Nautiloids
Straight-shelled orthocones and coiled nautiloids — cephalopods that ruled Ordovician seas and survive today as the living Nautilus.
Overview
Nautiloids are chambered, externally shelled cephalopods — cousins of the ammonites — that began as straight, cone-shelled swimmers in the earliest Palaeozoic seas and survive today as the living Nautilus. They include the giant orthocones, some of the first large predators on Earth.
Quick Facts
Key Characteristics
- •Chambered external shell, straight (orthoconic) to loosely coiled
- •Simple, gently curved sutures — far less complex than ammonites
- •Siphuncle running through the chambers to control buoyancy
- •Numerous tentacles without suckers; leathery hood closing the aperture
- •Some Ordovician orthocones reached several metres in length
Fossil Record
Thick calcitic shells preserve well in limestones — Ordovician rocks of the Midwest USA, the Baltic region and Morocco yield abundant orthocones; some beds are nearly paved with them. Coiled forms are common in later Palaeozoic strata.
Global Distribution
Global in Palaeozoic marine rocks; living nautiluses are restricted to the Indo-Pacific.
Evolutionary & Scientific Importance
Nautiloids were the first major cephalopod radiation, the apex predators of Ordovician seas, and their unbroken 490-million-year lineage makes Nautilus a celebrated 'living fossil' for studying buoyancy and chambered shell growth.