THE AGE AND ENVIRONMENT OF THE LATE DEVONIAN EAST GREENLAND TETRAPODS

The Famennian of East Greenland is internationally famous for the early tetrapods Ichthyostega and Acanthostega. The tetrapod-bearing Celsius Bjerg Group records significant short term climatic variation over the Milankovich eccentricity cycle, seen for example in the red-green colour banding of the Britta Dal Formation. Individual formations record longer-term climate trends.

The oldest formation, the Elsa Dal, is a sandy alluvial braid-plain system that has a sufficiently high sedimentation rate to preserve black mudstone layers. These yielded Famennian 2b spores. The overlying Aina Dal Fm marks a transition to meandering rivers with a significant proportion of more mud-dominated overbank deposits. Ichthyostega has been found in situ towards the top of the Aina Dal Fm. The Wimans Bjerg Fm formed in alternating lacustrine and inland playa conditions, with a high water table giving rise to the dark sediment colour.

The Britta Dal Fm sees a transition to a large, terminal alluvial fan accumulating extensive mud-rich flood plains. Intense vertisols formed on these plains. Occasional sheet sandstones formed in extensive, poorly-confined rivers created by occasional episodes of intense river discharge. The environment remained arid to semi-arid. In the Britta Dal Fm there is the well documented in situ occurrence of many articulated individuals of Acanthostega. These occur within a sheet sandstone. They were almost certainly carried downstream by a major flooding event and died at this location. We infer that they lived in small, more permanent parts of the river system which persisted for the years between major flood events.

The sediments of the overlying Stensiö Bjerg Fm show a transition back to less arid conditions overall. Intervals of organic-rich mudstone represent periods when stratified permanent lakes formed which contain the abundant remains of fish, plants and spores. The thickest of these mudstones is the overlying Obrutschew Bjerg Fm where 4-6 m of organic-rich sediments with limestones was deposited. It represents a single, deep, permanent lake of considerable size and stability. This lake is coincident with the Devonian-Carboniferous boundary.