Dinosaur tracks and possible lungfish aestivation burrows in a shallow coastal lake; lowermost Cretaceous, Bornholm, Denmark

Abstract

The Mesozoic succession of the island of Bornholm in the Baltic Sea has hitherto yielded only very few remains after vertebrate life. We describe a new find of a well-exposed vertebrate trample ground and large, densely spaced, vertical to inclined burrows from shallow lake deposits belonging to the Berriasian Skyttegård Member of the Rabekke Formation. The succession is terrestrial, but a coastal setting is indicated by the great abundance of pyrite concretions and framboidal pyrite. Organic geochemical and palynological data show that the lake was surrounded by a low-diversity gymnospermous vegetation. The trampling ground is exposed in a vertical section at the boundary between light grey, structureless or trough cross-bedded fluvial sand and dark brown, organic-rich lacustrine mud with densely spaced vertical rootlets, wood fragments and large pyrite concretions. The tracks are bowl-shaped, have an irregular flat or slightly conical base, are on average 22 cm deep and on average 43 cm wide in the upper part and 23 cm in the lower part, but some reach widths up to about 1 m and have a highly irregular shape. The underlying sand in some cases shows a faint stratification that is bent downwards adjacent to the mud-filled tracks, which have near-vertical and commonly overhanging margins. The morphology of the bowl-shaped depressions suggests that they were made by dinosaurs, most likely sauropods. They were clearly emplaced after deposition of the dark brown mud. Long, vertical to inclined, commonly slightly curved, J-shaped or spiraling cylindrical burrows with a diameter averaging 4.4 cm and lengths up to 45 cm occur densely spaced in the mud. They are unbranched, have no mud or lag linings. The top of the burrows is indistinct or not preserved, whereas the base is rounded, concave-up, commonly with a slightly enlarged chamber. The length is difficult to estimate precisely because of the missing top and because the burrows cross-cut and go in and out of the plane of exposure. The burrows have a passive infill of dark mud and, at some levels, of irregularly alternating laminae of white sand and mud. The sand may be of aeolian origin or may represent distal crevasse splays or washovers from adjacent rivers or the nearby sea. Skeletal remains have not been found in or associated with the burrows, which are similar to burrows referred to lungfish or crayfish, and they are with hesitation interpreted as representing lungfish aestivation burrows based on the morphological characteristics. This is only the second report of dinosaur tracks in Denmark and the first in-situ find where environmental conditions can be interpreted with some confidence. Possible lungfish burrows are here reported for the first time from the Danish Mesozoic. The overall rarity of vertebrate remains in the Mesozoic succession of Bornholm may reflect that the area was a horst island or submerged shoal during much of the Jurassic and Cretaceous and may not always have had direct land connection with surrounding land areas of the Baltic Shield or mainland Europe.