Faunal turnovers and trilobite morphologies in the upper CambrianLeptoplastusZone at Andrarum, southern Sweden

Abstract

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],

Faunal succession in the upper Cambrian (Furongian) Leptoplastus Superzone at Slemmestad, southern Norway

“…Leptoplastus raphidophorus occurs frequently in sections 1 and 2 and cooccurs in the former with E. latum, L. norvegicus and E. brevicauda. Such an assemblage has not been recorded previously; Terfelt et al (2011) Ahlberg (2003) reverted to five zones by leaving out the L. crassicornis Subzone, but subsequent ly Ahlberg et al (2006) reinstated this subzone. Terfelt et al (2008) abandoned the traditional zonation of the Furongian by elevating the subzones to zonal rank.…”

“…Basal stratotype: Andrarum, Caroli Shaft, Profile 6 sensu Westergaård (1922, p. 21), redescribed by Ahlberg et al (2006, fig. 3).…”

“…Henningsmoen (1957) outlined a Norwegian zonation, embracing the Leptoplastus raphido phorus, L. crassicornis, L. ovatus, L. angustatus and L. stenotus zones. The L. paucisegmentatus Zone, recognised in several districts in Sweden (Westergård, 1944(Westergård, , 1947Ahlberg et al, 2006), has not been recorded in…”

“…Here the Leptoplastus Superzone is preserved essentially intact and it is possible to reconstruct the succession of trilobites despite the structural overprinting. Henningsmoen (1957) (Westergård, 1944(Westergård, , 1947Ahlberg et al, 2006), has not been recorded in the Oslo area. However, the sequential order of the 'first appearance datum' (FAD) of the key species L. crassicornis, L. ovatus and L. angustatus differs between Slemmestad and Scania (Ahlberg et al, 2006 and present study), suggesting that the order of appearance reflects ecological control rather than chronostratigraphy.…”

Exotic trilobites from the uppermost Cambrian Series 3 and lower Furongian of Sweden

“…The subsequent major change in the faunal composition (e.g., Lauridsen and Nielsen 2005; Ahlberg et al 2006;Eriksson and Terfelt 2007) is reflected in the predominance of the Olenidae in the Furongian (e.g., Westergård 1922Westergård , 1946Westergård , 1947bHenningsmoen 1957a), i.e., a family of highly specialized, generally small trilobites, adapted to dysoxic, or occasionally even anoxic environments (e.g., Clarkson and Taylor 1995;Clarkson et al 1998;Fortey 2000;Schovsbo 2000Schovsbo , 2001Clarkson 2011). Monospecific assemblages are common, with rarely more than three species at one level (Westergård 1922(Westergård , 1947bClarkson and Taylor 1995).…”