''Harpoceras'' is an extinct genus of

ammonite Ammonoids are extinct, (typically) coiled-shelled cephalopods comprising the subclass Ammonoidea. They are more closely related to living octopuses, squid, and cuttlefish (which comprise the clade Coleoidea) than they are to nautiluses (family N ...

belonging to the family

Hildoceratidae Hildoceratidae is a family of Ammonoidea, ammonoid cephalopods from the Lower Jurassic, lower Pliensbachian (''Jamesoni'' zone) to lower Bajocian (maybe even upper Bajocian) substages, generally with strongly ribbed, involute shells. They are com ...

. These

cephalopod A cephalopod is any member of the molluscan Taxonomic rank, class Cephalopoda (Greek language, Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral symm ...

s existed in the

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...

period, during the

Toarcian The Toarcian is, in the International Commission on Stratigraphy, ICS' geologic timescale, an age (geology), age and stage (stratigraphy), stage in the Early Jurassic, Early or Lower Jurassic. It spans the time between 184.2 Megaannum, Ma (million ...

age from the Falciferum zone to the Commune subzone of the Bifrons zone.Sepkoski, Jac
Sepkoski's Online Genus Database – Cephalopoda
/ref> They were fast-moving

nekton Nekton or necton (from the ) is any aquatic organism that can actively and persistently propel itself through a water column (i.e. swimming) without touching the bottom. Nektons generally have powerful tails and appendages (e.g. fins, pleopods, ...

ic carnivores.Paleobiology Database - Harpoceras
2017-10-16. Harpoceras NT

Description

Shells of ''Harpoceras'' species show strong dimorphism in their size. While microconchs reach 24–51 mm in diameter, macroconchs shells width is 115–430 mm. They are moderately evolute to involute and compressed. Whorl sides are flat and there is strong keel. Ribs are falcoid or falcate and thus biconcave, strong and projected. Sometimes, ribs can be broad and flat topped on outer part of whorl and in some species they can be striate on inner part of whorl. Some species have midlateral groove, or series of undulating depressions on inner half of whorl.

Distribution

Fossils of species within this genus have been found in the

Lower Jurassic The Early Jurassic Epoch (in chronostratigraphy corresponding to the Lower Jurassic Series) is the earliest of three epochs of the Jurassic Period. The Early Jurassic starts immediately after the Triassic–Jurassic extinction event, 201.3 Ma&nb ...

rocks of Europe, Northern Africa, Russia, Japan, Borneo, New Zealand, Indonesia, North and South America (Argentina; El Cholo and

Los Molles Formation The Los Molles Formation is a geologic formation of Early to Middle Jurassic age, located at northern and central part of Neuquén Basin at Mendoza Shelf in Argentina Argentina, officially the Argentine Republic, is a country in the s ...

s). Two species, ''Harpoceras serpentinum'' and ''Harpoceras falciferum'', are

index fossil Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them.Hine, Robert. "Biostratigraphy." ''Oxford Reference: Dictionary of Biology ...

s used for stratigraphic correlation and dating of rocks of the

stage of the Lower Jurassic.

References

Hildoceratidae Ammonitida genera Toarcian life Early Jurassic ammonites of Europe Jurassic Germany Fossils of Germany Posidonia Shale Early Jurassic ammonites of North America Early Jurassic ammonites of South America Jurassic Argentina Fossils of Argentina Neuquén Basin Early Jurassic ammonites of Asia Early Jurassic ammonites of Africa Fossil taxa described in 1869 {{Ammonitina-stub