Mesolimulus walchi

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This horseshoe crab left behind a series of footprints when it died, seen as shallow, roughly parallel impressions on the surface of the slab. It is rare for a trace maker to be found as a body fossil at the end of its trackway. When that occurs, the trace fossils are known as “mortichina” or death tracks (Seilacher, 2008). Trace fossils like this one, associated with or left by horseshoe crabs, are generally assigned to the ichnotaxon Kouphichnium (Lomax & Racay, 2012).

There are several examples of death tracks left by horseshoe crabs and other crustaceans in the Solnhofen Limestone, and these are basically the only evidence of live animals at the bottom of the Solnhofen basins, shortly interacting with the substrate (Barthel et al., 1990; Seilacher, 2008). It is thought that these crustaceans were washed in and quickly died by asphyxiation, possibly because of hypersalinity of the bottom waters (Barthel et al., 1990; Seilacher, 2008; Lomax & Racay, 2012). Some of the Mesolimulus death tracks have been interpreted as showing signs of struggling behavior just before death (Lomax & Racay, 2012). Interestingly, all Solnhofen horseshoe crabs found at the end of their death tracks are juveniles (Barthel et al., 1990; Lomax & Racay, 2012).

This, and several other fossil horseshoe crabs are very similar to living horseshoe crabs, which, for this reason, are often called “living fossils” (Rudkin et al., 2008). Morphological stability is seen in many kinds of organisms. Fossil horseshoe crabs of this genus have been found in Germany, Spain, Russia, and Morocco (Paleobiology Database, n.d.).

Broader view of slab and specimen, with death tracks.

References:

Barthel K.W., Swinburne N.H.M., Conway Morris, S., 1990. Solnhofen: A Study in Mesozoic Palaeontology. Cambridge: Cambridge University Press.

Lomax, D.R. and Racay, C.A., 2012. A long mortichnial trackway of Mesolimulus walchi from the Upper Jurassic Solnhofen Lithographic Limestone near Wintershof, Germany. Ichnos, 19(3), pp. 175-183. doi:10.1080/10420940.2012.702704.

Paleobiology Database, No date. Mesolimulus. Available at: https://paleobiodb.org/classic/checkTaxonInfo?taxon_no=18932&is_real_user=1.

Seilacher, A., 2008. Biomats, biofilms, and bioglue as preservational agents for arthropod trackways. Palaeogeography, Palaeoclimatology, Palaeoecology, 270(3-4), pp. 252-257. doi:10.1016/j.palaeo.2008.07.011.