Late OrdovicianHolorhynchussuccession in the Siljan district, Sweden: facies, faunas and a latest Katian event

Abstract

The pentameride brachiopod genus Holorhynchus is a useful tool for correlation of the upper part of the Katian Stage in the Upper Ordovician. Holorhynchus occurs in the Boda Limestone, Siljan district, Sweden, but is generally only known from taxonomic descriptions. Here, we report in detail fossil occurrences relating morphology and sedimentary facies within the context of a major faunal turnover during the latest Katian. The specimens occur exclusively within the intercalated beds of pelmicritic mudstone and fossiliferous layers, which are down-slope storm deposits. The comparison of sedimentary features between the beds with and without Holorhynchus suggests that the animal inhabited fluid conditions on a muddy substrate, such as the channel environment reconstructed herein. The geological and autecological evidence suggests that the environmental conditions on the seabed were eutrophic. The opportunist Holorhynchus rapidly explored new habitats during a time of transition.

Warm-waterTcherskidiumfauna (Brachiopoda) in the Late Ordovician Northern Hemisphere of Laurentia and peri-Laurentia

“…The Alaskan shells mostly have incomplete preservation; however, judging from the preserved parts, the available specimens are generally smaller than the maximum shell size of typical H. giganteus but are comparable to the specimens from the Boda Limestone of Sweden (Shiino et al, 2015).…”

“…1) but have also been found in northern Baltica (Timan–northern Ural region, see Beznosova, 2014), southern Norway (St. Joseph, 1938; Cocks, 1982), southern Sweden, Estonia, and Lithuania (for a summary, see Rong et al, 1989). Holorhynchus , for example, is found in the mud-mound facies of the Boda Limestone in the Siljan district of Sweden (Shiino et al, 2015), as well as in successions in northern Sweden assigned to the intra-Iapetus Virisen Terrane (Kulling, 1933; Dahlqvist and Rasmussen, 2013). In Baltica, however, Tcherskidium is generally absent.…”

“…Here we argue that the Baltoscandian occurrences of Proconchidium and Holorhynchus would align with the predominantly late Katian Northern Hemisphere Tcherskidium fauna if Baltica were rotated about 80° clockwise, as reconstructed by Cocks and Torsvik (2005) for the 480–460 Ma interval, and positioned closer to the paleoequator. Aside from the uncertainties of the paleogeographic position of Baltica, the presence of Holorhynchus and associated pentamerides in the Boda limestone of Sweden has been interpreted by Shiino et al (2015) to be sporadic opportunistic colonization of the mudmound habitat. It is conceivable that this outlier of the Tcherskidium fauna invaded the reef ecosystem only during episodes of oceanic warming (e.g., see Fortey and Cocks, 2005) or followed a warm-water stream from the paleoequatorial region.…”

“…5.1–5.18, 5.22–5.25, 6, 7 (and comprehensive synonymy therein up to 2004).2008 Holorhynchus giganteus ; Beznosova, p. 34, fig. 3.1.2.2015 Holorhynchus giganteus ; Shiino et al, p. 28, figs. 3A–L, 4A–E.

Figure 9. Holorhynchus giganteus Kiær, 1902.

…”

“…6, fig. 5) and from the Siljan district (Shiino et al, 2015, fig. 3I), are well developed also in the Alaskan material (Fig.…”