Dactylioceras

Dactylioceras was a widespread genus of ammonites from the Lower Jurassic period, approximately 180 million years ago (mya).
Etymology.
The name "Dactylioceras" comes from the Greek "dactyl", meaning “finger”, and refers to the shell's branching ribs.
Description.
"Dactylioceras" are generally small, averaging in diameter. They have a strong, ribbed shell. The ribs are slightly inclined forward, running over the outer edge, and either simple or forking at outer end. Though they eventually died out 180 mya, their style of ribbing was copied by numerous subsequent ammonite genera until the whole group became extinct 66 million years ago with the dinosaurs.
Ecology.
"Dactylioceras" probably lived by scavenging on the sea floor. Mass mortality specimens of "Dactylioceras" are common, and perhaps suggest that these ammonites may often have died shortly after spawning. The dead shells were probably gently washed up into a shell bank on the margins of the Lower Jurassic seas. Flow tank experiments show that "Dactylioceras" was probably a slow swimmer.
Distribution.
"Dactylioceras" has been collected from almost every continent, and was one of the most successful ammonite lineages ever. They are abundant throughout Europe, with exceptionally fine specimens found in England and Germany. Like many other ammonites, the genus "Dactylioceras" is extremely important in biostratigraphy, being a key index fossil for identifying their region of the Jurassic.
Subgenera.
Currently, 3 or 4 subgenera are considered to be valid. Number differs on authors opinions.
Species.
Species within the genus "Dactylioceras" include:
"Dactylioceras commune" has shells reaching a diameter of . Usually the average diameter reach about , while the average width is .
In these small but strong shells ribs run straight or are slightly convex across the venter. These ribs are quite coarse on the outer whorls and finer on the inner whorls. The whorl section is as round as a circle.
Fossils of this species have been found in Lower Jurassic, Toarcian age of Canada, France, Italy, Russia, Serbia and Montenegro, United Kingdom and United States.