Ammonitina
Encyclopedia
The Ammonitina comprises a diverse suborder of ammonoid cephalopods that lived during the Jurassic and Cretaceous periods of the Mesozoic
Era. They are excellent index fossils, and it is often possible to link the rock layer in which they are found to specific geological time periods
.
The shells of Ammonitina are typically planospiral; coiled in a plane, symmetrical side to side. Shells vary in form, including those that are evolute, such that all whorls are exposed, and those that are strongly involute with only the outer whorl showing. They may be strongly ribbed, some bearing nodes and spines; others are entirely smooth. Some have broad rounded venters --the outer rim-- in others the venter is sharp and keel-like. Sutures are generally ammonitic, with intricately patterned saddle and lobes. However in some derived forms the suture becomes simplified,ceratitic, even goniatitic.
The Ammonitina are derived from the Phylloceratina, another ammonitid suborder which has its origin in the Ceratitida
of the Triassic
. As with the subclass, the closest living relatives of the Ammonitina are the Coleoidea
(octopus
, squid
, and cuttlefish
) and not the superficially similar modern Nautilus
Lower Jurassic superfamlies include the Psilocerataceae, Eocerocerataceae, and Hildocerataceae, which is in part Middle Jurassic. Superfamilies from the Middle and Upper Jurassic include the Stephanoceratacea, Perispinctaceae, and Haplocerataceae; the Perispinctaceae and Haploceratacea continued well into the Cretaceous. Exclusively Cretaceous superfamilies include the Desmoceratacea, Hoplitaceae, and the Acanthocerataceae.
The Eoderocerataceae of the Lower Jurassic gave rise in the middle of the epoch to the Hilderceratidae which in turn gave rise early in the Middle Jurassic to the Stephanocerataceae, Perispinctaceae, and Haplocerataceae. The Psiloceratacea from the Lower Jurassic stands alone.
The Cretaceous Desmocerataceae are derived from the Phylloceratina separately from Jurassic forms and give rise to the Hoplitaceae and to the Acanthocerataceae.
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...
Era. They are excellent index fossils, and it is often possible to link the rock layer in which they are found to specific geological time periods
Geologic time scale
The geologic time scale provides a system of chronologic measurement relating stratigraphy to time that is used by geologists, paleontologists and other earth scientists to describe the timing and relationships between events that have occurred during the history of the Earth...
.
The shells of Ammonitina are typically planospiral; coiled in a plane, symmetrical side to side. Shells vary in form, including those that are evolute, such that all whorls are exposed, and those that are strongly involute with only the outer whorl showing. They may be strongly ribbed, some bearing nodes and spines; others are entirely smooth. Some have broad rounded venters --the outer rim-- in others the venter is sharp and keel-like. Sutures are generally ammonitic, with intricately patterned saddle and lobes. However in some derived forms the suture becomes simplified,ceratitic, even goniatitic.
The Ammonitina are derived from the Phylloceratina, another ammonitid suborder which has its origin in the Ceratitida
Ceratitida
The Ceratitida is an order that contains almost all ammonoid cephalopod genera from the Triassic as well as ancestral forms from the Upper Permian, the exception being the phylloceratids which gave rise to the great diversity of post Triassic ammonites....
of the 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...
. As with the subclass, the closest living relatives of the Ammonitina are the Coleoidea
Coleoidea
Subclass Coleoidea, or Dibranchiata, is the grouping of cephalopods containing all the primarily soft-bodied creatures. Unlike its sister group Nautiloidea, whose members have a rigid outer shell for protection, the coleoids have at most an internal bone or shell that is used for buoyancy or support...
(octopus
Octopus
The octopus is a cephalopod mollusc of the order Octopoda. Octopuses have two eyes and four pairs of arms, and like other cephalopods they are bilaterally symmetric. An octopus has a hard beak, with its mouth at the center point of the arms...
, 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....
) and not the superficially similar 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...
Lower Jurassic superfamlies include the Psilocerataceae, Eocerocerataceae, and Hildocerataceae, which is in part Middle Jurassic. Superfamilies from the Middle and Upper Jurassic include the Stephanoceratacea, Perispinctaceae, and Haplocerataceae; the Perispinctaceae and Haploceratacea continued well into the Cretaceous. Exclusively Cretaceous superfamilies include the Desmoceratacea, Hoplitaceae, and the Acanthocerataceae.
The Eoderocerataceae of the Lower Jurassic gave rise in the middle of the epoch to the Hilderceratidae which in turn gave rise early in the Middle Jurassic to the Stephanocerataceae, Perispinctaceae, and Haplocerataceae. The Psiloceratacea from the Lower Jurassic stands alone.
The Cretaceous Desmocerataceae are derived from the Phylloceratina separately from Jurassic forms and give rise to the Hoplitaceae and to the Acanthocerataceae.