This blog post is about Paleozoic (probably Devonian) specimens of orthocone nautiloids popularly displayed in slabs of charcoal-colored limestone from Morocco and sold online via various websites. These orthocone specimens are from the same region where many trilobites have been found—(see my immediately previous post on “Trilobites of Morocco”). The Moroccan orthocone specimens have been quarried (no locality information given), and, then, cut (in most cases at two arbitrary angles) and then polished, in such a way so as to present “eye-catching” views that are used commonly for art décor displays, etc. These cut-and-polished cross-sections however destroy vital morphologic information needed to properly identify the genus and species of the specimens. Thus, these “prepared” specimens can only be identified as orthocone nautiloids or “Orthoceras” specimens. Most of the slabbed specimens show several individual specimens clustered together in an “preferred-parallel orientation,” caused by wave or current action that occurred shortly after death of the animals.
Orthocone nautiloids (a.k.a., orthocones) are extinct ocean-dwelling cephalopod mollusks with straight shells, ranging from a few inches long to, rarely, six feet long. The geologic time range of these carnivorous animals is Ordovician to Triassic age. They are most common in Ordovician, Silurian, and Devonian-age deposits, and their shells are found throughout the world (e.g., North America, Europe, Asia, Africa, and Australia).
Two views (side and anterior) of a commercially available model of an unidentified orthocone nautiloid (total length, including tentacles, 15.5 cm, width of extent of tentacles 8 cm) (sold as an "Orthoceras).
The name Orthoceras is the genus name that has been used historically for many orthocones [ortho = straight; ceras = horn means “horn”]. These animals are related to modern squids, the octopus, and also to the chambered Nautilus. The shell of Orthoceras is constructed internally similar to that of a Nautilus, but instead of having a coiled shell with chambers, Orthoceras has a long, slender conical cone divided into chambers. The chambered part (phragmocone) is located posterior to the living chamber, where most of the “soft-parts” of animal are located. The phragmocone is subdivided by simple watch-glass-shaped septa (thin walls). The chambers, separated by the septa, are partially filled or completely filled with cameral deposits (as mentioned above). A central tube (siphuncle) is present inside the shell of orthocone nautiloids, and this tube was used for the passage of gases that gave the shell its buoyancy. The animals used jet propulsion (squirting water) to move through the water.
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Classification details: from Sweet (1964):
Order Orthocerida
Superfamily Orthocerataceae
Family Orthoceratidae (with several subfamilies; two of these are):
Subfamily Orthoceratinae Mid Ordovician, northwestern Europe
Includes genus Orthoceras, as well as two other genera---they have no cameral deposits [cameral deposits consist of calcium carbonate that partly or completely fill the chambers of the shell, thereby stabilizing the animal and its shell during life].
Subfamily Michelinoceratinae Late Ordovician–Late Triassic, widespread. Includes genus Michelinoceras, as well as at least eight other genera---they have cameral deposits, which are generally well developed.
In order to differentiate the various orthocone genera, complete specimens are necessary, especially to see the critically important external features. Internal features are useful but, if the shells are sectioned, they must be cut perpendicular to the long axis of the shell (i.e., not at arbitrary angles that make the specimen attractive to customers who buy these fossils).
The genus Orthoceras Bruguiere, 1789 is based on specimens from northwestern Europe (including Sweden, Russia, Ukraine, Belarus, Estontia, and Lithuania). Orthoceras regulare (Schlotheim, 1820) is the type species of Orthoceras. This genus is restricted to Middle Ordovician strata; thus, any Devonian orthocone specimens from Morocco are not Orthoceras. Rather, they belong to several other named orthocone genera of Silurian to Devonian age that resemble Orthoceras. Michelinoceras Foerste, 1932, is just one, out of 10 or more, of these genera. It is distinguished mainly on the basis of surface ornamentation (Moore et al. 1952, p. 349).
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In summary, the “sliced” and polished orthocone nautiloid specimens for sale from Morocco do not show the necessary morphologic data needed in order to properly assign a genus name to them. In lieu of an accurate genus identification, the following designation should be used: “unidentifiable orthocone nautiloid.”
References
Moore, R.C., C.G. Lalicker, and A.G. Fischer. 1952. Invertebrate fossils. McGraw-Hill Book Company, 766 pp.
Sweet, W.C. 1964. Nautiloidea–Orthocerida. Treatise on Invertebrate Paleontology. Part K, Mollusca vol. 3. Geological Society of America, University of Kansas Press, 519 pp.
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