FAUNAL ANALYSIS OF MIOCENE MOLLUSKS



        One of the most important tasks performed by paleobiologists is the quantitative analysis of fossil communities. Only by evaluating the species composition of fossil communities is it possible to reconstruct the ecological setting within which these organisms lived.

        Ancient faunas may also have been organized in fundamentally different ways than modern faunas. Preliminary research on middle Miocene shark faunas in the Pungo River Formation of North Carolina suggests they were very different from that of modern shark communities (Yeh, Meng-Wan; unpubl. data). A comparison of shallow water (units 6 and 7; <70 m of water) and deep water (units 4 and 5; 150-200 m of water) sediments indicate a surprising homogeneity in species composition. This differs noticeably from modern shark communities where shallow and deep water faunas are much more dissimilar.

        Studies on many ancient communities indicate it is foolish to assume that they were organized in a manner similar to that of modern faunas. One of the most important areas of modern paleobiological research is the analysis of community structure.

        Communities can be described by several different attributes or characteristics;

While all of these attributes are important in describing the totality of a community, the most important and basic is to determine the species richness and species evenness (collectively referred to as species diversity) of the community. Highly diverse communities have a very large number of more or less equally abundant species. Communities of lower diversity have fewer species, a less equitable distribution of species, or both.

        Ecologists have developed a number of indices for quantifying species diversity. Unfortunately, there is an important precaution when investigating species diversity for paleocommunities, namely, fossil communities are biased subsets of the original community, with more durable taxa being more likely to survive fossilization. To offset this difficulty, paleobiologists use a different means of studying paleocommunities, community similarity indices. These indices are (slightly) less vulnerable to differences in durability, but are still best applied to taxa with taphonomically-similar durability.

        The two most widely used community similarity indices are the faunal resemblance index (FRI) and the relative abundance index (RAI). FRI is a relatively simple method of comparing faunas, and is calculated as:

where Nc = number of taxa in common, and Ns = number of taxa in the sample with fewer taxa.

        The RAI is more difficult to calculate, but gives a somewhat more reliable estimation of similarity between the two faunas. RAI is calculated as:

where Ic = number of individuals in the taxa in common, I1 = number of individuals in sample 1, and I2 = number of individuals in sample 2.

Both indices can have values ranging for 0 (= no similarity between faunas) to 100 (= identical faunas).
 

Laboratory Methods

        In this exercise you will evaluate two fossil mollusk communities from the middle Miocene Choptank Formation on Calvert Cliffs along Chesapeake Bay. These samples are named for the color of the enclosing sediments -- the blue horizon (= bluish-gray sediment) and the orange horizon (rusty-brown sediment). Clean as many specimens as possible from each horizon. Many of these specimens will be broken, but this will not affect the final analysis. Set aside any bivalve specimen that contains a hinge and any gastropod specimen that contains the spire. These specimens will be tabulated and analyzed. Discard any specimens that lack a hinge or spire, as that cannot be used.

        Sort the specimens in each horizon and separate into morphospecies. Use the field guide provided for the field trip to identify each morphospecies to species. See species key below.

Bivalves

Gastropods

Tabulate the number of individuals in each species for each of the two horizons.
 


Analysis

        Construct a dedicated spreadsheet to analyze your data. Use the data to determine the level of similarity between the two faunas. Calculate both FRI and RAI values for the data, based on the total number of individuals for each species.

        There are a number of recognized molluscan biofacies (= assemblage of species indicative of particular environmental regimes) from the Maryland Miocene (Figure 2; modified from R. E. Gernant. 1971. Invertebrate biofacies and paleoenvironments. Maryland Geological Survey. Guidebook no. 3: Environmental history of Maryland Miocene. R.E. Gernant, et al. (eds.), pp. 19-30).


 

Questions

Are the differences in your calculated community similarity indices consistent with the differences between these environmental differences?
 

What types of information can you get from the species richness, species diversity and species evenness values that you cannot get from the community similarity indices?