Final Footsteps

The 155 million year old death march of Mesolimulus walchi

155 million years ago, a dying horseshoe crab, crawled around in the soft mud at the bottom of a prehistoric tidal lagoon. The creature continued in a straight line for a short distance before curving around as it expired. This is not a hypothetical scenario. Why? Because of a very special fossil recovered from the Solnhofen Plattenkalk. Embedded on a plate of pale limestone, a question mark-shaped trail composed of two parallel lines of indentations curves over the surface. Towards the end, it becomes more erratic and disjointed before ending with their creator; a single fossilized horseshoe crab mineralised in perfect detail.

The fossil itself was small, but preserved a poignant yet mysterious story. Discovered in 2002 at Solnhofen, the fossil trackway, properly called mortichnia or 'death march,' was put on display in the Wyoming Dinosaur Centre in the US.

Since then, Dean Lomax of the Doncaster Museum and Art Gallery and Christopher Racay from the Wyoming Dinosaur Centre, have been studying it to uncover its secrets. What made this fossil special was that the tracks were over 9 metres in length. 'It's not particularly rare to find these horseshoe crabs at the end of short traces, but nothing quite as substantially large and scientifically important as this,' Racay said.

The creator at the end of the its death march

As a result, an incredibly detailed story emerges. The 9.7 metre death march begins rather abruptly. This led the researchers to believe that the horseshoe crab was flung into the depths of the lagoon by one of the countless storms which wracked the fringes of the ancient Tethys Ocean. The trail slowly narrowed, revealing that the crab (Mesolimulus walchi) had found itself on its back and righted itself before moving off across the mud at the bottom of sea.

Yet it was moving towards a hidden danger. Everything from fish to crustaceans teemed in the bright, nourishing waters close to the surface, but the depths were highly anoxic. Any corpse which sank into this toxic trap was quickly buried by mud and, because the absence of decomposing organisms, was preserved in perfect detail. The deeper waters were also deadly to living organisms, quickly suffocating them.

'We started to study the specimen closer and saw that the walking patterns and the animal's behaviour started to change. The leg impressions became deeper and more erratic, the telson (the long spiny tail) started being lifted up and down, up and down, showing that the animal was really being affected by the conditions,' Racay said. In the end the creature died, its body and the tracks filled by mineral rich mud to be rediscovered 155 million years later.

To find a trackway and its track-maker preserved together in the fossil record is extremely rare and exciting. Such discoveries offer evidence of the behaviour and environmental conditions, which in this case, led to the creature's moment of death. 'Working out who made a trackway is normally like detective work,' adds Dr Nic Minter from the University of Saskatchewan, Canada, 'as the suspect has been caught in the act.'