Archive for November 13th, 2011

Wooster’s Fossil of the Week: a baculitid ammonite (Cretaceous of Wyoming)

November 13th, 2011

This is a specimen I often place on my Invertebrate Paleontology course lab tests. It is the “straight” ammonite Baculites, which is common enough, but the shell and internal walls (septa) have dissolved completely away, leaving this strangely articulated set of internal molds. This past week, though, it didn’t fool any of my students — they all identified it correctly. They must have a very good paleontology professor.

This is a view of one of the “segments” of the baculitid specimen. It shows the sediment that was pressed up against one of the septa, which then dissolved away. You can barely see branching tunnels made by worms that crawled through the mud looking for deposited organic material, forming trace fossils.

Baculites (meaning “walking stick rock”) was a magnificent ammonite. Its proximal portion was coiled as in all ammonites, but most of the shell (conch) grew straight. They moved like miniature submarines parallel to the seafloor, diving down occasionally to capture prey with their tentacles. They could grow up to two meters long and so must have been impressive predators. The above internal mold of a baculitid is weathering from the Pierre Shale in South Dakota. On the left end the complex sutures (the junctions between septa and conch) are visible; on the right is the extended body chamber.

A happy John Sime (Wooster ’09) holds a nearly complete specimen of Baculites in the collections of the Black Hills Institute of Geological Research. We were on an Independent Study trip in June 2008 to South Dakota, Wyoming and Montana.

A reconstruction of Baculites (foreground) at the Black Hills Institute of Geological Research.

The genus Baculites was named in 1799 by the famous zoologist Jean-Baptiste Pierre Antoine de Monet, Chevalier de la Marck (1744-1829). In fact, Lamarck (as he is more usually known) was the first zoologist. He was a soldier as well as a scientist, and he had some of the earliest ideas about the evolution of life. I’m sure he would be proud of my students for their fossil identification skills!

Reference:

Lamarck J.-B. 1799. Prodrome d’une nouvelle classification des coquilles. Mém. Soc. Hist. nat. Paris, 74.