Horseshoe Crab Fossil With Trackway

THE PRESERVED FOSSIL AND "DEATH MARCH" OF AN ANCIENT HORSESHOE CRAB

Horseshoe crabs first appeared approximately 445 to 440 million years ago, at the boundary between the Ordovician and Silurian Periods of the Paleozoic Era. Often referred to as "living fossils," present-day Limulus horseshoe crabs look extremely similar to their Mesolimulus ancestors, including the Mesolimulus walchi specimen seen here.

This fossil horseshoe crab lived approximately 155 million years ago in what has now become the Solnhofen limestone of Bavaria, Germany. Examples of similar specimens from Solnhofen can be found in most prominent natural history museums and collections around the world. While the outline of the head, abdomen, tail, and opisthosomal spines of this specimen are all extremely well-defined, it is the 4-and-a-half-foot trace of this animal's mortichnial trackway, or "death march," that makes this fossil quite a rarity and something that many other collections cannot match.

Mortichnial trackways provide crucial information about the behavior of animals when they were alive. This is why trace fossils — also known as ichnofossils — are so valuable to researchers, museums, and collectors: whether it is the death marches of horseshoe crabs or the footprints of dinosaurs, they give us important information about ancient animal locomotion that skeletons and bones often cannot provide.

Presumably, this horseshoe crab fell into the oxygen-depleted environment of the ancient Solnhofen lagoon and quickly began asphyxiating as it walked along the muddy and stagnant bottom of this relatively lifeless environment. However, what killed the horseshoe crab is also what helped to preserve it: the anoxic environment of Solnhofen made it an ideal place to preserve fossils, without the deterioration, decay, and predation that hamper fossilization in oxygen-rich environments.