Belemnitida
Encyclopedia
Belemnitida is an extinct order
of cephalopod
s which existed during the Mesozoic era, from the Hettangian
age of the Lower Jurassic
to the Maastrichtian
age of the Upper Cretaceous
.
-like. They possessed ten arms of equal length studded with small inward-curving hooks used for grasping prey. However, they lacked the pair of specialized tentacle
s present in modern squid.
Belemnites (and other belemnoids
) were distinct from modern squids by possessing hard internal skeletons. The internal skeleton was composed of the guard or rostrum (plural: rostra), a heavy solid structure at the posterior of the animals. They were composed of calcite
or aragonite
. The rostrum is usually bullet-shaped and projects prominently backward, but in the suborder Belemnotheutina, it is only present as a thin layer. The rostrum is oftentimes the only remains of the animals preserved (often in very large numbers in a given area).
The rostrum is in turn attached to a chambered conical shell known as the phragmocone. At the tip of the phragmocone beneath the rostrum is a tiny spherical or cuplike nodule known as the protoconch, the remains of the embryo
nic shell. The space between the phragmocone and the rostrum is known as the alveolus (plural: alveoli). At the forward part of the phragmocone is a thin very fragile structure known as the proostracum (plural: proostraca). It is usually spoon-like in shape. It extends over the dorsal part of the mantle
.
Fossils which preserve the soft parts of belemnites indicate that like modern coleoids, they possessed an ink sac
, hard beaks, tail fins that were either apical or lateral, and large eyes. Well preserved specimens have even retained evidence of strong muscular fibers in the mantle, indicating that they were powerful swimmers like modern squids.
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
of 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 which existed during the Mesozoic era, from the Hettangian
Hettangian
The Hettangian is the earliest age or lowest stage of the Jurassic period of the geologic timescale. It spans the time between 199.6 ± 0.6 Ma and 196.5 ± 1 Ma . The Hettangian follows the Rhaetian and is followed by the Sinemurian.In Europe stratigraphy the Hettangian is a part of the time span in...
age of the Lower Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
to the Maastrichtian
Maastrichtian
The Maastrichtian is, in the ICS' geologic timescale, the latest age or upper stage of the Late Cretaceous epoch or Upper Cretaceous series, the Cretaceous period or system, and of the Mesozoic era or erathem. It spanned from 70.6 ± 0.6 Ma to 65.5 ± 0.3 Ma...
age of the Upper 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...
.
Description
Belemnites were superficially squidSquid
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...
-like. They possessed ten arms of equal length studded with small inward-curving hooks used for grasping prey. However, they lacked the pair of specialized tentacle
Tentacle
A tentacle or bothrium is one of usually two or more elongated flexible organs present in animals, especially invertebrates. The term may also refer to the hairs of the leaves of some insectivorous plants. Usually, tentacles are used for feeding, feeling and grasping. Anatomically, they work like...
s present in modern squid.
Belemnites (and other belemnoids
Belemnoidea
Belemnoids are an extinct group of marine cephalopod, very similar in many ways to the modern squid and closely related to the modern cuttlefish. Like them, the belemnoids possessed an ink sac, but, unlike the squid, they possessed ten arms of roughly equal length, and no tentacles...
) were distinct from modern squids by possessing hard internal skeletons. The internal skeleton was composed of the guard or rostrum (plural: rostra), a heavy solid structure at the posterior of the animals. They were composed of calcite
Calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
or aragonite
Aragonite
Aragonite is a carbonate mineral, one of the two common, naturally occurring, crystal forms of calcium carbonate, CaCO3...
. The rostrum is usually bullet-shaped and projects prominently backward, but in the suborder Belemnotheutina, it is only present as a thin layer. The rostrum is oftentimes the only remains of the animals preserved (often in very large numbers in a given area).
The rostrum is in turn attached to a chambered conical shell known as the phragmocone. At the tip of the phragmocone beneath the rostrum is a tiny spherical or cuplike nodule known as the protoconch, the remains of the embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
nic shell. The space between the phragmocone and the rostrum is known as the alveolus (plural: alveoli). At the forward part of the phragmocone is a thin very fragile structure known as the proostracum (plural: proostraca). It is usually spoon-like in shape. It extends over the dorsal part of the mantle
Mantle (mollusc)
The mantle is a significant part of the anatomy of molluscs: it is the dorsal body wall which covers the visceral mass and usually protrudes in the form of flaps well beyond the visceral mass itself.In many, but by no means all, species of molluscs, the epidermis of the mantle secretes...
.
Fossils which preserve the soft parts of belemnites indicate that like modern coleoids, they possessed an ink sac
Ink sac
With the exception of nocturnal and very deep water cephalopods, all coeloids which dwell in light conditions have an ink sac, which can be used to expel a cloud of dark ink to confuse predators. This sac is a muscular bag which originated as an extension of the hind gut...
, hard beaks, tail fins that were either apical or lateral, and large eyes. Well preserved specimens have even retained evidence of strong muscular fibers in the mantle, indicating that they were powerful swimmers like modern squids.