One Small Step for a Bug

We can use such remains to reconstruct extinct animals in life-like poses, and even infer their behaviour - but it is little more than informed speculation. Trace fossils, on the other hand, such as bite-marks and burrows, represent the remains of the activities of animals,  and are a  crucial source of information to ‘breathe life’ into extinct animals, revealing a snap-shot in time of their behaviour, providing direct evidence for their locomotion, and even indicating interactions between them. Trace fossils can be found in rocks that were deposited both in the sea and on the land. In fact, other evidence for life on land is rather scarce because dead bodies degrade more easily on land than in the sea.

The fossil record reveals that the arthropods - creatures such as spiders, insects, millipedes and crabs with segmented bodies, jointed limbs, and a hard outer skeleton - were the earliest animals on land. Even before they left the sea, arthropods were pre-adapted for life on land: their exoskeleton, that first evolved in the sea  for protection from predators, acted like a space-suit, shielding them from harmful ultra-violet rays and providing them with mechanical support, enabling them to lift their bodies clear of the water. The oldest body fossil of a land animal is a 430 million-year-old millipede from Scotland. Body fossils are extremely rare from this time in Earth history, but there is another source of evidence that supplies clues to when ancient life first stepped out of the sea: fossil footprints.

A few years ago there was a ground-breaking discovery in a quarry near Kingston, Ontario in Canada. Fossilised trackways, preserved in a coastal dune deposit, revealed that arthropods first conquered the land around 500 million years ago, some 50 million years earlier than first thought. Analysis of the Ontario trackways indicates that they were produced by a mysterious group of extinct arthropods called “euthycarcinoids”. These bizarre-looking bugs resemble a 30 cm long, flat woodlouse with a tail spine. However, they are probably not closely related to woodlice, recent research suggests that euthycarcinoids were, in fact, the ancestors of a large arthropod group that includes insects, millipedes and crustaceans (e.g. lobsters, crabs and woodlice). The trackways from Ontario indicate that several differently-sized euthycarcinoids were on the move at the same time. Who knows, but perhaps they represent the vanguard of an amphibious group-exodus from the sea, trekking across the dunes into an unknown land.

There is some evidence that other early arthropods - the eurypterids or giant sea scorpions - came ashore to protect their eggs from marine predators. In New York State hundreds of eurypterid fossils are found in the rocks.

Scientists used to think that a mass catastrophe caused them all to die at the same time, but there is a much more likely explanation; clues lie in the mating behaviour of modern horseshoe crabs, that congregate in large numbers once a year, by the light of the full moon on a high tide. Like all arthropods, eurypterids needed to periodically shed their exoskeleton, in a process called “moulting”. Thus, these abundant eurypterid fossils suggest that they too probably gathered together in large numbers to moult and then mate. Evidence from their trackways also indicates that eurypterids migrated en-masse into near-shore environments such as calm, tranquil lagoons to moult and then mate in relative safety, away from normal marine predators. Perhaps the earliest animals on land - the euthycarcinoids - first came ashore for a similar reason.

Our own ancestors, fish-like amphibians, first lumbered ashore a mere 370 million years ago. There they found a world teeming with early land plants and giant creepy-crawlies. With over a hundred-million-year head start in which to populate the planet before our ancestors arrived, those early arthropods filled an empty niche, evolving into monster-milli-pedes, super-sized scorpions, and colossal cockroaches. So, the next time you swat a fly or squish a spider, spare a thought for the true terrestrial pioneers - the arthropods - who left footprints in the sand as testimony to their conquest of the land half a billion years ago. It may have been one small step for a bug, but it was a giant leap for life on Earth.

This article first appeared in the University of Bristol Re:search magazine and is reproduced with permission. See http://eis.bris.ac.uk/~pslsw/ for more information.