Angelinoceras
Encyclopedia
Angelinoceras is a lituitid
from the Middle Ordovician that starts off with a open spiral of about 1.5 strongly compressed whorls followed by an orthoconic section that continues to expand for a length about equal to the diameter of the coiled portion before retaining a more or less consistent diameter. The juvenile spiral portion has a deep hyponomic sinus that becomes broad and shallow in the orthoconic adult.
Angelinoceras is most likely derived from Holmiceras
by a further opening of the initial spiral and a reduction in the expansion of the orthocone.
Lituitidae
The Lituitidae is a family of evolved tarphycerids characterized by a long orthoconic section that follows a coiled juvenile portion at the apex, along with a generally tubular siphuncle, which like that of the barrandeocerids is composed of thin connecting rings.-Taxonomic Position:Flower and...
from the Middle Ordovician that starts off with a open spiral of about 1.5 strongly compressed whorls followed by an orthoconic section that continues to expand for a length about equal to the diameter of the coiled portion before retaining a more or less consistent diameter. The juvenile spiral portion has a deep hyponomic sinus that becomes broad and shallow in the orthoconic adult.
Angelinoceras is most likely derived from Holmiceras
Holmiceras
Holmiceras is one of two ancestral lituitids from the late Early Ordovician . The other being Ancistroceras.Holmiceras begins with about 1.5 to 2 loosely coiled whorls followed by a rapidly expanding orthocone, much like Ancistroceras except for the apical whorls and that Holmiceras is known only...
by a further opening of the initial spiral and a reduction in the expansion of the orthocone.
References
- Flower, R. H. 1950. A Classification of the Nautiloidia. Jour Paleontology, V.24, N.5, pp 604–616, Sept.
- Furnish & Glenister, 1964. Nautiloidea -Tarphycerida. Treatise on Invertebrate Paleontology Part K, Mollusca 3 ...Nautiloidea