On the hunt for tetrapod footprints
Dr Alice M. Clement
The appearance of the first tetrapods, the limbed vertebrates which today dominate the land, represents one of the greatest ‘steps’ in evolution. Evidence for the first vertebrates with four limbs and digits (fingers and toes) can come from fossilised body parts, or from trace fossils, such as footprints.
The oldest widely accepted tetrapod trackway, and indeed the oldest widely accepted evidence for tetrapods in general, come from intertidal sediments from a quarry near Zachełmie, Poland. These prints, first announced in 2010, have been dated to the Middle Devonian, about 395 million years ago, and more than 18 million years older than the oldest tetrapod body fossils.
The trackways indicate they were made by flat-bodied, lizard-like creatures, some of which must have reached up to 2.5m in length. There are multiple trackways from several individuals, and most importantly, they clearly show the impressions of digits!
In Australia there have been some similarly impressive finds of trackways in Devonian rocks, including those from Glenisla and Genoa River in Victoria. The Genoa River tracks from far-eastern Victoria, are Late Devonian in age and were the first tetrapod trackways ever described from the “Age of Fishes” when first described in 1972. There is evidence of two different types of animals, clearly showing overlapping and alternating impressions of the fore and hind limbs clearly showing digits.
The Glenisla trackways are more problematic. The trackways were described in 1986, identified on a slab of rock in a homestead courtyard near the Grampians. However there remain persistent doubts over the precise age of the rock, with estimates ranging as far back as the Silurian Period, and the identity of the track maker has been questioned. If these really are tetrapod tracks and the Silurian date is correct, this represents the oldest evidence of limbed vertebrates in the world!
Professor John Long from Flinders University wanted to settle the controversy once and for all and so assembled a team of tetrapod trackway hunters to boldly venture into the remote wilderness of Victoria in search of fresh evidence. His team consisted of the walking encyclopaedia Dr Brian Choo, whizz-kid PhD student Ben King, venerable Swedish Professor Per Ahlberg, and myself.
Our first stop was the Grampians, where we were joined with an old friend of John’s, Peter Ward, a structural geologist and all-round handy camp guy. In addition to examining the original track-slab at Glenisla homestead, we visited two disused quarries and were able to pinpoint the true provenance of these enigmatic prints, along with the discovery of numerous other trace fossils made by various invertebrates. Of course, being in the Grampians, we were also rewarded with some spectacular views, incredible wildflowers and many friendly kookaburras and shingle-back lizards around our campsite.
After a few days we moved onto Melbourne for a much-need shower and for the chance to examine some old fossil samples collected decades ago from Genoa River, currently housed at The University of Melbourne Geology Department. The specimens contained tantalising material from a number of different types of fish and gave us great hope of finding new specimens in the field.
The Genoa River site is located in spectacular country within the Coopracambra National Park in far-eastern Victoria, some 50km northwest from Mallacoota Inlet. The National Park is a true wilderness and the playground of only the most determined hikers. For us it was either a 25km+ trek up and down cliffs through thick, temperate rainforest with all our gear and supplies, or a slightly more exhilarating transport option: GET TO THE CHOPPER!
Our weather-beaten but highly experienced pilot, Grant Shorland and his wife Ros, helped us reduce our load to just the essentials, and squeeze (just barely) into the 6-seater Écureuil. By following the course of the Genoa River, we were able to identify the area where we hoped to find fossils. However, the forest was so thick and the river often channelled between tall cliff faces, we struggled to find a suitable sandbank on which to be put down.
Even though we had seen the park from the air, it was still a surprise to experience how difficult it was to move through the undergrowth. We soon resorted to rock-hopping and eventually simply wading through the river itself. Genoa River was a spectacular site in which to spend four nights – we were lucky to share our camping spot with a plethora of skinks (Brian can tell you which species), beautifully-coloured water dragons, a few snakes, an easy-going platypus (that let us share his pristine swimming hole), and a deafening chorus of giant cicadas.
While new tetrapod-trackways remained elusive, we did manage to map the geology of the area, find some fish fossils and even an arthropod trackway suggesting we were on the right track! All of the team were in agreement that any future trips should take inflatable pack-rafts for ease of traversing the beautiful yet wild country of far-East Gippsland!