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Statistical Analysis of British Carboniferous Conodont Faunas

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Statistical Analysis of British Carboniferous Conodont Faunas Statistical analysis of British Carboniferous conodont faunas EDRIC C. DRUCE, FRANK HAROLD TREVOR RHODES, & RONALD LEYSHON AUSTIN CONTENTS I Introduction . 53 2 Statistical recognition of assemblages . 55 3 Method ..... 56 4 Interpretation of dendrograms .... 58 5 Detailed analysis of conodont associations . 58 6 Frequency of elements in individual samples. 66 7 Conclusions 67 8 References 69 SUMMARY Three lines of evidence are reviewed that determined. This indicates common occur- suggest conodont animals contained multi- rences of elements and can be supplemented element conodont assemblages in life. Re- by comparisons of total stratigraphic range and construction of the original multi-element relative frequency of elements. Thirteen groups assemblages of conodonts from the Cleistopora are recognized which are compared with (K) and Zaphrentis (Z) Zones of the North assemblage models based on assemblages Crop of the South Wales Coalfield is attempted preserved in situ on shale bedding planes. using the Centclass programme on a Control Group 8 compares with a basic duboisellid Data 36oo computer. Two methods are (Class C) type of assemblage. Groups 5 and described which are of use in assemblage 6 do not appear to represent meaningful reconstruction. Polythetic cluster analysis, first groups. All other groups represent either developed for use in ecological surveys, complete or partial Lochreia type Class A provides dendrograms from which the level of assemblages. association of individual elements may be i. Introduction THE FIRST SUGGESTION that a number of conodonts was associated in the original conodont animal was made by Hinde (1879 , pp. 361-364) who described what he concluded to be a natural assemblage from the Upper Devonian Genessee Shale of New York. As Lindstr6m (1964, p. 75) has pointed out, these specimens were strewn randomly over a shale surface, and it was a mere guess that they belonged together in life. Natural assemblages were first described independently by Schmidt (1934) from the lower Upper Carboniferous of the Rhineland, and by Scott (1934) from the lower Upper Carboniferous of Montana. The Lower Carboniferous/Upper Carboniferous boundary has yielded further natural assemblages 'in situ' on shale surfaces. These have been reported by Kraemer (194o), Schmidt & Mfiller Jl geol. So¢. vol. x~8, I972 , pp. 53-70. Figs. x-8. Printed in Northern Ireland. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/128/1/53/4884414/gsjgs.128.1.0053.pdf by guest on 29 September 2021 54 E. C. Druce, F. H. T. Rhodes & R. L. Austin (I964) , Scott (I934, i942 , i969) , Du Bois (i943) , and Rhodes (I952). The last paper, on assemblages from the Pennsylvanian of Illinois, deals in detail with the nature, interpretation, and classification of natural conodont assemblages. We also have assemblages from the Upper Carboniferous of Britain. Lange (I968) has described about seventy assemblages of late Devonian age, associated with bituminous matter. He recognized, five distinct types of association within this collection. Recently Melton & Scott (I97O) have described natural conodont assemblages of the genera Lewistownella and Scottognathus associated with impressions of soft parts that they interpret as the remains of a conodont bearing animal. Walliser (1964) recognized the repeated association of certain form species in the Silurian of Germany, and constructed model assemblage associations on the basis of common stratigraphic ranges and constant association ratios. He recognized ten natural groups and was able to show the mode of infraspecific evolution within a biological species. Recently the study of assemblages has been extended to the Ordovician. Multi-element species (BergstrOm & Sweet I966 ) or statistical assemblages (Webers 1966; Schopf 1966) have been recognized from the Middle Ordovician of North America. The criteria used to recognize these assemblages are stratigraphic distribution, relative abundance (Webers I966 , p. 7), size, colour, and secondary structural features as denticulation, attachment-surface morphology, and orna- mentation (Bergstri3m & Sweet x966). The last criterion has been used to recognize an assemblage from near the Middle-Upper Devonian boundary in Germany (LindstrOm & Ziegler i965). Kohut (I969) has used a rank-correlation statistical technique to confirm the validity of the groupings suggested by Bergstr6m & Sweet (i 966) on empirical grounds. Evidence of natural conodont assemblages has been extended by the discovery of fused units from the Carboniferous of England (Austin & Rhodes I969) , the Silurian of Indiana (Rexroad & Nicoll x964), (Pollock x968), and the Middle Ordovician of Canada (Barnes 1967). We have examined the sequence of conodonts from the Lower Carboniferous of Great Britain (Rhodes, Austin & Druce i969). All our samples were recovered from chemically disintegrated samples and only disjunct units were recovered. The success of Bergstr6m & Sweet (I 966) and Webers (1966) in recognizing natural assemblages by statistical methods and the presence of natural conodont assem- blages in black shales from Carboniferous strata persuaded us to examine our faunas statistically. There are thus three general lines of evidence which suggest that conodont animals bore a multi-element conodont assemblage in life: the 'in situ' assemblages known from certain rocks representing low-energy environments, elements from insoluble residues that are fused together, and individual species that have common stratigraphic range and relative frequencies. In our attempt to reconstruct the original multi-element assemblages of lower Carboniferous age, we make three fundamental assumptions. Firstly, our com- puter analysis is based upon the assumption that relative frequency of association of elements in insoluble residues provides a guide to original association. Secondly, Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/128/1/53/4884414/gsjgs.128.1.0053.pdf by guest on 29 September 2021 Statistical analysis of British Carboniferous conodontfaunas 55 we assume that, in the absence of any known Lower Carboniferous assemblages, we may use those of the Upper Devonian and especially those of the Upper Carboniferous to provide models for our present reconstructions. These have been used to interpret our computer-based groupings. Thirdly, we have assumed that the hypothetical assemblages we have reconstructed by these two methods may be tested by a comparison of the relative frequencies and stratigraphic ranges of their component elements. 2. Statistical recognition of assemblages Any method of classifying objects into sets involves two independent choices. Firstly, it may be divisive, dividing the original population into successively smaller groups, or it may be agglomerative, combining individuals into successively larger groups. Secondly, the classification may be monothetic, every group at every stage being defined by the presence or absence of particular attributes, or it may be polythetic, the groups being defined by general similarity of attribute structure. Lambert & Williams (r 966) have examined each of these four possible hierarchi- cal analyses in turn and conclude, in the case of the ecological analysis which most resembles our present situation, the most meaningful is the agglomerative, poly- thetic method. The strategy of fusion employed in our present programme is based on centroid sorting, where the most similar pairs of individuals are added together to produce new synthetic individuals, who are in turn compared with all others. Lambert and Williams have shown that, although this strategy requires more computer storage than a 'nearest neighbour' sorting, it does produce groups which grow in information content and become less sensitive to errors as the analysis proceeds. The programme developed by Dr Williams provides a direct print-out of the final dendrogram, with the component species numbers represented along the horizontal baseline, and the relative level of each subpopulation above the baseline reflecting its position in the overall hierarchy. The requirements of a 'good' dendrogram are that the groups should be well marked and differentiated at roughly comparable levels, and that they should be meaningful in the light of their independent data. Our results satisfy both these requirements. We wish to record, however, that similar analyses made upon samples from the D Zone of Scotland and the North Crop of the South Wales Coalfield gave ambiguous results. We interpret these as due to the smaller size of the Scottish and Welsh samples, ade- quate sample size being a fundamental requirement of this particular programme. This same programme has also been employed for the recognition of lithological facies (Veevers I969). The relative rarity of Devonian conodont assemblages has led us to base our model for these Mississippian faunas on typical Pennsylvanian Class A assem- blages (Rhodes I962 , p. W73 ). These generally consist of one or more pairs of platform type conodonts, arbitrarily designated as anterior, one or more pairs of arched blades, with or without a third denticulate process (such forms as Ozarko- dina and Hibbardella) and several pairs of 'posterior' elements, including such Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/128/1/53/4884414/gsjgs.128.1.0053.pdf by guest on 29 September 2021 56 E. C. Druce, F. H. T. Rhodes & R. L. Austin pick-shaped forms as Neoprioniodus, and such elongate blade-like forms as hindeo- dellids and
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