''Grammoceras'' is an

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

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 ...

found in Yorks Ravenscar, England, from

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 sediments. Its overall distribution is fairly worldwide. ''Grammoceras'' has a thin, discoidally evolute shell with simple, gently sigmoid ribbing and a low ventral keel. '' Dumortiera'' and '' Catulloceras'' from the same age are similar except that ''Dumortiera'' has a more rounded venter and ''Catulloceras'' has a subquadrate whorl section. ''

Asthenoceras ''Asthenoceras'' is a genus of ammonoid from the Middle Jurassic (Lower Bajocian) with dwarfish, evolute, smooth, compressed, discoidal, strongly keeled shell. ''Asthenoceras'' belongs to the Sonniniidae and may be a subgenus of the Lower Jurassi ...

'' from the slightly younger lower

Bajocian In the geologic timescale, the Bajocian is an age and stage in the Middle Jurassic. It lasted from approximately 170.9 ±0.8 Ma to around 168.2 ±1.2 Ma (million years ago). The Bajocian Age succeeds the Aalenian Age and precedes the Bathonia ...

may be a subgenus.

References

* Ammonitida Ammonites of Europe Jurassic ammonites Toarcian life Ammonite genera {{ammonite-stub