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.

3 Responses to “Wooster’s Fossil of the Week: a baculitid ammonite (Cretaceous of Wyoming)”

  1. Stephanie Jarvison 13 Nov 2011 at 11:19 am

    I remember that sample!! One of my favorites :)

  2. Mark Wilsonon 13 Nov 2011 at 11:25 am

    We sure enjoyed your visit back to Old Woo, Stephanie. Good luck with your thesis topic search!

  3. […] Please say hello to Pierrella larsoni Wilson & Taylor 2012 — a new genus and species of ctenostome bryozoan from the Upper Cretaceous (Campanian-Maastrichtian) Pierre Shale of Wyoming and South Dakota. I imagine it as a graceful little thing spreading delicately through the dark interiors of baculitid ammonite conchs on a muddy Cretaceous seafloor. Above is a fossil of Baculites formed when sediment filled the shell and lithified. The shell itself dissolved away, leaving the internal mold  of rock (or steinkern) as a kind of cast of the interior. (But don’t ever call it a “cast”!) Pierrella larsoni encrusted the inside surface of Baculites and is thus preserved as a series of connected teardrops on the outside of the internal mold. The specimen is from Heart Tail Ranch, South Dakota, and the scale bar is 10 mm. (Baculites was described in an earlier Fossil of the Week post.) […]

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