''Microderoceras'' is a flat sided, evolute, radially ribbed

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 ...

ammonite belonging to the ammonitid family

Eoderoceratidae Eoderoceratidae is the ancestral and most primitive family of the Eoderoceratoidea; lower Jurassic ammonite cephalopods, characterized by evolute, commonly serpenticonic, shells that had long body chambers and would have had no stable floating ...

and superfamily

Eoderoceratoidea Eoderoceratoidea is a superfamily of true ammonites (suborder Ammonitina) from the Lower Jurassic, comprising seven phylogenetically related families, characterized in general by having ribbed evolute shells that commonly bear spines or tubercle ...

. Its whorls are subquadrangular in section; venter on the outer rim, rounded; sides with two rows if distinct spines, tubercles on internal molds, formed at either end of the ribs. The lateral surface of the ribcage is concave toward both sides, the lateral side of the dorsal and the lateral side of the ventral surface. The ribs are divided into six pairs (1-3) of the following types: 2-3 is the only type. In the first type, the ribs on the ventral face are fused to the ribs on the ventral sides; the ribs on the dorsal face are fused to the ribs on the dorsal sides. The second type represents the ribbed body, the ribs are in the same arrangement as in the first type and the rib cage does not fuse to the dorsal surface. These shelled cephalopods, named by

Alpheus Hyatt Alpheus Hyatt (April 5, 1838 – January 15, 1902) was an American zoologist and palaeontologist. Biography Alpheus Hyatt II was born in Washington, D.C. to Alpheus Hyatt and Harriet Randolph (King) Hyatt. He briefly attended the Maryla ...

in 1871, are part of the now extinct

Ammonoidea Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttle ...

, which are distinct from the shelled

nautiloids Nautiloids are a group of marine cephalopods (Mollusca) which originated in the Late Cambrian and are represented today by the living ''Nautilus'' and ''Allonautilus''. Fossil nautiloids are diverse and speciose, with over 2,500 recorded species. ...

still found living today.

Distribution

Argentina, Canada and the United Kingdom.

References

;Notes ;Bibliography *Arkell ''et al.'', 1957. Mesozoic Ammonoidea, in Treatise on Invertebrate Paleontology, (Part L); Geological Soc. of America and University of Kansas press *Donovan, Callomon and Howarth 1981 Classification of the Jurassic Ammonitina; Systematics Association

Early Jurassic ammonites of North America Early Jurassic ammonites of Europe Early Jurassic ammonites of South America Sinemurian life {{Ammonite-stub