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

# Mollusks
Def. a "soft-bodied invertebrate , " is called a mollusc.
Def. a "body wall of a mollusc, from which the shell is secreted" is called a mantle.
Def. a "rasping tongue of snails and most other mollusks" is called a radula.
As a mollusk an ammonite may be expected to have
- a mantle with a cavity for breathing and excretion,
- a radula, and
- a structured nervous system.

# Cephalopods
Def. any "mollusc, , which includes squid, cuttlefish, octopus,  etc" is called a cephalopod.
An ammonite is expected to have cephalopod characteristics
- bilateral body symmetry,
- a prominent head, and
- a set of arms or tentacles (muscular hydrostats).

# Theoretical ammonites
Def. "the scientific study of squid (often extended to all cephalopods)" is called teuthology.
"Teuthology, a branch of malacology, is the study of cephalopods."
Def. "any of numerous flat spiral fossil shells of cephalopods" 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."
On the right are schematic drawings of four growth stages of Hoploscaphites nicolletii in lateral and transverse cross-sections:
- A is an embryonic shell called the ammonitella, scale bar 500 m,
- B is the first postembryonic stage called the neanic and the animal or shell is called the neanoconch, scale bar 1 mm,
- C is a juvenile, scale bar 5 mm, and
- D is an adult, scale bar is 1 cm.

# Agoniatites
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.

# Ammonites
These are ammonites of the suborder Ammonitida.

# Ceratites
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)."

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

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

# Lytocerates
Characteristics:
- loosely coiled,
- evolute,
- gyroconic,
- exposed whorls,
- whorls touching,
- subcircular to narrowly compressed whorls,
- broadly arched, or keeled venter,
- smooth or ribbed sides,
- aptychi are single valved and concentrically striated,
- suture saddle endings tend to be rounded but usually not phylloid,
- lobes tend to be more jagged with thorn-like endings, and
- complex moss-like suture endings with adventious and secondary subdivisions.

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

# Phyllocerates

# Prolecanites
"Type species by original designation of Librovitch 1957, Protocanites supradevonicus Schindewolf (1926)."
In the diagrams above are the suture patterns for various species holotypes:
- A - Protocanites gurleyi (Smith),
- B - Eocanites supradevonicus supradevonicus (Schindewolf),
- C - Eocanites semageominus (House),
- D - Eocanites wangyounensis(Ruan & He),
- E - Michiganites algarbiensis (Pruvost),
- F - Michiganites marshallensis (Winchell),
- G - Michiganites scalibrinii (Antelo),
- H - Michiganites greenei (Miller).

# Coleoids
The subclass Coleoidea has the cohort Belemnoidea which may contain shelled cephalopods.
The image second down on the right shows "the piglet squid , floating along with its tentacles waving above its head in the central Pacific Ocean near Palmyra Atoll."
The "squid  about 4,544 feet (1,385 meters) below the ocean surface  the exploration vehicle (E/V) Nautilus."
"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."
The "mantle is filled with ammonia, which the squid uses to control its buoyancy."
"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."
Helicocranchia is of the order Teuthida.

# Belemnoids
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."
"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 animals soft parts very rarely fossilise, leaving us with only the hard parts; the guard and the phragmacone."

## Aulacocerids
"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  are from these rocks in the Confusion Range and Burbank Hills of western Millard County."
For the study of the "shell morphology and ultrastructure in Hematites  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  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."
"Schematic diagram of the medial shell section in Hematites  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."

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

## Belemnitids
"Belemnites  have a worldwide distribution."
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.

## Diplobelids

## Belemnoteuthins

# Nautiloids
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  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)."
"Nautiluses first evolved in the Cambrian period and became significant marine predators during the Ordovician period."

## Actinocerids

## Ascocerids

## Bactrites

## Barrandeocerids

## Discosorids

## Ellesmerocerids

## Endocerids

## Nautilids
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:
- No common name (Allonautilus perforates),
- Crusty Nautilus (Allonautilus scrobiculatus),
- Palau Nautilus (Nautilus belauensis),
- Bellybutton Nautilus (Nautilus macromphalus),
- Chambered Nautilus (Nautilus pompilius), and
- White-patch Nautilus (Nautilus stenomphalus)".

## Oncocerids

## Orthocerids
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.

## Plectronocerids

## Pseudorthocerids

## Tarphycerids

# Anatomy
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).

# Predation
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)."
"A juvenile specimen of the ammonite Pseudaspidoceras  from the Early Turonian  of the Goulmima area in the Province of Er-Rachida in south-eastern Morocco shows clear evidence of predation by a tooth-bearing vertebrate."
"These  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."
"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."
"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)."

# Sizes
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.

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

# Danian
The beginning of the Danian age (and the end of the preceding Maastrichtian age) is at the CretaceousPaleogene extinction event at 66.0 Ma. The age ended 61.6 Ma, being followed by the Selandian age.
Post-"Cretaceous ammonites of the genus Hoploscaphites have been found at Stevns Klint in Denmark (Machalski & Heinberg, 2005; Machalski et al., 2009)."
"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 KPg 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."
"Scaphitid material from subunit IVf7 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 IVf7 are early Danian survivors."
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.

# Cretaceous
"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."
Scaphites hippocrepis is an index fossil for the Cretaceous.

# Late Cretaceous
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.

# Maastrichtian
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.
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.

# Edmontonian
Extends from 80.8 to 70.7 Mya.

# Judithian
Extends from 82.2 to 80.8 Mya.

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

# Haumurian
Extends from 84 to 65.5 Mya.

# Aquilan
Extends from 85.2 to 82.2 Mya.

# Santonian
Extends from 85.8  0.7 to 83.5  0.7 Mya.

# Piripauan
Extends from 86.5 to 84 Mya.

# Teratan
Extends from 89.1 to 86.5 Mya.

# Coniacian
Extends from 89.3  1.0 to 85.8  0.7 Mya.

# Senonian
Extends from 89.3 to 65.5 Mya.

# Emscherian
Extends from 89.5 to 83.5 Mya.

# Mangaotanean
Extends from 92.1 to 89.1 Mya.

# Turonian
Extends from 93.5  0.8 to 89.3  1.0 Mya.
Benueites is a Turonian ammonite genera from Nigeria.

# Cenomanian
These fast-moving nektonic carnivores lived during the Cenomanian stage of the Late Cretaceous (from 94.3 to 89.3 Ma).
Shells of Acanthoceras rhotomagensis may reach a diameter of about 36  (Expression error: Unexpected round operator. ). Their shells have ornate ribs.
Acanthoceras rhotomagensis fossils may be found in Western Europe and western North America.
Hibolites is a genus of belemnite, an extinct group of cephalopods of the Cenomanian.

# Arowhanan
Extends from 95.2 to 92.1 Mya.

# Lower Cretaceous

# Albian
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). Or, the range is from Albian to Santonian.
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. 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.
Kossmatella is an extinct genus of ammonoid cephalopods belonging to the family Lytoceratidae, were fast-moving nektonic carnivores that lived from Albian to Cenomanian age.
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.
Brancoceras is a rather small, strongly ribbed, acanthoceratacean ammonite from the Albian stage of the Lower Cretaceous:
- the shell is evolute with a subquadrate whorl section and rounded venter
- the suture forms a finely squiggly line with well-defined lobes and saddles
- Brancoceras (Eubrancoceras) aegoceratoides reached a diameter of at least 4.2 centimetres (1.6535433054 in)
- Brancoceras is representative of the subfamily Brancoceratinae, which makes up part of the Acanthoceratoidea (acanthoceratacean) family Brancoceratidae
- 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.
Anadesmoceras is an hoplitid ammonite from the lower Albian (upper Lower Cretaceous) of England, included in the subfamily Cleoniceratinae:
- a shell shaped more or less like a compressed Cleoniceras but with faint ornament only on the inner whorls
- the shell has bundled growth striae. The aperture is preceded by several wide sinuous constrictions.
Anacleoniceras is an extinct genus of cephalopod belonging to the Ammonite subclass lower Albian.
Aioloceras is an ammonite, order Ammonitida, from near the end of the Early Cretaceous:
- the shell is compressed with the outer whorl covering much of the previous
- sides are slightly convex, converge toward a narrowly ached venter
- inner whorls have sharp falcoid ribs, outer are smooth
- umbilical tubercles are lacking
- similar related forms include Neosaynella and Cleoniceras
- has been found in Albian (uL Cret) sediments in Madagascar, Patagonia, and possibly Queensland.

# Lower Albian
"Amberancient resins from treescommonly traps only some terrestrial insects, plants, or animals. Its very rare to find some sea animals in amber."
"This extraordinary assemblage, a true and beautiful snapshot of a beach in the Cretaceous, is just mind-blowing."
"The idea that theres a whole community of organisms in associationthat may prove more important in the long run."
"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."
"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."

# Aptian

# Barremian

# Hauterivian

# Valanginian

# Berriasian

# Early Cretaceous

# Jurassic
The Jurassic/Cretaceous boundary occurs at 144.2  2.6 Ma (million years ago).
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.
Perisphinctes tiziani is an index fossil for the Jurassic.

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

# Upper Jurassic

# Tithonian

# Kimmeridgian
Lithacosphinctes achilles is from the Kimmeridgian.

# Oxfordian

# Middle Jurassic

# Callovian
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.

# Bathonian

# Bajocian

# Aalenian
Leioceras opalinum is an ammonite from the Aalenian.

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

# Toarcian

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

# Sinemurian

# Hettangian
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).

# Triassic
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.

# Upper Triassic

# Rhaetian

# Norian

# Carnian

# Middle Triassic

# Ladinian

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

# Lower Triassic

# Olenekian

# Spathian
The Spathian is sometimes referred to as the Late Olenekian.
Olenekoceras meridianum is a "typical Late Olenekian  differs in its lithology from the same zone of Russian Island, where the Zhitkov Suite has been recognized (Zakharov, 1997; Zakharov et al., 2004)."

# Smithian
The Smithian is sometimes referred to as the Early Olenekian.

# Induan

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

# Permian
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).

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

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

# Mississippian
The Mississippian lasted from 359.2  2.5 to 318.1  1.3 Mb2k.
Prolecanites gurleyi is an index fossil of the Mississippian.

# Middle Mississippian
"This species has been consistently identified with the considerably younger, late Visean (late Holkerian to Asbian ) genus Beyrichoceras Foord, 1903 (type species, Goniatites obtusus Phillips, 1836) (eg, Gordon, 1965, p. 284."

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

# Upper Devonian

# Famennian
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.

# Frasnian

# Middle Devonian

# Givetian

# Eifelian

# Lower Devonian

# Emsian

# Pragian

# Lochkovian

# Early Devonian
Mimagoniatites is a genus of ammonites from the early Devonian.
"Shell  small to large size, evolute, thinly discoidal to discoidal. Whorl cross section of the first two whorls  approximately circular, in later whorls subtrapezoidal. Umbilicus  narrow to moderately wide, moderately large umbilical window ( 2.5, later up to 3.9). Growth line course  biconvex with prominent ventrolateral projection and deep ventral sinus."
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 , 398.5 million years".
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)".

# Silurian
The Silurian spanned 443.7  1.5 to 416.0  2.8 Mb2k.
Hexamoceras hertzeri is an index fossil for the Silurian.
Hexamoceras is a genus of the Nautiloidea.
"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'."

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

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

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

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

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

# Sciences
Classification of Baculites ovatus:
- Domain: Eukaryota
- Regnum: Animalia
- Subregnum: Eumetazoa
- Cladus: Bilateria
- Superphylum: Protostomia
- Phylum: Mollusca
- Classis: Cephalopoda
- Subclassis: Ammonoidea
- Ordo: Ammonitida
- Subordo: Ancyloceratina
- Superfamilia: Turrilitoidea
- Familia: Baculitidae
- Genus: Baculites
- Species: Baculites ovatus (Say, 1820)
The subclassis: Ammonoidea contains the ordines: Ammonitida, Ceratitida, Clymeniida, Goniatitida, and Prolecanitida.

# Acknowledgements
The content on this page was first contributed by: Henry A. Hoff.
Initial content for this page in some instances came from Wikiversity.