PAST - Case study 9 - Lower Silurian marine communities from Britain

Case study 9 - Lower Silurian marine communities from Britain

A consistent pattern of faunal changes through increasing water depth has been reported from many Llandovery (lower Silurian) successions throughout the world. Five communities are sequentially developed: the Lingula, Eocoelia, Pentamerus, Stricklandia and Clorinda communities occupied depth bands from intertidal environments to at least the edge of the continental shelf. These communities or ecogroups have proved useful in defining a sequence of depth zones or Benthic Assemblage Zones (BA) with increasing depth from BA1 to BA6. The geographic distribution of these communities in the Anglo Welsh area is illustrated below (modified from Clarkson 1993, fig. 7.18):

Data abstracted from Cocks & McKerrow (1984) are available in llandov.dat. The presence or absence of fourteen key taxa across the five depth-related communities, Lingula, Eocoelia, Pentamerus, Stricklandia and Clorinda is recorded. The following taxa were included in the analysis together with their approximate community assignments:
Lingula community: Lingula, bivalves (all), Stegerhynchus.
Eocoelia community:: Eocoelia, Tentaculites, Leptostrophia.
Pentamerus community:: Pentamerus or Pentameroides, streptelasmid corals, Atrypa or Protatrypa.
Stricklandia community: Stricklandia or Costistricklandia.
Clorinda community: Clorinda, Coolinia, Glassia, Eoplectodonta and Aegiria.

Jaccard, Dice and Raup-Crick coefficients

Open the file Llandov.dat, select all, and choose 'Dice/Jaccard' in the Statistics menu:

Discuss the mutual differences and similarities between each pair of associations. Which pairs of communities are most similar and are there any recognisable trends? Also try the Raup-Crick measure, and compare with Jaccard and Dice.

More information about Jaccard, Dice and Raup-Crick coefficients can be found in the manual.

Cluster analysis of taxa

Select 'Cluster' in the Multivar menu. Try different algorithms and distance measures, and compare the results. Below is shown a dendrogram based on the Paired Group clustering of the Rho correlation. Comment on the clusters of taxa - do they conform to the five communities recognized by Cocks & McKerrow (1984) or do you recognise other groupings?

More information about the clustering module can be found in the manual.

Detrended correspondence analysis of associations

Select 'Detrended correspondence analysis' in the Multivar menu. Remove the 'Rows (taxa)' option, but switch on 'Column labels', 'Column dots' and 'Detrending'. Set 'Y start' to -10 to see the complete plot:

The program has ordered the associations along two ordination axes. What is the order of associations along the principal axis (Axis 1)? Is this in correspondence with the environmental gradient?

More information about the detrended correspondence analysis module can be found in the manual.

Seriation

Choose 'Seriation' in the Multivar menu and select unconstrained seriation. Both the columns and rows are free to move. Is the conventional faunal gradient across the Llandovery community system confirmed?

More information about the seriation module can be found in the manual.

Cluster analysis of associations

In order to cluster associations, we need to transpose the matrix (Edit menu). Select 'Cluster' in the Multivar menu again. Comment on the dendrogram bearing in mind the depth-related trends discussed above.