Age: 343-337Ma
Viséan
Middle Mississippian Epoch
Carboniferous Period - Giant arthropods and amphibians, early reptiles, most plants fern or lycophyte-like, known for tropical forests and seas
Paleozoic Era - pre-Dinosaurs
Location: England
Lancashire
Hurst Green (Stonyhurst)
Dinckley
Dinckley Hall
The brook east of Dinckley Bridge
Rock Type: Bowland Shale Formation mudstone.
Species:
Orthoceras is a genus of extinct cephalopods belonging to the subclass Nautiloidea, characterised by its long, straight, conical shell. This genus thrived in marine environments during the Paleozoic Era and is found in rock formations worldwide. The fossils attributed to Orthoceras include some of the most recognisable and abundant remains of ancient marine life.
The shell of Orthoceras was composed of calcium carbonate, with a smooth or subtly ornamented surface and a tapered, cylindrical form. The internal structure featured a series of gas-filled chambers, separated by transverse walls known as septa. These chambers were connected by a central tube, the siphuncle, which allowed the animal to regulate buoyancy by controlling the levels of gas and fluid in the chambers. This is basically a type of phragmocone. This adaptation enabled Orthoceras to move efficiently through the water column, using jet propulsion to evade predators and hunt small prey.
Typically ranging from a few centimetres to over a metre in length, Orthoceras was an active predator. It likely fed on small marine organisms, such as trilobites and early crustaceans. Its tentacles, equipped with suction-like structures, were used to grasp prey and convey it to its beak-like jaws.
From an evolutionary perspective, Orthoceras represents an important early form of cephalopod. As one of the ancestors of both modern nautiloids and more advanced coleoids (such as squids and octopuses), it highlights the early diversification of cephalopods in Paleozoic oceans. The straight-shelled nautiloids, including Orthoceras, were eventually outcompeted by ammonoids and more flexible, soft-bodied cephalopods. However, their success during the Carboniferous period is evident from their widespread fossil record.
A cephalopod is a member of the molluscan class Cephalopoda, which includes creatures like squid, octopuses, cuttlefish, nautiluses, ammonites, orthocones, and belemnites. These exclusively marine animals are known for their bilateral body symmetry, prominent heads, and arms or tentacles—muscular hydrostats derived from the primitive molluscan foot.
Cephalopods first became dominant during the Ordovician period, primarily represented by primitive nautiloids. Today, the class includes two distantly related extant subclasses: Coleoidea (octopuses, squids, and cuttlefish) and Nautiloidea (Nautilus and Allonautilus). In coleoids, the shell is internalized or absent, while nautiloids retain their external shells. There are approximately 800 living species of cephalopods, with an estimated 11,000 extinct species. Some well-known extinct groups include Ammonoidea (ammonites) and Belemnoidea (belemnites). Because many cephalopods are soft-bodied, they are not easily fossilized.
Cephalopods are exclusively marine and have never adapted to freshwater habitats, likely due to biochemical constraints. They are widely regarded as the most intelligent of all invertebrates, possessing highly developed senses and large brains—larger than those of their molluscan relatives, the gastropods. The cephalopod nervous system is the most complex among invertebrates, and their brain-to-body mass ratio falls between those of endothermic and ectothermic vertebrates.
With the exception of the Nautilidae and certain deep-sea octopuses (suborder Cirrina), all known cephalopods possess an ink sac, which they use to expel a cloud of dark ink to confuse predators. This sac is a muscular extension of the hindgut, releasing almost pure melanin mixed with mucus to form a thick, smokescreen-like cloud. The ink is ejected via their funnel, using the same water jet propulsion system employed for locomotion. Early cephalopods likely produced jets by retracting their bodies into their shells, as Nautilus still does today.
The evolution of cephalopods is believed to have begun in the Late Cambrian, likely from a monoplacophoran-like ancestor with a curved, tapering shell, closely related to gastropods. The development of the siphuncle, a tube-like structure, allowed early cephalopods to fill their shells with gas, achieving buoyancy and differentiating them from putative ancestors like Knightoconus, which lacked a siphuncle. This buoyancy enabled cephalopods to rise off the seafloor and eventually develop jet propulsion, which furthered their evolution as top predators.
After the late Cambrian extinction, cephalopods diversified significantly during the Ordovician, filling newly available predatory niches and becoming a dominant presence in Paleozoic and Mesozoic seas. Initially, their range was limited to shallow, sublittoral regions in the low latitudes, often in association with thrombolites. Over time, they adopted a more pelagic lifestyle. By the mid-Ordovician, some cephalopods developed septa strong enough to withstand deeper water pressures, allowing them to inhabit depths greater than 100–200 meters.
The direction of shell coiling became a crucial evolutionary trait. Endogastric coiling, in which the ventral (lower) side is concave, limited size expansion, while exogastric coiling, where the ventral side is convex, allowed the large spiral shells familiar in fossil records. Ancient cephalopods, unlike most modern species, had protective shells. These early shells were straight and conical but later evolved into curved nautiloid shapes similar to modern Nautilus.
Competition from fish during evolutionary history is thought to have driven shelled cephalopods into deeper waters, exerting pressure towards shell loss. This adaptation gave rise to modern coleoids, which, despite losing buoyancy, gained greater maneuverability, allowing them to re-colonize shallow waters. Some straight-shelled nautiloids eventually evolved into belemnites, which in turn evolved into squid and cuttlefish. The loss of the shell may also have been a response to evolutionary pressures for increased mobility, giving cephalopods a more fish-like behavior.
