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Ammonoidea
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.
Ammonoidea
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.