Harpoceras falciferum
Harpoceras is an extinct genus of ammonites from the family Hildoceratidae, which lived during the Jurassic period, specifically in the Toarcian age, from the Falciferum zone to the Commune subzone of the Bifrons zone. These ammonites were fast-moving, nektonic carnivores, meaning they actively swam in the ocean and hunted prey.
The shells of Harpoceras species exhibit strong dimorphism in size. The smaller forms, known as microconchs, ranged from 24 to 51 mm in diameter, while the larger macroconchs had shell widths between 115 and 430 mm. The shells themselves were moderately evolute to involute, meaning the whorls either slightly exposed or overlapped previous whorls. The shells were also compressed with flat whorl sides and a pronounced keel running along the outer edge.
Ribbing on the shells varied between species but was typically falcoid or falcate, meaning the ribs were biconcave, strong, and curved. In some species, the ribs were broad and flat-topped on the outer whorl, while in others, they could appear striated on the inner whorl. Some species also featured a midlateral groove or a series of undulating depressions on the inner part of the shell.
Ammonoids are an extinct group of spiral-shelled cephalopods belonging to the subclass Ammonoidea, more closely related to modern coleoids (octopuses, squids, and cuttlefish) than to shelled nautiloids like Nautilus. They first appeared during the Devonian period (around 409 million years ago) and went extinct shortly after the Cretaceous-Paleogene extinction event (66 million years ago). Originating from bactritoid nautiloids, ammonoids are commonly referred to as "ammonites," although this term technically applies to the order Ammonitida, the last surviving group of ammonoids from the Jurassic until their extinction.
Ammonites are prized as index fossils, as their presence helps geologists link rock layers to specific time periods. Their fossilized shells typically appear as tightly coiled planispirals, but some variations, such as helically coiled or non-spiraled forms (heteromorphs), have been discovered.
The name "ammonite" derives from the spiral shape of their fossilized shells, which resemble coiled ram's horns. The term comes from Pliny the Elder, who called them "ammonis cornua" ("horns of Ammon") after the Egyptian god Ammon, often depicted with ram’s horns. Many ammonite genera have names ending in -ceras, from the Greek word "kéras" meaning "horn."
Ammonoids are distinguished from nautiloids by the structure of their septa, the walls dividing the chambers of their shells (phragmocone). Their suture lines—the intersection of septa with the outer shell—are more complex, featuring lobes and saddles. Three main suture patterns define ammonoids: Goniatitic – Characterized by undivided lobes and saddles, typical of Paleozoic ammonoids. Ceratitic – Lobes with subdivided tips (saw-toothed) and rounded saddles, common in Triassic ammonoids. Ammonitic – Highly subdivided lobes and saddles, found in Jurassic and Cretaceous ammonoids.
The soft body of ammonoids occupied the largest chamber at the end of the shell coil, while earlier chambers, filled with gas, aided in buoyancy. A tube called the siphuncle connected these chambers, allowing the ammonoid to regulate buoyancy by emptying water from the chambers through osmotic processes. Their shells were generally planispiral, with varying degrees of overlap between the whorls. This degree of overlap is reflected in two main shell types: Involute shells (e.g., Anahoplites) have outer whorls that largely cover earlier whorls. Evolute shells (e.g., Dactylioceras) have minimal overlap, with a large umbilicus exposing earlier whorls.
Ammonoids exhibited various shell forms, each suggesting different lifestyles and levels of hydrodynamic efficiency. Some major forms include: Oxycone – Narrow, involute shells with sharp keels, adapted for rapid swimming. Serpenticone – Evolute, discoidal shells; flattened for efficient acceleration, likely planktonic or nektonic. Spherocone – Broad, globular shells, suited for vertical migration in the water column. Platycone and Discocone – Intermediate forms between oxycones, spherocones, and serpenticones.
These shapes likely influenced how ammonoids swam, with some species (such as Oxynoticeras) thought to have been efficient swimmers, while others were slower bottom-dwellers.
Ammonoids likely lived in the upper 250 meters of the water column and are often found in rocks deposited in open-water conditions. Fossil evidence suggests they may have fed on plankton, with some ammonites showing remains of small molluscs and isopod larvae in their buccal cavities. Like modern cephalopods, ammonoids may have defended themselves by ejecting ink, a feature occasionally preserved in fossil specimens.
The chambered structure of ammonite shells, known as the phragmocone, consisted of progressively larger chambers (camerae) divided by septa. The living animal occupied the last chamber, continuously adding new chambers as it grew. This chambered design helped maintain buoyancy and control movement within the water column. Ammonites with shells that diverged from the typical planispiral shape are known as heteromorphs, featuring more open or non-spiral coiling.
In medieval Europe, ammonite fossils were believed to be petrified snakes, known as "snakestones" or "serpentstones," often associated with legends of saints such as St. Hilda of Whitby and St. Patrick. Traders would sometimes carve or paint snake heads on the fossils to enhance their resemblance to snakes, selling them as objects of mythological or healing significance.
