April 2016

Relic of a lost forest

My son and I were camping and he found this in the creek (near Bretti, NSW). It looks like fish scales.

Answer

Leptophloeum sp., found near Bretti NSW. Image courtesy: Adrian KervilleLeptophloeum australe, Burdekin Basin Qld.

The specimen in your photo looks like a fossil plant called Leptophloeum. This plant was a giant lycopod. Today lycopods are small, herblike plants but during the Devonian and Carboniferous they grew into the first large trees. Your specimen is part of the trunk of one of these trees. The ‘scales’ are places where leaves were attached.

Giant lycopods are relatively common fossils from the Devonian and Carboniferous periods (between 299-419 million years ago). They’re commonly associated with the ‘coal swamps’ that characterise the Carboniferous period in the northern hemisphere (Australian coal is much younger, mostly from the Permian and Triassic periods).

Your specimen was found in an area with rocks of Permian age, but your specimen is most likely much older. I suspect that it came from rocks of Devonian age further upstream. My suspicions are supported by the rounded shape of your specimen, which suggests that it was transported by water.

When your fossil was a living plant, the seas around what would one day become Australia were filled with armoured fish and massive coral reefs. The swamps where Leptophloeum grew may also have hosted early tetrapods, the ancestors of all land-dwelling vertebrates.

It’s been suggested that a drying climate during the late Carboniferous and early Permian periods caused the decline and extinction of the giant lycopod trees.

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