Help
Classification
Ammonites are a type of cephalopod mollusk. Cephalopods also include squids, octopods, cuttlefish, and the chambered nautilus. Mollusks or Molluscs are the group that cephalopods are contained in. Clams, snails, and oysters are also mollusks.
Orders
There are 8 recognized orders of ammonites, although there really should be 9. These orders are listed below from primitive to advanced.Agoniatida
(a-GON-ee-uh-TITE-id-uh)Agoniatitida is the most primitive group of ammonites. It is sometimes called "Anarcestina." Agoniatitida is named after
Agoniatites
.
Clymeniida
(CLIE-meen-ee-ih-duh)Clymeniida is another primitive group of ammonites. They were killed off by the Devonian extinction. They are unusual because their siphuncle is in the same position as it is in nautiloids! Clymeniida is named after
Clymenia
.
Gonaititida
(GON-ee-uh-TITE-id-uh)Goniatitida is the most advanced primitive order of ammonites. They are more ammonite-like than Agoniatitida or Clymeniida. Goniatitic sutures are named after this group. Goniatitida is named after
Goniatites
Prolecanitida
(PRO-leh-CAN-thid-uh)Prolecanthida is a fairly small order of ammonites. They were part of the second stage of ammonite diversity and more advanced than the previously mentioned orders. Prolecanitida is named after
Prolecanites
(which still needs an entry).
Ceratitida
(SARE-uh-TITE-id-uh)Goniatitida is the most advanced primitive order of ammonites. They are more ammonite-like than Agoniatitida or Clymeniida. Goniatitic sutures are named after this group. Goniatitida is named after
Goniatites
Phylloceratitida
(FIE-loe-SARE-at-id-uh)Phylloceratida is the first of the third explosion of ammonite diversity. They were the only known ammonite order to make it pass the Triassic Extinction. They are named after their leaf-like sutures. Phylloceratida is named after
Phylloceras
Lytoceratida
(LITE-oh-SARE-at-id-uh)Lytoceratida is the next group of "third generation" ammonites, appearing right after the Triassic extinction. or Clymeniida. Goniatitic sutures are named after this group. Goniatitida is named after
Goniatites
Ammonitida
(AM-uh-NITE-id-uh)Ammonitida is the most advanced order of ammonites. They are by far the largest group of ammonites. Along with Lytoceratida and Phylloceratida, Ammonitida died out in the Cretaceous extinction event. Ammonitic sutures are named after this group. Ammonitida is named after
"Ammonites"
, which is no longer a valid taxon.
"Ancyloceratida"
(an-KIE-loe-SARE-at-id-uh)"Ancyloceratida" is not a recognized group. They are also called heteromorphs. They are very unsettling, because they have been classified in both Ammonitida and Lytoceratida, although neither for any very good reason. Ancyloceratina is a valid suborder. Ancyloceratina is named after
Ancyloceras
The World of Ammonites
Ammonites lived in prehistoric oceans. They would have preyed on small fish and other swimming animals, or fed on small planktonic organisms. In turn, they would have been hunted by mosasaurs (marine lizards that looked like monitor lizards with flippers), other marine reptiles, and large fish. Large ammonites like Parapuzosia and Tropaeum may also have eaten their smaller relatives. Read on for more detail.Hunting
Ammonites had tough jaws that could cut through their victims. Their tentacles could easily grasp and probably manipulate items. Ammonites used jet propulsion to move. This would have made them fast, but they would have trouble seeing where they were going.Being Hunted
Many predators would have had trouble with the hard shells of ammonites, and this served as a deterrent for smaller fish. Ammonites also could move fast. However, there still were a wide range of predators that targeted ammonites. Mosasaurs (discussed above) were known for their frequent predation on ammonites, but their hunting strategies were different. Some, like Platecarpus would have bitten the ammonite to stop it, then targeted its soft parts, or just simply keep biting it until it broke. Others like Globidens would have just crushed through the hard parts with their powerful jaws. See here for more on mosasaurs.Why did Ammonites die out?
Ammonites reproduced by releasing large amounts of planktonic eggs that floated near the surface. Nautiloids, on the other hand, reproduced by laying a few eggs that were left on the seabed. Ammonites definitely had a more effective way of reproduction, but it put them at risk to mass extinction events. When the meteor or comet struck the planet at the end of the Cretaceous, it would have exploded and scattered dust around the world. This would have killed most oceanic organisms in the upper layers of water, but not have affected deep-water swimmers as much. This would have made it much harder for ammonite eggs to survive, but not have affected nautiloids as badly.Anatomy
The following list contains hopefully all the confusing terms that you might come across relating to anatomy.
● Ammonitic Suture: Complex, frilled suture markings. Compare with: Goniatitc suture, Ceratitic suture.
● Aperture: The opening of an ammonite shell.
● Aptychus: (pl. Aptychi) Ammonite mouthparts, also called ammonite jaws.
● Camera: (pl. Camerae) A chamber in the ammonite shell.
● Ceratitic Suture: Intermediate form of folded suture.
● Coil: Spiral-shaped regions of the shell.
● Connecting Tube: Thin tube separating the siphuncle from the shell chambers.
● Convolute: See Involute
● Discoidal: Disk-shaped, also called the “typical” coiling pattern. Seen in Dactylioceras, Goniatites, Ceratites, and the ammonite in the image above. Compare with heteromorphic.
● Evolute: The shell’s coils do not overlap. Compare with involute.
● Flank: The side of an ammonite.
● Goniatitic Suture: Very simple form of folded suture.
● Helix: A spiral pattern similar to half a DNA molecule or an RNA molecule. Helical refers to something shaped like a helix.
● Heteromorphic: An ammonite with an unusual shape, also used to refer to the suborder Ancyloceratina.
● Involute: The shell’s coils overlap. Compare with evolute.
● Living Chamber: Largest chamber of the shell. This is where the ammonite would have lived.
● Nautilitic Suture: Very simple suture with no major folds.
● Protoconch: The first and smallest chamber of the shell.
● Radial Ribs: Ridges running along the outside of the shell.
● Rib: See Radial Ribs.
● Septa: (pl. Septae) Walls separating the camerae.
● Shell Wall: The outside of the shell.
● Siphuncle: Tube running through the camerae. In life it would have transported gasses around the camerae to control the ammonite’s buoyancy.
● Suture: Structural lines running around the shell.
● Tubercle: Bump.
● Umbilicus: Depressions formed by the outer coil. Present on both sides of the shell. Umbilical refers to the umbilicus.
● Venter: Bottom of the shell (in coiled ammonites it’s the external edge). Ventral refers to the venter.
○ Ventral Groove: Groove along the venter.
○ Ventral Keel: Ridge along the venter.
Time
*Based on Wikipedia's time scale chart. Not to scale.
