ORTHOCONE: STRAIGHT SHELLED CEPHALOPODS

Orthocone Nautiloid Fossil

An orthocone is an unusually long, straight shell of a nautiloid cephalopod. You have likely seen them from Ontario or Morocco. These straight-shelled nautiloids rules our seas during the Ordovician, nothing else was even close in size to them. 

To put that into context, they would have been more than two times longer than the tallest person you know. 

During the 18th and 19th centuries, all shells of this type were named Orthoceras, creating a wastebasket taxon, but it is now known that many groups of nautiloids developed or retained this type of shell.

An orthocone can be thought of as a nautilus but with a pencil-straight, uncoiled shell. You have likely seen living nautilus in the sea if you are very lucky or on social media, if your curiosity has you streaming cephalopod posts. Living nautilus are chunky and coiled with a wee squid-like body living within their shells that they use for protection and the air within for buoyancy to move through the water. Their ancestors were not dissimilar. For a long while, we thought that these marine lovelies represented the most primitive form of nautiloid, but we now know that the earliest nautiloids had shells that were slightly curved. 

An orthoconic form evolved several times amongst cephalopods. Amongst nautiloid cephalopods, we 
see this in the primitive ellesmerocerids, the endocerids — apex predators of the Ordovician who dined on trilobites, molluscs and brachiopods, in the generally straight-shelled actinocerids, the orthoceratoids (perhaps the last unexplored wilderness in the Cephalopoda).

Orthocone Fossil

We see this form again in the rather smallish order bactritids (relatively speaking within the vast array of the Class Cephalopoda).

These lovely straight-shelled fossils are found in Late Cambrian to Late Triassic outcrops but they were most common in the early Paleozoic. Revivals of the orthocone design later occurred in other cephalopod groups, notably baculitid ammonites in the Cretaceous Period. 

Orthocone nautiloids range in size from wee little fellas less than 25 mm (1 in) to a massive 5.2 metres or 17 feet long in the case of the giant endocerids of the Ordovician. Never underestimate just how large a cephalopod can get. If our oceans remain fertile, I expect we'd see one larger than a city block one day.