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. D. (Dactylioceras) Hyatt, 1867: type species: Ammonites communis Sowerby, 1815 D. (Orthodactylites) Buckman, 1926: type species: Dactylioceras directus Buckman, 1926 D. (Iranodactylites) Repin, 2000: type species: Dactylioceras (Iranodactylites) ketevanae Repin, 2000 D. (Eodactylites) Schmidt-Effing, 1972: type species: Dactylioceras pseudocommune Fucini, 1935. This subgenus is not recognized by some authors, while some other prefer to maintain it. Species Species within the genus Dactylioceras include: D. (O.) aequistriatum Zieten, 1830 D. (D.) alpestre Wiedenmayer, 1980 D. (D.) amplum Dagis, 1968 D. (O.) andaluciensis Jiménes & Rivas, 1991 D. (O.) anguiforme Buckman, 1928 D. (O.) anguinum Reinecke, 1818 D. (D.) annuliferum Simpson, 1855 D. (D.) arcus Buckman, 1926 D. (D.) athleticum Simpson, 1855 D. (D.) attenuatum Simpson, 1855 D. (O.) chilense Hillebrandt and Schmidt-Effing, 1981 D. (O.) clevelandicum Howarth, 1973 D. (D.) commune Sowerby, 1815 D. (D.) comptum Dagis, 1968 D. (D.) consimile Buckman, 1926 D. (D.) crassescens Simpson, 1855 D. (O.) crassifactum Simpson, 1855 D. (O.) crassiusculosum Buckman, 1912 D. (D.) crassiusculum Simpson, 1855 D. (D.) crassulum Buckman,