Narona decaptyx: A Fossil Vampire

13.5 mm long specimen of Narona decaptyx, from Landau et al. (2012).

Narona decaptyx was described by Brown & Pilsbry (1911) from a single small, fusiform fossil shell, 11 mm in length, from the Gatun Formation of Panama, north of Panama City. They regarded the formation as probably Oligocene in age but Landau et al. (2012) later referred N. decaptyx to the upper Miocene. Until it's redescription by the latter, this species was only known from Brown & Pilsbry's original holotype; Landau et al. described further material from the Bocas del Toro region to the west of the original locality. Both the sites from which N. decaptyx are known are on the Caribbean coast of Panama.

Narona decaptyx is a member of the Cancellariidae, the nutmeg snails. Cancellariids are one of the smaller families of the great neogastropod radiation, the group that also includes such forms as whelks and cone shells. They generally have a more or less developed sculpture of criss-crossing spiral and axial ribs; the latticed pattern this produces is formally referred to as 'cancellate' and provides the source of the family's name. The most distinctive feature of Cancellariidae is their radula, a slender ribbon of long, flexible teeth arranged in a single row. How this radula functioned was long a mystery. Dissections of the gut of cancellariids failed to find any trace of solid food, and it was suggested they may be adapted to some form of suctorial feeding. Some authors suggested that cancellariids might feed by slurping up micro-organisms. Then, in the 1980s, one species of cancellariid Cancellaria cooperi was observed feeding on sleeping electric rays. The snail would cut incisions in the ray's skin, presumably with its radula, before inserting its proboscis to slurp up the fish's blood (O'Sullivan et al. 1987). Other cancellariids have been observed feeding on fluids from other invertebrates such as benthic molluscs or their egg masses.

Cancellaria cooperi feeding on an electric ray, copyright Clinton Bauder.

The genus Narona to which N. decaptyx belongs is now restricted to the north Pacific. In this respect, it is not unique. Cancellariids are poorly represented in the modern Caribbean fauna with only six species known from the sea's shallow waters but they were much more diverse there in N. decaptyx's time. However, following the rise of the isthmus of Panama, many cancellariid taxa once found widely in the tropical Americas became extinct for whatever reason on the eastern side of the divide. This happened regularly enough that the term 'paciphile' has been coined for referring to such taxa. A number of distinct waves of paciphile extinctions have been identified in the Caribbean cancellariid fossil record and they have been used to identify distinct chronological zones. Narona decaptyx became extinct as part of the GNPMU (Gatunian Neogene Paciphilic Molluscan Unit) 1 period. Other Narona species persisted in the Caribbean and Gulf of Mexico for longer, surviving into the Pliocene, but eventually they too succumbed to whatever dampened this family's prospects in the region.

REFERENCES

Brown, A. P., & H. A. Pilsbry. 1911. Fauna of the Gatun Formation, Isthmus of Panama. Proceedings of the Academy of Natural Sciences of Philadelphia 63 (2): 336–373.

Landau, B., R. E. Petit & C. M. da Silva. 2012. The family Cancellariidae (Mollusca: Gastropoda) in the Neogene of the Bocas del Toro region, Panama, with the description of seven new species. Journal of Paleontology 86 (2): 311–339.

O'Sullivan, J. B., R. R. McConnaughey & M. E. Huber. 1987. A blood-sucking snail: the Cooper's nutmeg, Cancellaria cooperi Gabb, parasitizes the California electric ray, Torpedo californica Ayres. Biological Bulletin 172 (3): 362–366.