33 One-eared floating scallop

Mollusca, Bivalvia: Monotis spp.

Monotis is an extinct group of scallops that had a worldwide distribution in the Late Triassic. Monotis shells have a distinctive shape with a spoon-shaped ear separated from the anterior wing by a deep byssal notch and strong ribs interspersed with finer ones. The species of Monotis had quite thin shells relative to their size and probably lived attached to floating seaweeds at the ocean surface. Monotis has a cosmopolitan distribution and often occurs in dense shell beds. It must have been very common in suitable environments where the dead shells settled and thickly covered the sea floor.

The top image shows a section of a shellbed composed almost entirely of fossil Monotis intermedia shells. The image along the bottom left shows an individual Monotis subcircularis shell. The bottom right shows former logos of the Geological Society of New Zealand showing Monotis in a block of sandstone against a geology hammer. Image credit: JH Robinson (photographs), J Koster, D Gregg, G Warren, P Suggate, K Kingma, G Coates (GSNZ logo). Permission to include the logo designs kindly provided by the Geoscience Society of New Zealand. For a history of the logo please see “Origin of the Geological Society of New Zealand’s logo” by Don Gregg and David Smale, page 47 in Hayward BW (2005) Geological Society of New Zealand 1955-2005: Our first 50 years. Geological Society of New Zealand Miscellaneous Publication 118. 74pp.

The first European professional geologist to work in New Zealand, Ferdinand von Hochstetter, collected Monotis fossils from the Nelson area during his short but productive visit in 1859 and took them back to Austria where they were described by paleontologist KA Zittel. Once all placed in Monotis richmondiana, later research has recognised five genera and up to 14 different species but it is challenging for a non-specialist to tell them apart.

Monotis is possibly the best-known and most widely recognised fossil in New Zealand. Tens of millions of individual specimens form shellbeds that can sometimes be traced for many kilometres at localities from South Otago through Southland to Nelson and the Waikato. The Warepan Stage has its type locality near the tiny settlement of Warepa in South Otago and rocks can be immediately identified as of Warepan age (217 to 208.4 million years when the Monotis shellbed is present. Isolated shellbeds are also found in rocks of the Torlesse terrane in Canterbury and Wellington.

The Geological Society of New Zealand (now Geoscience Society of New Zealand) was set up in 1955. Seven years later the fledgling society decided it needed a logo, and in 1964 the first version of the logo was published: a block of rock with a stylised Monotis shell resting on a geological hammer.

—Written by Jeffrey H Robinson