Redescription of Opsipedon Gracilis Heer (Crustacea, Tanaidacea) from the Middle Jurassic of Northern Switzerland, and the Palaeoenvironmental Significance of Tanaidaceans

Abstract

ABSTRACT. Opsipedon gracilis Heer, 1865, is the oldest valid name for a fossil tanaidacean crustacean. The type material, which was originally described as coming from the Lower Jurassic`Insect marls' (Insektenmergel, Hettangian), is actually of Middle Jurassic age (Opalinum Clay, Aalenian). A redescription of this species is given, and its position within the suborder Apseudomorpha is established. A new model is proposed which relates the occurrence of fossil tanaids with palaeo-oxygenation, both long-term average values and intensity of (seasonal) uctuations. Many tanaidaceans occur in oxygen-depleted benthic environments, but they cannot tolerate anoxia for more than a few days. Therefore, tanaidaceans are completely wiped out in sedimentary basins that experienced anoxia. After such an event, recolonization proceeds very slowly because tanaidaceans do not possess a planktonic larval stage, and their rate of dispersal as juveniles and adults is exceedingly low. Therefore, fossil tanaidaceans have great potential for aiding distinction between more or less stable dysoxic environments, and environments with recurrent bottom-water anoxia (poikilaerobic biofacies). In the latter case, no fossil tanaidaceans occur.

A well‐preserved isopod from theMiddleJurassic of southernGermany and implications for the isopod fossil record

“…Although fossils of isopods are more common than those of any other peracarid crustacean group, this fossil record has been until now of limited value, quite in contrast to the situation in tanaidaceans (Schram et al . ; Etter ). The earliest isopod, Hesslerella shermani Schram, dates from the Carboniferous and is a marine member of the Phreatoicidea (Schram ; Wilson and Edgecombe ).…”

Three new tanaid species (Crustacea, Peracarida, Tanaidacea) from the Lower Cretaceous Álava amber in northern Spain

“…A* the time, this included C. emersoni, Jurapseudes friedericianus (Malzahn, 1965), and J. acutirostris (Sachariewa-Kowatschewa and Bachmayer, 1965). Subsequently, Etter (2004) described the long over looked Opsipedon gracilis Heer, 1865 and assigned that taxon to the jurapseudoids as well. C. emersoni and J. friedericianus were known to be dikonophoric and other species of jurapseudoids were considered to be so as well.…”

“…This is so despite the fact that if the nature of modern tanaidaceans is any indicator, this is a group characterized by small animals. However, as Etter (2004) wrote, the potential of tanaidaceans to form fossils must be quite high. They live in self-made tubes or burrowed tunnels in muddy sediments, spend most of their life within or near those tubes, and seem to prefer low oxygen and low energy environ ments.…”