The pitfalls of extrapolating modern depth ranges to fossil assemblages: new insights from Middle Jurassic brittle stars (Echinodermata: Ophiuroidea) from Switzerland

Abstract

Depth reconstruction based on the extrapolation to fossil assemblages of present-day depth ranges of closely related groups is one of the most widely used approaches in palaeobathymetry. Here, we assess the ophiuroid fauna of the Bajocian to Bathonian (Middle Jurassic) Hauptrogenstein Formation and coeval formations in Switzerland with respect to the depth ranges of extant members of the groups identified. In addition to previously known taxa, we describe three new species, one assignable to the extant genus Ophiotholia within the family Ophiomycetidae (resurrected herein), and two belonging to new genera within the family Ophiacanthidae. The Hauptrogenstein ophiuroid fauna is shown to display a striking similarity to modern bathyal brittle star assemblages. In combination with taphonomic evidence of the autochthonous nature of the ophiuroid occurrences, the direct extrapolation of present-day depth ranges, as performed in various previous studies, would imply the Hauptrogenstein Formation to have been deposited in a bathyal setting. This, however, is in stark contrast with the generally accepted, sedimentology-based concept of this unit as a very shallow, high-energy carbonate platform deposit. Evidently, direct extrapolation of modern depth distribution patterns fails to provide a reliable palaeobathymetrical assessment here. In this respect, the case of the Hauptrogenstein ophiuroid fauna serves as a remarkable example to stress the pitfalls of assemblage-based palaeodepth estimates: (1) depth distribution patterns might not be controlled by water depth, or not even by a factor directly related to depth, (2) habitat preferences of a group might have changed through time without being reflected by morphological modifications and (3) shifts in depth ranges might occur due to the rise or extinction of groups interacting with the organism in question. Thus, extrapolation of present-day depth ranges to ancient communities can only produce reliable palaeodepth estimates if there is a mechanistic explanation why organisms are confined to a particular depth.