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
""quote" -- author, date, source.
BIG QUESTION:How does the fossil record document the changes in form and behavior over the life cycle of ancient animals?
The Naming of Names
Linnean taxonomy has its own special set of grammatical rules:
Genera have one word names (e.g., Panthera, Homo, Ginkgo,
The genus name is always Capitalized and italicized (or
underlined if you don't have access to italics);
Species have two word names, the first part of which is the same as the genus
name (e.g., Panthera leo, Homo sapiens, Ginkgo biloba,
The genus name is ALWAYS Capitalized, the second part ("trivial nomen" or "epithet") is
ALWAYS in lower case, and the name is ALWAYSitalicized or
The complete trinomial for a species includes the full species name PLUS the
family names of the original describers of the species, and the year of description.
Example: Tyrannosaurus rex Osborn 1905
If the species was originally in a different genus, but was lumped into a previously named
species OR split into an entirely new genus, the author name and date of the original
species is put in parentheses:
Example: Anatosaurus annectens Lull and Wright 1942 is typically lumped into the
same genus as Edmontosaurus saskatchewanensis Lambe 1917. So the first species would
now be written out as: Anatosaurus annectens (Lull and Wright 1942).
Species names can be abbreviated by using only the first letter of the genus name,
followed by a period (NEVER by a hyphen): H. sapiens and T. rex are
correct; H. Sapiens or T-Rex are WRONG!!;
Each genus has a type species: the one species uniquely associated with that genus;
All taxon names other than species have one word names, which are Capitalized;
all taxon names other than genera and species are in roman letters (i.e., they are never
italicized/underlined): Dinosauria, Tyrannosauridae, Animalia; not Dinosauria,
tyrannosauridae, or animalia.
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).
Taxonomists who consider a particular set of specimens to represent many taxa are
called splitters; those who consider a particular set to represent few taxa are
If a taxonomist feels that some specimens of a genus belong to an as-yet unnamed
species, they can split these specimens off as a new species (which a new type specimen);
On the other hand, if a taxonomist considers that two previously named species are
not distinct enough from each other to truly be distinct species (that is, the taxonomist
regards the two names as synonyms), they may lump them together:
In these cases, the Rule of Priority is used: whichever of the names was
published first (the senior synonym), even if only by days, is the name that must be used;
The junior synonym is then abandoned
The same case applies to genera: if two genera are thought to represent the same genus, the first named genus name is the one that is used.
For those interested in a website concerning some unusual Linnaean species names, click here.
Four main sources of information for forming behavior hypotheses:
Analogy to modern animals (i.e., horns in modern mammals compared to horns in ceratopsids)
Phylogenetic distribution of behaviors in modern animals (i.e., behaviors shared by crocs & birds would be expected in all dinosaurs)
Of particular note, archosaurs and other diapsids are profoundly visual animals, using many types of visual displays (static ones like colors
or crests, and moving ones like dances and headbobs). Almost certainly, extinct dinosaurs would have done the same.
Furthermore, archosaurs are very "talky": both birds and crocodilians have large repertoires of calls and signals. Almost assuredly non-avian
dinosaurs did the same.
Biomechanics to determine whether certain behaviors were feasible under laws of physics
Sedimentary record where unusual preservation suggests certain behaviors (i.e., bone beds made up almost exclusively of
centrosaurine dinosaurs suggests these dinosaurs lived in herds)
Some behaviors to consider:
Displays: displays can have various "messages"
Sexual (aka courtship) displays: attract potential mates
Territorial displays: defend territory (which might also include mates)
Defensive display: more generally, to warn off potential threats
Specific Recognition: features distinctive to each species. Especially common where multiple closely-related forms live in the same
Combat: as in display
Locomotion: adaptations for weight bearing vs. speed
Feeding: different types of carnivory; different herbivorous specializations
Important to consider the difference between intraspecific and interspecific
Intraspecific: within a species
Interspecific: between species
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:
Kin Selection: or, "Blood is Thicker Than Water." From a gene's perspective, protecting or nurturing close relatives with the same genes as you can be as effective in
spreading additional copies of that gene. So behaviors that favor cooperation between kin might be selected for if those behaviors wind up
promoting the transmission of those genes (in comparison to populations which do not have the cooperative behaviors).
Reciprocal Altruism: or, "You Watch My Back, and I'll Watch Yours." In some circumstances behaviors can be selective where individuals
keep a look out for each other (or similar type of altruism (helpfulness)) so long as they get the same benefit from other members of that population.
For example, if there are "cheaters" (ones that take advantage of others looking out for them, but that don't waste their own energy keeping a lookout)
than those populations may get additional predation (because of the predators who attacked when the cheaters could have seen them and warned others.)
If there are other populations of the same species in which the reciprocal altruistic behavoirs are present, those populations will have fewer losses
and so prosper relative to the populations with cheaters.
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.
Sexual Dimorphism: when the two sexes (at least as adults) have distinctive forms. Difficulty in testing this in the fossil record:
Are you looking at fossils of two sexes in the same species, or two different species?
Many sexual display features (and sex organs!) are soft tissue, so they would only rarely fossilize
If you have only a few individuals, are you looking at true dimorphism, or just end members in a continuous spectrum?
Some things to look for in potential cases of sexual dimorphism:
One morph (often the male) is rarer
One morph (often the male) is showier
Distinction between alleged males and females is less pronounced prior to sexual maturity
Changes to dimorphic forms is rapid once sexual maturity is reached
Frequency of the two morphs remains relatively constant in strata where the species is found.
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:
Likely to be most pronounced when related species are sympatric (since they live together, there is greater need to distinguish A from B)
Differences should be in obvious traits, not subtle internal ones