Orthocerida
Encyclopedia
Orthocerida is an order of extinct nautiloid
cephalopod
s also known as the Michelinocerda that lived from the Early Ordovician
possibly to the Late Triassic
. A fossil found in the Caucasus
suggests they may even have survived until the Early Cretaceous
. They were most common however from the Ordovician to the Devonian
.
") or gently curved. In life, these animals may have been similar to the modern squid
, except for the long shell. The internal structure of the shell consists of concavo-convex chambers
linked by a centrally-placed tube called a siphuncle
. There is a tendency for the chambers to develop cameral deposits
, which were used as ballast to balance the long gas-filled shell. Depending on the family
, the siphuncle has orthochoanitic or cyrtochoanitic septal necks. The shell surface may be (depending on the species or genus) smooth, transversely ribbed, or ornamented by a network of fine lirae. Fossil
s are common and have been found on many continents, including the Americas
, Africa
, Europe
, and Asia
.
shell resting horizontally in the water, although some workers have suggested they floated more passively among the plankton or rested on the sea floor. Like modern cephalopods they would have used jet-propulsion for locomotion. However, the long bulky shell and relatively weak muscle attachments make it unlikely that they were as agile as ammonoids or modern cephalopods. They most likely fed on trilobite
s and small arthropods.
is based on characters found in the shell, principally in the nature of the siphuncle. It is not without its problems however since features can appear repeatedly by the process of homeomorphy (convergent evolution
), making certain taxa appear to have a much longer stratigraphic range than in actuality.
A recent study of very well-preserved embryonic shells
of the family Pseudorthoceratidae
from the Mississippian (Lower Carboniferous) Imo Formation of Arkansas
(Kröger and Mapes 2004) reveals that the morphologic diversity of the early growth stages of these creatures is more diverse than would be expected. Although this indicates that the Pseudorthoceratidae may be in need of revision, it also shows the value of embryonic shell morphology
in understanding orthocerid phylogeny.
.
However, several workers have disputed the monophyly of the Orthocerida as traditionally defined, and some have criticized it as a "nightmare for taxonomists" lacking clearly defined characters. Orthocerida as usually understood may thus be a polyphyletic group, having arisen as several lineages from early Ordovician
cephalopods. Some workers have split off the Pseudorthocerida
and Dissidocerida
as separate orders, the latter on the grounds that it arose from a different baltoceratid ancestor. The pseudorthocerids are thought to be distinct because their protoconch
and septal necks are so different.
Orthocerids flourished in the Paleozoic
Era, giving rise to such intriuging forms as the ascocerids
and Lituites
. Moreover, the spherical protoconch
, or first chamber
, of some orthocerids suggests they were ancestors to the Bactritida
, small orthocones that spawned both the ammonoids and coleoids. Fossilized radula
s from orthocerids also suggest a closer affinity with modern coleoids than with Nautilus. This implies that some orthocerids, such as the Lamellorthoceratidae, Arionoceratidae, Michelinoceratidae, and Sphaerorthoceras, belong in the same "Neocephalopoda
" clade as coleoids and ammonoids.
There is some dispute when the orthocerids finally became extinct. Although they are often said to have survived into the Triassic Period, the two genera that date from that period may actually be pseudorthocerids. In that case, the last orthocerids may date only to the Permian. However, the discovery in the Caucasus of a possible orthocerid from the Early Cretaceous
Period suggests that they may have endured much longer as a ghost lineage
.
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...
s also known as the Michelinocerda that lived from the Early 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...
possibly to the Late Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
. A fossil found in the Caucasus
Caucasus
The Caucasus, also Caucas or Caucasia , is a geopolitical region at the border of Europe and Asia, and situated between the Black and the Caspian sea...
suggests they may even have survived until the Early 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...
. They were most common however from the Ordovician to 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...
.
Shell form
The shell is usually long, and may be straight ("orthoconicOrthocone
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....
") or gently curved. In life, these animals may have been similar to the modern 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...
, except for the long shell. The internal structure of the shell consists of concavo-convex chambers
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...
linked by a centrally-placed tube called 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...
. There is a tendency for the chambers to develop 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...
, which were used as ballast to balance the long gas-filled shell. Depending on the family
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
, the siphuncle has orthochoanitic or cyrtochoanitic septal necks. The shell surface may be (depending on the species or genus) smooth, transversely ribbed, or ornamented by a network of fine lirae. Fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
s are common and have been found on many continents, including the Americas
Americas
The Americas, or America , are lands in the Western hemisphere, also known as the New World. In English, the plural form the Americas is often used to refer to the landmasses of North America and South America with their associated islands and regions, while the singular form America is primarily...
, Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
, Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
, and Asia
Asia
Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...
.
Ecology
Orthocerids may have swum near the sea bed with their buoyantBuoyancy
In physics, buoyancy is a force exerted by a fluid that opposes an object's weight. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus a column of fluid, or an object submerged in the fluid, experiences greater pressure at the bottom of the...
shell resting horizontally in the water, although some workers have suggested they floated more passively among the plankton or rested on the sea floor. Like modern cephalopods they would have used jet-propulsion for locomotion. However, the long bulky shell and relatively weak muscle attachments make it unlikely that they were as agile as ammonoids or modern cephalopods. They most likely fed on trilobite
Trilobite
Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period , and they flourished throughout the lower Paleozoic era before...
s and small arthropods.
Taxonomy
Orthocerid taxonomyTaxonomy
Taxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...
is based on characters found in the shell, principally in the nature of the siphuncle. It is not without its problems however since features can appear repeatedly by the process of homeomorphy (convergent evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
), making certain taxa appear to have a much longer stratigraphic range than in actuality.
A recent study of very well-preserved embryonic shells
Protoconch
A protoconch is an embryonic or larval shell of some classes of molluscs, e.g., the initial chamber of an ammonite or the larval shell of a gastropod...
of the family Pseudorthoceratidae
Pseudorthoceratidae
Pseudorthoceratidae is an extinct family of actively mobile aquatic carnivorous cephalapods belonging to the subclass Orthoceratoidea endemic to what would be North America, Asia, and Europe during the Silurian living from 460.5—251 Ma, existing for approximately .In life, these animals may have...
from the Mississippian (Lower Carboniferous) Imo Formation of Arkansas
Arkansas
Arkansas is a state located in the southern region of the United States. Its name is an Algonquian name of the Quapaw Indians. Arkansas shares borders with six states , and its eastern border is largely defined by the Mississippi River...
(Kröger and Mapes 2004) reveals that the morphologic diversity of the early growth stages of these creatures is more diverse than would be expected. Although this indicates that the Pseudorthoceratidae may be in need of revision, it also shows the value of embryonic shell morphology
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
in understanding orthocerid phylogeny.
Evolutionary history
The orthocerids probably arose from the Baltoceratidae, a family of 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:...
.
However, several workers have disputed the monophyly of the Orthocerida as traditionally defined, and some have criticized it as a "nightmare for taxonomists" lacking clearly defined characters. Orthocerida as usually understood may thus be a polyphyletic group, having arisen as several lineages from early 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...
cephalopods. Some workers have split off the Pseudorthocerida
Pseudorthocerida
Pseudorthocerida are generally straight longiconic nautiloids with a subcentral to marginal cyrtochoanitic siphuncle composed of variably expanded segments which may contain internal deposits that may develop into a continuous parietal lining.. Cameral deposits are common and concentrated ventrally...
and Dissidocerida
Dissidocerida
The Dissidocerida comprise an order of Early Ordovician to the Early Silurian orthoceratoid cephalopods in which the siphuncle has a continuous lining or a longitudinal rod-like structure within....
as separate orders, the latter on the grounds that it arose from a different baltoceratid ancestor. The pseudorthocerids are thought to be distinct because their protoconch
Protoconch
A protoconch is an embryonic or larval shell of some classes of molluscs, e.g., the initial chamber of an ammonite or the larval shell of a gastropod...
and septal necks are so different.
Orthocerids flourished in the Paleozoic
Paleozoic
The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...
Era, giving rise to such intriuging forms as the ascocerids
Ascocerida
The Ascocerida are comparatively small, bizarre nautiloids known only from Ordovician and Silurian sediments in Europe and North America, uniquely characterized by a deciduous conch consisting of a longiconic juvenile portion and an inflated breviconic adult portion that separate sometime in...
and Lituites
Lituites
Lituites 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...
. Moreover, the spherical protoconch
Protoconch
A protoconch is an embryonic or larval shell of some classes of molluscs, e.g., the initial chamber of an ammonite or the larval shell of a gastropod...
, or first chamber
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...
, of some orthocerids suggests they were ancestors to 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...
, small orthocones that spawned both the ammonoids and coleoids. Fossilized radula
Radula
The radula is an anatomical structure that is used by molluscs for feeding, sometimes compared rather inaccurately to a tongue. It is a minutely toothed, chitinous ribbon, which is typically used for scraping or cutting food before the food enters the esophagus...
s from orthocerids also suggest a closer affinity with modern coleoids than with Nautilus. This implies that some orthocerids, such as the Lamellorthoceratidae, Arionoceratidae, Michelinoceratidae, and Sphaerorthoceras, belong in the same "Neocephalopoda
Neocephalopoda
Neocephalopods are a group of cephalopod mollusks that include the coleoids and all extinct species that are more closely related to extant coleoids than to the nautilus. In cladistic terms, it is the total group of Coleoidea...
" clade as coleoids and ammonoids.
There is some dispute when the orthocerids finally became extinct. Although they are often said to have survived into the Triassic Period, the two genera that date from that period may actually be pseudorthocerids. In that case, the last orthocerids may date only to the Permian. However, the discovery in the Caucasus of a possible orthocerid from the Early Cretaceous
Early Cretaceous
The Early Cretaceous or the Lower Cretaceous , is the earlier or lower of the two major divisions of the Cretaceous...
Period suggests that they may have endured much longer as a ghost lineage
Ghost lineage
A ghost lineage is a phylogenetic lineage that is inferred to exist but has no fossil record....
.