The closest relatives of tetrapods are the elpistostegalians, a small group of fossil forms including genera such as Panderichthys, Tiktaalik and Elpistostege. Dating to the end of the Middle Devonian and beginning of the Late Devonian, they are too late to be tetrapod ancestors themselves, but they are likely informative about the ancestral body form and lifestyle. The most perfect and complete elpistostegalian specimen is a whole Elpistostege from Miguasha in Québec, Canada, described by Richard Cloutier and colleagues in 2020. The body is complete but divides up into a series of separate blocks. In September 2022, Richard was visiting France and we had the opportunity to take the block containing the pelvic fins to ESRF for synchrotron microtomography. The scans reveal not only the pelvic fin skeletons but also a beautifully preserved hindgut fill containing the digested remains of its last meals. Once they have been studied we will have a much better understanding of the lifestyle of this enigmatic animal.
During July and August 2022, a team expedition to Greenland spent almost four weeks camped on the upper slopes of Celsius Bjerg (Mount Celsius), Ymer Ø (Ymer Island), on the east coast of Greenland. The participants were Per Ahlberg, Grzegorz Niedzwiedzki, Martin Qvarnström, Henning Blom (all Uppsala University), John Marshall (Southampton University) and Alex Chavanne (field manager). We were exploring the very youngest Devonian strata and the Devonian-Carboniferous boundary, which are exceptionally well represented in the East Greenland Devonian Basin.
The Devonian-Carboniferous boundary is especially important because it coincides with a major mass extinction, which also marks the beginning of "Romer's Gap" - a 20-million-year period with a very poor fossil record of tetrapods. The East Greenland Devonian Basin represents a lowland environment in the middle of Euramerica (a continent that comprised present-day North America and Europe), more than 1000 km from the nearest coast. It is famous for yielding Devonian tetrapods such as Ichthyostega and Acanthostega, but these come from slightly older strata than those we were exploring.
The expedition was extremely successful. We discovered an entirely new tetrapod assemblage from just below the D-C boundary, containing what appear to be at least 3-5 new tetrapods - which would equal the number of Devonian tetrapods discovered in Greenland during the past 90 years. This material will be studied with synchrotron microtomography at the European Synchrotron Radiation Facility (ESRF). We also found important plant material and made a detailed sedimentary log across the D-C boundary.
In May 2022, a team comprising Per Ahlberg, Grzegorz Niedzwiedzki, Hannah Byrne, Martin Qvarnström (all Uppsala University), Chris Berry (Cardiff University), and Neil Davies and Yorick Veenma (Cambridge University) visited Valentia Island in County Kerry, Republic of Ireland. The Valentia Slate Formation, which outcrops on the island and adjacent mainland, records a Middle Devonian (385 million-year-old) subtropical floodplain. It is famous for containing the second oldest tetrapod trackways in the world, after those of Zachełmie in Poland. The Valentia Island tracks were discovered and first described by Iwan Stössel (1995). We are re-studying them using new techniques, but the main purpose of this trip was to investigate the fossil flora and the depositional environment in order to gain a better understanding of the environment. We were able to show that the land was traversed by meandering rivers, and that cut-off river channels were initially inhabited by fish before they silted up and the resulting moist soils were colonised by land vegetation. The plants are represented by fossil roots, so we don't know what they looked like above ground, but they may have included quite large trees. The diversity of invertebrate trace fossils (trackways and burrows of arthropods) is also greater than we had thought.
Three papers on the Valentia Slate Formation fossil assemblage are currently in preparation or awaiting publication.