Acrioceratidae is a family of heteromorph

ammonites 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 cuttlefish) ...

included in the

Ancyloceratoidea Ancyloceratoidea, formerly Ancylocerataceae, is a superfamily of typically uncoiled and loosely coiled heteromorph ammonoids Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These ...

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 ''Acrioceras'' is an extinct genus of cephalopods belonging to the ammonite subclass. with and (1996), ''Mollusca 4 Revised , Cretaceous Ammonoidea'', vol. 4, in ''Treatise on Invertebrate Paleontology'', Part L (Roger L. Kaesler et el. eds. ...

'' 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'' (or ''TIP'') published by the Geological Society of America and the University of Kansas Press, is a definitive multi-authored work of some 50 volumes, written by more than 300 paleontologists, and co ...

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