Faunal dynamics and microevolutionary investigations in the Upper CambrianOlenusZone at Andrarum, Skåne, Sweden

Publisher Website
Google Scholar
Abstract
The Upper Cambrian Homagnostus obesus/Olenus Biozone is well exposed in the old quarries at Andrarum, and is typically represented by unfossiliferous black shales with fossiliferous carbonate concretions. Near the north end of the “Great Quarry”, however, the shales themselves are replete with fossils, and in these a 1.8 m section has been logged to establish how trilobite populations fluctuated through time. Successive bedding planes were examined, at 1-cm intervals where possible, and for each surface trilobite exuviae, with rare complete specimens, were counted within a 5×5 cm quadrat. Olenus and Homagnostus are very common, but their abundance fluctuates dramatically; they co-occur in the upper part of the sequence but are otherwise almost mutually exclusive. This suggests that they had different ecological requirements. The rare Glyptagnostus and the bradoriid “ostracode” Cyclotron are confined to particular levels; the latter genus is found where no other fossils are present and may have been tolerant of very low oxygen levels. Eight horizons or bands are defined, based upon occurrence and abundance of the faunas; these bands are sometimes separated by clays, possibly relics of storm events. The logged section partially overlaps with that of Kaufmann. In our shale succession, Olenus truncatus, O. wahlenbergi and O. attenuatus are present in sequence, and the lengthening and narrowing of the pygidium, and the development of lateral spines in O. attenuatus as recorded by Kaufmann, are confirmed.