30.1: Diversity vs. abundance

Figure \(\PageIndex{1}\): Look at the diversity and abundance of Ordovician fossils in this limestone. How would you count everything in this rock sample? (Photo by K. Layou)

The fossil record is full of different characters that provide rich details about the story of Earth’s story–multiplying bacteria, scuttling trilobites, darting fish, stomping T. rex, flowering plants. Certain taxa follow one another through time and the vertical sequence of strata in a principle defined by William Smith as faunal succession. Major appearances and disappearances of vertebrate groups were used in part to create the first versions of the geologic time scale. But a deeper consideration of the fossil record can yield many fascinating questions about the history of life.

Counting taxa

First, there is the consideration of diversity–typically this is defined as the number of taxa present within some defined timeframe, geographic area, or stratigraphic layers. Let’s break that down a bit. Recall a taxon is one level of the Linnean taxonomic hierarchy–domain, kingdom, phylum, class, order, family, genus, or species. Paleobiologists can ask questions at a variety of levels–when did the major phyla originate? Why do some genera have more species than others? All of this is dependent on the traits that place an organism within the hierarchy. At higher taxonomic levels, key traits are relatively easy to identify. For example, all arthropods have a segmented body, or all echinoderms exhibit 5-fold (pentameral) symmetry. But as you dig down deeper into the lower taxonomic levels, defining criteria become less clear, particularly at the species level.

Figure \(\PageIndex{2}\): Sexual dimorphism in spiders. Larger female on left, smaller male on right. (Creative Commons Attribution 2.5 Generic license.)

The species concept is a long-discussed, little-resolved aspect of biodiversity. There are numerous ways taxonomists, biologists, paleontologists, and evolutionary scientists have defined a species–we will only highlight a few of the most common here. The biological species concept, which most people are familiar with, is based on reproductively isolated populations of individuals–for example, fish in mountain lakes at high elevations. However, this definition is highly biased toward sexually reproducing eukaryotes. There are many eukaryotic organisms, and entire domains of prokaryotes that reproduce asexually. This definition is also problematic for the fossil record, as we cannot directly observe which fossil organisms were able to successfully lreproduce with each other. Paleobiologists, therefore, commonly apply the morphological species concept, using fossilizable characteristics of shape and form to define species. Some specimens in museums are often described as the “type” specimen of species because that fossil was used to formally describe the species. Can you think of any issues with this method of defining a species? There are definitely a few–consider the issue of sexual dimorphism (difference in males and females of a species, say in size, presence of antlers), or individual age (size of juveniles versus adults), or even ecophenotypic variation (variation in shell thickness in calm vs. rough currents). There is also the tendency of the taxonomist at work to be a “lumper” or a “splitter” in terms of recognizing nuances of specimens. Some organize specimens into fewer, more inclusive groups (lumpers), while others default to defining a greater number of more exclusive groups (splitters). The evolutionary species concept focuses on a clade, or single lineages of a population and its descendants that are distinct from others. Again, this is challenging to directly apply to the fossil record, and will still be dependent on fossil morphological analysis and rRNA molecular clock data.

Counting individuals

Figure \(\PageIndex{3}\): Turritella sp. snails, Yorktown Formation, Virginia. Full snails are about 2.5 cm long from apex to aperture. (Photo by K. Layou)

Another consideration of understanding life in the fossil record is fossil abundance. While thinking about diversity provides a list of all the species present in a sampling unit, paleobiologists may also want to consider how many individuals of each species exist as well. There are many ways to collect abundance data and the method used is typically dependent on the hypothesis being tested. In some cases, relative abundance counts are appropriate. This is often done using the terms rare, common, and abundant, but these terms should be given some guideline amounts. For example, rare might be 1-3 specimens; common 4-10 specimens; abundant, more than ten specimens. The number of specimens using this method will vary from study to study though. In many cases, quantitative analyses of ecological paleocommunities require absolute specimen counts. Once again, depending on the fossils, the study needs to define how counts are made. For whole individuals, like a snail, it is pretty easy to say one shell equals one individual. But for bivalved shells like clams, or complex vertebrate skeletons, it may be impossible to say if two shell or bone fragments are from one or more individuals. Additionally, many arthropods like trilobites molt their exoskeletons as they grow, so one individual could leave behind many exoskeletons! Therefore, attention to details like the number of right vs. left valves or individual skulls may be helpful.