Oxynoticeratidae
Encyclopedia
Oxynoticeratidae is a family of true ammonites (order Ammonitida
) from the Lower Jurassic
. They are part of the superfamily Psilocerataceae
, characterized by mostly involute, oxyconic shells with narrow venter and compressed, lanceolate whorl section. Suture is ammonitic. Ribbing is feeble, hardly functional and often absent.
The Oxynoticeratidae evolved rapidly. They developed shorter body chambers than in the ancestral Arietitidae
, resulting in more stable floating positions. Their mostly smooth, knife-edge discoidal forms would have allowed for quick, though probably no sustained, movement through the water.
Oxynoticeratids have a broad, worldwide distribution but a narrow stratigraphic one, being known only from the Upper Sinemurian
and Lower Pliensbachian
, a span of some 4 or 5 million years during which their shells changed little in form.
Ammonitida
The Ammonitida is an order of more highly evolved ammonoid cephalopods from the Jurassic and Cretaceous time periods, commonly with intricate ammonitic sutures....
) from 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...
. They are part of the superfamily Psilocerataceae
Psilocerataceae
The Psilocerataceae is a superfamily of Early Jurassic ammonoid cephalopods proposed by Alpheus Hyatt in 1867, assigned to the order Ammonitida....
, characterized by mostly involute, oxyconic shells with narrow venter and compressed, lanceolate whorl section. Suture is ammonitic. Ribbing is feeble, hardly functional and often absent.
The Oxynoticeratidae evolved rapidly. They developed shorter body chambers than in the ancestral Arietitidae
Arietitidae
Arietitidae is a family of true ammonites that make up part of the superfamily Psilocerataceae. They comprise medium-size to large or gigantic genera which in general are strongly ribbed, tuberculate in some, with keeled or grooved and keeled venters, and well differentiated ammonitic sutures...
, resulting in more stable floating positions. Their mostly smooth, knife-edge discoidal forms would have allowed for quick, though probably no sustained, movement through the water.
Oxynoticeratids have a broad, worldwide distribution but a narrow stratigraphic one, being known only from the Upper Sinemurian
Sinemurian
In the geologic timescale, the Sinemurian is an age or stage in the Early or Lower Jurassic epoch or series. It spans the time between 196.5 ± 2 Ma and 189.6 ± 1.5 Ma...
and Lower Pliensbachian
Pliensbachian
The Pliensbachian is an age of the geologic timescale or stage in the stratigraphic column. It is part of the Early or Lower Jurassic epoch or series and spans the time between 189.6 ± 1.5 Ma and 183 ± 1.5 Ma . The Pliensbachian is preceded by the Sinemurian and followed by the Toarcian.The...
, a span of some 4 or 5 million years during which their shells changed little in form.