Biodiversity of the Silurian osteostracans of the East Baltic

Abstract

The sculpture and histology of osteostracan head shields and trunk squamations from the Silurian of Estonia are described, illustrated and used for the identification of disarticulated microremains collected from outcrop sections and numerous drill cores in Estonia and Latvia over the last 40 years. The scattered osteostracan material contains thousands of specimens of scales and shield fragments. The sculpture and histology of species of the previously identified eight genera in the region (Tremataspis, Dartmuthia, Saaremaaspis, Oeselaspis, Aestiaspis, Thyestes, Procephalaspis, Witaaspis) are redescribed, together with Ateleaspis, found in Estonia for the first time. The sculpture on the cornua of several taxa is described for the first time. The new taxa Tremataspis perforata sp. nov., Dartmuthia procera sp. nov., Eldaaspis miklii gen. et sp. nov., Tahulaspis ordinata gen. et sp. nov., Tahulaspis praevia gen. et sp. nov., Meelaidaspis gennadii gen. et sp. nov. and Ohesaareaspis ponticulata gen. et sp. nov. are established, based upon sculpture and histological characteristics of the exoskeleton. The supraoral field with denticles of that field are described for the first time in Oeselaspis pustulata, as is the supraoral plate with buccal denticles in Meelaidaspis gennadii gen. et sp. nov. Thin sections of all taxa (except of Witaaspis) have been studied. This work sheds light on East Baltic osteostracan biodiversity from the Maasi Beds of the Jaagarahu Stage, Sheinwoodian, lower Wenlock up to the Ohesaare Stage, upper Přidoli, Silurian.

End-Wenlock terminal Mulde carbon isotope excursion in Gotland, Sweden: Integration of stratigraphy and taphonomy for correlations across restricted facies and specialized faunas

“…This diversity peak is more or less contemporaneous in both the Eastern Baltic region and Gotland (Sweden), but equally diverse faunas have been reported from younger sediments of the East Baltic (Blom et al, 2002;Märss et al, 2007Märss et al, , 2014. Thelodonts seem to have spread across a wide range of environments (Märss et al, 2007), but facies associations of anaspids and osteostracans indicate that these groups preferred lagoon and shoal areas (Märss and Einasto, 1978;Sansom, 2008;Märss et al, 2014). Sansom (2008) suggested that our understanding of early osteostracan evolution is largely obscured by the taphonomic bias resulting from their seemingly specific environmental preferences during the Silurian.…”

“…Furthermore, there are a number of remains with osteostracan and anaspid affinities that are too worn or too fragmentary to be identified. The majority of the vertebrates found in this study have already been described from Gothemshammar or figured in open nomenclature (Martinsson, 1966;Gross, 1968;Fredholm, 1990;Blom et al, 2002;Märss et al, 2014), and our descriptions focus on the new scale types and sculpture patterns belonging to taxa not reported from Gothemshammar before.…”

“…13A-E) following the suggestion by Janvier (1985). However, we do not follow this revision, based the more recent work by Märss et al (2014). The specimens in Gross (1968: Figs. 11A-E, ?12F-G, 13A-E) are instead referred to Th.…”

“…Märss, 1989;Turner, 1999). However, towards the Wenlock/Ludlow boundary diverse faunas containing many thelodont, osteostracan, and anaspid taxa have been found in several areas of the Baltic Basin (Fredholm, 1990;Blom et al, 2002;Märss et al, 2007Märss et al, , 2014. This diversity peak is more or less contemporaneous in both the Eastern Baltic region and Gotland (Sweden), but equally diverse faunas have been reported from younger sediments of the East Baltic (Blom et al, 2002;Märss et al, 2007Märss et al, , 2014.…”

“…However, towards the Wenlock/Ludlow boundary diverse faunas containing many thelodont, osteostracan, and anaspid taxa have been found in several areas of the Baltic Basin (Fredholm, 1990;Blom et al, 2002;Märss et al, 2007Märss et al, , 2014. This diversity peak is more or less contemporaneous in both the Eastern Baltic region and Gotland (Sweden), but equally diverse faunas have been reported from younger sediments of the East Baltic (Blom et al, 2002;Märss et al, 2007Märss et al, , 2014. Thelodonts seem to have spread across a wide range of environments (Märss et al, 2007), but facies associations of anaspids and osteostracans indicate that these groups preferred lagoon and shoal areas (Märss and Einasto, 1978;Sansom, 2008;Märss et al, 2014).…”