The continuum in soft-bodied biotas from transitional environments: a quantitative comparison of Triassic and Carboniferous Konservat-Lagerstätten

Abstract

The exceptionally preserved Triassic (Anisian) Grès à Voltzia fauna from the Vosges of northeastern France is compared with four major Carboniferous Konservat-Lagerstätten. The problem of comparing faunas with different types of preservation and degrees of taxonomic determination is addressed with a newly devised similarity coefficient. This coefficient quantifies and combines data from different levels in the taxonomic hierarchy and allows comparisons between biotas in a consistent direction regardless of relative diversity. The rank order of similarity between the four Carboniferous Konservat-Lagerstätten and the Grès à Voltzia is as follows: 1. Mazon Creek (Westphalian D) of Illinois; 2. Glencartholm (Visean) of southern Scotland; 3. Bear Gulch (Namurian) of Montana; and 4. Blanzy-Montceau (Stephanian) of France. These occurrences represent conditions transitional between nearshore fully marine and fresh water. The Grès à Voltzia fauna is significantly closer to the Mazon Creek fauna than to the others; the taxonomic overlap with Blanzy-Montceau and Bear Gulch is limited.Stratigraphic age has an insignificant influence on the result; indeed, the fauna closest in age to the Grès à Voltzia, that of Blanzy-Montceau, is least similar. Taphonomic factors are important in determining the range of organisms preserved. The Glencartholm fauna is represented only by forms with either mineralized or robust chitinous skeletons, implying a greater degree of decay prior to the onset of diagenetic mineralization than in the other Konservat-Lagerstätten. Environment, however, is the major control on similarity. The Mazon Creek biota, like that of Grès à Voltzia, represents settings transitional between terrestrial and marine-influenced delta. Groups common to both Konservat-Lagerstätten include medusae, brachiopods, polychaetes, bivalve and gastropod molluscs, limulids, scorpions, spiders, branchiopods, ostracodes, malacostracans, cycloids, euthycarcinoid and myriapod uniramians, insects, fish, and tetrapods. There is a striking continuity between the faunas of Carboniferous and Triassic transitional sedimentary environments. Groups that were adapted to fluctuating conditions (e.g., shifting salinity) show strong congruence at the family and lower levels and were little affected by Permian extinctions. The major taxonomic contrasts are in the eumalacostracans and insects: many of the groups represented in the Grès à Voltzia appeared in the Permian and radiated across the Permo-Triassic boundary as Paleozoic forms became extinct.