Nautiloids / thinker with tentacles
Curved chambers and visible siphuncle
Témiscamingue (Québec) - Collection Société d'histoire du
THINKER WITH TENTACLES
First Period of Appearance
During the Upper Cambrian, 520 million years ago.
The nautiloids are octopus endowed with a shell with multiple chambers. The animal lives in the last chamber, more spacious. The shell grows regularly of one chamber every lunar cycle for the nautilus : its direct descendants.
A siphon goes through the empty chambers to allow air pumping. Doing so, they can rise in the sea water, feed at the water surface and then take shelter in the depth by expelling its air. They were at the Ordovician the first carnivorous marine animal to swim freely and were the first to have a central brain in it's head well oxygenated by 4 gills.
The Ordovicien is characterized by large size nautiloid that could reach 10 meters in length.
Internal cast of a Nautiloid siphuncle
Mann island (Québec) - Collection Société d'histoire du Témiscamingue
Témiscamingue is renowned for its wide variety of nautiloids dating from the Ordovician (-460 million years) and the Silurian (-420 million years). Many of the same genus or species are also found in Hudson Bay, in the Saint Lawrence lowlands and in Michigan, which allows to extrapolate that the Ordovician and Silurian sea covered all of that territory. The Plectoceras, a newly discovered rare specie, confirm this hypothesis.
Also, the coiled shelled Plectoceras, like his actual descendant nautilus, shows that the appearance of new characteristic was not always made gradually. In a gradual change, the completely straight shell of the majority of nautiluses should have gradually rolled itself before it came to be perfectly coiled. On the contrary, already at the Ordovician (-460 million years) this coiled model already existed while a majority of the shells are straight and even some of them started to show curbs. At that period, all shaped existed, but natural selection kept the most adapted: the perfectly coiled shelled nautiloids.
To help you identify your fossil, take some time to look at the identification keys developed by the Dolomieu Institute (French Only) or the Kentucky Geological Survey of the University of Kentucky :
Kentucky Geological Survey
Home page: http://collections.obs.ujf-grenoble.fr
Identification keys: http://collections.obs.ujf-grenoble.fr/paleontologieenseignementclef.html
We also like National Audubon Society's Field Guide to Fossils.
For more information
Author: we are currently looking to know who is the author of this practical work.
Travaux pratiques de Paléontologie des invertébrés 2012 - p. 1 Les céphalopodes sont des mollusques dont la coquille est cloisonnée intérieurement.