THE HYDROSTATICS OF PALEOZOIC ORTHOCONIC CEPHALOPODS (NAUTILOIDEA) WITH IMPLICATIONS FOR EARLY COLONIZATION OF THE PELAGIC ZONE

During the Early Ordovician, diversification of nautiloid cephalopods led to an increased occupation of the pelagic zone. Small longiconic and cyrtoconic nautiloids (Ellesmerocerida) dominated during the Upper Cambrian and almost exclusively inhabited the neritic zone. In the Early Ordovician, new orders appeared (including the Endocerida and Orthocerida, among others) and rapidly diversified. Understanding the hydrostatic properties of these morphotypes is essential to better understand the nautiloid modes of life during their early biodiversification.

Three-dimensional models were constructed that resemble the typical morphologies of the Ellesmerocerida, Orthocerida, and Endocerida. The conditions for neutral buoyancy were found by computing the appropriate cameral volume in the phragmocone that would permit the total mass of each model to equal the mass of water displaced. This involves creating 3D models for each material of unique density to measure their attributed masses. The orientation and stability for each model were also computed by measuring the influence of each material on the total mass distribution.

The Ellesmerocerid 3D model was created by reconstructing the Cambrian endogastric cyrtocone, Plectronoceras. This model would require nearly twice the available cameral volume to be neutrally buoyant, which supports the interpretation of this morphotype being restricted to the benthos in a neritic habitat. The Orthocerid and Endocerid 3D models were all able to attain neutral buoyancy due to sufficient phragmocone to living chamber volume ratios permitting open water occupation as vertical migrants. Cameral deposits (in the Orthocerid models), endosiphuncular deposits (in the Endocerid models), and the position of the siphuncle in both were investigated and found to have little influence on orientation, which was vertical for all models. However, stability is heavily dependent upon the distribution of cameral liquid and cameral/endosiphuncular deposits suggesting that stability reduction, coupled with active locomotion, may allow deviation from a vertical position.