Evolutionary history of cephalopods
Encyclopedia
The cephalopod
s have a long geological history, with the first nautiloid
s found in late Cambrian
strata, and purported stem-group representatives present in the earliest Cambrian lagerstätten.
The class developed during the middle Cambrian, and underwent pulses of diversification during the Ordovician
period to become diverse and dominant in the Paleozoic
and Mesozoic
seas. Small shelly fossils such as Tommotia
were once interpreted as early cephalopods, but today these tiny fossils are recognized as sclerite
s of larger animals, and the earliest accepted cephalopods date to the Middle Cambrian Period. During the Cambrian, cephalopods are most common in shallow near-shore environments, but they have been found in deeper waters too. Cephalopods were thought to have "undoubtedly" arisen from within the tryblidiid
monoplacophoran clade. However genetic studies suggest that they are more basal, forming a sister group to the Scaphopoda but otherwise basal to all other major mollusc classes. The internal phylogeny of Mollusca, however, is wide open to interpretation – see mollusc phylogeny.
to some gastropods was used in support of this view. The development of a siphuncle
would have allowed the shells of these early forms to become gas-filled (thus buoyant) in order to support them and keep the shells upright while the animal crawled along the floor, and separated the true cephalopods from putative ancestors such as Knightoconus
, which lacked a siphuncle. Negative buoyancy (i.e. the ability to float) would have come later, followed by swimming in the Plectronocerida
and eventually jet propulsion in more derived cephalopods.
However, because chambered shells are found in a range of molluscs – monoplacophora and gastropods as well as cephalopods – a siphuncle is essential to ally a fossil shell conclusively to the cephalopoda. Chambered gastropods can be distinguished from cephalopod shells by the absence of a siphuncle, the irregular spacing of septa, the layering of the shell and (in younger or unmetamorphosed rocks) its microstructure, and the relatively thick width of the shell. The earliest such shells do not have the muscle scars which would be expected if they truly had a monoplacophoran affinity. Additionally, the discovery that Nectocaris pteryx
, which did not have a shell and appeared to possess jet propulsion in the manner of "derived" cephalopods, complicated the question of the order in which cephalopod features developed – provided it is in fact a cephalopod and not an arthropod
.
Understanding of early cephalopod origins is by necessity biased by the available fossil material, which on the whole consists of shelly fossils. Critical fossils are detailed below; since their stratigraphic age has guided the interpretation of the fossils, they are listed in descending order of age.
and Eoclarkoceras, none of the 30+ Cambrian cephalopod genera are known to have survived into the Ordovician. Cambrian cephalopods differ from their descendants by account of their small size (a few centimetres in length); long, tapering shells; smooth shell surfaces; closely spaced septa; and lack of deposits in their body chamber; several more specific features are also only seen in certain groups of Cambrian cephalopod.
is a Cambrian
monoplacophoran thought to represent an ancestor to the cephalopods. It had a chambered, conical shell, but lacked a siphuncle
. Although earlier molluscan fossils are also septate, Knightoconus is the latest septate mollusc before the first sipunculate cephalopods – a point that has been taken to prove its relevance to the Cephalopoda. The absence of this siphuncle has been taken as evidence against cephalopod ancestry – how, it is argued, could a siphuncle evolve to penetrate existing septa? The prevailing argument suggests that a strand of tissue remained attached to the previous septum as the mollusc moved forwards and deposited its next septum, producing an obstacle to the complete closure of the septum and becoming mineralised itself. 10 or more septa are found in mature individuals, occupying around a third of the shell – septa form very early and have been found in specimens as small as 2 mm in length. Septa are uniformly spaced, which is inconsistent with a gastropod affinity. Unlike monoplacophoran fossils, there is no evidence of muscle scarring in Knightoconus fossils.
is arguably the earliest known crown-group cephalopod
, dating to the Upper Cambrian.
Its 14 known specimens hail from the basal Fengshan Formation (north-east China) of the earliest Fengshanian stage. None of the fossils are complete, and none show the tip or opening of the shell. Approximately half of its shell was filled with septa; 7 were recorded in a 2 cm shell. Its shell contains transverse septa separated by about half a millimetre, with a siphuncle
on its concave side. Its morphology matches closely to that hypothesised for the last common ancestor of all cephalopods, and the Plectronocerida
have been said to be the ancestors of the Ellesmerocerids, the first "true cephalopods".
s with a snorkel-like tube on one surface. The snorkel has been seized upon as characteristic of a cephalopod-like water circulatory system, or perhaps as a precursor to the siphuncle. However, neither of these theories have been borne out.
, which were quite small organisms; their shells were slightly curved, and the internal chambers were closely spaced. The siphuncle penetrated the septa with meniscus-like holes. This marks an important difference from the earlier cephalopods, whose siphuncle was positioned at the edge of the septum and against the shell wall. On the basis of muscle scars preserved in such genera as Paradakeoceras
and Levisoceras
, these animals are reconstructed with a straight body and dorsal shell, with the head at the anterior, concave surface of the shell, and the funnel – consisting of a pair of folds in the foot – at the rear — not juxtaposed with the head as in later, oncocerid-like forms.
Early cephalopods had fine shells that could not cope with the pressures of deep water. In the mid Tremadoc, these were supplemented by larger shells around 20 cm in length; these larger forms included straight and coiled shells, and fall into the orders Endocerida (with wide siphuncles) and Tarphycerida
(with narrow siphuncles).
By the mid Ordovician these orders are joined by the Orthocerids, whose first chambers are small and spherical, and Lituitids, whose siphuncles are thin. The Oncocerids also appear during this time; they are restricted to shallow water and have short exogastric conchs. The mid Ordovician saw the first cephalopods with septa strong enough to cope with the pressures associated with deeper water, and could inhabit depths greater than 100–200 m. The wide-siphuncled Actinocerida and the Discocerida both emerged during the Darriwilian. The direction of coiling would prove to be crucial to the future success of the lineages; endogastric coiling would only permit large size to be attained with a straight shell, whereas exogastric coiling – initially rather rare – permitted the spirals familiar from the fossil record to develop, with their corresponding large size and diversity. (Endogastric mean the shell is curved so as the ventral or lower side is longitudinally concave (belly in); exogastric means the shell is curved so as the ventral side is longitudinally convex (belly out) allowing the funnel to be pointed backwards beneath the shell.)
Curved shells brought a number of benefits. Firstly, minerals are not required in as large quantities, as each successive whorl builds on the one before. Also, the organism is more stable (its centre of mass coincides with its centre of buoyancy) and more manoeuvrable.
Early cephalopods were likely predators near the top of the food chain. In the Early Palaeozoic, their range was far more restricted than today; they were mainly constrained to sub-littoral regions of shallow shelves of the low latitudes, and usually occur in association with thrombolites. A more pelagic habit was gradually adopted as the Ordovician progressed. Deep-water cephalopods, whilst rare, have been found in the Lower Ordovician – but only in high-latitude waters.
was a cephalopod. However discoveries of more detailed fossils showed that Volborthella’s small, conical shell was not secreted but built from grains of the mineral silicon dioxide
(silica); neither was it septate. This illusion was a result of the laminated texture of the organisms' tests. Therefore, Volborthella’s classification is now uncertain.
, which was reclassified based on a depressed groove that forms a band around the shell, which is similar to a feature seen in Hypseloconus. The septa in this genus are either closely or irregularly spaced.
The ancestors of coleoids (including most modern cephalopods) and the ancestors of the modern nautilus, had diverged by the Floian Age of the Early Ordovician Period, over 470 million years ago. We know this because the orthocerids were the first known representatives of the neocephalopoda,
were ultimately the ancestors of ammonoids and coleoids, and had appeared by the Floian. It is widely held that the Bactritida
, an Silurian–Triassic group of orthocones, are paraphyletic to the coleoids and ammonoids – that is, the latter groups arose from within the Bactritida. An increase in the diversity of the coleoids and ammonoids is observed around the start of the Devonian period, and corresponds with a profound increase in fish diversity. This could represent the origin of the two derived groups.
Unlike most modern cephalopods, most ancient varieties had protective shells. These shells at first were conical but later developed into curved nautiloid shapes seen in modern nautilus
species.
It is thought that competitive pressure from fish forced the shelled forms into deeper water, which provided an evolutionary pressure towards shell loss and gave rise to the modern coleoids, a change which led to greater metabolic costs associated with the loss of buoyancy, but which allowed them to recolonise shallow waters. However, some of the straight-shelled nautiloids evolved into belemnites, out of which some evolved into squid
and cuttlefish
. The loss of the shell may also have resulted from evolutionary pressure to increase manoeuvrability, resulting in a more fish-like habit. This pressure may have increased as a result of the increased complexity of fish in the late Palaeozoic, increasing the competitive pressure. Internal shells still exist in many non-shelled living cephalopod groups but most truly shelled cephalopods, such as the ammonites, became extinct at the end of the Cretaceous
.
The earliest cephalopods,Ordovician orthocone nautiloids are the first for which trace fossil evidence is available like Nautilus and some coeloids, appeared to be able to propel themselves forwards by directing their jet backwards. Because they had an external shell, they would not have been able to generate their jets by contracting their mantle, so must have used alternate methods: such as by contracting their funnels or moving the head in and out of the chamber.
and Francis Creek shale.
Cephalopod
A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...
s have a long geological history, with the first nautiloid
Nautiloid
Nautiloids are a large and diverse group of marine cephalopods belonging to the subclass Nautiloidea that began in the Late Cambrian and are represented today by the living Nautilus. Nautiloids flourished during the early Paleozoic era, where they constituted the main predatory animals, and...
s found in late Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
strata, and purported stem-group representatives present in the earliest Cambrian lagerstätten.
The class developed during the middle Cambrian, and underwent pulses of diversification during the Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
period to become diverse and dominant in the Paleozoic
Paleozoic
The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...
and Mesozoic
Mesozoic
The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...
seas. Small shelly fossils such as Tommotia
Tommotia
Tommotia is a small shelly fossil from the Early Cambrian Period. Originally, only a cone-shaped shell was recognized, which was originally thought to be an early cephalopod, with either squid-like tentacles or a snail-like foot...
were once interpreted as early cephalopods, but today these tiny fossils are recognized as sclerite
Sclerite
A sclerite is a hardened body part. The term is used in various branches of biology for various structures including hardened portions of sponges, but it is most commonly used for the hardened portions of arthropod exoskeletons....
s of larger animals, and the earliest accepted cephalopods date to the Middle Cambrian Period. During the Cambrian, cephalopods are most common in shallow near-shore environments, but they have been found in deeper waters too. Cephalopods were thought to have "undoubtedly" arisen from within the tryblidiid
Tryblidiida
The Tryblidiida are a group of monoplacophora containing the only extant representatives: a total of 29 species are alive today, inhabiting the ocean at depths of between 175 and 6500 metres .- History of discoveries :...
monoplacophoran clade. However genetic studies suggest that they are more basal, forming a sister group to the Scaphopoda but otherwise basal to all other major mollusc classes. The internal phylogeny of Mollusca, however, is wide open to interpretation – see mollusc phylogeny.
Traditional view of origins
The cephalopods were once thought to have evolved from a monoplacophoran-like ancestor with a curved, tapering shell, and to be closely related to the gastropods (snails). The similarity of the early shelled cephalopod PlectronocerasPlectronoceras
Plectronoceras is the earliest known shelly cephalopod, dating to the Upper Cambrian.Its 14 known specimens hail from the basal Fengshan Formation of the earliest Fengshanian stage. None of the fossils are complete, and none show the tip or opening of the shell. Approximately half of its shell...
to some gastropods was used in support of this view. The development of a siphuncle
Siphuncle
The siphuncle is a strand of tissue passing longitudinally through the shell of a cephalopod mollusk. Only cephalopods with chambered shells have siphuncles, such as the extinct ammonites and belemnites, and the living nautiluses, cuttlefish, and Spirula...
would have allowed the shells of these early forms to become gas-filled (thus buoyant) in order to support them and keep the shells upright while the animal crawled along the floor, and separated the true cephalopods from putative ancestors such as Knightoconus
Knightoconus
Knightoconus antarcticus is a Cambrian monoplacophoran thought to represent an ancestor to the cephalopods. It had a chambered, conical shell, but lacked a siphuncle...
, which lacked a siphuncle. Negative buoyancy (i.e. the ability to float) would have come later, followed by swimming in the Plectronocerida
Plectronocerida
Plectronocerida is a primitive order from which subsequent cephalopod orders are ultimately derived.-Occurrence:Plectronoceratids are known from the Upper Cambrian of China and North America...
and eventually jet propulsion in more derived cephalopods.
However, because chambered shells are found in a range of molluscs – monoplacophora and gastropods as well as cephalopods – a siphuncle is essential to ally a fossil shell conclusively to the cephalopoda. Chambered gastropods can be distinguished from cephalopod shells by the absence of a siphuncle, the irregular spacing of septa, the layering of the shell and (in younger or unmetamorphosed rocks) its microstructure, and the relatively thick width of the shell. The earliest such shells do not have the muscle scars which would be expected if they truly had a monoplacophoran affinity. Additionally, the discovery that Nectocaris pteryx
Nectocaris
Nectocaris pteryx is a species of possible cephalopod or arthropod affinity, known from the Middle Cambrian Burgess Shale.Nectocaris was a free-swimming, predatory or scavenging organism, possibly occupying a niche similar to the arrow worms...
, which did not have a shell and appeared to possess jet propulsion in the manner of "derived" cephalopods, complicated the question of the order in which cephalopod features developed – provided it is in fact a cephalopod and not an arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
.
Early shelly record
Cambrian cephalopods
With the exception of the shelly genera EctenolitesEctenolites
Small, slender, cylindrial members of the Ellesmeroceratidae that resemble small but proportionally narrower Ellesmeroceras. Septa, as typical for ellesmerocerids, are close spaced with shallow lobes on either flank. The body chamber is proportionally long, the shell itself slightly compressed...
and Eoclarkoceras, none of the 30+ Cambrian cephalopod genera are known to have survived into the Ordovician. Cambrian cephalopods differ from their descendants by account of their small size (a few centimetres in length); long, tapering shells; smooth shell surfaces; closely spaced septa; and lack of deposits in their body chamber; several more specific features are also only seen in certain groups of Cambrian cephalopod.
Knightoconus
KnightoconusKnightoconus
Knightoconus antarcticus is a Cambrian monoplacophoran thought to represent an ancestor to the cephalopods. It had a chambered, conical shell, but lacked a siphuncle...
is a Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
monoplacophoran thought to represent an ancestor to the cephalopods. It had a chambered, conical shell, but lacked a siphuncle
Siphuncle
The siphuncle is a strand of tissue passing longitudinally through the shell of a cephalopod mollusk. Only cephalopods with chambered shells have siphuncles, such as the extinct ammonites and belemnites, and the living nautiluses, cuttlefish, and Spirula...
. Although earlier molluscan fossils are also septate, Knightoconus is the latest septate mollusc before the first sipunculate cephalopods – a point that has been taken to prove its relevance to the Cephalopoda. The absence of this siphuncle has been taken as evidence against cephalopod ancestry – how, it is argued, could a siphuncle evolve to penetrate existing septa? The prevailing argument suggests that a strand of tissue remained attached to the previous septum as the mollusc moved forwards and deposited its next septum, producing an obstacle to the complete closure of the septum and becoming mineralised itself. 10 or more septa are found in mature individuals, occupying around a third of the shell – septa form very early and have been found in specimens as small as 2 mm in length. Septa are uniformly spaced, which is inconsistent with a gastropod affinity. Unlike monoplacophoran fossils, there is no evidence of muscle scarring in Knightoconus fossils.
Plectronoceras
PlectronocerasPlectronoceras
Plectronoceras is the earliest known shelly cephalopod, dating to the Upper Cambrian.Its 14 known specimens hail from the basal Fengshan Formation of the earliest Fengshanian stage. None of the fossils are complete, and none show the tip or opening of the shell. Approximately half of its shell...
is arguably the earliest known crown-group cephalopod
Cephalopod
A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...
, dating to the Upper Cambrian.
Its 14 known specimens hail from the basal Fengshan Formation (north-east China) of the earliest Fengshanian stage. None of the fossils are complete, and none show the tip or opening of the shell. Approximately half of its shell was filled with septa; 7 were recorded in a 2 cm shell. Its shell contains transverse septa separated by about half a millimetre, with a siphuncle
Siphuncle
The siphuncle is a strand of tissue passing longitudinally through the shell of a cephalopod mollusk. Only cephalopods with chambered shells have siphuncles, such as the extinct ammonites and belemnites, and the living nautiluses, cuttlefish, and Spirula...
on its concave side. Its morphology matches closely to that hypothesised for the last common ancestor of all cephalopods, and the Plectronocerida
Plectronocerida
Plectronocerida is a primitive order from which subsequent cephalopod orders are ultimately derived.-Occurrence:Plectronoceratids are known from the Upper Cambrian of China and North America...
have been said to be the ancestors of the Ellesmerocerids, the first "true cephalopods".
Yochelcionellids
The Yochelcionellids have given rise to the "snorkel hypothesis". These fossils are aseptate helcionellidHelcionellid
The Helcionellids are fossil shells of very small marine mollusks that are preserved as shells. These animals are first found about in the late Nemakit-Daldynian age, which is the earliest part of the Cambrian period. A single species persisted to the Early Ordovician...
s with a snorkel-like tube on one surface. The snorkel has been seized upon as characteristic of a cephalopod-like water circulatory system, or perhaps as a precursor to the siphuncle. However, neither of these theories have been borne out.
Ellesmerocerida
The earliest true cephalopod order to emerge was the EllesmeroceridaEllesmerocerida
The Ellesmerocerida is a order of primitive cephalopods belonging to the subclass Nautiloidea with a widespread distribution that lived during the Late Cambrian and Ordovician.-Morphology:...
, which were quite small organisms; their shells were slightly curved, and the internal chambers were closely spaced. The siphuncle penetrated the septa with meniscus-like holes. This marks an important difference from the earlier cephalopods, whose siphuncle was positioned at the edge of the septum and against the shell wall. On the basis of muscle scars preserved in such genera as Paradakeoceras
Paradakeoceras
Paradakeoceras is an early Ordovician cephalopod belonging to the nautiloid order Ellesmerocerida.- Morphology :The shell of Paradakeoceras is an expanding endogastric cyrtocone with the siphuncle situated near the shell's concave margin....
and Levisoceras
Levisoceras
Levisoceras is an early Ordovician ellesmerocerid cephalopod.- Morphology :Levisoceras is a distinctive ellesmeroceratid that combines rapid expansion a strong endogastric curvature. The cross section is compressed with the height increasing more rapidly than the width. The siphuncle is ventral and...
, these animals are reconstructed with a straight body and dorsal shell, with the head at the anterior, concave surface of the shell, and the funnel – consisting of a pair of folds in the foot – at the rear — not juxtaposed with the head as in later, oncocerid-like forms.
Early Ordovician diversity
The Ellesmerocerids were the only shelled cephalopods that survived the end-Cambrian extinction, and all subsequent cephalopods, which diversified throughout the Ordovician period, are thus thought to be derived from these forms.Early cephalopods had fine shells that could not cope with the pressures of deep water. In the mid Tremadoc, these were supplemented by larger shells around 20 cm in length; these larger forms included straight and coiled shells, and fall into the orders Endocerida (with wide siphuncles) and Tarphycerida
Tarphycerida
The Tarphycerida were the first of the coiled cephalopods. They are found in marine sediments from the Lower Ordovician to the Middle Devonian. Some like Aphetoceras and Estonioceras are loosely coiled, gyroconic, others like Campbelloceras, Tarphyceras, and Trocholites are tightly coiled, but...
(with narrow siphuncles).
By the mid Ordovician these orders are joined by the Orthocerids, whose first chambers are small and spherical, and Lituitids, whose siphuncles are thin. The Oncocerids also appear during this time; they are restricted to shallow water and have short exogastric conchs. The mid Ordovician saw the first cephalopods with septa strong enough to cope with the pressures associated with deeper water, and could inhabit depths greater than 100–200 m. The wide-siphuncled Actinocerida and the Discocerida both emerged during the Darriwilian. The direction of coiling would prove to be crucial to the future success of the lineages; endogastric coiling would only permit large size to be attained with a straight shell, whereas exogastric coiling – initially rather rare – permitted the spirals familiar from the fossil record to develop, with their corresponding large size and diversity. (Endogastric mean the shell is curved so as the ventral or lower side is longitudinally concave (belly in); exogastric means the shell is curved so as the ventral side is longitudinally convex (belly out) allowing the funnel to be pointed backwards beneath the shell.)
Curved shells brought a number of benefits. Firstly, minerals are not required in as large quantities, as each successive whorl builds on the one before. Also, the organism is more stable (its centre of mass coincides with its centre of buoyancy) and more manoeuvrable.
Early cephalopods were likely predators near the top of the food chain. In the Early Palaeozoic, their range was far more restricted than today; they were mainly constrained to sub-littoral regions of shallow shelves of the low latitudes, and usually occur in association with thrombolites. A more pelagic habit was gradually adopted as the Ordovician progressed. Deep-water cephalopods, whilst rare, have been found in the Lower Ordovician – but only in high-latitude waters.
Fossils mistaken for cephalopods
A number of fossils have historically been considered to represent components of the cephalopods' history, but been reinterpreted on the basis of additional material.Volborthella
When it was discovered in 1888, it was thought that the early Cambrian VolborthellaVolborthella
Volborthella is an animal of incertain classification, whose fossils pre-date . It has been considered for a period a cephalopod. However discoveries of more detailed fossils showed that Volborthella’s small, conical shell was not secreted but built from grains of the mineral silicon dioxide , and...
was a cephalopod. However discoveries of more detailed fossils showed that Volborthella’s small, conical shell was not secreted but built from grains of the mineral silicon dioxide
Silicon dioxide
The chemical compound silicon dioxide, also known as silica , is an oxide of silicon with the chemical formula '. It has been known for its hardness since antiquity...
(silica); neither was it septate. This illusion was a result of the laminated texture of the organisms' tests. Therefore, Volborthella’s classification is now uncertain.
Shelbyoceras
Because the characters differentiating monoplacophora from cephalopods are few, several monoplacophora have been mistaken for cephalopod ancestors. One such genus is ShelbyocerasShelbyoceras
Because the characters differentiating monoplacophora from cephalopods are few, several monoplacophora have been mistaken for cephalopod ancestors. One such genus is Shelbyoceras, which was reclassified based on a depressed groove that forms a band around the shell, which is similar to a feature...
, which was reclassified based on a depressed groove that forms a band around the shell, which is similar to a feature seen in Hypseloconus. The septa in this genus are either closely or irregularly spaced.
Kirengellids
The Kirengellids are a group of shells that, whilst originally aligned to the monoplacophoran ancestry of the cephalopods, have been reinterpreted as brachiopods.Coleoidea
were ultimately the ancestors of ammonoids and coleoids, and had appeared by the Floian. It is widely held that the Bactritida
Bactritida
The Bactritida form a small order of more or less straight-shelled cephalopods that first appeared during the Emsian Stage of the Devonian Period and persisted until the Carnian Stage of the Triassic Period...
, an Silurian–Triassic group of orthocones, are paraphyletic to the coleoids and ammonoids – that is, the latter groups arose from within the Bactritida. An increase in the diversity of the coleoids and ammonoids is observed around the start of the Devonian period, and corresponds with a profound increase in fish diversity. This could represent the origin of the two derived groups.
Unlike most modern cephalopods, most ancient varieties had protective shells. These shells at first were conical but later developed into curved nautiloid shapes seen in modern nautilus
Nautilus
Nautilus is the common name of marine creatures of cephalopod family Nautilidae, the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina. It comprises six living species in two genera, the type of which is the genus Nautilus...
species.
It is thought that competitive pressure from fish forced the shelled forms into deeper water, which provided an evolutionary pressure towards shell loss and gave rise to the modern coleoids, a change which led to greater metabolic costs associated with the loss of buoyancy, but which allowed them to recolonise shallow waters. However, some of the straight-shelled nautiloids evolved into belemnites, out of which some evolved into squid
Squid
Squid are cephalopods of the order Teuthida, which comprises around 300 species. Like all other cephalopods, squid have a distinct head, bilateral symmetry, a mantle, and arms. Squid, like cuttlefish, have eight arms arranged in pairs and two, usually longer, tentacles...
and cuttlefish
Cuttlefish
Cuttlefish are marine animals of the order Sepiida. They belong to the class Cephalopoda . Despite their name, cuttlefish are not fish but molluscs....
. The loss of the shell may also have resulted from evolutionary pressure to increase manoeuvrability, resulting in a more fish-like habit. This pressure may have increased as a result of the increased complexity of fish in the late Palaeozoic, increasing the competitive pressure. Internal shells still exist in many non-shelled living cephalopod groups but most truly shelled cephalopods, such as the ammonites, became extinct at the end of the Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
.
Organ origins
The tentacles of the ancestral cephalopod developed from the mollusc's foot; the ancestral state is thought to have had five pairs of tentacles which surrounded the mouth. Smell-detecting organs evolved very early in the cephalopod lineage.The earliest cephalopods,Ordovician orthocone nautiloids are the first for which trace fossil evidence is available like Nautilus and some coeloids, appeared to be able to propel themselves forwards by directing their jet backwards. Because they had an external shell, they would not have been able to generate their jets by contracting their mantle, so must have used alternate methods: such as by contracting their funnels or moving the head in and out of the chamber.
Exceptional preservation
The preservation of cephalopod soft parts is not entirely unusual; soft-bodied fossils, especially of coeloids (squid), are relatively widespread in the Jurassic, but phosphatized remains are unknown before this period. On the other hand, soft parts – including a possible ink sac — are known from the Paleozoic Hunsrück SlateHunsrück Slate
The Hunsrück Slate is a Lower Devonian lithostratigraphic unit, a type of rock strata, in the German regions of the Hunsrück and Taunus. It is a lagerstätte famous for exceptional preservation of a highly diverse fossil fauna assemblage.-Geology:...
and Francis Creek shale.