A recent article in the journal Ichnos describes the fossilized mortichnia (“death march”) of a Jurassic-age juvenile horseshoe crab, Mesolimulus walchi. Discovered in 2002 in Germany’s Solnhofen limestone deposits famous for their exquisite preservative qualities, the track is extraordinary in several ways. At 32 feet, it’s the longest of its kind ever discovered, and the track contains an unusually complete chronology of the horseshoe crab’s last minutes: from the impact print of its fall into the then-tropical lagoon, to the crab’s fossilized corpse itself.
The scope and detail of this find has provided paleontologists with a chance to study the 150-million-year-old crab’s final moments in special detail. Like a kind of prehistoric cinema, the trail captures not only the crab’s motion through time, but provides compelling insights into its physio- and psychological state as it walked further into the anoxic (oxygen-deprived) and extremely saline Jurassic lagoon. The path not only shows the way in which M. walchi walked, but preserved footprints, telson (tail) drag traces, and prosoma (head) prints, giving scientists insight into both body structure and how the organism moved. How it fell into the lagoon is unknown; the authors surmise that it might have been swept in during a heavy storm. Among the information that this track preserves is a depiction of the crab’s early efforts to right and reorient itself, as well as its gradual decline into unconsciousness and ultimately death. The opportunity to link traces of ancient behavior to a specific, individual specimen or species is a rare and invaluable one for paleontologists. Often scientists will uncover just the track or just the organism, but together, these two provide a new depth of information. In the words of lead study author Dean Lomax, at Doncaster Museum and Art Gallery in England, no fossilized coupling of track and track-maker “quite as substantially large and scientifically important as this” has been found before now.
Read more about this discovery at Sci-News.com.
Read the original article in the journal Ichnos.
Watch a YouTube video of the intertidal wanderings of Mesolimulus‘ modern equivalent, a juvenile Hong Kong horseshoe crab Tachypleus tridentatus, below.