Wooster’s Fossils of the Week: Very common orthocerid nautiloids from the Siluro-Devonian of Morocco
Mark Wilson November 3rd, 2013
If you’ve been to a rock shop, or even googled “fossil”, you’ve seen these beautiful and ubiquitous objects. They are polished sections through a nautiloid known as “Orthoceras“. We put quotes around the genus name because with these views it is nearly impossible to identify the actual genus, so “Orthoceras” becomes the go-to term for unknown orthoconic (straight) nautiloids. We also do not know exactly where in Morocco these fossils come from, but chances are they were dug out of the Orthoceras Limestone (Siluro-Devonian) exposed near Erfoud in the Ziz Valley near the edge of the Sahara Desert. They are easily excavated, take a nice polish, and look good from almost any angle of cut. People bring these to me often to ask about their origin, so let’s do a Fossil of the Week about the critters.
These fossil nautiloids consisted in life of a long, straight conical shell with internal chambers pierced by a long tube. The shells were originally made of aragonite, but almost all have been replaced and recrystallized with calcite. A squid-like animal produced the shell. Most of its body was in the large body chamber at the open end of the cone. They were effective nektic (swimming) predators during the Paleozoic Era around the world. In some places (like Morocco) nautiloids were so common that their dead shells carpeted shallow seafloors. Nautilus is a living descendant.
In this closer cross-sectional view of a Moroccan “Orthoceras“, we can identify the critical parts. A = a chamber (or camera); B = the siphuncle (tube running through the center of the shell); C = a septum that divides one chamber from another; D = an orthochoanitic (straight) septal neck of shell that runs briefly along the siphuncle. The white to gray material is crystalline (“sparry”) calcite that filled the empty shell after death and burial.
By the way, you can buy “Orthoceras healing stones“. A quote from that site: “Fossils are believed to increase life span, reduce toxins, anxiety, stress, balance the emotions, make one more confident. Containing supernatural and physical healing powers. They promote a sense of pride and success in business. Healers use fossils to enhance telepathy and stimulate the mind. Traditionally, fossils have been used to aid in reducing tiredness, fatigue, digestive disorders, and rheumatism.” No wonder paleontologists are always the very image of health and wealth!
The genus Orthoceras was named in 1789 by the French zoologist (and physician) Jean Guillaume Bruguière (1749–1798). The only image I could find of him is the small one above. Bruguière earned a medical degree from the University of Montpellier in 1770, but like many aspiring naturalists, he never practiced. He traveled very widely for an 18th Century scientist, usually to pursue living and fossil mollusks on various expeditions. That he was a Republican in revolutionary France probably saved his head, but he lost his income in the turmoil. Most of his descriptions of fossil taxa appeared in print decades after he died on a voyage back from Persia. Of all his taxonomic contributions, the genus Orthoceras is the most widely known.
Histon, K. 2012. Paleoenvironmental and temporal significance of variably colored Paleozoic orthoconic nautiloid cephalopod accumulations. Palaeogeography, Palaeoclimatology, Palaeoecology 367–368: 193–208.
Kröger B. 2008. Nautiloids before and during the origin of ammonoids in a Siluro-Devonian section in the Tafilalt, Anti-Atlas, Morocco. Special Papers in Palaeontology 79, 110 pp.
Lubeseder, S. 2008. Palaeozoic low-oxygen, high-latitude carbonates: Silurian and Lower Devonian nautiloid and scyphocrinoid limestones of the Anti-Atlas (Morocco). Palaeogeography, Palaeoclimatology, Palaeoecology 264: 195-209.