2006 06 20: Faunal turnovers and trilobite morphologies in the upper Cambrian Leptoplastus Zone at Andrarum, southern Sweden. Lethaia , Vol. 39, pp. 97 Á110. Oslo. ISSN 0024-1164.The Furongian (upper Cambrian) Leptoplastus Zone marks a time of critical changes in the evolution of olenid trilobites. This zone, unexposed at Andrarum in Skåne, southern Sweden, has been re-excavated and the sequence of faunas and sediments logged in detail. The faunal succession accords with that previously described from borehole cores by Westergård, and the subzones of L. paucisegmentatus , L. raphidophorus , L. crassicornis , L. ovatus , L. angustatus , and L. stenotus have been recognized. In the first two subzones the olenid assemblages are monospecific. At the base of the L. crassicornis Subzone more than one species is present and morphotypes with long genal spines appear for the first time. Faunal turnover is rapid, but the incoming of new species is invariably linked to an abrupt change in sedimentation, or follows an unfossiliferous interval; species either arose or migrated in after a time of environmental perturbation. Particular faunal associations are often confined to discrete sedimentary packages though some species may range through a succession of sedimentary changes. Leptoplastus crassicornis has very long genal spines, adapted for resting on the sea floor; it may have competed with the coeval, and very similar, L. angustatus . Subsequently, L. angustatus is accompanied by the stout-bodied, short-spined L. ovatus , which presumably occupied a different niche within the same environment. Leptoplastus stenotus is convergent on the much earlier L. paucisegmentatus , and likewise is found as a monospecific assemblage, presumably being adapted to a similar niche. I Andrarum , biostratigraphy, Cambrian , Leptoplastus Zone , palaeoecology, Skåne , Sweden, trilobites .Per Ahlberg [per.ahlberg@geol.lu.se],
In the Ordovician Northern Belt of the Southern Uplands, basal volcanics (Arenig) are followed by cherts (Llanvim-?Llandeilo), then by graptolite shales (LlandelbLower Caradoc), and finally by Caradoc greywackes. Within the greywackes (Kukcolm Formation) are a number of occurrences of fossiliferous conglomerate and overlying mudstone; these can be traced along-strike for some 30 km.The conglomerates, and especially the mudstones, yield rich mid-Caradoc shelly assemblages; brachiopods (20 spp.), trilobites (14 spp.), gastropods, bivalves, bryozoans, and the first known Palaeozoic scleractiniamorph coral.These occurrences are interpreted as mass-flow deposits derived by downslope movement from a now-vanished shelf to the North, and may belong either to a single gigantic debris flow event, or to a series of smaller, but roughly contemporaneous flows.Strong faunal similarities to faunas at Girvan (western Scotland), and Tyrone (Northern Ireland), lying North of the Southern Upland Fault may suggest sinistral strike-slip movement of no more than a few hundred kilometres.
This paper completes the description of intact and three-dimensional Ctenopyge species from the upper Cambrian Peltura minor Zone in Västergö tland, central Sweden. All these species are present together, on the same bedding planes. The most abundant species, Ctenopyge (Eoctenopyge) angusta Westergård, 1922 has previously been described, and an almost complete ontogeny worked out. C. (Ctenopyge) gracilis Henningsmoen, 1957 is a small trilobite with nine thoracic segments and very long, thin curving and subparallel thoracic spines; the genal spines partially encircle the body. Two axial spines at the rear are of considerable length. When reconstructed in side view, the posterior thoracic spines rise upwards as an inclined fan, but when relaxed the tips of all the thoracic and axial spines come to lie in the same plane as the horizontal genal spines. An almost complete ontogeny is described for this species, and individuals show an evident spinosity from an early stage, but the body size at which thoracic segments are liberated is highly variable. C. (Ctenopyge) ahlbergi n. sp. is a larger, robust and broad species distinguished by long, stout genal spines, ten thoracic segments, and a very spiny body with the first three to four spines expanded into lateral flanges. A degree 6 meraspis shows these flanges already developing. C. (Ctenopyge) rushtoni n. sp has likewise ten thoracic segments, and has stout, broad-based and tapering spines. Incomplete meraspides 6 and 7 are known for this species. In both C. (Ctenopyge) ahlbergi and C. (Ctenopyge) rushtoni there are also two axial spines at the rear, and the extended body would have had a similar rising tail fan to that of C. (Ctenopyge) gracilis. C. (Mesoctenopyge) tumida is also present as a single large adult and several smaller holaspides. In this species the first thoracic segment is confirmed as bearing a pair of long curving spines, somewhat smaller than the encircling genal spines. The remaining thoracic spines are straight and sharp, and evidently longer in young holaspides. There is a single long axial spine on the last segment. No adult pygidium has been found.Some comments on the diversity of the fauna as a whole and the range of functional types are appended.
ABSTRACT. The genus Ctenopyge is known mainly from disarticulated sclerites and from rare complete specimens attened in shales. Hitherto, very few specimens have been found preserved intact and in three dimensions. In a recently discovered fauna, however, in the Peltura minor Subzone in Va Èstergo Ètland, central Sweden, there occur several species of Ctenopyge, of which many are complete and superbly preserved; moreover they occur at all stages of growth. Of these the abundant Ctenopyge (Eoctenopyge) angusta Westerga Êrd, 1922 is described and reconstructed here as an adult, and the entire ontogeny is documented for all post-protaspid growth stages. Many characters typical of the adult, such as the long genal spines and the caudal spine, develop very early in ontogeny, and the relative dimensions of the cranidium do not greatly change during growth. Macropleural spines, however, develop later. The transitory pygidium, relatively large and shield-shaped in the early meraspid, later becomes very small as the ten thoracic segments are liberated; a median spine develops on the last thoracic segment only at the holaspid stage. Instar groupings can be clearly distinguished for the early stages. Recurrent associations of sclerites are interpreted as moulting con®gurations. As reconstructed, the genal spines are horizontal and parallel with the extended thorax; an adaptation which presumably allowed the trilobite to rest on the sea¯oor.
The upper Cambrian trilobite Olenus wahlenbergi Westergård, 1922 occurs abun- dantly in stinkstone concretions in the Alum Shales at Andrarum, in Skåne, southern Sweden (Olenus/Homagnostus obesus Zone). All growth stages from anaprotaspis onwards are represented, though the early stages are comparatively uncommon and complete specimens are rare. Fine details of structure are preserved, suitable for scanning electron microscopic study.The protaspis has five axial rings, which in the adult become divided transversely and reduced to three, and the preglabellar field expands markedly in the holaspis. The librigena broadens during development and the eye, with its widely separated lenses is retained on the librigena throughout meraspid development. The visual surface dehisces during ecdysis from the early holaspid onwards, due to emplacement of the ocular suture. The hypostome apparently changes from a conterminant to a natant condition in the early holaspis, which is consistent with the growth of the preglabellar field at this stage. The transitory pygidium has a slightly serrated margin throughout development, but becomes entire when all thoracic segments have been liberated.Cuticular sculpture changes dramatically throughout ontogeny. Large (Type A) tubercles appear on the fixigena in early meraspides; these become prominent and coalesce by the end of meraspid development. They are abruptly replaced by smaller (Type B) tubercles in the early holaspid, which link to form a rough network in mature specimens. Comparable Type A tubercles in early meraspid librigenae give rise to a strong polygonal caecal sculpture in holaspids, while a radial sculpture on the broadening anterior border develops independently. The significance of these cuticular changes remains obscure. A median occipital organ is described for the first time in the Olenidae.
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