Mid-cautleyan (Ashgill Series) trilobites and facies in the English lake distinct

Abstract

During Zones 2 and 3 of the Cautleyan Stage (Ashgill Series) a marine transgressive sequence of conglomerates, sandstones (Longsleddale Formation), calcareous siltstone (Stile End and High Pike Haw Formations), mudstone (Torver Formation) and interbedded argillaceous limestone and calcareous mudstone (Applethwaite Formation) was deposited in the southern Lake District. Three tribolite associations are recognized within these rocks: a Proetid Association, found in the sandstones and calcareous siltstone; an Illaenid Association in the mudstone of the Torver Formation and at local, restricted horizons within the Applethwaite Formation; a Calymenid Association, found in the interbedded mudstone and limestone. It is suggested that strong causal relationships, both positive and negative, existed between the trilobites and facies type. Absences of particular trilobite groups do not appear, in the main, to be artifacts of sampling and, along with presences and abundances, are considered important to the understanding of the distribution of trilobites and their relationships with the facies type. Antipathetic relationships are noted between Prionocheilus and other calymenids; and between Tretaspis and both proetids and encrinurids. Because the trilobite associations could be recognised in the field prior to statistical analysis, it is possible to compare and contrast quantitative measures which summarize the relationships among localities and taxa. The following measures are calculated from abundances: correspondence, covariance (taxa) or average Euclidean distance squared (localities), and correlation coefficient (taxa) or standardized average Euclidean distance squared (localities). Of these the principal axes of the correspondence matrix separate the three associations much further than the other two measures. Considered as comparisons of taxa, correspondence values may offer a compromise, desirable in quantitative ecology, between emphasis on common taxa (covariance) and on rare taxa (correlation coefficient). When only occurrences are used, the trilobite associations are scattered more broadly and taxa behave more independently. The greater degree of interdependence of taxa when abundances are used is considered helpful to characterization of association in this study.