Trilobites and bradoriid arthropods from the Middle and Upper Cambrian at Gudhem in Västergötland, Sweden

Abstract: Middle and basal Upper Cambrian strata are well exposed in an old quarry at Gudhem in the Falbygden area of Västergötland, south-central Sweden. The exposures consist of finely laminated alum shale with scattered stinkstone (orsten) lenses, up to 2.3 m in diameter. Four sections have been measured and sampled in order to establish the succession of trilobite species. Fossils are generally preserved only in the stinkstone, and not in the shale. Trilobites, including agnostids, and bradoriid arthropods are generally common, whereas lingulate brachiopods and hyoliths are minor faunal constituents. In the northeastern wall of the quarry the rock sequence is about 3.6 m thick, and includes the Middle-Upper Cambrian boundary. The lower and middle part of this section contains agnostids indicative of the Lejopyge laevigata Zone. These include, e.g., Lejopyge laevigata, Diplagnostus planicauda, and Peronopsis insignis. A bradoriid arthropod, Anabarochilina primordialis (Linnarsson, 1869), is common throughout the L. laevigata Zone. Stinkstone lenses in the topmost part of the section contain Agnostus pisiformis in abundance, indicating the A. pisiformis Zone. A conglomeratic limestone (the Exporrecta Conglomerate) was previously exposed at the base of the section (Wallerius 1895, 1930). In the southwestern wall of the quarry the rock sequence is 1-3 m thick. A fairly diverse fauna, including abundant L. laevigata, L. armata, Hypagnostus sulcifer, Diplagnostus planicauda and Anabarochilina primordialis along with Agnostus neglectus, A. pater, Acrocephalites stenometopus, Toxotis pusilla, Proceratopyge conifrons and Peronopsis insignis, shows that this section is referable to the L. laevigata Zone. In the northwestern part of the quarry, this zone is overlain by the A. pisiformis Zone. As in the northeastern wall, unfossiliferous alum shales occur between stinkstones containing the L. laevigata and A. pisiformis faunas. Hence there is an interval (~70 cm) of uncertainty concerning the zonal and series boundaries. At the northwestern entrance to the quarry there are exposures of the Hypagnostus parvifrons Zone. A section here consists of a more or less coherent limestone bed at the top, underlain by at least 4 m of unfossiliferous alum shales. The limestone bed has yielded H. mammillatus in abundance, as well as H. parvifrons and fragmentary specimens of Paradoxides paradoxissimus.