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

Spring Semester 2018
"What is It?": Identifying Fossils and the Nature of Species

Many skulls of the dire wolf Canis dirus discovered at the La Brea Tar Pits, California

"Of what use are the great number of petrifactions, of different species, shape and form which are dug up by naturalists? Perhaps the collection of such specimens is sheer vanity and inquisitiveness. I do not presume to say; but we find in our mountains the rarest animals, shells, mussels, and corals embalmed in stone, as it were, living specimens of which are now being sought in vain throughout Europe. These stones alone whisper in the midst of general silence." -- Aphorism 132, Philosophia Botanica (1751), Carolus Linnaeus

"I am fully convinced that species are not immutable; but that those belonging to what are called the same genera are lineal descendants of some other and generally extinct species, in the same manner as the acknowledged varieties of any one species are the descendants of that species." -- Introduction, On the Origin of Species by Means of Natural Selection (1859), Charles Darwin

"The usual concept of species can be stated as follows (Mayr 1970): "Species are groups of interbreeding natural populations that are reproductively isolated from other such groups." This concept is grandly called "the biological species concept." But that is an arbitrary appropriation of a term with a more general and earlier meaning. I will instead use the term "reproductive species concept."" -- "Ecological species, multispecies, and oaks" (1991), Leigh Van Valen

BIG QUESTION: How do we identify fossils? What are species?

Parts is Parts: Homology, Analogy & Comparative Anatomy
In order to recognize how organisms are similar, or different, we need to compare its body parts. The important thing is to recognize the equivalent body parts: no sense in comparing a leg with a tail, or a jaw with a stomach. It had been noted by early anatomists that related organisms were built on the same "body design" (in German, Bauplan or "building plan"). In each of these, the underlying structure was repeated from organism to organism: these parts are considered to be homologous.

Homologous structures are the same body part, but might be shaped or modified differently. The wing of a bat, the front leg of a horse, the flipper of a whale, and the arm of a human are all homologous, and have the same basic parts: a single upper arm bone, a pair of bones below; some rounded wrist bones; some long bones in the palm of the hand; and series of long bones down each finger. But even though they are homologous, they have different functions.

In contrast, structures that have the same function but are derived from different body parts are analogous. The wings of bats are modified arms, but the wings of insects are modified gill flaps.

The science of comparative anatomy was developed to describe, compare, and contrast the homologous structures of different kinds of organisms. Given a language of comparative anatomy, we can show how two specimens are similar and how they are different. And we can characterize different types of organisms based on their distinctive combination of features.

Taxonomy: the Naming of Names
Taxon (pl. taxa): a named group of organisms.

Naturalists have long noted that there exist units of natural diversity, species, in which the members share certain distinctive features with each other. Traditionally, each culture had its own name for the animals, plants, and other organisms in their region. But EACH culture had its own set of names, so the same type of animal might have many different names. During the 1600s and 1700s, methods were proposed for a formal scientific set of names. Following the work of Carl von Linne' (Linnaeus) in the 1700s (most specifically, the rules he set down in the Systema Naturae ("System of Nature") in 1758; later workers added and modified the system (primarily with the addition of new "ranks")), species were recognized as one unit within a nested hierarchy of larger clusters of organisms: taxa (singular, taxon; literally, "named thing").

Some of the Linnaean rules:

Linnaean taxonomy has its own special set of grammatical rules:

Type Specimens and Type Species: Another aspect of Linnaean taxonomy is that each species must have a particular type specimen. This is a particular individual preserved specimen (extant animal) or fossil (extinct animal) that is the "name holder" for that species. A type specimen is specifically referred to in the original description and diagnosis of the species. It need not be the most complete specimen known at the time (although that helps, as the more complete it is, the better the chance a less-complete individual can be compared to it!). The type specimen plus all the additional (referred specimens) are collectively called the hypodigm. Ultimately, if a species is regarded as being "valid" (that is, representing a real species in Nature), the type specimen is the only individual that is absolutely certain to belong that that species.

Similarly, each genus has a particular type species. This is the particular species to which the genus name is linked. If a genus is valid, the type species is the only species that is absolutely guaranteed to be within that genus.

As an example, CM 9380 (in the collections of the Carnegie Museum of Natural History) is the type specimen of Tyrannosaurus rex, and Tyrannosaurus rex is the type species of the genus Tyrannosaurus.

Parataxonomies: There is a formal set of names for some types of fossils that is parallel to, but independent of, the biological nomenclature of actual species and genera and the like. These are parataxonomies. For instance, there is an "ootaxonomy" of "oospecies" and "oogenera" and "oofamilies" of fossil eggs, and a whole complex of ichnospecies for trace fossils. These are even given italicized Latinate names and use rules of priority and the like. But these are names of the eggs, burrows, footprints, etc., and NOT of the organisms that produce them.

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).

Here are some cases:

Sometimes, by accident, two taxa wind up with the same name. These are said to be homonyms. In this case, the senior (earlier proposed) of the two names occupies the name (i.e., it gets to keep it!). The junior homonym needs a new name: maybe there is another name already proposed that could be used, but if not it needs a new name. For instance, a dinosaur was given the name Syntarsus in 1969; unfortunately, a modern beetle was given that name back in 1869! So the beetle occupies Syntarsus, and the dinosaur wound up being renamed Megapnosaurus in 2001.

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

But, What ARE Species?
What is a species? Above we see the rules for these names, but it doesn't tell us about what it is being named.

Linnaeus' "species" were taxa like lions, tigers, black bears, etc. These were assemblages of individuals that share certain attributes:

Darwin did not regard species as a distinct "kind" of biological entity. Instead, he considered them as essentially the same thing as geographic or stratigraphic variations (see these below), but ones in which extinction has removed the intermediate forms that otherwise would blend into the closest living relative group.

20th Century biologist Ernst Mayr (and most contemporary biologists) formalized their definition of a species as a "an array of populations which are actually or potentially interbreeding, and which are reproductively isolated from other such arrays under natural conditions". (Almost certainly you learned some version of this in high school and BSCI classes.) It works pretty well for the first pass: it emphasizes isolation, and thus species would represent pools of shared genes which do not get mixed with their closest relatives. Mayr and his followers refer to this as the "biological species concept" (or "BSC"), but as paleontologists Leigh Van Valen (see quote above) pointed out, this is an over-reach on the part of its proponents, and it is better termed a "reproductive species concept".

But there are some problems with this. For one: hybrids (crosses between two separate species) do occur naturally, and many of these are actually fertile! And for paleontologists: we can't test interfertility between populations because they are dead!

Other problems with the BSC exist, too:

This is part of what is called the species problem: it is damned difficult to find a good, useful criterion that can be broadly applied to our ideas of what species are and how we recognize their boundaries. Biologists have tried to come up with hard-and-fast rules about how to recognize species, and although they have named many such "species concepts", these tend to cluster around two major different ideas:

Although related, they really aren't talking about the same things necessarily.

There are other species definitions and concepts that people have tried to apply, but none have been able to universally encapsulate the diversity out there.

In real life, species do seem to have "fuzzy boundaries", and the distinction between different closely related species on the one hand and clusters of variation within a species are nearly impossible to tell. In fact, biologists go through shifts of fashion towards increasing splitting (the former idea) and lumping (the latter) over time. Currently the fashion is towards splitting: consequently, whereas in much of the 20th Century we recognized only one species each of African elephant, gorilla, orangutan, Nile crocodile, and orca, early 21st Century taxonomists recognize two or more. (On the flip side, dinosaur paleontologists seem to be following the opposite trend, lumping once-separate species and genera into each other).

As with many things, we run into problem with typological thinking: the idea that there are ideal types of things, and that we judge a specimens membership in a group by how well it conforms from that type. Instead, we find that variation is the reality. So we need to use population-based thinking. (Next lecture we will add tree-based thinking.)

Ultimately, for paleontologists we are stuck looking only at shapes (and in fact, only the shapes of those hard parts that survive fossilization).

The question then becomes: how different do two individuals, or two populations, have to be for us to consider them different species? This is actually a terribly difficult question even with living organisms!! There are several sources of variation:

In fact, the recognition that species were NOT absolute kinds, but instead have "fuzzy" boundaries that blend into each other, is one of the main clues to the discovery of evolution.

Identification and Display Features
Many animals engage in various types of display:

Some of these forms of display result in preservable "showy" features of the anatomy that might help us identify species more easily, or ironically confound us into thinking that two different sexes represent two different species!

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:

Identification and Ontogeny
Another potentially confounding issue is ontogeny. As organisms grow they might look profoundly different. Some undergo varying degrees of metamorphism: consider tadpoles vs. frogs, or caterpillars vs. butterflies. Such examples also occur in the fossil record: without sufficient information and samples of the various growth stages in the proper sedimentological context, we might mistake different growth stages as different species.

But it need not be so profound a change to be an issue. For some fossil species there remain debates over whether smaller individuals of a particular group from a formation are the juveniles of the larger species, or are smaller species that lived sympatrically (in the same time and place). Without a large enough sample size and sufficient numbers of intermediate stages, this might be very difficult to resolve.

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Last modified: 7 February 2018

Growth series of the Cambrian trilobite Elrathia kingii