An ammonoid is an extinct cephalopod mollusk with a flat-coiled spiral shell.

This ammonitic ammonoid shows the septal surface (especially at right) with its undulating lobes and saddles. Credit: Jonathan350.{{free media}}

An ammonite may be an ammonoid that belongs to the order Ammonitida, typically having elaborately frilled suture lines.

An ammonitic ammonoid in the images on the right shows the septal surface (especially at right) with its undulating lobes and saddles.



Def. a "soft-bodied invertebrate[1] [of the phylum Mollusca][2], [typically with a hard shell of one or more pieces]"[3] is called a mollusc.

Def. a "body wall of a mollusc,[4] from which the shell is secreted"[5] is called a mantle.

Def. a "rasping tongue of snails and most other mollusks"[6] is called a radula.

As a mollusk an ammonite may be expected to have

  1. a mantle with a cavity for breathing and excretion,
  2. a radula, and
  3. a structured nervous system.



Def. any "mollusc, [of the class Cephalopoda][7], which includes squid, cuttlefish, octopus, [nautiloids][8] etc"[9] is called a cephalopod.

An ammonite is expected to have cephalopod characteristics

  1. bilateral body symmetry,
  2. a prominent head, and
  3. a set of arms or tentacles (muscular hydrostats).

Theoretical ammonites

The schematic drawings show four growth stages in the ontogeny of Hoploscaphites nicolletii. Credit: Hugo Bucher, Neil H. Landman, Susan M. Klofak, and Jean Guex.{{fairuse}}

Def. "the scientific study of squid (often extended to all cephalopods)"[10] is called teuthology.

"Teuthology, a branch of malacology, is the study of cephalopods."[11]

Def. "any of numerous flat spiral fossil shells of cephalopods"[12] is called an ammonite.

We "describe the overall mode of growth of ammonoids with reference to Nautilus, the only externally shelled cephalopod that is still extant. Ammonoids are, in fact, phylogenetically more closely related to coleoids than they are to Nautilus (Engeser, 1990; Jacobs and Landman, 1993; Chapter 1, this volume). However, the retention of an external shell in ammonoids implies that these extinct forms shared with Nautilus basic similarities in their processes of growth, although not necessarily a similarity in their rate of growth or age at maturity."[13]

On the right are schematic drawings of four growth stages of Hoploscaphites nicolletii in lateral and transverse cross-sections:

  1. A is an embryonic shell called the ammonitella, scale bar 500 µm,
  2. B is the first postembryonic stage called the neanic and the animal or shell is called the neanoconch, scale bar 1 mm,
  3. C is a juvenile, scale bar 5 mm, and
  4. D is an adult, scale bar is 1 cm.[13]


This is an example of Agoniatites obliquus, from Srbsko, Czech Republic. Credit: Hectonichus.{{free media}}
An example of Agoniatites nodiferus is from Srbsko, Czech Republic. Credit: Hectonichus.{{free media}}
Agoniatites vanuxemi is the only species of Ammonoid found in the Hamilton Group, Mahantango Formation. Credit: Michael Tomczyk.{{free media}}

Agoniatites are also known as Anarcestes.

Agoniatites vanuxemi, on the lower right, is the only species of Ammonoid found in the Hamilton Group, Mahantango Formation.


This is an example of Parapuzosia seppenradensis, the largest known ammonite, with a diameter of 1.80 m. Credit: Gunnar Ries.{{free media}}

These are ammonites of the suborder Ammonitida.


This is an example of Ceratites nodosus from the Upper Muschelkalk, Meißner-Formation. Credit: Didier Descouens.{{free media}}

A ceratite may be an ammonoid of an intermediate type, typically with partly frilled and partly lobed suture lines.

"The Ceratitida, which is the dominant ammonoid order of the early Mesozoic and one of the major orders of Ammonoidea, ranged from early Permian to the end of Triassic times, and has an almost worldwide distribution (Hewitt et al., 1993; Page, 1996)."[14]


This example of Clymenia laevigata is from the Upper Devonian. Credit: Wikipek.{{free media}}

Any clymeniid may be an ammonoid with a dorsal siphuncle; i.e., a siphuncle on the inside of the coil rather than the outside.


This ammonite fossil exhibits goniatitic sutures. Credit: Rama.{{free media}}

An ammonoid like the one on the right typically with simple angular suture lines is referred to as a goniatite.


Fossil shell is of Lytoceras cornucopia from Isère (France). Credit: Hectonichus.{{free media}}


  1. loosely coiled,
  2. evolute,
  3. gyroconic,
  4. exposed whorls,
  5. whorls touching,
  6. subcircular to narrowly compressed whorls,
  7. broadly arched, or keeled venter,
  8. smooth or ribbed sides,
  9. aptychi are single valved and concentrically striated,
  10. suture saddle endings tend to be rounded but usually not phylloid,
  11. lobes tend to be more jagged with thorn-like endings, and
  12. complex moss-like suture endings with adventious and secondary subdivisions.


Fossils of Nipponites mirabilis are exhibited in the National Museum of Nature and Science, Tokyo, Japan. Credit: Momotarou2012.{{free media}}

Nipponites mirabilis on the right may be from the Upper Cretaceous.


Fossil shells of Phylloceras serum are from Alpes-de-Haute-Provence, France. Credit: Hectonichus.{{free media}}


These are suture patterns of the Prolecanitidae. Credit: Michael R. House.{{fairuse}}

"Type species by original designation of Librovitch 1957, Protocanites supradevonicus Schindewolf (1926)."[15]

In the diagrams above are the suture patterns for various species holotypes:

  1. A - Protocanites gurleyi (Smith),
  2. B - Eocanites supradevonicus supradevonicus (Schindewolf),
  3. C - Eocanites semageominus (House),
  4. D - Eocanites wangyounensis(Ruan & He),
  5. E - Michiganites algarbiensis (Pruvost),
  6. F - Michiganites marshallensis (Winchell),
  7. G - Michiganites scalibrinii (Antelo),
  8. H - Michiganites greenei (Miller).[15]


An example of Trachyteuthis hastiformis is a fossil. Credit: FerdiBf.{{free media}}

The subclass Coleoidea has the cohort Belemnoidea which may contain shelled cephalopods.


This piglet squid has a mantle full of ammonia. Credit: Nautilus.{{fairuse}}

The image on the right shows "the piglet squid [Helicocranchia pfefferi], floating along with its tentacles waving above its head in the central Pacific Ocean near Palmyra Atoll."[16]

The "squid [was spotted] about 4,544 feet (1,385 meters) below the ocean surface [from] the exploration vehicle (E/V) Nautilus."[16]

"Is that a squid? I think it's a squid. It's like a bloated squid with tiny tentacles and a little hat that's waving around. And it looks like it's got a massive, inflated mantle cavity. I've never seen anything quite like this before."[17]

The "mantle is filled with ammonia, which the squid uses to control its buoyancy."[16]

"This Nautilus expedition is an effort to explore the deep ocean waters of the Marine National Monument, near Kingman Reef, Palmyra Atoll and Jarvis Island, which are among the most remote U.S.-controlled territories."[16]

Helicocranchia is of the order Teuthida.



The cohort Belemnoidea has five extinct orders. Any one of these may contain cephalopods with an external shell.

"Belemnites (Belemnitida) were squid-like animals belonging to the cephalopod class of the mollusc phylum, and therefore related to the ammonites of old as well as to the modern squids, octopuses and nautiluses."[18]

"Now extinct, their fossils are found in rocks of Jurassic and Cretaceous ages, with a few species hanging on into the early part of the Tertiary. The animal’s soft parts very rarely fossilise, leaving us with only the hard parts; the guard and the phragmacone."[18]


Hematites barbarae external shell is from Pendleian age rocks. Credit: R. H. Flower and M. Gordon Jr.{{fairuse}}
Schematic diagram is of the medial shell section in Hematites. Credit: Larisa A. Doguzhaeva, Royal H. Mapes & Harry Mutvei.{{fairuse}}

"Pendleian age rocks in the Chainman Shale include the upper beds of the Camp Canyon Member and the Willow Gap Limestone Member. The fossil cephalopods [an example of Hematites barbarae is shown above] are from these rocks in the Confusion Range and Burbank Hills of western Millard County."[19]

For the study of the "shell morphology and ultrastructure in Hematites [more] than 30 specimens of this genus were collected by the second author from the Upper Mississippian in Arkansas. The data obtained confirm the detailed description of the external shell morphology [diagrammed on the right] in the genus published by FLOWER & GORDON (1959) and GORDON (1964), and it also includes new information on the conotheca structure, conotheca rostrum/mantle attachment, “living” chamber length, and morphology of the adoral portion of the rostrum."[20]

"Schematic diagram of the medial shell section in Hematites [on the right shows] the truncation of the initial portion of the phragmocone which is plugged by the central rod structure (crs) and by the additional septum (as). Scale bar: 1 mm. as = additional septum; c = conotheca; r = rostrum; s = septum; sn = septal neck; t = place of truncation."[20]


This is a fossil of Phragmoteuthis conocauda. Credit: User:Ghedoghedo.{{free media}}

The image on the right suggests that Phragmoteuthis conocauda does not have an external shell.


This diagram describes some anatomy of a belemnoid. Credit: Charles William.{{fairuse}}
These are pyritic belemnite phragmacones. Credit: Charles William.{{fairuse}}
A rostrum is a posterior bullet shaped section of an internal shell and is the fossil that is most commonly found. Credit: Charles William.{{fairuse}}
The image shows opalization of a guard (rostrum) of belemnite genus Peratobelus: Cairn Hill mine, Coober Pedy, Coober Pedy - Everard Range Regions, South Australia, Australia. Credit: Parent Géry.{{free media}}

"Belemnites [...] have a worldwide distribution."[21]

Shells or shell-like structures are the phragmacone in the image on the left and the rostrum, the second image on the left, which have been found apparently internal to the soft body. The second image down on the left shows rostrums from Passatoteuthis auricipitis Lang, Jurassic, Lower Lias, found in Gloucestershire.

The image second down on the right shows a rostrum from the genus Peratobelus, found in the Cairn mine, South Australia.






This is an individual of Nautilus pompilius. Credit: appealtoreason.{{free media}}
The images include an anatomical diagram of Nautilus. Credit: David Darling.{{fairuse}}

Def. a cephalopod mollusk with a light external spiral shell that is white with brownish bands on the outside and lined with mother-of-pearl on the inside is called a nautiloid.

Nautiloidea is another subclass of cephalopods.

"Nautilus [included in the diagram on the left] is one of the few surviving animals resembling the primitive or original cephalopods. The fossilized shells of these extinct forms, called ammonites (A), are quite common. (B) is a deep-sea species Nautilus pompilius that lives in tropical waters. To the right is a section through Nautilus showing the shell (1) and siphuncle (2) wound in a spiral. Immediately behind the tentacles lies the mouth (4) leading to the intestine (7). Nautilus has an advanced nervous system with a brain (3) and respires by means of gills (6) that are located in the mantle cavity. It swims by forcing a jet of water out of its mantle cavity and through the siphon (5)."[22]

"Nautiluses first evolved in the Cambrian period and became significant marine predators during the Ordovician period."[22]
















This is Nautilus, the only extant exterior-shelled cephalopod. Credit: J. Baecker.{{free media}}
Here are a couple of Nautilus macromphalus. Credit: Pierre Sylvie.{{free media}}

An individual example of the genus Nautilus is on the right.

A couple of Nautilus macromphalus are on the left, photographed during a night dive, at 15 meters, near Lifou, Sandal wood bay, New Caledonia.

"The six living species of nautiluses are:

  1. No common name (Allonautilus perforates),
  2. Crusty Nautilus (Allonautilus scrobiculatus),
  3. Palau Nautilus (Nautilus belauensis),
  4. Bellybutton Nautilus (Nautilus macromphalus),
  5. Chambered Nautilus (Nautilus pompilius), and
  6. White-patch Nautilus (Nautilus stenomphalus)".[22]




Orthoceras species (artist's impression) is a fossil nautiloid from the middle Ordovician. Credit: Nobu Tamura.{{free media}}
This is a fossil of Orthoceras currens. Credit: Alberto Salguero.{{free media}}

Centered at the top is an artist's impression of an Orthoceras species from the middle Ordovician.

On the left is a fossil of Orthoceras currens.








An Ammonite shell cut longitudinally to show septa. Credit: John Alan Elson.{{free media}}
This is an internal mold of a Baculites. Credit: Wilson44691.{{free media}}
Picture is of a fossil ammonite aptychus. Credit: H. Sulzer.
Ammonite, Perisphictes, has aptychi. Credit: Antonov.{{free media}}
An Agoniatite is illustrated with a central siphuncle. Credit: P4en.{{free media}}
Diagrams are of the internal shell structure (left) and measurements (right) of an early ammonoid shell. Credit: Yasunari Shigeta, Yuri D. Zakharov and Royal H. Mapes.{{fairuse}}

Cut in the plane of the spiral (medial or median cut), the shell reveals the chambers inside.

On the left is an internal mold from a Baculites individual. The original aragonite of the outer conch and inner septa has dissolved away, leaving this articulated internal mold. Baculites is an ammonite from the Late Cretaceous of Wyoming.

The tissue used to close the chamber to the outside is called an aptychus. Perisphictes on the lower left has aptychi.

Agoniatites have a central siphuncle as shown in the illustration on the right with septal necks pointing to the rear (retrochoanitic).

The diagrams on the lower left show median sections where the siphuncle is in a ventral position. Measurements to characterize an ammonite are indicated in the right-hand diagram. The abbreviations are for ammonitella (am), caecum (c), initial chamber (ic), primary constriction (pc), prosiphon (ps), siphunclar tube (s), proseptum (first septum, s1), primary septum (second septum, s2), third septum (s3), maximum initial chamber size (A), minimum initial chamber size (B), ammonitella size (D), and ammonitella angle (E).[14]


Fossil shell of ammonite Placenticeras whitfieldi from Black Hills, South Dakota at Peabody Museum of Natural History, Yale. Credit: Hectonichus.{{free media}}
Pseudaspidoceras madagascariensis (Basse, 1954), specimen OUM KX17236, is from the Early Turonian of Goulmima in south-eastern Morocco. Credit: Andrew S. Gale, William James Kennedy and David Martill. {{fairuse}}

The fossil shell of ammonite Placenticeras whitfieldi on the right shows punctures caused by the bite of a mosasaur.

"In the late Cretaceous, it is the mosasaurs that have been identified as ammonite predators, beginning with the study of Kauffman and Kesling (1960), who described a 300 mm diameter Placenticeras (first illustrated by Fenton and Fenton in 1958) from the Late Campanian Pierre Shale of South Dakota that had been bitten, in their interpretation no less than 16 times, by what they concluded to be a platycarpine mosasaur (we suggest that the mosasaur was playing with its prey, as do contemporary cetaceans)."[23]

"A juvenile specimen of the ammonite Pseudaspidoceras [in the image on the left] from the Early Turonian [Late Cretaceous] of the Goulmima area in the Province of Er-Rachida in south-eastern Morocco shows clear evidence of predation by a tooth-bearing vertebrate."[23]

"These [teeth punctures] are interpreted as the product of a single bite by a mosasauroid, probably a Tethysaurus."

"All of the convincing well-documented examples of mosasaur-bitten ammonite shells are thus from North America, the overwhelming majority from the Late Campanian of the northern part of the Western Interior of the United States and Alberta in Canada."[23]

"The Goulmima occurrence is the only convincing record of mosasauroid attack on an ammonite outside North America, and of the latter, the overwhelming majority are restricted to the Late Campanian of the northern interior. The only adequately documented putative occurrence outside of the interior, in the Early Maastrichtian Rosario Formation of Baja California, Mexico, may not in fact be by a mosasaur, although there is evidence of mosasauroid attack on two Campanian nautiloids from San Diego County in California."[23]

"Given the above, we see no evidence to support the view that there was coevolution between ammonites and mosasaurs, nor that mosasaurs were "The ecologically dominant predators of Cretaceous marine seas" as proposed by Kauffman (1990)."[23]


This is the world's largest known ammonite Parapuzosia seppenradensis. Credit: Xocolatl.{{free media}}

On the right is an image of the world's largest known ammonite, Parapuzosia seppenradensis (originally Pachydiscus seppenradensis) discovered in Seppenrade, Germany. The partial fossil specimen has a shell diameter of 1.95 metres (6.4 ft). But, the living chamber was incomplete. The shell diameter may have been about 2.55 metres (8.4 ft) when it was alive.





The Paleocene dates from 65.5 ± 0.3 x 106 to 55.8 ± 0.2 x 106 b2k.


Hoploscaphites constrictus johnjagti subsp. nov., adult macroconchs are ammonites from the Danian. Credit: Marcin Machalski.{{fairuse}}

The beginning of the Danian age (and the end of the preceding Maastrichtian age) is at the Cretaceous–Paleogene extinction event at 66.0 Ma. The age ended 61.6 Ma, being followed by the Selandian age.[24]

Post-"Cretaceous ammonites of the genus Hoploscaphites have been found at Stevns Klint in Denmark (Machalski & Heinberg, 2005; Machalski et al., 2009)."[25]

"The maximum age for Danian scaphitid survivors from the Cerithium Limestone at Stevns Klint, Denmark, has recently been estimated to be around 0.2 Ma following the K–Pg boundary event (Machalski and Heinberg in press). Assuming the Cretaceous– Paleogene boundary at 65.4 ± 0.1 Ma (Jagt and Kennedy 1994), the present study covers more than 4 Ma of the final stages in scaphitid evolution."[26]

"Scaphitid material from subunit IVf−7 at the very top of the Meerssen Member [...] traditionally regarded to be uppermost Maastrichtian, has recently been reassigned to the lowermost Danian, based on microfossil and strontium isotope evidence (Smit and Brinkhuis 1996). According to Jagt et al. (2003), the scaphitid and baculitid ammonites preserved in subunit IVf−7 are early Danian survivors."[26]

Above center are Hoploscaphites constrictus johnjagti subsp. nov., adult macroconchs, ammonites from the Danian: A. MGUH 27366, lowermost Danian, Stevns Klint, Denmark, in apertural (A1), lateral (A2, A3), and ventral (A4) views.



The Cretaceous/Cenozoic boundary occurs at 65.0 ± 0.1 Ma (million years ago).[27]



"The Cretaceous period is the third and final period in the Mesozoic Era. It began 145.5 million years ago after the Jurassic Period and ended 65.5 million years ago, before the Paleogene Period of the Cenozoic Era."[28]

Scaphites hippocrepis is an index fossil for the Cretaceous.[29]

Late Cretaceous

Photograph is of a fossil cast of a Baculites grandis shell taken at the North American Museum of Ancient Life. Credit: Ninjatacoshell.{{free media}}
Plesiacanthoceras wyomingense is from the late Cretaceous in Wyoming, USA. Credit: Ryan Somma.{{free media}}

In the top center is a 2.7 cm section of a polished shell with 6 sutures. It is from the extinct cephalopod Baculites compressus; Cretaceous, 100 million years old, Bearpaw Formation, Montana, USA.

The lower center is a fossil cast of a Baculites grandis shell taken at the North American Museum of Ancient Life.

On the right is an example of Plesiacanthoceras wyomingense from the late Cretaceous in Wyoming, USA. It is exhibited in Smithsonian National Museum of Natural History: Hall of Fossils.



The Lancian ranges from 66.8 to 65.5 Ma (K/T boundary).


Photograph is of a fossil ammonite Jeletzkytes spedeni. Credit: Dlloyd..{{free media}}
Fossil is of Baculites ovatus, an extinct mollusc. Credit: Ghedoghedo.{{free media}}
This is an example of Discoscaphites iris an ammonite from the Owl Creek Formation (Upper Cretaceous), Owl Creek, Ripley, Mississippi. Credit: Mark A. Wilson.{{free media}}
Biostratigraphic chart of the Upper Cretaceous (Campanian and Maastrichtian) of the Western Interior of North America showing ammonite and inoceramid zones. Credit: Neil H. Landman, W. James Kennedy, Neal L. Larson, Joyce C. Grier, James W. Grier, and Tom Linn.{{fairuse}}
Baculites baculus type specimen to allow field identification present in the Cretaceous of the Western Interior Seaways. Credit: Cretaceous Atlas of Ancient Life.{{fairuse}}

Extends from 70.6 ± 0.6 to 65.5 ± 0.3 Mya.

The specimen on the left is Jeletzkytes spedeni from the Maastrichtian (Upper-Cretaceous) Fox Hills Formation, locality - South Dakota, USA. Matrix free specimen is 7.5 cm (3") in diameter, displaying pearly aragonite preservation of the shell.

The center photo is of Baculites ovatus, at the Naturalis Museum, Leiden.

Baculites ovatus apparently occurs in the Ripley Formation.[30]

Discoscaphites iris on the right is an ammonite from the Owl Creek Formation (Upper Cretaceous), Owl Creek, Ripley, Mississippi USA.

The McNairy Formation found in Illinois is also from the Upper Cretaceous Maastrichtian.

Baculites baculus, type specimen in the image at lower right, lower boundary of zone and lower boundary for Maastrichtian dated to 70.6 ± 0.6 Ma and upper boundary dated to 70.00 ± 0.45 Ma.[31]



The Kirtlandian ranges from 75.6 to 72.8 Ma.


Didymoceras cheyennense (Meek & Hayden, 1856) is a fossil ammonite from the Cretaceous of South Dakota, USA. Credit: James St. John.{{free media}}
Piceance Basin stratigraphy, ammonite zones are for the Campanian only (Edmontonian not labelled) and inferred ages. Credit: Michael H. Hofmann, Anton Wroblewski and Ron Boyd.{{fairuse}}

Edmontonian extends from 80.8 to 70.7 Mya.

The fossil shown on the right has a bizarre and irregularly shaped shell. Such examples are called heteromorph ammonites.

Classification: Animalia, Mollusca, Cephalopoda, Ammonoidea, Ancyloceratina, Nostoceratidae.

"The Williams Fork Formation represents a period spanning seven ammonite zones from the Didymoceras cheyennense to the Baculites baculus (Meek and Hayden, 1861) Zone (Newman, 1987). Based on the timescale of Obradovich and Cobban (1975), these seven ammonite zones represent approximately three million years, correlative with the late Campanian to early Maastrichtian (Harland et al., 1990). Lillegraven and Ostresh (1990) correlated these seven ammonite zones to the Edmontonian North American Land Mammal Age (NALMA)."[32]

Other ammonite zones include Baculites compressus from the Fruitland Formation above the Didymoceras cheyennense zone.[33] "Baculites compressus age is from a bentonite bed in the Bearpaw Shale, Big Horn County, Montana (Obradovich, 1993 and written communication, Obradovich, 1996)."[33]

The stratigraphic column on the left shows ammonite zones between Didymoceras nebrascense and the Maastrichtian with inferred ages.

Bentonite "marker beds are found throughout the lowermost Williams Fork Formation and the underlying Cameo Coal interval [...] the Yampa ash marker bed, is found near the top of the Cameo Coal interval, suggesting a depositional age equal to or younger than 72.2 ± 0.1 Ma for the lower Williams Fork Formation [...] and time equivalent to the previously mentioned Baculites reesidei zone (Brownfield and Johnson 2008) [...]."[34]



Extends from 82.2 to 80.8 Mya.


Stratigraphic column for the Tuscaloosa is shown below the Eutaw. Credit: Richard B. Powers, USGS.{{free media}}
This is a 2.7 cm section of Baculites compressus. Credit: Kevmin.{{free media}}
A specimen of Placenticeras ammolite from the Bearpaw Formation. Credit: James St. John.{{free media}}

The Bearpaw Formation is famous for its well-preserved ammonite fossils. These include Placenticeras meeki and Placenticeras intercalare, and the baculite Baculites compressus.[35]

Extends from 83.5 ± 0.7 to 70.6 ± 0.6 Mya.

The Baylis Formation, Post Creek Formation and the Tuscaloosa Formation are Upper Cretaceous from the Campanian.



Extends from 84 to 65.5 Mya.



Extends from 85.2 to 82.2 Mya.



Extends from 85.8 ± 0.7 to 83.5 ± 0.7 Mya.



Extends from 86.5 to 84 Mya.



Extends from 89.1 to 86.5 Mya.



Extends from 89.3 ± 1.0 to 85.8 ± 0.7 Mya.



Extends from 89.3 to 65.5 Mya.



Extends from 89.5 to 83.5 Mya.



Extends from 92.1 to 89.1 Mya.



Extends from 93.5 ± 0.8 to 89.3 ± 1.0 Mya.

Benueites is a Turonian ammonite genera from Nigeria.[36]



These fast-moving nektonic carnivores lived during the Cenomanian stage of the Late Cretaceous (from 94.3 to 89.3 Ma).[37][38]

Middle Cenomanian


The hierarchy of ammonites in the Middle Cenomanian is shown on the left below beginning at 95.73 ± 0.61.[39]

Subdivisions of the Middle Cenomanian
Period Western Interior Ammonite
Taxon Range Zones
Age Ma Species image
>94.71 ± 0.49
Plesiacanthoceras wyomingense is from the late Cretaceous in Wyoming, USA. Credit: Ryan Somma.{{free media}}
<94.96 ± 0.50
Acanthoceras amphibolum Zone is Middle Cenomanian starting at 94.96 ± 0.50 Ma. Credit: Cretaceous Atlas of Ancient Life.{{fairuse}}
<95.73 ± 0.61
Acanthoceras bellense Zone is Middle Cenomanian starting at 95.73 ± 0.61 Ma. Credit: Cretaceous Atlas of Ancient Life.{{fairuse}}
<95.73 ± 0.61
Acanthoceras muldoonense Zone is Middle Cenomanian. Credit: Cretaceous Atlas of Ancient Life.{{fairuse}}
<95.73 ± 0.61
Acanthoceras granerosense Zone is Middle Cenomanian. Credit: Cretaceous Atlas of Ancient Life.{{fairuse}}
<95.73 ± 0.61
Conlinoceras tarrantense Zone is Middle Cenomanian. Credit: Cretaceous Atlas of Ancient Life.{{fairuse}}
Fossil shell of Acanthoceras rhotomagensis from France, on display at Gallery of Paleontology and Comparative Anatomy in Paris. Credit: Hectonichus.{{free media}}

Shells of Acanthoceras rhotomagensis may reach a diameter of about 36–50 centimetres (14–20 in). Their shells have ornate ribs.[40][41]

Acanthoceras rhotomagensis fossils may be found in Western Europe and western North America.[42]

Acanthoceras rhotomagensis fossils occur in the Middle Cenomanian just above the boundary with the Lower Cenomanian.[23]

The "highly fossiliferous marl, 1 m in thickness, is the Cast Bed of Price (1877), [where the] lowest record of Acanthoceras rhotomagensis (Brongniart)" occurs.[23]

Lower Cenomanian

Fossil is of Hibolites hastatus. Credit: Ghedoghedo.{{free media}}

Hibolites is a genus of belemnite, an extinct group of cephalopods of the Lower Cenomanian.[43]

The Lower Cenomanian extends from 99.6 ± 0.9 Ma to 95.73 ± 0.61 Ma.[44]



Extends from 95.2 to 92.1 Mya.

Lower Cretaceous



Scaphites is from the upper Albian. Credit: DanielCD.{{free media}}
Mortoniceras from South Africa is from the middle Albian. Credit: H. Zell.{{free media}}
Puzosia is from the lower Albian. Credit: Antonov.{{free media}}

Puzosia is a genus of Desmoceratidae (desmoceratid) ammonites, and the type genus for the Puzosiinae, which lived during the middle part of the Cretaceous, from early Aptian to Maastrichtian (125.5 to 70.6 Ma).[45] Or, the range is from Albian to Santonian.[46]

Otohoplites is a genus of ammonite that lived in the Early Albian whose fossils were found in Svalbard, Denmark, England, France, Austria, Poland, Russia and Kazakhstan, evolved from Hemisonneratia and gave rise to genus Hoplites.[47] Shells belonging to this genera are rather inflated to compressed and have zigzaging, or looped ribs that ends in oblique ventrolateral clavi; usually, ribs are zigzaging through venter; macroconchs have smooth body chamber and rounded venter.[48]

Kossmatella is an extinct genus of ammonoid cephalopods belonging to the family Lytoceratidae, were fast-moving nektonic carnivores[49] that lived from Albian to Cenomanian age.[46]

Cleoniceras included in the subfamily Cleoniceratinae is a rather involute, high-whorled hoplitid from the Lower to basal Middle Albian of Europe, Madagascar, and Transcaspian region, where the shell has a generally small umbilicus, arched to acute venter, and typically at some growth stage, falcoid ribs that spring in pairs from umbilical tubercles, usually disappearing on the outer whorls.[50]

Brancoceras is a rather small, strongly ribbed, acanthoceratacean ammonite from the Albian stage of the Lower Cretaceous:[51][52]

  1. the shell is evolute with a subquadrate whorl section and rounded venter
  2. the suture forms a finely squiggly line with well-defined lobes and saddles
  3. Brancoceras (Eubrancoceras) aegoceratoides reached a diameter of at least 4.2 centimetres (1.7 in)
  4. Brancoceras is representative of the subfamily Brancoceratinae, which makes up part of the Acanthoceratoidea (acanthoceratacean) family Brancoceratidae
  5. stratigraphic range is rather narrow, extending only from the upper Lower to the Middle Albian.

Arcthoplites is an extinct genus of cephalopod belonging to the Ammonite subclass from the lower Albian.[46]

Anadesmoceras is an hoplitid ammonite from the lower Albian (upper Lower Cretaceous) of England, included in the subfamily Cleoniceratinae:

  1. a shell shaped more or less like a compressed Cleoniceras but with faint ornament only on the inner whorls
  2. the shell has bundled growth striae. The aperture is preceded by several wide sinuous constrictions.[46][51]

Anacleoniceras is an extinct genus of cephalopod belonging to the Ammonite subclass lower Albian.[46]

Aioloceras is an ammonite, order Ammonitida, from near the end of the Early Cretaceous:[46][51]

  1. the shell is compressed with the outer whorl covering much of the previous
  2. sides are slightly convex, converge toward a narrowly ached venter
  3. inner whorls have sharp falcoid ribs, outer are smooth
  4. umbilical tubercles are lacking
  5. similar related forms include Neosaynella and Cleoniceras
  6. has been found in Albian (uL Cret) sediments in Madagascar, Patagonia, and possibly Queensland.

Lower Albian

High-resolution scans revealed the ammonite's internal structure, subgenus Puzosia (Bhimaites) that emerged more than a hundred million years ago and lived until at least 93 million years ago. Credit: Bo Wang.{{fairuse}}
An amber-encased ammonite juvenile belongs to the subgenus Puzosia (Bhimaites), in 99-million-year-old amber. Credit: Bo Wang.{{fairuse}}

"Amber—ancient resins from trees—commonly traps only some terrestrial insects, plants, or animals. It’s very rare to find some sea animals in amber."[53]

"This extraordinary assemblage, a true and beautiful snapshot of a beach in the Cretaceous, is just mind-blowing."[54]

"The idea that there’s a whole community of organisms in association—that may prove more important in the long run."[55]

"If you were scuba-diving in a shallow marine setting, you absolutely would have seen ammonites. They would be as common as seeing some snails crawling around."[56]

"Based on its internal shell structure, the amber-encased ammonite is a juvenile that belongs to the subgenus Puzosia (Bhimaites), which makes a lot of sense in 99-million-year-old amber."[53]











Early Cretaceous



This is an example of Neophyllites antecedens showing suture marks. Credit: Günter Knittel.{{fairuse}}

The Jurassic/Cretaceous boundary occurs at 144.2 ± 2.6 Ma (million years ago).[27]

Perisphinctes tiziani is an index fossil for the Jurassic.[29]

Late Jurassic

Kosmoceras cromptoni is from the Late Jurassic. Credit: Daderot.{{free media}}

On the right is an example of Kosmoceras cromptoni from the Late Jurassic, Chippenham, England.

Upper Jurassic





Orthosphinctes (Lithacosphinctes) achilles is in the Museum of Toulouse. Credit: Jean Fontayne.{{free media}}

Lithacosphinctes achilles is from the Kimmeridgian.



Middle Jurassic



Peltoceras solidum is an ammonite from the Callovian. Credit: Wilson44691.{{free media}}
Kosmoceras medea is from the Callovian. Credit: Hectonichus.{{free media}}
Kosmoceras proniae is sized using 1 PLN coin. Credit: Ag.Ent.{{free media}}

On the right is an image of Peltoceras solidum, an ammonite from the Matmor Formation (Jurassic, Callovian), Makhtesh Gadol, Israel.

On the left is an example of Kosmoceras medea.

Another species of Kosmoceras is on the lower right, specifically Kosmoceras proniae.






Leioceras opalinum, Graphoceratidae; has a diameter: 4.5 cm; Lower Aalenian, Middle Jurassic; between Ohmenhausen and Reutlingen, Germany. Credit: H. Zell.{{free media}}

Leioceras opalinum is an ammonite from the Aalenian.

Lower Jurassic

Uptonia jamesoni is from the lower Jurassic. Credit: Eduard Solà Vázquez.{{free media}}

Uptonia jamesoni from the lower Jurassic is in the family Polymorphitidae, superfamily Eoderocerataceae, order Ammonitida, subclass Ammonoidea, class Cephalopoda.




Pleuroceras spinatum is from the Pliensbachian. Credit: Didier Descouens.{{free media}}

Pleuroceras spinatum (Bruguière 1789) is of the family Amaltheidae. It is a pyritic specimen. The biozone index is to the end of Pliensbachian.

The International Commission on Stratigraphy (ICS) has assigned the First Appearance Datum of Bifericeras donovani and of genus Apoderoceras the defining biological marker for the start of the Pliensbachian Stage of the Jurassic, 190.8 ± 1.0 million years ago.[57]


Lithostratigraphy of the Lower and Middle Lias in Middle Atlas (A) and the Middle Atlas Causse (B). Credit: J. Auajjara and J. Boulègue.{{fairuse}}
Photograph is of the ammonite Asteroceras obtusum. Credit: Dlloyd.{{free media}}
Asteroceras obtusum is an ammonite from the Early Jurassic. Credit: Nobu Tamura.{{free media}}

"During the Lower Lias, deposition of lower and middle formations (Hettangian-Sinemurian) occurs under a low bathymetry."[58]

On the left is a photograph of Asteroceras obtusum from the Jurassic Lower Lias Formation, Obtusum Zone. Locality is Lyme Regis, Dorset, England. Complete calcified specimen measures 11.5 cm (4.5") in diameter, in a limestone matrix.


Psiloceras spelae tirolicum has its first occurrence at the Triassic-Jurassic boundary as geochron for the base of the Jurassic. Credit: Axel von Hillebrandt et al.{{fairuse}}
Fossil shell of Psiloceras planorbis from Germany, on display at Galerie de paléontologie et d'anatomie comparée in Paris. Credit: Hectonichus.{{free media}}
This is an example of Psiloceras psilonotum from the Hettangian. Credit: Günter Knittel.{{fairuse}}
This fossil of Neophyllites imitans is from the Jurassic Hettangian. Credit: Günter Knittel.{{fairuse}}

Psiloceras psilonotum, Psiloceras spelae tirolicum and Psiloceras planorbis are from the Hettangian.

The Triassic/Jurassic boundary occurs at 205.7 ± 4.0 Ma (million years ago).[27]


This is an example of Psiloceras tilmanni from the Jurassic. Credit: Günter Knittel.{{fairuse}}

Although the example of Psiloceras tilmanni is from the Jurassic. Its lowest occurrence is in the New York Canyon section of Nevada USA which may be Triassic.

Trophites subbuliatus is an index fossil for the Triassic.[29]

Upper Triassic








Middle Triassic





Ussuriphyllites amurensis (Kiparisova) is from the Lower-most Anisian, Atlasov Cape area. Credit: Alexander M. Popov.{{fairuse}}

An example of Ussuriphyllites amurensis (Kiparisova) is on the right. It is from the Lower-most Anisian, Atlasov Cape area.[59]

Lower Triassic





Olenekoceras meridianum (Zakharov) is found in the Upper Olenekian, Atlasov Cape area. Credit: Alexander M. Popov.{{fairuse}}

The Spathian is sometimes referred to as the Late Olenekian.[60]

Olenekoceras meridianum is a "typical Late Olenekian [fossil which] differs in its lithology from the same zone of Russian Island, where the Zhitkov Suite has been recognized (Zakharov, 1997; Zakharov et al., 2004)."[59]


This ammonoid fossil is a syntype of Wyomingites aplanatus (White 1879) from the Triassic of S.E. Idaho. Credit: Kevin Bylund.{{fairuse}}

The Smithian is sometimes referred to as the Early Olenekian.[60]





The Paleozoic era spanned 542.0 ± 1.0 to 251.0 ± 0.7 Mb2k.



The Permian lasted from 299.0 ± 0.8 to 251.0 ± 0.4 Mb2k.

The Permian/Triassic boundary occurs at 248.2 ± 4.8 Ma (million years ago).[27]



The Carboniferous began 359.2 ± 2.5 Mb2k and ended 299.0 ± 0.8 Mb2k.



The Pennsylvanian lasted from 318.1 ± 1.3 to 299.0 ± 0.8 Mb2k.

Reticuloceratidae, subtaxa: Aphantites, Gaitherites, Marianoceras, Melvilloceras, Surenites, Ugamites, Verneuilites, age range: 318.1 to 314.6 Ma, Distribution: Carboniferous of Canada (2: Nunavut collections), the Russian Federation (13), United States (25: Arkansas), Uzbekistan (3).[61][62]

Lower Pennsylvanian


Aphantites aenigmaticus ranges from 314.2 Ma to 313 Ma.[63]



The Mississippian lasted from 359.2 ± 2.5 to 318.1 ± 1.3 Mb2k.

Prolecanites gurleyi is an index fossil of the Mississippian.[29]

Middle Mississippian


"This species has been consistently identified with the considerably younger, late Viséan (late Holkerian to Asbian [late Meramecian to early Chesterian]) genus Beyrichoceras Foord, 1903 (type species, Goniatites obtusus Phillips, 1836) (eg, Gordon, 1965, p. 284."[64]



The Devonian spanned 416.0 ± 2.8 to 359.2 ± 2.5 Mb2k.

Upper Devonian



Fossil is of Platyclymenia intracrostata Credit: Wikipek.{{free media}}
This is another example of Clymenia laevigata. Credit: Hectonichus.{{free media}}

A specimen of Clymenia laevigata from the Upper Devonian Famennian of Poland is on the right.

On the left is a fossil of Platyclymenia intracrostata also from the Famennian of Poland.



Middle Devonian






Lower Devonian








Early Devonian


Mimagoniatites is a genus of ammonites from the early Devonian.

"Shell [is] small to large size, evolute, thinly discoidal to discoidal. Whorl cross section of the first two whorls [is] approximately circular, in later whorls subtrapezoidal. Umbilicus [is] narrow to moderately wide, moderately large umbilical window (< 1 mm). Whorl expansion rate increases remarkably from the second whorl on (> 2.5, later up to 3.9). Growth line course [is] biconvex with prominent ventrolateral projection and deep ventral sinus."[65]

The lower boundary of the genus is "LD3C--LD3D: Anetoceras Range Zone top, 405.5 million years" and the upper boundary is "CZB maureri--sulc.antiqua Zone [19,30], 398.5 million years".[65]

Geographic distribution: "Devonian of Algeria (2 collections), Canada (1: Nunavut), China (7), the Czech Republic (5), Germany (3), Morocco (13), the Russian Federation (1), Spain (4), Turkey (3), United States (1: Pennsylvania)".[66]



The Silurian spanned 443.7 ± 1.5 to 416.0 ± 2.8 Mb2k.

Hexamoceras hertzeri is an index fossil for the Silurian.[29]

Hexamoceras is a genus of the Nautiloidea.[67]

"Rolfe made the important observation that 'Other genera are pre-Devonian and hence cannot be ammonoid aptychi, but Ruedemann's suggestion that aptychi "would naturally also have existed in the Ordovician and Silurian cephalopods" has been largely overlooked'."[68]


A Nautiloidea fossil Discoceras boreale species fossil found at Herøya, Porsgrunn, Norway. Credit: c:user:Kittil hagen.{{free media}}

The Ordovician lasted from 488.3 ± 1.7 to 443.7 ± 1.5 Mb2k.

The Nautiloidea fossil Discoceras boreale has been found in Middle and Upper Ordovician sediments in Northern Europe, Baffin Island in Canada, Yunnan and Hubei provinces in China, and in Punjab, India.

Upper Ordovician

This is an internal mold of a nautiloid from the Upper Ordovician of northern Kentucky. Credit: Wilson44691.{{free media}}

The image on the right is an over-encrusted, internal mold of a nautiloid from the Upper Ordovician of northern Kentucky.



The Cambrian lasted from 542.0 ± 1.0 to 488.3 ± 1.7 Mb2k.

Middle Cambrian


"The fossils date to the early Cambrian period and are about 522 million years old, according to the researchers, who found the fossils on the Avalon Peninsula of Newfoundland, Canada. Until now, the oldest cephalopod on record was a shelled creature known as Plectronoceras cambria, which lived about 30 million years after the recently discovered, yet-to-be-named cephalopod."[69]

The finding suggests "that cephalopods emerged at the very beginning of the evolution of multicellular organisms during the Cambrian explosion."[70]

"The fossils show that this ancient creature had a cone-shape shell that was subdivided into different chambers. These chambers were connected by a siphuncle — an internal tube seen in shelled cephalopods, including extinct ammonites and modern-day nautiluses — that pumps fluids and gases through the different chambers to help the animal adjust its buoyancy."[69]

"By evolving a siphuncle, cephalopods became the first known organisms to be able to actively move up and down in the water. With this ability to move, early cephalopods picked the open ocean as their chosen habitat."[70]

"The fossils were discovered on the ancient micro-continent of Avalonia, which encompassed parts of eastern Newfoundland and Europe."[70]

"Although an early Cambrian origin of cephalopods has been suggested by molecular studies, no unequivocal fossil evidence has yet been presented. Septate shells collected from shallow-marine limestone of the lower Cambrian (upper Terreneuvian, c. 522 Ma) Bonavista Formation of southeastern Newfoundland, Canada, are here interpreted as straight, elongate conical cephalopod phragmocones. The material documented here may push the origin of cephalopods back in time by about 30 Ma to an unexpected early stage of the Cambrian biotic radiation of metazoans, i.e. before the first occurrence of euarthropods."[71]

"We recently redescribed the Middle Cambrian organism Nectocaris pteryx known from 92 specimens from the Burgess Shale (Smith & Caron 2010). [This] new material allowed us to identify new features consistent with a cephalopod affinity."[72]


This may be an ammonite fossil. Credit: Halvard : from Norway.{{free media}}


  1. Each of the ammonoids has a set of genes producing a distinct suture mark.
  2. Ammonoids are alive today.
  3. Morphological descriptions should be sufficient to identify unknown ammonites at the species level.



Classification of Baculites ovatus:

  1. Domain: Eukaryota
  2. Regnum: Animalia
  3. Subregnum: Eumetazoa
  4. Cladus: Bilateria
  5. Superphylum: Protostomia
  6. Phylum: Mollusca
  7. Classis: Cephalopoda
  8. Subclassis: Ammonoidea
  9. Ordo: Ammonitida
  10. Subordo: Ancyloceratina
  11. Superfamilia: Turrilitoidea
  12. Familia: Baculitidae
  13. Genus: Baculites
  14. Species: Baculites ovatus (Say, 1820)

The subclassis: Ammonoidea contains the ordines: Ammonitida, Ceratitida, Clymeniida, Goniatitida, and Prolecanitida.

See also



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