Bellerophont Mollusks: A Paleozoic Enigma

Bellerophonts have a shell that is coiled in a single plane (no spire). Their shell superficially resembles that of a nautiloid cephalopod, except that a bellerophonts lack the chambers that characterize nautiloids. Bellerophonts have a spiral band (selenizone) that encircles the middle of the shell. This band indicates successive growth positions of where the anal slit was located on the shell. Some gastropods (e.g., pleurotomariids (= the deep-sea "slit shells)," abalones, scissurellids, and some limpets) have also a selenizone. Interestingly, most of these other shells, unlike bellerophonts, have aragonite shells with mother-of-pearl luster.

The geologic range of bellerophonts is broadly given as Late Cambrian to Early Triassic, but their main range is Ordovician to Permian. There are several families and subfamilies of bellerophonts, as well as many genera. Two of these genera (Knightites and Pharkidonotus) are shown below.

Specimen C4 (width 7.7 mm): Three views (dorsal, aperture, and left side) of Knightites (Cymatospira) sp. of Pennsylvanian age:

Specimen 319 (width 18 mm): Three views (dorsal, aperture [filled with silt], and right side) of Knightites multicoronuatus of Pennsyslvanian age from the Ozawkie Limestone in Kansas. Genus Knightites ranges in age  from Devonian to Middle Permian. The slit band, which extends down (as shown here) along the middle of the shell, is bordered on both sides by several prominent nodes (weathered somewhat on this specimen). These nodes grade into narrow, closely spaced ridges along the sides of the shell:

Specimen 96 (width 14.4 mm): Three views (dorsal, aperture, and right side) of Pharkidonotus percarinatus of Pennsylvanian age from Grayford, Wise County, Texas: 

The largest known bellerophontid is approximately 190 mm width, from the Carlsbad Caverns area in New Mexico, and of Permian age. This specimen lived in a reef environment.

Bellerophonts represent a group whose undoubted evolutionary relationship with other mollusks remains indeterminate. Are bellerophonts gastropods? Are they part of an even more primitive group of mollusks? There are more questions than answers, when it comes to trying to understand the origin of bellerophonts.