Geisonoceras: Prehistoric Animal of the Week

This week we will be checking out an invertebrate which leaves behind some of the most beautiful fossils ever!  Enter Geisonoceras!

Life reconstruction by Christopher DiPiazza.

Geisonoceras was a cephalopod mollusk, related to modern octopus, squid, cuddlefish, nautolus, and the now extinct, yet highly successful ammonites.  Like them it would have possessed a soft, muscular body with tentacles and a sharp beak, as well as a hard outer shell.  It was most likely a predator, hunting trilobites and other arthropod prey.  Geisonoceras is a genus that contained many species which spanned over much of the world's oceans in a broad time long before the first dinosaurs, from roughly 460 to 390 million years ago.  This ranged across two two periods in prehistoric times called the Ordovician and Devonian.  The shells of this amazing mollusk range in size from a few inches to several feet long.  The name Geisonoceras translates to "Geison Horn".  A geison is a long architectural structure seen on ancient Greek buldings.  Geisonoceras possessed a long, slender, horn-shaped shell so the name fits! 

A rather large Geisonoceras fossilized shell on display at the Mesalands Dinosaur Museum in New Mexico.  If you look closely you can see that the stand for this fossil also contains smaller specimens within it!

The shell of Geisonoceras was long and pointed, and would have grown with it from birth.  However, the animal only lives in the first few chambers of the shell, as it is divided into walled sections that become more numerous, making the shell longer as the animal ages.  The shell also contained a hollow tube that spanned its length called a siphuncle.  This structure could be filled with water to control the overall density of the animal, allowing it to control its longitudinal movement in the water. 

That is all for this week!  As always feel free to comment below or on our facebook page!  Stay tuned for more coverage of Gary and I's trip to New Mexico for Triassic field work!

References

Walter A Sweet, 1964. Nautiloidea -Orthocerida, Treatise on Invertebrate Paleontology Part K, Endoceratoidea, Actinoceratoidea, Nautiloidea. Geological Society of America and Univ Kansas Press. Teichert and Moore (Eds)