A tiny circle only 10mm across in a fragment of silty mudstone. The distinctive pattern is highlighted by iron oxide and is clearly biological. It was found within the volcanic breccia in the Hornsby – Thornleigh diatreme system.

It is a fossil, but a fossil of what? How old is it and what is it doing in an igneous rock where we do not usually expect to find fossils?

It is a mould of a single disk from the stem of a crinoid or sea lilly. Related to modern sea stars and sea urchins these first appeared in the mid-Cambrian (about 510 million years ago). They had maximum diversity in the Permian when they were important parts of limestone reefs, survived several mass extinction events, and still live in marine waters. They are filter feeders anchored to the sea floor with a flexible skeleton composed of the mineral calcite (CaCO₃). On death the individual breaks up and the hard parts are commonly fossilised.

This one was found with other fossils that we can identify as early Permian (circa 273 – 271million years ago) and the whole assemblage is like that in the Wandrawandian Siltstone at Ulladulla. We can’t give this single disk a generic name as there are 12 known species described from the sedimentary rocks on the south coast.

What is it doing in a Jurassic diatreme that is younger than the marine sediments on which it lived? Those diatremes erupted up through the full thickness of all the rocks in the Sydney Basin and the breccia that fills the vent included broken fragments of every unit it passed through.

A marine fossil in an igneous rock displaced upwards perhaps 2km and 70 or 80 million years out of time!