Ancient currency or petrified food?
Sometimes, human history and natural history are neatly entwined, as was recently discovered when researching a pair of disc-shaped fossil specimens.
Single-celled giants, the Pyramids and more ...
The fossils are giant Foraminifera, belonging to the genus Nummulites. Foraminifera, or forams, are a mostly marine, mostly microscopic group of single-celled animals that have a chambered "shell". Think of an amoeba and give it a house.
The disc-shaped shell of Nummulites, composed mostly of calcium carbonate, is really a tight coil of chambers which are added as the animal grows. Below is an image of 40 million year old (Eocene age) Auckland Museum specimens, collected from Bracklesham Bay in West Sussex, England.
Nummulites is a giant compared to most of its cousins, measuring between 1.5 and 5 cm - remember they consist of only a single cell.
New Zealand giants
New Zealand, too, has had some large fossil foram species in the past. For example, in our collections, we have this block of 20 million year old Miogypsina limestone from the Kaipara Harbour, featuring hundreds of the lentil-sized fossils. It is thought that the large size of Nummulites and Miogypsina is made possible because of a symbiotic relationship they have with photosynthetic algae, which live inside the 'shell'.
Building the link
So that's the natural history part - here's the link to human history.
At Giza, near Cairo, the ancient Egyptians built the bulk of their pyramids using a Nummulites limestone (also of Eocene age). The limestone blocks were quarried from outcrops right next to the pyramids.
A single-cell currency?
From European folklore, these fossils have been dubbed 'angel's money', as it was thought that the ancient Egyptians used these disc-shaped fossils as currency. Yes, they make perfect coins, don't they? Maybe you can picture it with the engraved profile of your favourite monarch (or pharaoh).
Our first written account of these Foraminifera was by the Greek historian Herodotus in about the 5th century BC, after he noticed these round, flat 'shells' in the Great Pyramids.
Another Greek historian and geographer, Strabo, travelled to Egypt in about 25 BC and became aware of these lentil-shaped stones, and was told they were supposedly petrified food waste left by the labourers who built the pyramids. He thought this unlikely, as he had observed similar round, flat stones in his home country (modern day Turkey) and instead considered them to be them to be merely stones shaped by water movement.
It's all in the name
There has been some musing over where the name for the foram Nummulites came from - was it perhaps to do with the theory that these fossils were once used as currency? After all, the word 'nummulus' is the Latin diminutive of 'nummus' meaning coins or money. But who was it who adopted the term first?
Herodotus and Strabo were both Greek, however Strabo's home country was by then part of the Roman Empire, so the Latin word for coin would have been known to him. It was Lamarck in 1801 who actually described and named the genus of forams as Nummulites, derived presumably from 'nummulus'. Note, too, that the term 'numismatics', meaning the study or collection of currency, has the same root.
So, in summary, this little giant, named from the Latin word nummus for coin, shares this etymology with the study of coins (numismatics), and may have once been used as coins by an ancient civilisation that built one of the wonders of the world ... out of fossil coins!
Today, there are still some very large living, tropical Foraminifera, such as Marginopora and Amphistegina. The former is nowadays pedalled to tourists in the form of jewellery, in exchange for more modern currency.
Cite this article
Schlumpf, Heidi, and Grenfell, Hugh.
Ancient currency or petrified food?. Auckland War Memorial Museum - Tāmaki Paenga Hira. First published: 3 September 2015. Updated: 12 November 2019.