“Fossilised worm casts” are trace fossils, and I have found them in stones deposited on the beaches between Orepuki and Riverton at the bottom of the South Island of New Zealand. In the first Post in this series, I noted how these stones are especially associated with Gemstone Beach, near Orepuki. In the second Post, I introduced the idea of “trace” fossils. This Post looks at the shapes of the traces of the “worm casts” in more detail.
The traces on these fossil worm cast stones exhibit different shapes. A handful of the stones are crowded with different shaped traces:
Other stones have only one trace, or a partial trace, or a very faint trace:
It appears that a micro-faultline that has split this trace:
On the occasional stone, the trace is noticeably raised in relation to the rest of the stone:
However, on the vast majority of the stones, the trace has worn away at the same rate as the rest of the stone.
Let’s look more closely at the shape of the traces. One of the stones I collected has these shapes on it:
Are “A” and “B” essentially the same shape, “B” being a bit more stretched out somehow? There are also a number of much smaller and less complex shapes “C” – are these made by a different animal altogether? Or are they parts of shapes similar to “A” and “B”?
In the next stone (below), “D” seems similar to “B” in the previous stone, but “E” and “F” appear to be different from these. “E” is not segmented, and “F” is not lineal. Unless “E” is in effect a side view of “D”, and “F” is a view of “D” from the top or from below. It is important to be aware that a trace will look different depending on whether it is lying along the surface of a stone or “emerging” from it.
The next trace shape seems to change along its length, being clearly segmented at the bottom but much less so at the top. Is the very top end part of the same trace? Or is it a different one altogether?
On this stone (below), is “G” the same kind of trace as “H”, with “H” being a bit more worn away? Or are they different traces altogether?
And is this next shape different from all the others?
Some of the stones have blank featureless stripes on them, such as “I” on the stone below:
Is such a stripe a worn, or less-worn, version of the segmented trace to the left of it? Or was a different kind of animal or activity involved?
What are we to make of the trace shapes on this stone (below)? Are the traces all different or are they variations of the same shape? Or are they parts of one shape that has been partly worn away? If they are different shapes, might they have still been made by the same animal?
Finally, an example of complexity. The stone below contains a number of trace shapes, many of which we have already seen in the stones above. However, the lineal feature marked “J” is a long complex shape not previously encountered. How would we make sense of it? What kind of animal would have made such marks, and how?
The next Post will present the most common shape of the traces on the fossilised worm cast stones I have collected. The Post after that will try to identify this trace based on the literature that examines in detail the kinds of traces left behind by animals on the floor of the sea.
