Geoff Chapple once walked with the geologist Nick Mortimer along the Southland coast between Colac Bay Oraka and Riverton Aparima. “We looked for the tattooed rock, the trace fossils that Māori call mokomoko, and it took some time but we did find them, wetting down the face of a rock layer to reveal 270 million-year-old traces of burrowing worms that took the purple of their starting layer down into the pale depths beneath, or dragged their pale layer into some purple darkness below, working their primitive palette, thousands of small finger painters out of the Permian…” (page 253 of Geoff Chapple “Terrain: Travels Through A Deep Landscape”, 2015). Not all trace fossils along this coast are purple, in fact they are a range of colours, with green most common. Below are photos of 12 I have found on the Te Waewae Bay coast during one week of my fossicks in February.
“Trace fossils are what is left of the activity of some ancient critter”, writes retired kiwi geologist Brian Ricketts in his blog “Geological Digressions”. He provides an excellent general introduction to trace fossils here. As I have noted in a previous Post, trace fossils are called “ichnofossils” – they are identified by their shapes (not by what made the shapes as this may not always be known). The term is derived from the Greek word “ichnites” meaning “footprint”. An “ichnogenus” is a group of trace fossils with similar characteristics (a “genus” is an intermediate category between “species” and “family”). “Ichnogenus Protovirgularia” are trace fossil shapes consisting of “a small keel-like trail which is composed of an elevated median line and lateral wedge-shaped appendages alternating on both sides” (source), including lines of chevron shapes (as in many of the stones I have found on Gemstone Beach and along the Te Waewae Bay coast). They were given this name in 1850 by Frederick McCoy, an influential Irish palaeontologist, who believed he was seeing the actual fossil of an ancestor of Virgularia mirabilis, the slender sea-pen, hence the term “proto-virgularia” (“proto” meaning original or earliest). It was not until 1958 that the shape was identified instead as a trace fossil (and one which was made by quite a different animal).
There are some geologists who specialise in the study of trace fossils, and some of them have a keen interest in protovirgularia shapes. There is a diagram (see below, middle photo) that is from a 2010 article on protovirgularia found in Patagonia, Argentina, published in the “Journal of Paleontology”. There is a lot of variety in the forms of this trace shape. Forms #3, #4 and #5 on the diagram look at first sight to be close to the well-defined chevron shaped traces often found in the stones I collect. For more, see my blog Post “The Fossilised Worm Cast Stones of Gemstone Beach and Riverton – Part Four: Ichnogenus Protovirgularia and the Permian Rocks of the Eglinton Valley”.
The next Post in the “Southern Sojourn 2023” Series features a banded stone with an unexpected top. The first Post in the Series is here. The Index to the Series is here.
