“Fossils are the preserved evidence of past life. They may include organic remains such as wood, shells, bones and teeth that have been buried, mineralised, and turned to stone” (Te Ara Encyclopedia of New Zealand). Fossils are very important to geology because they are used to date sedimentary rock strata, just like pottery and other artifacts are used to date archaeological layers. Fossils don’t survive the forces that produce metamorphic rocks nor the extreme heat associated with volcanic rocks, so they are found only in sedimentary rocks.
Fossil worm cast stones, like those above, introduced in the first Post in this series, are examples of trace fossils (also known as “ichno-fossils“), which are different from most other types of fossils. In the book, “A Photographic Guide to Fossils of New Zealand“, written by Hamish Campbell, Allan Beu, James Crampton, Liz Kennedy and Marianna Terezow (2013), it is pointed out that trace fossils are not the organic remains of dead organisms. “They include marks, traces, tracks, burrows and deposits that relate to animal and plant behaviour, dead or alive” (page 9). The authors go on to state that trace fossils “record an animal’s moving, exploring, escaping, hiding, breathing, hunting feeding, excreting, reproducing, growing, playing fighting, dying, or resting” (page 10).
Take as an example of a trace fossil, the footprints which record the movement of a dinosaur. In Bolivia is a limestone cliff on the face of which can be seen the footprints of at least eight different dinosaur species, left as trace impressions over 68 million years ago. A news article in “The Guardian” in 2011 explains how the footprints were made: “The creatures’ feet sank into the soft shoreline in warm damp weather, leaving marks that were solidified by later periods of drought. Wet weather then returned, sealing the prints below mud and sediment. The wet-dry pattern was repeated seven times, preserving multiple layers of prints. The cherry on the cake was added when tectonic activity pushed the flat ground up to a brilliant viewing angle.” See the photos below.
Dinosaur trace fossil footprints have also been found in New Zealand, in northwest Nelson. In 2009, GNS Geologist Greg Browne came across 70-million year old dinosaur footprints in sandstone around the shores of Westhaven (Whanganui) Inlet. They were the very first dinosaur footprints to be recognised in New Zealand as well as providing the first evidence of dinosaurs in the South Island.
There is an excellent YouTube clip in which Greg Browne shows the dinosaur trace fossil footprints he discovered, discusses how they were made, and points out the information provided by such fossils. According to a 2016 news report, it is thought that the footprints were most likely made by the colossal sauropod dinosaurs, the largest animals to have walked on Earth, growing to about 40 metres in length and weighing more than 100 tonnes (there is a depiction of a sauropod below the next video clip, the photo on the left). But there can often some doubt over exactly what dinosaur makes trace fossil footprints. We don’t have live dinosaurs whose contemporary footprints we can study!
In May this year, a significant trace fossil find in Otago, New Zealand, happened when Michael Johnson found some moa footprints that had been exposed in a streambed. Floods had scoured away the bed to expose the footprints, which had been made in clay.
Those who have studied dinosaur trace fossil footprints have pointed out that different substrates (sand, mud, silt etc.) and environments (land, stream, coast etc.) offer different chances of preserving them. An excellent diagram is in the photo below right.
Similar issues arise with the trace fossils of small marine animals like those whose markings have been left on the stones of the south coast. In “A Photographic Guide to Fossils of New Zealand” are only two trace fossil entries, on page 64, the first of which looks similar to the fossil worm casts found at Gemstone Beach and Riverton (see below). In the second entry, it is noted that the animal responsible for the traces is unknown despite the distinctiveness of the trace.
Another New Zealand example of a trace fossil can be found in Jocelyn Thornton’s (2013) “The Field Guide to New Zealand Geology”. On page 64 she refers to “the feeding traces of a wormlike creature, possibly something like a sea-pen”, these traces being “rows of grey arrowheads” and “swirling patterns” (see photo of text extract below). A sea-pen actually doesn’t look like a worm (see photos below) but it has a “peduncle” to anchor itself in sand or mud. A “peduncle” is a stalk-like part by which the sea-pen is attached to its substrate, by burrowing into it. As a New Zealand guide to sea-pens puts it: “Virtually all sea pens attach to the soft substrata of benthic sediments by an unbranched rootlike and sausage-shaped muscular peduncle enabling them to stand erect” (page 3 of “Pennatulacea (Sea Pens) Descriptions for the New Zealand Region“, 2014, by Gary Williams, Di Tracey and Erika Mackay). I guess it is the peduncle that will leave burrow-like traces behind, but it’s difficult to see them as having much length or going in a curving direction.
.jpg){kind=link}
_at_night.jpg){kind=link}
Nevertheless, this example from Thornton’s book raises again that important point about the kind of trace fossils I am interested in (the kind of traces left by marine worm-like animals). The point is that there is a distinction between the trace itself and the animal that makes the trace. What we have available to us are the traces – we then have to work out what animals made the traces (just as the dinosaur researchers have to do). And it may be that more than one kind of marine animal can make the same kind of trace. And can different traces be made by the same animal?
Furthermore, other things than animals can make “traces” on the sea floor or on rocks. Three different kinds of “pseudofossils”, commonly mistaken for trace fossils, are referred to in “A Photographic Guide to Fossils of New Zealand”. These are: sedimentary features such as ripples, caused by sea currents acting on sand, silt and mud (see photo below left); mineral growths, such as iron and manganese hydroxide minerals growing on the surface of a crack in a rock (see photo below middle); and structural features, such as clay minerals having a preferred orientation leading to regular patterns in a rock (see photo below right).
For more on pseudofossils, see Western Australia Dept of Mines, Industry Regulation and Safety and Wikipedia.
In the next Post in this series, I will examine the shapes of the traces found in the fossilised worm cast stones of Gemstone Beach and Riverton. Later Posts will then try to make sense of these shapes.
6 thoughts on “The Fossilised Worm Cast Stones of Gemstone Beach and Riverton – Part Two: What is a Trace Fossil? From Dinosaur Traces to Sea-Pen Traces”