Death march ("mortichnium") of a horseshoe crab

It's a "trackway" - fossilized footprints of the last steps of a creature that died 150 million years ago, as reported by the BBC:

The fossil trackway of the animal's last moments - known as a mortichnia, or death march - was discovered in the lithographic limestone of Bavaria in Germany in 2002, where spectacular fossils of the famous feathered dinosaur Archaeopteryx have also been found.

"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 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. 

New word for the day: "mortichnia."  However, after (very brief) research, I've used the word "mortichnium" in the title, because I think "mortichnia" is plural, based on the last paragraph on this page of this book on trace fossils, written in 2007.

"Although such markings are exceptional, I would like to call them mortichnia - perhaps an adequate act upon finishing this book and a happy affair that has lasted sixty-five years."