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

Spring Semester 2014

Ancestors, "Missing Links", and Transitions: The Fossil Record of Evolution

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.

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: 27 March 2014