September 2015

Tiny trilobites

About a month ago my father and I travelled to Mt Isa in search of trilobite fossils. While we found many high quality specimens, we also stumbled upon these fossils a few kilometres away from the trilobite site. We have not seen anything like these before and I hope you can shed some light on them. The area in which they were found is known as the Beetle Creek Formation.

Answer

agnostid trilobites agnostid trilobites

The specimens in your photos are agnostid trilobites, which are rather bizarre tiny trilobites known from the Cambrian and Ordovician periods (between 541 and 444 million years ago). Numerous species have been documented from Cambrian rocks in western Queensland, but I think most of your specimens look like Ptychagnostus seminula. This species lived during the middle Cambrian.

Agnostids differ from most trilobites by having a very large final segment (technically called the pygidium) and very few trunk segments (commonly two or three, in comparison to more than 60 in some contemporary trilobites). Many agnostids (including your specimens) lack eyes, and this is thought to indicate that they lived in deep water. As with other arthropods, agnostids needed to periodically shed their exoskeleton as they grew. Your specimens are isolated heads and tails (technically cephalons and pygidiums), suggesting that they are discarded exoskeletons.

The last agnostid trilobites were victims of a major mass extinction at the end of the Ordovician period (444 million years ago). This extinction was the second-most destructive in Earth’s history, causing the loss of more groups of animals than the Cretaceous-Palaeogene extinction that wiped out the dinosaurs (with the exception of birds of course!). Climatic change in the late Ordovician is thought to be the likely cause of the extinction.

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