Wooster’s Fossils of the Week: Nummulitid foraminiferans (Eocene of the United Arab Emirates)

July 3rd, 2011

The Great Pyramids of Egypt are made primarily of a yellowish limestone. About 40% of that limestone is made of the fossil type pictured above. These are foraminiferans (single-celled organisms with shells) that lived by the countless billions during the Eocene (56 to 34 million years ago) in the Tethys Ocean. They are called Nummulites from the Latin nummulus, which means “little coin”. They have the honor of having been first described by the 5th Century BCE Greek historian Herodotus, who noticed them in the Pyramid stones. (He thought, by the way, that they were fossilized lentils.)

(From R.A. Lydekker (1894), Life and Rocks.)

If you slice a nummulitid test (what we call a foraminiferan shell) like a bagel, the inside is revealed to be a long spiral. The single-celled organism built the shell by progressively adding to this spiral, making the largest foraminiferan tests ever known, some over 15 cm in diameter. (Most foraminiferans have tests about the size of a pinhead.)

I collected our Wooster Nummulites during fieldwork with my colleague Paul Taylor near Al Ain in the United Arab Emirates. I thought at the time that I was picking up two species: a small one and a significantly larger one. I later learned that these are two versions of the same species. Nummulites reproduced alternately sexually and then asexually. This alternation of generations is seen today in many living foraminiferans, and we see an analogue in some plants like ferns.

Nummulitids are the stars of a strange story in paleontology. A British zoologist who was at the time assistant keeper of invertebrates at the British Museum (Natural History) wrote a book (The Nummulosphere, 1912) and a series of papers with the astonishing thesis that all rocks were actually made of foraminiferan tests — all rocks! Even basalt, granite and meteorites are organic in origin. The late Stephen Jay Gould told the tale of Randolph Kirkpatrick (1863-1950) memorably in his compilation of essays The Panda’s Thumb (Norton, 1980). Kirkpatrick’s work is well worth reading for his incredible arguments. (Lava, for example, is melted “siliceous nummulitic rock” recycled back to the surface.) The tortured logic to show the remains of Nummulites tests in igneous rocks is oddly entertaining.

“Fig. A [Plate XXI]. Section of rotten trachyte [a volcanic rock] permeated with sulphur from interior of upper crater of Tenerife, x 4.5. The coils of a much-blasted nummulite in perpendicular section are visible to the trained vision.”

 

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