The depositional environment and taphonomy of the Homerian “Aulacopleura shales” fossil assemblage near Loděnice, Czech Republic (Prague Basin, Perunican microcontinent)

Abstract

Excavation of Joachim Barrande's classic fossil locality of the ``Aulacopleura shales{''} exposed on Na Cemidlech Hill near Lodenice reveals that most specimens were recovered from a 1.4 m interval exposed in ``Barrande's pits{''}. These are located at the eastern end of a 0.4 km trench dug in the mid 1800's to expose the interval along strike. Over an hundred bedding planes occur within the 1.4 m interval and thousands of articulated trilobites have been collected at the site. Individual bed surfaces vary in the density size and taxonomic composition of the fossils contained. Some preserve a diverse benthic shelly fauna others are almost exclusively dominated by the trilobite Aulacopleura koninckii and a third variety is apparently barren of all shelly fossils. Isolated sclerites of A. koninckii are rare and on almost all bedding surfaces exoskeletons are predominantly partially articulated and lack both alignment and sclerite fragmentation. The occurrence of A. koninckii conforms in many ways to the characteristics of a Type I trilobite lagerstatte of Brett et al. (20:12). The presence of enrolled A. koninckii suggests that final burial may have resulted from relatively rapid obrution although the condition of partial articulation indicates that many carcasses or exuviae partially disaggregated before burial. The mean size and density of A. koninckii specimens varies markedly among bedding planes with some assemblages entirely comprised of juveniles suggesting that notably dense trilobite clustering was not restricted only to reproductively mature individuals. The presence of multiple clusters of different mean specimen size partly accounts from the unusually comprehensive record of the articulated meraspid and holaspid ontogeny of this species at this locality. Limited bioturbation suggests a dysoxic substrate and the olenimorphic form and distribution of A. koninckii combined with a lack of encrustation or predation upon it suggests that this species may have periodically bloomed in abundance at particular oxygen concentrations that largely excluded other skeletonized benthos. Some bedding plane assemblages might represent mass mortality events perhaps as available oxygen passed below levels necessary to maintain the metabolism of A. koninckii.