29 Dunedin’s ammonite named for geology great

Mollusca, Cephalopoda: Kossmaticeras bensoni

One of the ammonoids that swam in Late Cretaceous oceans around Zealandia was Kossmaticeras bensoni. This large ammonoid had a shell that could reach up to 30 cm in diameter and is locally famous in Ōtepoti Dunedin for two reasons. The first reason is the frequency of discovery in concretions in the Fairfield sand quarry (a privately-owned quarry where access permission must first be sought before entering). The second reason is the name.

Natural mould of the shell from the ammonoid Kossmaticeras bensoni (specimen OU 4679). Kossmaticeras bensoni can be identified from its large size and the detail on the outside of the shell (i.e. ornamentation). Image credit: JH Robinson.

Kossmaticeras bensoni was named by Robert (Bob) Henderson in 1970 as part of a major study of Late Cretaceous ammonites in New Zealand. The species name bensoni was chosen to honour William Noel Benson, Professor and former Head of the Department of Geology at the University of Otago.

The holotype of Kossmaticeras bensoni is part of the Geology Museum collections. For every named species of organism on the planet, somewhere in the world there will be a single individual housed within a taxonomic collection that is considered the principal example of that species (the holotype). Every other individual that is identified as belonging to the same species has been compared to the holotype. There are some exceptions to this rule of biological classification, but nevertheless, protecting holotypes is one of the most important roles of the University of Otago Geology Museum, and of many other museums and taxonomic collections around the world.

Did you know there is a 3D printable model of this fossil for you to view online and download? Follow this link to learn more: 3D model—Dunedin’s ammonite named for geology great.

Kossmaticeras bensoni can be identified from its large size and a combination of features ornamenting the shell including the approximately 70 ribs (i.e. radial ridges) on each half turn (i.e. half whorl) of the shell. In the well-cemented concretions in Fairfield Greensand at Fairfield quarry, Kossmaticeras and other molluscs are preserved as carbonised and three dimensional structures from which all original calcareous shell has been leached away. Accordingly, while surface details are preserved, none remain for internal structures – the septa, chambers and semiporous tube. Clusters of up to five individuals have been preserved together, suggesting that this species was abundant in these shallow seas. On one memorable field trip, a quarry digger driver dug out a row of concretions, lined them up at the foot of the quarry face and proceeded to crack them open for watching students. Dozens of such concretions, many with ammonites, are housed in the research collections in the South Basement, awaiting further study.

Species of Kossmaticeras have now been recognised in each of the former fragments of Gondwana and also in Canada, revealing Kossmaticeras to have had a broad distribution across Late Cretaceous oceans.

—Written by Daniel B Thomas