GEOL 204 Dinosaurs, Early Humans, Ancestors & Evolution:
The Fossil Record of Vanished Worlds of the Prehistoric Past

Spring Semester 2015

Bringing Fossils to Life: Methods of Paleobiology


Paleo CSI
Paleontology and forensic science (e.g., "crime scene investigation") have a lot of similarities. Both involve reconstructing past events from the limited evidence (often circumstantial) to find one (or often several) likely explanations. This is just as true for trying to reconstruct how ancient organisms lived: that is, paleobiology. Paleontology has many different uses: reconstructing past environments, past events, sequence in geologic time, etc. But for many people, paleobiology (i.e., "bringing fossils to life") is the main draw.

So how do we go about this?

Step 1: Identify the Parts: Determining what particular organs, structures, etc. you are seeing in a fossil can be complicated. In some cases, they are animals or plants which are no longer extant, and may have parts that do not correspond exactly in size and shape to the homologous parts in living relatives. And in some cases, they may even have parts that are NOT homologous to the parts of any living organism! Hence, the need to use comparative anatomy to establish what parts you see in the fossils.

Step 2: Classify the Organism: Once you know the parts, you can compare it to already-known specimens. It might very closely match the anatomy of species previously established from fossils; in this case, the fossil is most likely a new specimen of that previously-named species. Or it might differ to some degree from the known fossils: in this case, the paleontologist might make the case that fossil belongs to a new species, a new genus, or indeed some other previously unknown branch of the Tree of Life.

Step 3: Restore the Missing Parts (including Soft Tissue): No fossil will be entirely complete: after all, most are missing soft parts, and even exceptionally good fossils are missing significant portions of the hard parts. So we need to fill in the gaps. For the hard parts this can often be done by mirror imaging the equivalent part on the other side of the body (for a bilaterally-symmetrical organism, at least) or by comparing it to the missing part of other fossils of the same species or closely-related species (but make sure to scale it up or down in size appropriately!).

In the case of soft tissues, this is where comparisons to close living relatives and the use of Lagerstätten becomes important. For some organisms, marks on the hard tissues allows us to reconstruct soft tissues which are closely associated with them. An example would be the muscles in vertebrates. Muscles attach to bones, so for fossil species we can fill in the missing parts based on the muscle pattern of living relatives. The same can go for structures like eyeballs, brains, etc., which have a close association with hard tissues.

In other cases, though, it is more difficult. And indeed there are many organisms where we honestly have to be skeptical about treating any one reconstruction as better supported than a host of other equally-likely hypotheses.


Reconstructing Behavior
Trying to determine how a fossil organism lived requires many different lines of evidence. Some are more secure than others, but all have their role.

Analogies: The oldest method of all is the use of analogies: finding living organisms with similar anatomical structures and inferring a similar life habit. For instance, sauropod dinosaurs have elongated necks, as do giraffes. Giraffes feed high in trees as well as on the ground, so some have argued that sauropods did the same.

Analogies can be weak, however, because often the same structure may be used a number of different ways by the same living group, or by different ways in different living groups. Therefore, they should be treated cautiously.

That said, analogies can be useful. For instance, in one formation from the Early Cretaceous of China there are a set of pterosaur (flying reptile) fossils that are very similar, except that some have tall crests and others have broad pelvic regions (hips) (and in at least one of the latter type, there is an egg present in its body). Thus, a very reasonable explanation is a sexually-dimorphic species rather than two closely-related species.

Trace Fossils: Body fossils are great, but trace fossils are produced by organisms while they were still alive, and give insights into behavior that body fossils do not. For instant, trackways can be used to calculate the speed of locomotion of extinct animals, or whether they travelled in herds or not. Bite marks and other feeding traces, gut contents, and coprolites allow paleontologists to determine which organisms ate which kind of food.

Phylogenetic Inference: As we saw before, we can use the extant phylogenetic bracket to reconstruct soft tissue structures in fossil organisms. We can do the same with behavior. Behaviors shared between the extant bracket (or between an extant relative and securely indicated by an extinct bracket) allows for a Type I inference; behaviors present in just one of the extant bracket may be present in the extinct form, and are only Type II inferences; and those which are not present in either of the extant bracket are Type III inferences and can only be supported by evidence independent of phylogeny.

Biomechanics: Organisms live in the world of Physics, and thus we can use the rules of mechanics to judge the physical abilities of a fossil organism. We know the mechanical strength and properties of tissues such as bone, tendon, muscle, wood, etc. Given the known hard part size, and the inferred soft tissue structures, we can calculate whether a given type of motion or force or the strength thereof would allow for any particular behavior. (Of course, organisms are not REQUIRED to live at the extremes of their biomechanical ability, but this technique at least allows us to see which range of behaviors might be allowable for a particular species.)


A Case Study: The Life and Times of Tyrannosaurus rex
Below is a recent lecture I gave about inferring the life habits of the giant carnivorous dinosaur Tyrannosaurus rex:


The Naming of Names
Linnean taxonomy has its own special set of grammatical rules:

Because there is disagreement about the features used to define a particular species or genus, different biologists and paleontologists will sometimes disagree about which specimens belong in a particular species, and which species belong in a particular genus (and so forth).

For those interested in a website concerning some unusual Linnaean species names, click here.


Displays
Four main sources of information for forming behavior hypotheses:

Some behaviors to consider:

Important to consider the difference between intraspecific and interspecific displays:

Visual displays have a great potential for making it into the fossil record, if the part of the animal used for visual display has a hard-part component (or is preserved in a Lagerstatte).


Why Behave? The Logic of Behavior
What role does display have? In the case of defensive and territorial displays, they can be a non-lethal means of getting a point across. Many animals might growl, hiss, spit, rattle, etc. and a would-be attacker leaves them alone: this is beneficial to both the defender and the attacker. In the case of sexual displays, this can be a means of assessing potential fitness of a mate without having to mate with them first.

There is evidence that like some modern animals, certain fossil species lived in groups (herds, flocks, whatever): that is, they were gregarious. There are different types of evidence for group living. The best evidence are beds mostly containing fossils of multiple individuals of different ages of a single species buried at the same time. This would suggest that the died together, and thus very likely lived together. Secondarily, trace fossil beds might show that many individuals passed through a region at the same time.

This gives the advantage of protection (more eyes to spot predators; larger group may scare off some predators; etc.) and (for carnivores) ability to attack as a group (may allow for strategies that a single hunter couldn't use). However, it means that more mouths feeding from the same food sources (since each species is ultimately its own biggest competitor). Different closely related species today might have very different strategies: e.g., lions are social group hunters, while tigers and leopards are mostly loners.

So why (and when) would natural selection favor living together cooperatively, if individuals of the same species are in competition for the same resources? Two main reasons that--in some circumstances--cooperative group living might be favored:

Sexual strategies: male and female animals have different priorities in terms of reproduction. Males can in principle fertilize many many individuals, while females typically have fewer sex cells (eggs) available at any given time. With less cells to use, females often are "choosier" in terms of mates. So many species evolve displays in which males somehow "show off" (in terms of physical features, ritual motions, combat between rivals, etc.) and females evaluate the display.

For example:

Sexual Dimorphism: when the two sexes (at least as adults) have distinctive forms. Difficulty in testing this in the fossil record:

Some things to look for in potential cases of sexual dimorphism:

In very rare cases the eggs or embryos have been found inside a fossil, which rather unambiguously shows them to be female. Otherwise, there can be circumstantial evidence. For instance, if the species has crests, horns, etc., and these are some rarer showier crests, these might more likely be male.

An alternative to sexual displays for showy structures, however, is specific recognition systems (SRS). In this cases, different species have unique characteristics within their ecosystem to recognize other members of the species from all other species they encounter. For cases of olfactory and aural SRS we are lost with regards to fossils. But we have potential with visual SRS.

Things to look for in potential SRS:


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Last modified: 23 February 2015