Bactrites is a genus of straight-shelled (orthoconic) cephalopod with a long, narrow shell. It gives its name to the order Bactritida, which ranges from the Early Devonian to the Late Triassic, roughly 407 to 231 million years ago. That span includes the Pennsylvanian rocks I collect. However, I have never found a bactritid in Parks Township, and none are in my catalog. This page is a record of what I was looking for and how to tell one apart.

What sets bactritids apart: the siphuncle

The siphuncle is a tube of living tissue that runs the length of a cephalopod’s shell. It passes through each of the walls (septa) that divide the shell into chambers. By moving fluid into and out of the chambers, the animal adjusts its buoyancy and its depth in the water.

Where the siphuncle sits is one of the key identifying features:

  • In many straight-shelled cephalopods from my area, such as Pseudorthoceras, the siphuncle runs through or near the center of the shell.
  • In bactritids, it runs along the shell’s lower (ventral) edge, hugging the outer wall. This is called a ventral marginal siphuncle.

Ammonoids, which are thought to have descended from bactritids, also have a ventral marginal siphuncle. So on a straight shell, a siphuncle at the margin points toward a bactritid. In a coiled shell it’s not diagnostic.

Diagram of a coiled cephalopod shell with the siphuncle shown in yellow running through the chambers.
The siphuncle (yellow) in a coiled cephalopod, where it runs through the middle of the shell. In bactritids it runs along the shell’s outer edge instead.
Original picture © BIODIDAC 2004.

Ctenobactrites

Ctenobactrites Shimansky, 1951 is a bactritid genus. It differs from Bactrites by its coarse transverse lirae: raised ridges that wrap around the shell and grow taller as the shell lengthens. The shell is orthoconic, and the genus is rarely reported from Pennsylvanian rocks of North America. Mapes (1979) reviewed the North American species and described C. lesliensis.

The best-known American species, C. isogramma (Meek, 1871), has an awkward history. Several authors have looked for Meek’s holotype without success, so the species rests on descriptions and later material.

Fragments of Ctenobactrites can fool you. The coarse ridges resemble growth lines on a bivalve, and a broken piece can pass for part of a gastropod shell. On closer examination, the straight, chambered shell and the siphuncle give it away.

Shimansky (1954) divided the genus into subgenera based on the shape of the lirae, rounded or flattened. Mapes (1979) found that these features vary between individual specimens and did not recognize the subgenera.

References

  • Holland, C.H., 2003. Some observations on bactritid cephalopods. Bulletin of Geosciences 78(4): 369–372.
  • Mapes, R.H., 1979. Carboniferous and Permian Bactritoidea (Cephalopoda) in North America. University of Kansas Paleontological Contributions, Article 64.
  • Miller, A.K., and Unklesbay, A.G., 1947. The cephalopod fauna of the Conemaugh Series in western Pennsylvania: supplement. Annals of the Carnegie Museum 30: 319–330.
  • Shimansky, V.N., 1951. On the question of the evolution of the Upper Paleozoic straight cephalopods. Doklady Akademii Nauk SSSR.
  • Sturgeon, M.T., Windle, D.L., Mapes, R.H., and Hoare, R.D., 1997. Pennsylvanian cephalopods of Ohio. Part 1: Nautiloid and bactritoid cephalopods. Ohio Division of Geological Survey Bulletin 71, pp. 84–85, 98–99.

Further reading