As reported by Nick Crumpton, a fossil “death march” of a horseshoe crab was found in the Solenhofen limestone—the same formation that yielded the famous transitional fossil Archaopteryx. The fine-grained sediments from what was once a quiet lagoon produced exquisite preservation, and in this case we have what is interpreted as the final walk of a horseshoe crab flung into the lagoon by a storm (the storm part is speculative) 150 million years ago. Here’s the animal:
Note the phenotypic similarity to modern horseshoe crabs, a similarity which makes this animal a famous “living fossil.” Of course they’re not externally identical to modern ones, and we know nothing about the changes in its anatomy, biochemistry, or simply DNA sequence, which presumably has changed via the molecular clock in the last 150 million years.
After it purportedly landed in the lagoon, the crab began to walk, and made 9.7 meters before it died. (Remember, horseshoe crabs are in the subphylum Chelicerata, not Crustacea, so they’re more closely related to spiders and scorpions than to “real” crabs, which are crustaceans.)
As the BBC reports:
The fossil records an entire walk, and the researchers believe that the abrupt beginning of the trace can be explained by the animal being “flung” into the lagoon during a storm, although they cannot be certain of this interpretation. . .
“The lagoon that the animal found itself in was anoxic, so at the bottom of these lagoons there was no oxygen and nothing was living,” Mr Lomax [Dean Lomax of the Doncaster Museum and Art Gallery] told the BBC.
“This horseshoe crab [Mesolimulus walchi] found itself on the lagoon floor and we can tell by looking at the trace that the animal righted itself, managed to get on to its feet and began to walk,” he explained.
However, the anoxic conditions of the lagoon floor quickly proved fatal to the arthropod and it soon began to struggle.
“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,” he said.
And here’s the fossil trackway, also preserved in those optimal conditions. The animal moved from right to left in the picture (tracing is below), and you can see the fossil at the end.
RIP, ancient arthropod.