A HETEROMORPH AMMONITE WITH AN UNUSUAL “TWIST”

This post is an addendum to my previous post on heteromorph ammonites that I provided a few weeks ago. This new post focuses on a single hetermorph: Pravitoceras sigmoidale Yabe, 1902, which is a nostoceratid ammonite belonging to family Diplomoceratidae. 

This hand-colored plastic model of Pravitoceras sigmoidale is 8 cm height and 5 cm wide. The color scheme is fictional. A former graduate student of mine recently discovered that this unusual ammonite is now available for purchase online, and he sent me one. I thank him for his astuteness and generosity.  

This species is known only from a few formations of Late Cretaceous age (latest Campanian Stage) in southwestern Japan. Most specimens are found in nodules within dark gray, massive mudstone. It has been found with the following other ammonites:

Hypophylloceras sp., Pachydiscus sp., Patagoiosites alaskensis, and Baculites sp.

Pravitoceras sigmoidale is unusual because it has an unprecedented bend in its living chamber (= the “body chamber”. That is to say, its living chamber, where the bulk of the soft parts of the animal resided, is uncoiled and turned 180 degrees in the opposite direction of the rest of this ammonite’s coiled shell. Techincally speaking, the orientation of its living chamber is referred to as a “retroversal U-shape hook.”

Some specimens of P. sigmoidale are also unusual because they can have some anomiid bivalves attached to one or both the sides of the living chamber of this ammonite. The presence of these attached bivalves on both sides of the ammonite shell indicate that this ammonite did lie on the sea floor but did not drag its living chamber. Fully mature individuals of this ammonite lived for a long time after having formed the retroversal hook because several generations of anomiids have been found colonizing the body chamber on some specimens.

The above sketch shows a complete shell of P. sigmoidale with anomiid bivalves (each indicated by the letter "a") attached on the “retroversal U-shape hook.” Arrow shows the position of the last septum = the back of the living chamber.

For more information about this ammonite, see:

Matsunaga et al. 2008. First discovery of Pravitoceras signmoidale…Paleontological Research, v. 12, no. 2, pp. 309–319.

doi:10.2517/prpsi/12.309

Misakei et al. 2014. Hetermorph ammonites with commensal bivalves…Palaeontology 57, pt. 1, pp. 77–95.

doi: 10.1111/pala. 12050