Early Ordivicain - Late Triassic
Kingdom: Animalia
Phylum: Mollusca
Class: Cephalopoda
Subclass: Nautiloidea
Genus: Orthoceras
Most of what we know about Orthoceras today is based on fossils and observing the modern nautilus. This group of Nautiloids had the largest about of diversity, lived the longest, and were the most abundant. Their shell could be up to 15 ft. long with possibly hundreds of tentacles.
The shell is divided by septal walls that build chambers. These chambers are full of gas and connected by a tube called the siphuncle. The siphuncle would take water out of the a chamber while gas from the bloodstream fills the chamber. This would control the animal’s buoyancy. This can be seen in fossils we find today. Although we have no soft body fossils today we can can see a hole on the shell. This is called the hyponomic sinus and is believed to where the hyponome is located. The hyponome is the organ that squeezed out water to act as a water jet to propel the animal forward. The smaller orthoceras would benefit from their structure, the conical shape would be aerodynamic so they could quickly move around. The larger ones though would be very awkward and clumsy trying to get around. Steering would be difficult for this animal with such a long shell. Many scientists believe this is an example as to why they started having coiled shells instead of long conical ones. These animals could move forwards and backward depending on how the hyponome was pointed. They would go forward while looking for food and would go backwards when they felt threatened and needed to escape since it was much faster. There is inconclusive evidence to suggest there was sexual dimorphism in this group. They think the males had a larger aperture than females. They fed mostly on arthropods, gastropods, and trilobites so the fact that they were slow and inefficient swimmers was not an issue. Once jawed fish joined the ocean their swimming ability became an issue as the competition increased. This supports the theory as to why their shells became more coiled with time.