Mystery Object: A shell of its former self

Identifying obscure and bizarre objects is all in a day’s work for staff in the Queensland Museum Discovery Centre! Today Steve Wilson, Information Officer, joins us to share the mystery object of the month for December.

A mantis-shaped object. How did it form?

This specimen was found in a shallow pool in the Einasleigh Uplands region of north Queensland. The general shape of a praying mantis is clearly visible, as though the unfortunate insect had somehow been turned to stone. Close examination reveals that the surface of the object is covered by tiny florets of crystals. These are a mineral called calcite, which is a form of calcium carbonate (this is the same material as limestone, mollusc shells and coral skeletons). How could such a specimen have formed? Is it a fossil, many millions of years old?

The mantis is completely encrusted by calcite crystals.
Up close, the calcite crystals resemble tiny cauliflower florets.
The World Heritage-listed travertine pools at Pamukkale in Turkey are also the result of encrustation by calcite crystals, albeit on a larger scale. Image credit: Talaroo hot springs.

The answer is…

This extraordinary specimen began to form when an unlucky praying mantis put a foot wrong and fell into a travertine pool. It may appear like an old fossil, but the process took only a matter of months, or a few years at most, for its body to become encased in crystals.

When water, rich with dissolved calcium carbonate, percolates to the surface the mineral solidifies on contact with solid objects to form a coating. If it is in a concentrated solution, the process can be quite rapid. This can be the case when the internal linings of pipes become coated and blocked from artesian bore water. Travertine pools are formed from this mineral-rich water.

The bottom and edges of the pool become coated with calcium carbonate, and as the water flows over the rim, often into a series of pools below, the minerals build up into a thick coating. Over time these can develop into impressive and attractively sculptured structures like a tier of ornamental pools.

This mantis was found in just such a travertine pool in the Einasleigh Uplands region of north Queensland. It would have died quickly as the water is geothermally heated. The water was nearly boiling when it percolated to the surface, cooling as it flowed down through a series of pools.

This is the process by which the mantis, found in one of the lower, cooler pools, became encrusted. Other organic material such as twigs are similarly affected. We do not know how much of the original insect remains inside this mineral cast. However the crystalline form, reflecting the shape of the original living insect, opens the question as to whether such a relatively young item can be defined as a fossil.

A living False Garden Mantis (Pseudomantis albofimbriata), happily un-encrusted. Photo: Jeff Wright, Queensland Museum.

Visit the Discovery Centre on Level 4 of the museum

Visit the Discovery Centre to see this Mystery of the Month this December. Do you have an interesting question or mystery object? Our helpful and knowledgeable staff can answer your questions through our Ask an Expert inquiry service.