48

Mollusca, Gastropoda, Epitoniidae: Cirsotrema lyratum

Small white gastropods with a distinctive pattern of spiral costae in each whorl are quite common in limestones and greensands of the Waitaki Geopark although the type specimen for the species, Cirsotrema lyratum, comes from Aotea Harbour south of Raglan, in the Waikato.

Collected by two of New Zealand’s most illustrious 19th century geologists, Ferdinand Hochstetter and Julius von Haast on 24 March 1859, the holotype was sent back to Vienna where it was studied and formally described by Karl Alfred von Zittel, a well-respected German paleontologist of the mid 19th century in one of the first major publications on New Zealand fossil shells.  It is now preserved in the collections of the Naturhistorisches Museum Wien in Vienna, Austria (NHMW), together with a number of other type specimens from various parts of New Zealand. Many New Zealand paleontologists have made a pilgrimage to Vienna to see for themselves the originals of important fossils far from home.

Zittel’s name for the little shells was Scalaria lyrata but they were transferred to the cosmopolitan genus, Cirsotrema, in 1897 and have remained there ever since. Note that the species name changed from lyrata to lyratum to correspond to the ending of the new genus. The species name means in the shape of a lyre (musical instrument).

Sometimes regarded as deep-water snails, the species of Cirsotrema living in New Zealand seas today live in sandy environments from shallow water down to around 200 m.  Like other members of their family, they are parasitic on sea anemones.

Interestingly, the creamy-white shells of Cirsotrema are often the only gastropods preserved at some fossil sites. The reason for this is that they have shells composed of stable calcite which is preserved when all gastropods with aragonitic shells have been dissolved away.

There is some controversy about just how many species of Cirsotrema are present in the New Zealand fossil record. There is considerable variation in the number of costae (ribs) in each whorl, the width of the costae and the prominence of the spiral cords.

Beu and Maxwell (1990) listed some 15 species, ranging in age from Eocene to Recent. However, the late Andrew Grebneff who had a particular interest in these little snails, considered that there was just a single, highly variable species found throughout New Zealand. However, at present not enough well-preserved specimens which still retain the diagnostic protoconch (the first-formed part of the shell at the apex) have been studied to prove this one way or another.

– Written by Daphne E Lee