First record of Gyrosteus mirabilis (Actinopterygii, Chondrosteidae) from the Toarcian (Lower Jurassic) of the Baltic region.

Jahn J Hornung,Sven Sachs

doi:10.7717/peerj.8400

Abstract

An isolated hyomandibula from a lower Toarcian carbonate concretion of the Ahrensburg erratics assemblage (Schleswig-Holstein, northern Germany) represents the first record of a chondrosteid fish from the Lower Jurassic of the southwestern Baltic realm. Except for its smaller size, the specimen is morphologically indistinguishable from corresponding elements of Gyrosteus mirabilis from the Toarcian of Yorkshire, England. This find, which probably originates from the western Baltic basin between Bornholm Island (Denmark) and northeastern Germany, markedly expands the known range of this chondrosteid taxon across the northern part of the strait connecting the Boreal Sea with the Tethys Ocean during the Early Jurassic. For the first time the extension of the paleogeographic range of a chondrosteid species beyond its type area is documented, which can contribute to future studies of vertebrate faunal provincialism during the Lower Jurassic in Europe.

Highlights

The Ahrensburg erratics assemblage, named after its main occurrence in Pleistocene glacigenic deposits near the town of Ahrensburg, Schleswig-Holstein, northern Germany, provides a high diversity of mainly Lower Jurassic invertebrate and vertebrate fossils, often in exceptional quality (Lehmann, 1967, 1971; Lierl, 1990; Sachs et al, 2016)
The petrographic characters of the grayish, concretionary, sandy calcareous marlstone suggest an origin from the lower falciferum zone, lower Toarcian, Lower Jurassic (Lierl, 1990)
The rare and only incompletely known G. mirabilis is here described for the first time from the Toarcian of the southwestern Baltic realm

Summary

Introduction

The Ahrensburg erratics assemblage, named after its main occurrence in Pleistocene glacigenic deposits near the town of Ahrensburg, Schleswig-Holstein, northern Germany, provides a high diversity of mainly Lower Jurassic invertebrate and vertebrate fossils, often in exceptional quality (Lehmann, 1967, 1971; Lierl, 1990; Sachs et al, 2016). They open a precious window into the marine fossil communities of the southwestern Baltic and adjacent regions, in which autochthonous surface exposures of Lower Jurassic strata are rare and mostly covered by Quarternary deposits.

Results

Discussion

Conclusion