Rioceras
Encyclopedia
Rioceras is an extinct orthocerid
cephalapod genus of the family Baltoceratidae
that lived in what would be North America during the Early Ordovician
from 480—472 mya, existing for approximately 8 million years.
by Flower (1964) and Hook and Flower (1977), but reassigned to the Ellesmeroceratidae by Kröger et al (2007)
Whether Rioceras is assigned to the Baltoceratidae and therefor to the Orthocerida or to the Ellesmeroceratidae depends on the definition of each. Flower placed Rioceras in the Baltoceratidae when he proposed the genus since it lacks diaphragms in the siphuncle which for him was a defining character of the family. Kröger removed Rioceras to the Ellesmeroceratidae on the basis of its inwardly thicked connecting rings which produce concave segments, a defining character of that family as emended. Flower did point out in his original diagnosis that Rioceras has moderately thick connecting rings "of ellesmeroceratoid aspect"
") or gently curved. In life, these animals may have been similar to the modern squid
, except for the long shell.
Orthocerida
Orthocerida is an order of extinct nautiloid cephalopods also known as the Michelinocerda that lived from the Early Ordovician possibly to the Late Triassic . A fossil found in the Caucasus suggests they may even have survived until the Early Cretaceous...
cephalapod genus of the family Baltoceratidae
Baltoceratidae
Baltoceratidae is an extinct family of orthoconic cephalopods belonging to the subclass Nautiloidea endemic to what would be Asia, Australia, Europe, North America, and South America during the Ordovician living from about 480–460 mya, existing for approximately .-Taxonomy:Baltoceratidae was...
that lived in what would be North America during the Early Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
from 480—472 mya, existing for approximately 8 million years.
Taxonomy
Rioceras was named by Flower (1964). Its type is Rioceras nondescriptum. It was assigned to BaltoceratidaeBaltoceratidae
Baltoceratidae is an extinct family of orthoconic cephalopods belonging to the subclass Nautiloidea endemic to what would be Asia, Australia, Europe, North America, and South America during the Ordovician living from about 480–460 mya, existing for approximately .-Taxonomy:Baltoceratidae was...
by Flower (1964) and Hook and Flower (1977), but reassigned to the Ellesmeroceratidae by Kröger et al (2007)
Whether Rioceras is assigned to the Baltoceratidae and therefor to the Orthocerida or to the Ellesmeroceratidae depends on the definition of each. Flower placed Rioceras in the Baltoceratidae when he proposed the genus since it lacks diaphragms in the siphuncle which for him was a defining character of the family. Kröger removed Rioceras to the Ellesmeroceratidae on the basis of its inwardly thicked connecting rings which produce concave segments, a defining character of that family as emended. Flower did point out in his original diagnosis that Rioceras has moderately thick connecting rings "of ellesmeroceratoid aspect"
Morphology
The shell is usually long, and may be straight ("orthoconicOrthocone
An orthocone is a usually long straight shell of a nautiloid cephalopod. During the 18th and 19th centuries, all shells of this type were named Orthoceras, but it is now known that many groups of nautiloids developed or retained this type of shell....
") or gently curved. In life, these animals may have been similar to the modern 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...
, except for the long shell.