Expedition Day 21 – The black lake

We had our first look at the rocks yesterday after dinner. We climbed up from our camp at 820 m to the snow line on Celsius Bjerg and fanned out: Grzegorz concentrating on the sandstone blocks to look for footprints; Henning and Ben for outcrops of black shale that mark the Obrutschew Bjerg Formation. This is one of the main rock units we are interested in and represents a deep lake that was not only populated by early bony fishes, but also spans the geological boundary between the Devonian and Carboniferous periods 358 million years ago. This timeframe is important for the extinction of archaic bony fishes known as placoderms and the first major radiations of early bony fish (actinopterygians) and shark (chondrichthyans) ancestors.

The shale itself splits easily into wafer thin dinner plate sized sheets. We used chisels for this – the plan being to work through enough shale to amass a collection of complete fish fossils and other identifiable bones.

After our first taste of the rocks we headed back to camp for dinner.

Met up with Grzegorz who had found some tracks of giant millipedes, as well as some other curious traces that might be tetrapod footprints. We collected the block for later analysis back in Uppsala.

The next morning was a hard three-hour trek downslope, across the steep concave face of the mountain, and back up the parallel ridge. We had also climbed over an uneven slope strewn with massive sandstone boulders.

We had taken this long way round because a vast sheet of ice and snow covered part of the mountain face. Lasse had checked this the evening before and thought that it was too dangerous to cross in case of avalanche.

One the trek we found a few bones of the “last” placoderm, Groenlandaspis. This weird box-like fish would have lived on the bottom of rivers 358 million years ago. After death the plate-like bones of its skull and forelimb came apart and were washed downstream to preserve as fossils.

The ancient lake fish were what we were really after though. Problem was that the site was covered by snow. We had to scratch around looking for blocks of shale. After locating three promising outcrops we dug out large blocks the size of dinner-plates up to car tires and moved them over to a central flat area of sandstone to start splitting. The shale was wet and each paper-thin sheet needed to dry out in the sun before we could examine it properly. A bit like drying wet sheets of paper.

The results were spectacular.

Entire layers covered in tiny scales, bones and complete fish. We collected numerous examples of Cuneognathus – our early bony fish – and some enigmatic spines belonging to an unknown acanthodian – a cartilaginous fish probably ancestral to sharks and their relatives. This is one of the key fossils we are after.

The haul was carefully wrapped in aluminum foil to stop it from flaking and packed in bubble wrap and tape. Each parcel was strapped flat against our backs in the packs and padded with spare clothes and paper.

We then started the long hike back.

We cut along the snowline but got a point where there had been a small avalanche – too risky. We had to climb down into the valley again and clamber all the way along and back up to camp.

Made it home at 20.30 too tired to eat.

Grzegorz had found some more possible footprints in the Britta Dal Formation. This is the rock unit that produced both Ichthyostega and Acanthostega – two of the most famous early tetrapods. Did we have traces of their lost wanderings 358 million years ago? We would have to assess the finds in the morning and decide exactly what had been found.

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