Subcollina
Encyclopedia
Subcollina is an evolute to serpenticonic stephanoceratacean
ammonite
from the Middle Jurassic (Baj
) of Mexico, placed in the family Spiroceratidae.
The whorl section of Subcollina is subquadrate, slightly depressed in the inner whorls, slightly compressed at the mature aperture. The inner whorls are covered with fine, dense, primary ribs that become wide-spread and thicker on the outer whorls, terminating in ventrolateral tubercles from which two or sometimes three secondary ribs extend that cross the venter in a regular zig-zag pattern.
Subcollina is probably derived from the Stephanoceratidae
. Some stephanoceratids also have the zig-zag pattern of Subcollina. Related genera include Parastrenoceras
, Strenoceras, and Spiroceras.
Note that the described zig-zag pattern in the secondary ribs as they cross the venter may be hard to make out in fig 32 of Sandoval and Westermann (1986)
Stephanocerataceae
Stephanocerataceae is a superfamily of middle Jurassic ammonoid cephalopods within the order Ammonitida containing diverse forms, generally with sharp ribbing and complex suture lines...
ammonite
Ammonitina
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...
from the Middle Jurassic (Baj
Bajocian
In the geologic timescale, the Bajocian is an age or stage in the Middle Jurassic. It lasted from approximately 171.6 Ma to around 167.7 Ma . The Bajocian age succeeds the Aalenian age and precedes the Bathonian age....
) of Mexico, placed in the family Spiroceratidae.
The whorl section of Subcollina is subquadrate, slightly depressed in the inner whorls, slightly compressed at the mature aperture. The inner whorls are covered with fine, dense, primary ribs that become wide-spread and thicker on the outer whorls, terminating in ventrolateral tubercles from which two or sometimes three secondary ribs extend that cross the venter in a regular zig-zag pattern.
Subcollina is probably derived from the Stephanoceratidae
Stephanoceratidae
Sephanoceratidae is a family of planulate and coronate ammonites within the Stephanocerataceae. Shells are evolute so that all whorls are exposed and have strong ribbing that bifurcates, that is splits in two, on the flanks. Many have tubercles at the point of bifurcation...
. Some stephanoceratids also have the zig-zag pattern of Subcollina. Related genera include Parastrenoceras
Parastrenoceras
Parastrenoceras is an extinct genus from the ammonoid cephalopod order Ammonitida, included in the family Spiroceratidae that lived during the Bajocian stage of the early Middle Jurassic....
, Strenoceras, and Spiroceras.
Note that the described zig-zag pattern in the secondary ribs as they cross the venter may be hard to make out in fig 32 of Sandoval and Westermann (1986)