Vertebrate remains and conodonts in the upper Silurian Hamra and Sundre formations of Gotland, Sweden

Abstract

A long history of geological research on the island of Gotland, Sweden, has resulted in a detailed biostratigraphy based on conodonts for the Gotland sedimentary succession, but the relation between the Hamra and Sundre formations, the youngest strata on southern Gotland, has remained poorly resolved. These formations have also remained relatively poorly described in terms of vertebrates compared to other parts of the succession. A survey of museum collections and newly sampled material reveal that the taxonomical compositions and richness of vertebrate faunas remain similar compared to the underlying Burgsvik Sandstone and Oolite members. However, the relative abundance of the respective groups changes: Paralogania ludlowiensis and rare osteostracan remains of Tahulaspis sp. only occur in samples from the lower Hamra Formation, while Thelodus sculptilis becomes more common in samples from Sundre Formation. Conodont and isotope data give support to previous suggestions that the Hamra and Sundre formations may be largely isochronous, and it is possible that the observed differences in vertebrate faunas reflect changes in depositional setting. This interval on Gotland has been suggested to represent a hiatus in the East Baltic sections, where younger strata show an increased importance of acanthodians in the vertebrate faunas. Gotland could therefore give insights into the early stages of this diversification of gnathostomes during late Silurian times. However, this has to be done in combination with data from other areas, as well as with a review and revision of the scalebased taxonomy of Silurian acanthodians from the Baltic Basin.

Revision of thelodonts, acanthodians, conodonts, and the depositional environments in the Burgen outlier (Ludlow, Silurian) of Gotland, Sweden

“…crispa has also been found in sample G72-17CB from Glasskär 1 (probably Burgen Oolite Formation). This species is also known from earlier occurrences in the Baltic area (Viira & Aldridge, 1998;Bremer et al, 2020), although Märss and Männik (2013) suggested that these may represent specific morphologies of Oz. snajdri and considered them Oz.…”

“…sculptilis (Fredholm, 1989) and poracanthodid acanthodians (Fredholm, 1989;Eriksson et al, 2009) from Burgen outlier indicate a late-, rather than mid-Ludlow age of these samples (see Märss and Männik, 2013). Indeed, these taxa are more readily found in younger strata on Gotland (Bremer et al, 2020). Furthermore, the samples from Burgen outlier lack the thelodonts Thelodus carinatus Pander, 1856 and Phlebolepis elegans Pander, 1856 that are typical for faunas found in older strata on Gotland (e.g., Fredholm, 1989;Eriksson et al 2009).…”

“…Jeppsson et al (2006) assigned only the Sundre Formation to the Oz. crispa Biozone, although the taxon has been identified by L. Jeppsson in samples from the Hamra Formation (Bremer et al, 2020). The Hamra Formation was placed by Jeppsson et al (2006) in the Oz.…”

“…The Ludlow-aged strata placed in the Burgsvik Formation (Fig. 1) in Gotland, Sweden, represents an important archive of the evolution of early vertebrates (Fredholm, 1989(Fredholm, , 1990Märss, 1992;Bremer & Blom, 2015;Bremer et al, 2020) and plants (see Mehlqvist et al, 2015 and references therein), and of global sea-level and palaeoclimate perturbations (Samtleben et al, 2000;Calner, 2005;Kozłowski & Munnecke, 2010;Jeppsson et al, 2012). The formation follows the onset of the Lau Event (Jeppsson et al, 2006;Jarochowska et al, 2020a), which is one of several episodes to be coupled with major faunal turnovers and extinctions (Jeppsson, 1987;Koren, 1987;Kaljo & Märss, 1991;Eriksson et al, 2009) and changes in eustatic sea level (Loydell, 2007;Kozłowski & Munnecke, 2010).…”

Neotypes for some upper Silurian acanthodian taxa from the Baltic Sea Region and the Welsh Borderland

“…3C-D) as the neotype for Gomphonchus volborthi. The geographical and stratigraphic distribution of the species is as follows: upper Silurian, lower Přidoli, Ludlow Bone Bed; Welsh Borderland, U.K. (Vergoossen 1999a); Ludlow to lower Přidoli, ?När to Sundre formations, Gotland, Sweden (Bremer et al 2019b); Ludlow to lower Přidoli, Öved Sandstone Formation, southern Sweden; upper Ludlow, Uduvere Beds of the Paadla RS and Tahula Beds of the Kuressaare RS, Estonia; uppermost Ludlow/lower Přidoli, Winnica Formation, Holy Cross Mountains, Poland (Bremer et al 2018); upper Silurian, lower Přidoli, erratics, the Netherlands (Vergoossen 1999c(Vergoossen , 2002; possibly Ludlow or Přidoli, reworked deposit in Quaternary gravels, Mazury Quarry, near Minsk, Belarus (Plax 2019). (Lehman, 1937) Radioporacanthodes biblicus was regarded by Gross (1947) as a junior synonym of Poracanthodes porosus Brotzen, 1934(now Radioporacanthodes porosus, see Vergoossen 1997), but the species was resurrected by Vergoossen (2002).…”