It can often be difficult for the non-expert like myself to identify a stone. Consulting books or the internet about different rock and mineral types can be frustrating because you can often find three or four possibilities that might match your stone. Moreover, your stone might have some characteristics that appear to be unique and not present in any of the “pure” types included in the books. There may be clues in how locals refer to a stone, though even they can be mistaken. You may be lucky enough to come across a reference to your stone by an expert somewhere, or you might even be fortunate enough to contact an expert who can help you.
This Post is part of a series on what are called “fossilised worm cast stones” found on Gemstone Beach and the beaches of Riverton in Southland. I have not come across any authoritative reference to them anywhere in the literature or online. Until now. Recently I received information on them from two different sources. This Post reports on the first source, and the next Post (not yet available) reports on the second. (The first Post in the Series can be found here.)
A FOSSIL TRAIL, ICHNOGENUS PROTOVIRGULARIA, POSSIBLY FROM NUCULA, A SMALL BIVALVE
In early July I emailed the following seven photos to someone I know in a University Department of Geology.
“The stones contain what are probably fossilised traces of worms”, I wrote. “I would like to identify the kind of rocks the stones come from, what kind of worm is involved, and when they were active (in other words, when did they start becoming fossilised).” My inquiry was passed on to another geologist with expertise in ichnology (the study of trace fossils) who replied: “Yes, a trail, probably ichnogenus Protovirgularia, usually attributed to a small bivalve such as Nucula. I’ve seen it in float boulders in the Eglinton and very similar pebbles on the East shore of Te Anau, where it is likely from the Permian.”
It seems to me that five points follow from this information:
1) The traces on the stones are trails . If this term is taken strictly and narrowly, then a “trail” is left behind on the surface of sediment by the movement of an animal. A “trail” is not a “burrow” nor a “cast” (excretion). However, a looser definition of a “trail” could include “burrow” and “cast”, so it is important to keep an open mind on this.
2) The trace is called ichnogenus Protovirgularia. My initial research into this term reveals the following. “Ichnogenus” is a category of taxonomy (a way of grouping similar things) for trace fossils. A “genus” is an intermediate category between “species” and “family”. In relation to the term “Protovirgularia”, “Virgularia” is a genus of sea pen in the family Virgulariidae (sea pens were discussed in Part Two in this series of Posts), and “Proto” means original, primitive, first or earliest. For a number of weeks I was not able to find out why this group of trace fossils was given the term “Protovirgularia” – I thought that maybe it was initially thought to have been made by a sea pen. It is reasonably well-established now that in the vast majority of cases the pattern is made by bivalves, though it can be made by other invertebrates (this may be the subject of a later Post). I was eventually able to find a copy online of the original article that first identified protovirgularia. It was written by Frederick McCoy (Professor of Geology and Mineralogy, Queen’s College, Belfast) in 1850 in the “Annals and Magazine of Natural History“. On page 272, McCoy wrote: “…In the form of its axis and the structure of the transversely ridged celluliferous pinnae, both in the curved extended, and in the straight contracted states, it perfectly resembles the recent Virgularia mirabilis…” (my underlining).
In his 1850 article, McCoy also referred to Protovirgularia dichotoma which varied a little from the basic Protovirgularia he first described.
Below are photos of Virgularia mirabilis, also known as the slender sea-pen. The photo of a dead one on the far right below shows the resemblance noted by McCoy to the fossil he had observed, and he believed it to be of a primitive ancestor of this animal. It was not until 1958 that the fossil was identified as a trace fossil made by a different animal.
One academic source defines ichnogenus Protovirgularia as a small keel-like trail which is composed of an elevated median line and lateral wedge-shaped appendages alternating on both sides (see also the entry in FossilID). The following description of Protovirgularia dichotoma is very interesting in relation to the trace fossils found at Riverton and Gemstone Beach because of reference to chevrons (v-shaped patterns). This source describes Protovirgularia dichotoma trace fossils as “horizontal or subhorizontal cylindrical burrows, trapezoidal, almond-shaped, or triangular in cross-section, distinctly or indistinctly bilobated [having two lobes]. Internal structure, if preserved, is formed by successive pads of sediment that may be expressed as ribs on the exterior. Ribs arranged in chevron-shaped, biserial pattern along external or internal dorsal part.” Below are examples.
The photo below contains four examples of protovirgularia trace fossil specimens from Japan, discussed in a 2011 article in the academic journal “Palaeogeography, Palaeoclimatology, Palaeoecology”. The variations within the protovirgularia grouping is shown. Note too that these are three-dimensional traces in contrast to the trace in the Gemstone Beach and Riverton stones that have been subject to “flattening” by the smoothing action of the river and ocean.
The diagram below is from a 2010 article on protovirgularia found in Patagonia, Argentina, published in the “Journal of Paleontology”. Again, the significant variety in the forms of this trace shape is demonstrated. Forms #3, #4 and #5 on the diagram look at first sight to be closest to the well-defined chevron shaped traces often found in the Gemstone Beach and Riverton stones.
3) Despite its name linking it to sea pens, it seems that this type of fossil trail, “ichnogenus Protovirgularia”, is usually thought to be left behind by a small bivalve. A “bivalve” is an aquatic mollusc which has a compressed body enclosed within a hinged shell, such as a clam, oyster, mussel, or scallop. Bivalves have inhabited the Earth for over 500 million years. They especially flourished in the Mesozoic and Cenozoic eras (the last 250 million years). The excellent British Geological Survey entry on bivalves as fossils shows how they are able to burrow into sediment. One article in the journal “Ichnos” in 2010, again on protovirgularia from Argentina, gives the following account of the link between the trace and bivalves: it is a locomotion trace produced by bivalves, a burrow resulting from the rhythmic action of a muscular cleft-foot (page 42). Nucula is a genus of very small saltwater clams, according to Wikipedia (see photo, below left, of Nucula in Te Papa Museum and, below right, Auckland Museum).
These clams are very ancient, having been around for the last 450 million years or so, and have been very common around the world. They can reach a size of up to 30 millimetres and live in the muddy sand close to the sediment surface at an oceanic depth of 20 to 200 metres. It is thought that the movement of Nucula leaves behind the “ichnogenus Protovirgularia” trace shape. However, I assume there is also the possibility that maybe something else could leave that trace shape.
4) The trace fossil shapes found in the stones at Orepuki and Riverton have also been found on float boulders in the Eglinton River and in pebbles on the East shore of Lake Te Anau. The Eglinton River flows into Lake Te Anau at a middle point along the lake’s eastern shore, having made its way there from the north-north-east:
A “float” rock is a loose piece of rock that is not connected to an outcrop, that is, it has been removed and transported from its place of origin, especially by gravity, ice, water or people. This often makes it difficult for a geologist to identify float rocks. In relation to the terms “boulders” and “pebbles”, when it comes to the sizes of float rocks, geologists use a set of categories. One such set is as follows: A “grain” (e.g., of sand) has a diameter of 2 to 4 millimetres, a “pebble” is between 2 and 64 millimetres, a “cobble” is 64 to 256 millimetres in diameter, and a “boulder” is larger than 256 millimetres.
5) These trace fossils were likely laid down in the Permian period. The Permian spans 47 million years from the end of the Carboniferous period about 300 million years ago to the beginning of the Triassic period about 250 million years ago. The world at the time was dominated by two continents known as Pangaea and Siberia (the latter also known as Angaraland), surrounded by a global ocean called Panthalassa. The New Zealand area was on the southeast part of Pangaea and with it moved around the southern part of the globe, including over the South Pole area.
This is how a University of Waikato website, by the Biology and Earth Sciences Departments, describes the Permian context of New Zealand and some of its local geological happenings:
By Permian times, the continents were moving even closer together than during the Carboniferous as the northern and southern supercontinents of Laurasia and Gondwana began to assemble into a single great landmass, called Pangaea. The Permian period was the final period of the Paleozoic era and is named after the province of Perm, Russia, where rocks of this age were first studied.
The global geography of the Permian included massive areas of land and water. Models indicate that the interior regions of this vast continent were probably dry, with great seasonal fluctuations, because of the lack of the moderating effect of nearby bodies of water, and that only some parts of the supercontinent received rainfall throughout the year. The ocean itself still has little known about it. There was extensive glaciation in southern parts of the landmass, shown by glacial striations on Permian rocks from what are now Africa, South America, and Antarctica, and extensive deposits of windblown soil indicate a very dry climate. However, there are indications that the climate of the Earth shifted at this time, and that glaciation decreased, as the interiors of continents became drier.
One of the most striking transitions in the evolution of life occurred when mammals evolved from one lineage of reptiles. This transition began during the Permian…
Permian rocks are widely distributed in New Zealand’s South Island and also occur in the Northland region of the North Island, where they are the oldest known rocks. In some places the deep sedimentary marine series that was laid down is up to 20 kilometres thick and one of the most complete Permian sequences preserved anywhere in the world. The biggest build up of volcanic rocks made the Takitimu Mountains near Redcliff, Waiau Valley, where the pile of tuff layers is 14 kilometres thick. Unfortunately Permian fossil outcrops are hard to find, but at Arthurton, near Gore, complete shells of Atomodesma, a bivalve, can be found. And of course a whole leaf of Glossopteris plant has been found at Productus Creek…
There is no mention of the Permian trace fossils of Southland but the rocks in which they occurred, which would likely have bordered on the Waiau Valley, including the Eglinton Valley, are identified in general terms.
As Te Ara The Encyclopedia of New Zealand notes, the end of the Permian period marks earth’s largest ever extinction – two-thirds of plant and animal life died out, probably as a result of a sudden climate change.
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The next Post in this Series (not yet available) looks at another, maybe related, explanation for the “fossilised worm cast stones” found on Gemstone Beach and the beaches of Riverton in Southland.
Other Posts on these stones include “Photo-Book ‘The Trace Fossil Stones of Gemstone Beach'”, towards the end of “Little Wing Boutique, Riverton: Gemstone Beach Polished TumbleStones For Sale”, “FB Group Posts: 5, 6 & 7 May 2021 – Banded Argillite, Trace Fossil Stones and Shades-of-Grey Quartzites” and “I” is for “Ichnogenus Protovirgularia”.
TumbleStone 
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