Comparing comments on the ‘spiral death tracks’ of fossil juvenile horseshoe crabs found at Solnhofen with the normal spiral foraging trails of juvenile horseshoe crabs in their natural habitat on an intertidal mudflat in Hong Kong.

This post is one in a series of posts which compares the depositional environment of the late Jurassic lithographic limestones at Solnhofen in Germany based on extracts from Solnhofen: A Study in Mesozoic Palaeontology with observations made at a modern inter-tidal mudflat at Ha Pak Nai, Deep Bay, New Territories, Hong Kong and proposes an inter-tidal mudflat origin for the examples cited.

This post compares references to the presence of spiral death tracks made by horseshoe crabs at Solhofen - Eichstatt with photographs of spiral foraging tracks made by juvenile horseshoe crabs in their natural habitat on intertidal mudflats in Hong Kong.

Extract from: Solnhofen: A Study in Mesozoic Palaeontology

Species: Mesolimulus walchi

“A very few, relatively tolerant animals were able to survive the hypersaline milieu for a short time, perhaps a matter of hours, but all their traces end in the death of the beast. Washed into the stagnant, salty basin, they were merely able to crawl in a disoriented spiral before collapsing in their tracks. Most famous and comparatively common are the tracks of the horseshoe crab, Mesolimulus [See Figiures 5.5 and 5.7].” (Barthel et al - Pages 77 & 79).

“The other type of trace is the spiral death track with the body at the end.” (Barthel et al - Page 79).

“Limulids burrow in the mud to shallow depths in search of small invertebrates for food. They may spend short periods out of the water, and they are particularly tolerant to fluctuations in the salinity or temperature of the water. Consequently, it is not surprising they were one of the few animals still alive when they reached the poisonous lagoon floor, being responsible for some of the most famous spiral tracks or ‘death marches’ which terminate with the body at the centre.” (Barthel et al - Page 140).

“… the juvenile horseshoe crab, Mesolimulus and the crustacean Mecochirus were still alive for a short time on the lagoon floor. These creatures produced the famous spiraling trails with the body lying at the centre, interpreted as the last, disorientated crawl of a moribund animal before it collapsed in its tracks” (Barthel et al - Page 58).

Comments on juvenile horseshoe crab habitats and ecology in Hong Kong

There are four species of horseshoe crab living today, Limulus polyphemus,Tachypleus tridentatus, Tachypleus gigas and Carcinoscorpius rotundicauda. L. polyphemus  occurs along the east coast of the USA and Mexico. The other three species occur in South-East Asia. T. tridentatus and C. rotundicauda occur in Hong Kong.

When juvenile T. tridentatus and C. rotundicauda forage in Hong Kong, they leave spiral foraging trails. Such spiral trails represent normal feeding patterns and rather than being ‘disorientated’, they are suggestive of a systematic search for food, ensuring the same ground is not covered twice. In fact when searching for juvenile horseshoe crabs on intertidal mudflats, one of the first signs to look for is the presence of spiral foraging trails.

Spiral foraging and movement trails are therefore the norm, not the exception, so ‘spiral death tracks’ are likely the result of normal activity, albeit one that is fatal in its outcome.

Photographs of spiral foraging trails of juvenile T. tridentatus on an intertidal mudflat

 in Hong Kong

References

Barthel, K.W., Swinburne, N.H.M., and Conway Morris, S. (1994). Solnhofen: A Study in Mesozoic Palaeontology. Cambridge University Press, Cambridge.