Eoderoceratidae is the ancestral and most primitive family of the

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 tuberc ...

; lower Jurassic

ammonite 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 ...

cephalopods, characterized by evolute, commonly serpenticonic, shells that had long body chambers and would have had no stable floating position; and thus resemble contemporary

Psiloceratoidea Psiloceratoidea is a superfamily of Early Jurassic ammonoid cephalopods proposed by Hyatt in 1867, assigned to the order Ammonitida. They were very successful during Hettangian and Sinemurian. Last of them, family Cymbitidae and genera '' Hypox ...

. Spines, or tubercles, are typically found in two rows on the inner and outer parts of the whorl sides, joined by radial ribs. These are often more developed on the inner and middle whorls, becoming less so or absent on the outer. Sutures are highly complex. The Eoderoceratidae can be divided into two subfamilies, the Xiphoceratinae which is the earliest and in which there is an early maximum development of spines of the inner whorls, and the Eoderoceratinae. Two other subfamilies were included in the Treatise 1957 but are now regarded as families in their own right, These are the

Phricodoceratidae Phricodoceratidae is a family in the Eoderoceratoidea, aberrant ammonites from the Lower Jurassic characterized by a large adult size and a marked change of shell form and ornament with growth. Shells are stoutly ribbed, early growth stage is rou ...

and

Coeloceratidae Coeloceratidae is a family of ammonites belonging to the Eoderoceratoidea that lived during the Early Jurassic. Shells are evolute, tending to be broadly discoidal with depressed whorls bearing primary and secondary ribs that branch from outer la ...

Genera that have been attributed to the Xiphoceratinae are ''

Bifericeras ''Bifericeras'' is a Lower Jurassic ammonite belonging to the family Eoderoceratidae, and sometimes placed in the subfamily Xipherceratinae. Whorls are strongly depressed, but still evolute in coiling. The early growth state is prolonged, and sm ...

'', '' Microderoceras'', and '' Xiphoceras''. Those attributable to the Eoderoceratinae include ''

Crucilobiceras ''Crucilobiceras'' is an ammonoid cephalopod genus from the Lower Jurassic belonging to the eoderoceratoidean family Eoderoceratidae. ''Cruciliboceras'' has an evolute shell, such that all whorls are well exposed, with persistent radial ribbing ...

'', ''

Eoderoceras ''Eoderoceras'' is an evolute, round whorled ammonite from the Lower Jurassic with an outer row of distinct spines, and in some, an inner row of tubercles, on either side; ribs only on the inner whorls. ''Eoderoceras'', named by Leonard Spath i ...

'', '' Neomicroderoceras'', and ''

Promicroceras ''Promicroceras'' is an extinct ammonite genus from the upper Sinemurian ( Lower Jurassic) of Europe, named by Leonard Spath in 1925. ''Promicroceras'' is included in the family Eoderoceratidae, which is part of the ammonitid superfamily Eoderoc ...

'' All can be regarded simply as eoderoceratids.

References

*Arkell ''et al.'', 1957. Mesozoic Ammonoidea, ''in'' Treatise on Invertebrate Paleontology, (Part L); Geological Soc. of America and University of Kansas press. *Donovan, D.T. Callomon and Howarth 1981 Classification of the Jurassic Ammonitina; Systematics Association

Ammonitida families Eoderoceratoidea Sinemurian first appearances Early Jurassic extinctions {{Ammonitina-stub