Geoscientist Online

Howard Falcon-Lang*, using research funds awarded by the Society, has been on the trail of the earliest reptiles

Geoscientist 20.09 September 2010

The evolution of reptiles in mid-Carboniferous times transformed the Earth’s terrestrial ecosystems forever. Land-based vertebrates no longer had to stay close to ponds to reproduce. Instead the amniotic egg functioned as a sort of ‘portable pond’ and could be laid even in the most arid of environments. The evolution of reptiles signalled the opening of a totally new frontier for life on Earth. Soon the race was on to colonise the great interior drylands of the supercontinent Pangaea. It was the beginning of the diverse land-based ecosystems we know today.

Much remains to be learned about the origin of reptiles. Famously, the oldest reptile skeletons come from crumbling cliffs of Joggins, Nova Scotia, which fringe the Bay of Fundy in eastern Canada. Their discovery, 150 years ago, owes much to the prowess of Charles Lyell. In September 1852, Lyell spent a few days fossil hunting at Joggins, while en route to Boston to deliver the Lowell Lectures. One foggy afternoon, he and his young companion William Dawson (later Principal of McGill and well-known palaeontologist), stumbled across a fossil tree broken open on the shore. Inside the tree were bones. Although these turned out to be the remains of amphibians, Dawson later discovered other skeletons in a different tree while exploring the same spot in 1859. He named this beast Hylonomus lyelli in honour of his mentor. After more than century lying in dusty museum drawers, in 1963, this animal was finally confirmed by Bob Carroll as a true reptile– and the world’s oldest at that.

Since Carroll’s groundbreaking work, abundant vertebrate fossils have continued to drop out of the Joggins cliffs (now a UNESCO World Heritage Site). However, this amazing fossil assemblage poses as many questions as it answers. For example, we now know that both major lineages of reptiles – synapsids and sauropsids - occur in the Early Pennsylvanian Joggins Formation. This implies that reptiles had already existed for some time before Hylonomus fatefully crawled into that hollow tree-stump, 315 million years ago. Unfortunately, the fossil record is silent about this earlier phase of reptile evolution. In fact we have to travel back a further 15-20 million years before another diverse vertebrate fossil site is encountered: the famous late Viséan locality at East Kirkton, Scotland. However, despite various false alarms, no true reptiles have yet been found at this deeper stratigraphic level.

With fossil bones and teeth so rare, how can palaeontologists shed light on the evolutionary events that led up to the appearance of reptiles? One approach is to study the footprints they left behind. Tracks are far more abundant than bone. Furthermore they tell us much about the ecology of the animals that made them. As luck would have it tracks are very common in the Carboniferous rocks of the Bay of Fundy. William Logan, the early Canadian geologist, first spotted tracks in Tournaisian rocks at Horton Bluffs, Nova Scotia, in 1841, and since then there has been a torrent of discoveries through the entire Carboniferous succession. New fossils are constantly falling from the coastal cliffs of this part of eastern Canada, hewn by the legendary power of the Fundy tides – the world’s highest.

My interest in fossil reptile tracks started in 2006. I was working on the remote Maringouin Peninsula of southern New Brunswick, which projects down into the Bay of Fundy, near Sackville. The Geological Society had kindly awarded me the Tyrell Fund to study some of the fossil soils developed in these early Pennsylvanian red beds. As I walked back to my car one evening, I passed some large blocks of sandstone that had recently toppled out of the cliff. I could hardly believe my eyes! The rocks were covered in spectacular footprints. All the more amazing was the fact that I had walked past this very spot earlier in the day and had missed the discovery completely. Only in the evening was the sunlight glancing at just the right angle to pick them out.

Being principally a fossil plant expert, tracks were a bit of a novelty to me. I showed them to my colleague Mike Benton, a vertebrate palaeontologist. By carefully comparing the footprints with the fragmentary skeletons of Carboniferous animals, we were able to show that the track-maker was distinctly reptile-like. Specifically, the footprints indicated that all four feet of the animal comprised five slender scale-covered toes, terminating with a claw. Furthermore, coming from rocks that pre-dated those at Joggins, these tracks provided the earliest evidence yet for the evolution of reptiles. We published our findings in the Journal of the Geological Society and the work attracted quite a bit of media attention at the time (http://news.bbc.co.uk/1/hi/sci/tech/7047065.stm). It also triggered a certain amount of controversy, some disputing whether these were really the tracks of reptiles, or merely reptile-like animals.

Joggins revisitied

Thanks to second award from Tyrell Fund, in July 2009 I have been able to return to the Bay of Fundy to explore for more tracks. I worked with Randy Miller of the New Brunswick Museum and the sedimentologist, Martin Gibling, based at Dalhousie University. At one new site called Tynemouth Creek, near St. Martin’s, southern New Brunswick, we located about 15 slabs of early Pennsylvanian sandstone covered in tracks. Some may be reptilian, while small amphibians were probably responsible for others. What is interesting about this particular site is that the tracks are impressed into the deposits of dryland river plains. The bedding surfaces are covered with the raindrop impressions and deep mud cracks suggesting torrential downpours followed by seasonal droughts. Fossil soils also suggest seasonal aridity. Thus, here we have the opportunity to explore the ecology of the animals that inhabited the driest part of the mid-Carboniferous tropics.

With such an abundance of fossil tracks available for study, we now plan to analyze the structure and ecology of these early Pennsylvanian communities. We also want to delve back into the literature and deeper into the stratigraphic succession to investigate questions the fossil record of bones and teeth simply can’t answer: When did reptiles evolve? When did they first become abundant? Into what niches did they diversify? What environmental factors influenced their evolution? Although long neglected in earlier studies of reptile evolution, fossil tracks may hold the key to many of these unanswered questions – and if nothing else, graphically illustrate that these ‘first impressions’ really do count!

• If you wish to apply for a research fund of the Society, please visit www.geolsoc.org.uk/grants

Further reading

• FALCON-LANG, H.J. 2006. A history of research at the Joggins Fossil Cliffs, the world’s finest Pennsylvanian section. Proceedings of the Geologists’ Association 117 (3) : 377-392.
• FALCON-LANG, H.J. & CALDER, J.H. 2004. UNESCO World Heritage and the Joggins cliffs of Nova Scotia. Geology Today 20 (4), 140-144.
• FALCON-LANG, H.J., BENTON, M.J. & STIMSON, M. 2007. Ecology of the earliest reptiles inferred from basal Pennsylvanian trackways. Journal of the Geological Society, London 164 (6): 1113-1118.
• FALCON-LANG, H.J., GIBLING, M.R., BENTON, M.J., MILLER, R.F., BASHFORTH, A.R. 2010. Diverse tetrapod trackways in the Lower Pennsylvanian Tynemouth Creek Formation, southern New Brunswick, Canada. Palaeogeography, Palaeoclimatology, Palaeoecology, doi:10.1016/j.palaeo.2010.06.020
• KEIGHLEY, D., PARK, A., CALDER, J.H., WALDRON, J., FALCON-LANG, H.J., BENTON, M.J. 2008. Discussion on ecology of earliest reptiles inferred from basal Pennsylvanian trackways. Journal of the Geological Society, London 165 (5): 1113-1118.

* Department of Earth Sciences, Royal Holloway, University of London, Egham, Surrey, TW20 0EX, UK. E-mail: [email protected]