42 Spectacular crab from Caversham Sandstone

Arthropoda, Decapoda: Lobocarcinus pustulosus

This large crab was initially found with just the worn tips of the chelae poking out on the surface of a fallen block of Caversham Sandstone at the base of a cliff near Cornish Head, Waikouaiti, Otago. Careful preparation was required to uncover the rest of the crab and reveal details of what turned out to be a nearly complete, fully articulated specimen of a new species of a crab not previously reported from the Pacific region.

Dorsal view of the Waikouaiti fossil crab Lobocarcinus pustulosus (specimen OU 41147). In this view bubbly detail on the carapace for which the crab is named can be clearly seen. The white structure down the midline of the carapace is a serpulid worm that attached to the crab while it was buried on the sea floor. Image credit: JH Robinson.

The family Cancridae in which the crab is placed is characterised by having a carapace that is wider than it is long, three or four spines on the frontal region, and up to 10 spines on the long convex anterolateral margin,. These are all features seen on the Waikouaiti specimen. Named Lobocarcinus pustulosus because of the strongly pustulose ornament on the pentagonal carapace, the body of the dead crab must have lain on the sea floor for some time as it is encrusted by a large serpulid worm tube as well as several tiny coiled Spirorbis serpulids.

The Caversham Sandstone is a thick, generally well-cemented calcareous sandstone that forms spectacular cliff outcrops on the coast north and south of Ōtepoti Dunedin. Macrofossils are not particularly common and are generally scattered randomly throughout the 120 metre thick formation. The fallen block from which the crab was extracted is from a poorly cemented, silty to sandy limestone bed probably tens of metres above the base. This was the first crab fossil to be reported from this formation although a few more examples have been collected since 1996.

—Written by Daphne E Lee