Taphocoenoses of Arthropods on Saltpan Floodplains of the Oshana Region / Namibia (Arthropoda: Solifugae, Scorpiones, Chilopoda, Diplopoda, Insecta)

Abstract

A taphocoenosis was found (Steffan 1991) in the saltcrust of the floodplain of a saltpan (Okandeka Pan) north of the Etosha Pan. From the resulting collection, 3 species of Solifugae (3 individuals), 3 spp of Scorpiones (10 ind), 1 sp of Chilopoda (6 ind), 1 sp of Diplopoda (2 ind), and 85 spp of Insecta (580 ind) were identified (A Mursch). All of them were of non-aquatic origin, i.e: allochthonous. The possible ways of active or passive introduction onto the floodplain are discussed for the various groups as well as the probable manner of their selection from the neighbouring biocoenosis. The overrepresentation of reproductives of eusocial insect species (Isoptera: Termitidae, Hodotermitidae; Hymenoptera: Formicidae: Dorylinae) which are on the wing during distinct seasons only, suggests that their embedding in the saltcrust happened soon after this seasonal event. Most of the other subfossilized arthropods might have invaded the floodplain at the same time, either actively or unvoluntarily by air, or they have been washed by rainfloods from the surrounding shoulders of the saltpan onto the floodplain, dead or still alive. There are two embedding zones, an outer one where mainly larger arthropods (Arachnida, Myriapoda, Coleoptera) with strong and heavy exoskeletons are to be found in spots, and an inner embedding zone, where small insects, insect and plant particles are assembled in larger layers. Most of the large and middle-size insects were embedded in the 0,3-1,0 cm thick saltcrust, with their bodies in parallel, and almost parallel to the northern shore-line. This means, that before conservation in the salt-crust they must have floated at the surface of shallow water, must have been drifted by wind and waves towards the northern shore, and with this movement must have been sorted out as by size and weight. With the gradual water decline, they were left on the muddy ground earlier or later, and with further drying and crystallising of the surface layer, became tightly embedded into the salt-crust. A consideration of the actual initial phases of fossilisation reported here, i.e: the manner of selection of species and specimens from the nearby biocoenotic resource, as well as of their various ways of active invasion or passive transportation into the taphotope, and their being sorted out by water movements, might contribute to the understanding of fossilisation processes in terrestrial habitats of the past. To evaluate possible similarities, this actuo-taphocoenosis was partly compared with palaeo-taphocoenoses primarily described from Tertiary times.