Cutting Edge

August 27, 1999

Fossilised arthropod trackways provide 'palaeo-Polaroids', or snapshots, of the lives of earth's earliest living creatures.

Behind the hype and the incredible claims from the "Jurassic Park generation" concerning the reintroduction of dinosaurs to the modern-day biosphere, there are many palaeontologists working hard to answer some serious questions about the evolution of life on earth.

One of the most important of these concerns the earliest colonisation of land by animals. Long before the earliest vertebrates, the true terrestrial pioneers were various creepy-crawlies, arthropods such as centipedes, scorpions and their spider-like relatives, known from fragmentary fossils derived from a few, now famous localities.

Trace fossils - evidence of animal activity rather than remains of the organism itself - include items such as the fossilised trails of arthropods, and provide a unique insight into early life on land.

As traces can be preserved in environments that exclude body fossils, they can also be used to keep track of some of the earliest groups, for example the myriapods (eg centipedes), which are known from their trackways long before their earliest body fossils.

Research carried out by me and Professor Derek Briggs at the University of Bristol, funded by the Leverhulme Trust, is investigating the trackways of early arthropods in order to analyse the age, environmental distribution and diversity of these communities and trace the movement of arthropods onto land.

Our research developed from ideas and approaches that I devised during my PhD, at the University of Manchester, on the palaeobiology and biomechanics of the eurypterids (extinct giant sea-scorpions, some of which grew to two metres in length), which were among the earliest animals to venture ashore.

Arthropod trackways record the land fall, 450 million years ago or even earlier, of the arthropods, and their subsequent diversification into a range of environments. The search for early life on land has taken me to the western Cape of South Africa, where I met local farmers who used trackway specimens about 470 million years old as wash-stones.

Trackways about 410 million years old appear in lake-shore settings within continental interiors, and those from 290 million years ago are very abundant and widespread. In the Robledo mountains of southern New Mexico I have worked on the most abundant and diverse assemblage of terrestrial trace fossils in the world, from tidal flat deposits 290 million years old.

Trace fossils may also be thought of as "fossil behaviour"; trackways, in particular, are "palaeo-Polaroids" or snapshots of the activities of extinct animals, providing direct evidence for their locomotory styles.

Modern palaeontology employs all of the modern technologies that other sciences enjoy. Our research is using computer modelling of extinct arthropods to analyse the producers of the trackways and elucidate the biomechanics of the arthropods. Such modelling allows generic arthropod body plans to be animated by various walking patterns, enabling speed-stability relationships to be assessed and theoretical trackways to be generated, which allows direct comparisons with the trace fossil evidence.

Research into the terrestrialisation of life is still in progress. Indeed, our work on trace fossils indicates that we are only just beginning to scratch the surface of the true nature and diversity of life at the birth of terrestrial ecosystems. There is no doubt that the future will reveal more about the past.

Simon Braddy is a postdoctoral researcher at the department of earth sciences, University of Bristol.

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