More than 100 million years ago, a harsh storm washed an unlucky horseshoe crab into a toxic lagoon. The doomed crustacean scrambled and plodded along the muddy bottom before succumbing, leaving behind a now-fossilized death march that ends with the crab's body.
Such a find is "extremely rare," the researchers write on Aug. 29 in Ichnos: An International Journal for Plant and Animal Traces. Once plopped into the oxygen-free, highly salty lagoon, the creature did not live long, the researchers say; even so, it did live long enough to try to escape, creating a track extending 32 feet (9.7 meters) long with an average width of 1.7 inches (4.25 centimeters).
The horseshoe crab, Mesolimulus walchi, measured 5 inches (12.7 cm) long and 2.7 inches (6.9 cm) wide and was likely a juvenile when it died. It has a rounded shieldlike shape that narrows to a trapezoid and finally to a long, pointed telson — the crab's third body segment, or its tail, which is used for steering and righting itself.
The fossilized animal and its death track were discovered near the village of Wintershof, Bavaria, Germany, in 2002; the layer of limestone in which they were found dates to about 150 million to 145 million years ago. [See Images of the Crab's Death March]
From the "footprints" and telson marks, Dean Lomax of the Doncaster Museum & Art Gallery in the U.K. and Christopher Racay of the Wyoming Dinosaur Center came up with a most-likely scenario for the crustacean's death.
"[W]e did not find any other specimens, per say, though we compared the specimen with other similar 'death tracks,' though these others were not complete and nowhere near as extensive," Dean Lomax told LiveScience in an email.
At the start of the fossil trackway, the surface looks trampled and includes several imprints from the animal's "feet" and telson, along with circular depressions. The researchers think the animal dropped through the water column and flipped upside down, as this is the most stable position from a hydrodynamic perspective. When it hit the bottom, the horseshoe crab would've been on its back and likely had to struggle to turn upright, explaining the disturbed surface there.
Throughout parts of the trackway, imprints suggest various movements, including gentle meandering, sudden angular turns of the body and straight walking locomotion. In the second part of the trackway, the researchers noticed the horseshoe crab made two 90-degree turns, making small partial spirals and using the front of its body, called the prosoma, to do these spins.
Toward the end of the trackway, however, the horseshoe crab begins to show signs of asphyxiation from the lagoon's anoxic (no oxygen) conditions. "[T]he telson drag marks and imprints become less uniform and more erratic," they write. "[T]his may show evidence the limulid has become distressed and disorientated due to the toxic environment of the lagoon as it tries to escape."
While the researchers can't be sure an ancient storm dumped the crab to its death, they think it's the most likely possibility. Another idea, though more unlikely, they write, is that a predator such as a pterosaur dropped the horseshoe crab into the lagoon. With no predation marks found on the crab, Lomax and Racay dismiss this hypothesis.
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