Acrioceratidae is a family of heteromorph

ammonites 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 ...

included in the

Ancyloceratoidea Ancyloceratoidea, formerly Ancylocerataceae, is a superfamily of typically uncoiled and loosely coiled heteromorph Ammonoidea, ammonoids established by Alpheus Hyatt in 1900, that may contain as many as 11 families, depending on the classificat ...

comprising ancyloceratid-like forms that start off with a coiled juvenile section, followed by a straight or curved shaft ending in a hook. Two described genera are included, '' Acrioceras'' and '' Dissimilites''. The Acrioceratidae form a link, or evolutionary transition, between the loosely coiled Crioceratidae and the commonly tuberculate and heavy hooked

Ancyloceratidae Ancyloceratidae is a family of heteromorphic Ammonitida, ammonites that lived during the Early Cretaceous. Their shells begin as a loose spiral with whorls not touching which then turns into a straight shaft that ends in a J-shape hook or bend ...

. Although resembling ''Acrioceras'' in general form, '' Toxancyloceras'' is included in the Ancyloceratidae where it resides as a transitional form. The primary morphological distinction between the Acrioceratidae and Ancycloceratidae is that the Acrioceratidae generally lack the tubercles and spines characteristic of the ancyloceratids. They differ from the ancestral Crioceratidae in that, like the Ancyloceratidae, they are truly heteromorphic (crioceratids aren't) with distinct growth phases. The

Treatise on Invertebrate Paleontology The ''Treatise on Invertebrate Paleontology,'' published from 1953–2007 by the Geological Society of America and the University of Kansas, then 2009–present by the University of Kansas Paleontological Institute, is a definitive multi-authore ...

1957, Part L, included ''Acrioceras'', the nominate genus, in the Ancyloceratidae, with ''Dissimilites'' considered synonymous.

References

* W.J. Arkell ''et al.'', 1957. Mesozoic Ammonoidea; Treatise on Invertebrate Paleontology, Part L Mollusca 4. Geological Society of America and University of Kansas Press. *Alexander Lukeneder and Susanne Lukeneder The Barremian heteromorph ammonite ''Dissimilites'' from northern Italy: taxonomy and implications. Acta Palaeontologica Polonic

Ancyloceratoidea Ammonitida families Hauterivian first appearances Early Cretaceous extinctions {{ammonitida-stub