The conodont fauna in the Middle Ordovician Eoplacognathus pseudoplanus Zone of Baltoscandia

Abstract

Conodont faunas from the Eoplacognathus pseudoplanus Zone in five main areas of Middle Ordovician rocks in Sweden are described and discussed and compared with faunas of comparable age from neighbouring areas. The investigation is based on 118 samples which together yielded 175 114 conodont elements. The Swedish conodont faunas are intermediate between shallower-water faunas from the eastern part of Baltica and deeper-water faunas towards the west. Correlation with the Yangtze platform of south-central China is fairly easy, and the occurrence of key taxa, such as Histiodella, makes it also possible to correlate some levels with Laurentia. Although faunal lists for the Swedish localities are almost identical, the conodont taxa have their relative abundance maxima in separate areas: species of Protopanderodus and Periodon are most common in deeperwater settings, Eoplacognathus, Baltoniodus and Microzarkodina characterize intermediate areas, and Semiacontiodus is typical of shallow subtidal environments.

Conodont biostratigraphy and global correlation of the middle Darriwilian-lower Sandbian (Ordovician) Las Aguaditas Formation, Precordillera of San Juan, Argentina

“…According to Löfgren (1999aLöfgren ( , 1999bLöfgren ( , 2003Löfgren ( , 2004 Semiacontiodus and Cornuodus are associated with shallowest settings in central Baltoscandia. In contrast, in the same intervals Costiconus costatus, and the highest abundance of Periodon were documented, which are reliable indicators of relatively deep water (Barnes and Fåhraeus, 1975;Lindström, 1976;Pohler, 1994).…”

“…This zone correlates with strata in the Yangtze valley of south-central China (Ni and Li, 1987;Zhang, 1998a), New Zealand (Zhen et al, 2011b) as well as at diverse localities in Baltoscandia, such as some from Estonia (Viira, 1974;Viira et al, 2001;Löfgren, 2004;Viira, 2011;Männik and Viira, 2012), Norway (Rasmussen, 2001), and Poland (Dzik, 1994). Since stratigraphically late forms of E. pseudoplanus were recovered, they enable a precise correlation with the Kunda-Aseri stage boundary interval in Sweden (Löfgren and Zhang, 2003;Mellgren and Eriksson, 2009).…”

“…The Periodon and Histiodella lineages have great potential for global correlation (Stouge, 1984(Stouge, , 2012Stouge et al, 2011;Chen et al, 2006;Stouge and Zhao, 2006) and support a correlation with equivalent strata in Canada (Uyeno and Barnes, 1970;Barnes and Poplawski, 1973;Pyle et al, 2003;Stouge, 2012), USA (Bauer, 2010;Graves and Ellison, 1941;Bradshaw, 1969), Norway (Rasmussen and Stouge, 1995;Rasmussen, 2001), Sweden (Stouge and Bagnoli, 1999;Löfgren, 2004;Mellgren and Eriksson, 2009), Estonia (Löfgren, 2004), China (e.g., Zhang, 1998a;Chen et al, 2006;Zhang and Gao, 1991;Zhao et al, 2000;Stouge and Zhao, 2006;Stouge et al, 2011;Zhen et al, 2011a), Australia (Zhen and Percival, 2004), New Zealand (Zhen et al, 2009;Zhen et al, 2011b), and Kazakhstan (Zhylkaidarov, 1998;Tolmacheva et al, 2004).…”

“…The presence of E. pseudoplanus and D. tablepointensis may be used for the definition of an association zone for the Precordilleran region as they have similar ranges but different environmental requirements. The index species Microzarkodina hagetiana and M. ozarkodella, which define the lower and upper subzones of the Eoplacognathus pseudoplanus Zone in the Baltoscandic region (Zhang, 1998b;Löfgren, 2004) have also been identified in the Precordillera. The upper boundary of this biozone is missing from the top of the lower member of the Las Aguaditas Formation because a stratigraphic gap separates the lower and middle members, indicated by the presence of Sandbian conodonts of the Pygodus anserinus Zone in the middle member.…”

“…The index species Histiodella kristinae defines the lower subzone of the E. suecicus Zone in the Precordillera (Albanesi, 1998). However, in Scandinavia H. kristinae appears earlier in the record, in an interval where E. suecicus is not yet present (Zhang, 1998b;Löfgren, 2004). Also, this species was recovered along with its ancestor H. holodentata, which indicates early forms of H. kristinae as the evolving species.…”