs (unable to
move well), but have worn teeth (were feeding): suggests altricial. Vacated nests show crushed shell fragments.
- Some other baby dinosaurs (such as troodontids) have fully formed joints prior to
hatching: would have been able to move from day 1. Vacated nests show the intact lower half of the egg.