Orthoceras
Encyclopedia
Orthoceras is a genus
of extinct nautiloid
cephalopod
. This genus is sometimes called Orthoceratites. Note it is sometimes misspelled as Orthocera, Orthocerus or Orthoceros .
Orthoceras fossils are common and have a global distribution, occurring in any marine rock, especially limestone
s.
These are slender, elongate shells with the middle of the body chamber
transverse
ly constricted, and a subcentral orthochoanitic siphuncle
. The surface is ornamented by a network of fine lirae . Many other very similar species are included under the genus Michelinoceras
.
These orthocone
cephalopods are conspicuous in the fossil record for their occasional but persistent appearances in monospecific assemblages dense enough to be rock-forming.
Based on studies of size distributions of the orthocone shells, some scientists have concluded that these assemblages likely represent post-mating mass deaths, as are common among modern cephalopods (though not modern nautiloids) and indeed among many semelparous species. However, such studies have not been entirely convincing and do leave the door open for alternate interpretations.
These assemblages, are known mostly from Ordovician
rocks but do occur later as well, at least into the Devonian
. Well-known examples occur in Morocco, Scandinavia, the Alps, and Iowa (USA).
The Baltic island of Öland
off the southern coast of Sweden has many quarries that yield orthocone
nautiloids of great beauty. For centuries Öland has supplied greater Europe with material for floors, stairs and grave stones. This hard limestone is durable and the fossil inclusions make it very desirable. Occasionally the chambers of the fossil shells are colored differently. The ground water minerals that percolated the strata during diagenesis
determines the color. Greens and browns are most common.
Orthoceras grew to a length of 6 inches (15 centimeters) and fed upon small animals.
s with a straight-shell, called an "orthocone
" . But later research on their internal structures, such as the siphuncle
, cameral deposits
, and others, showed that these actually belong to a number of groups, even different orders.
In the authoritative Treatise on Invertebrate Paleontology
, the name Orthoceras is now only used to refer to the type species
O. regulare
from the Middle Ordovician
of Estonia
, Lithuania
, Sweden
and parts of the former Soviet Union
such as Ukraine and Belarus. The genus might include a few related species.
cephalopods
are often confused with the superficially similar Baculites and related Cretaceous
orthoconic ammonoids. Both are long and tubular in form, and both are common items for sale in rock shops (often under each others' names). Both lineages evidently evolved the tubular form independently of one another, and at different times in earth history. Orthoceras lived much earlier (Middle Ordovician
) than Baculites (Late Cretaceous
) The two types of fossils can be distinguished by many features, most obvious among which is the suture line: simple in Orthoceras, intricately foliated in Baculites and related forms.
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
of extinct 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...
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...
. This genus is sometimes called Orthoceratites. Note it is sometimes misspelled as Orthocera, Orthocerus or Orthoceros .
Orthoceras fossils are common and have a global distribution, occurring in any marine rock, especially limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
s.
These are slender, elongate shells with the middle of the body chamber
Body chamber
The body chamber, also called the living chamber, is the outermost or last chamber in the shell of a nautiloid or ammonoid cephalopod. The body of the animal occupies the living chamber, apart from the siphuncle which extends through the rest of septa to provide buoyancy....
transverse
Transverse plane
The transverse plane is an imaginary plane that divides the body into superior and inferior parts. It is perpendicular to the coronal and sagittal planes....
ly constricted, and a subcentral orthochoanitic 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...
. The surface is ornamented by a network of fine lirae . Many other very similar species are included under the genus Michelinoceras
Michelinoceras
Michelinoceras is the oldest known genus of the Michelinocerida, more commonly known as the Orthocerida, characterized by long, slender, nearly cylindrical orthocones with a circular cross section, long camerae, very long body chambers, and a central or near central tubular siphuncle free of...
.
Monospecific assemblages
Orthocone
An orthocone is a usually long straight shell of a nautiloid cephalopod. During the 18th and 19th centuries, all shells of this type were named Orthoceras, but it is now known that many groups of nautiloids developed or retained this type of shell....
cephalopods are conspicuous in the fossil record for their occasional but persistent appearances in monospecific assemblages dense enough to be rock-forming.
Based on studies of size distributions of the orthocone shells, some scientists have concluded that these assemblages likely represent post-mating mass deaths, as are common among modern cephalopods (though not modern nautiloids) and indeed among many semelparous species. However, such studies have not been entirely convincing and do leave the door open for alternate interpretations.
These assemblages, are known mostly from 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...
rocks but do occur later as well, at least into the Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
. Well-known examples occur in Morocco, Scandinavia, the Alps, and Iowa (USA).
The Baltic island of Öland
Öland
' is the second largest Swedish island and the smallest of the traditional provinces of Sweden. Öland has an area of 1,342 km² and is located in the Baltic Sea just off the coast of Småland. The island has 25,000 inhabitants, but during Swedish Midsummer it is visited by up to 500,000 people...
off the southern coast of Sweden has many quarries that yield orthocone
Orthocone
An orthocone is a usually long straight shell of a nautiloid cephalopod. During the 18th and 19th centuries, all shells of this type were named Orthoceras, but it is now known that many groups of nautiloids developed or retained this type of shell....
nautiloids of great beauty. For centuries Öland has supplied greater Europe with material for floors, stairs and grave stones. This hard limestone is durable and the fossil inclusions make it very desirable. Occasionally the chambers of the fossil shells are colored differently. The ground water minerals that percolated the strata during diagenesis
Diagenesis
In geology and oceanography, diagenesis is any chemical, physical, or biological change undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface alteration and metamorphism. These changes happen at relatively low temperatures and pressures...
determines the color. Greens and browns are most common.
Orthoceras grew to a length of 6 inches (15 centimeters) and fed upon small animals.
History of the name
Originally Orthoceras referred to all nautiloidNautiloid
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 with a straight-shell, called an "orthocone
Orthocone
An orthocone is a usually long straight shell of a nautiloid cephalopod. During the 18th and 19th centuries, all shells of this type were named Orthoceras, but it is now known that many groups of nautiloids developed or retained this type of shell....
" . But later research on their internal structures, such as the 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...
, cameral deposits
Camerae
Camerae are the spaces or chambers enclosed between two adjacent septa in the phragmocone of a nautiloid or ammonoid cephalopod. These can be seen in cross-sections of a nautilus shell and in the polished cross-sections of ammonites...
, and others, showed that these actually belong to a number of groups, even different orders.
In the authoritative Treatise on Invertebrate Paleontology
Treatise on Invertebrate Paleontology
The Treatise on Invertebrate Paleontology published by the Geological Society of America and the University of Kansas Press, is a definitive multi-authored work of some 50 volumes, written by more than 300 paleontologists, and covering every phylum, class, order, family, and genus of fossil and...
, the name Orthoceras is now only used to refer to the type species
Type species
In biological nomenclature, a type species is both a concept and a practical system which is used in the classification and nomenclature of animals and plants. The value of a "type species" lies in the fact that it makes clear what is meant by a particular genus name. A type species is the species...
O. regulare
from the Middle 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...
of Estonia
Estonia
Estonia , officially the Republic of Estonia , is a state in the Baltic region of Northern Europe. It is bordered to the north by the Gulf of Finland, to the west by the Baltic Sea, to the south by Latvia , and to the east by Lake Peipsi and the Russian Federation . Across the Baltic Sea lies...
, Lithuania
Lithuania
Lithuania , officially the Republic of Lithuania is a country in Northern Europe, the biggest of the three Baltic states. It is situated along the southeastern shore of the Baltic Sea, whereby to the west lie Sweden and Denmark...
, Sweden
Sweden
Sweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....
and parts of the former Soviet Union
Soviet Union
The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....
such as Ukraine and Belarus. The genus might include a few related species.
Confusion with Baculites
Orthoceras and related orthoconic nautiloidNautiloid
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...
cephalopods
are often confused with the superficially similar Baculites and related 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...
orthoconic ammonoids. Both are long and tubular in form, and both are common items for sale in rock shops (often under each others' names). Both lineages evidently evolved the tubular form independently of one another, and at different times in earth history. Orthoceras lived much earlier (Middle 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...
) than Baculites (Late 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...
) The two types of fossils can be distinguished by many features, most obvious among which is the suture line: simple in Orthoceras, intricately foliated in Baculites and related forms.
See also
- Baculites - another type of extinct, straight-shelled cephalopod.
- HamitesHamitesHamites is a genus of heteromorph ammonite that evolved late in the Aptian stage of the Early Cretaceous and lasted into the Cenomanian stage of the Late Cretaceous. The genus is almost certainly paraphyletic but remains in wide use as a "catch all" for heteromorph ammonites of the superfamily...
- LituitesLituitesLituites is an extinct nautiloid genus from the Middle Ordovician and type for the Lituitidae, a tarphycerid family that in some more recent taxonomies has been classified with the orthocerids and listed under the order Lituitida...
- Belemnite
