Lytoceratidae

Lytoceratidaeis a taxonomic family ofammonoidcephalopods belonging to the suborderLytoceratina,characterized by very evolute shells that generally enlarge rapidly, having whorls in contact but mostly overlapping very sightly, or not at all.

Lytoceratidae Temporal range:
Fossil shells ofLytoceras cornucopiafrom I sắc re (France), on display atGalerie de paléontologie et d'anatomie comparéein Paris
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Cephalopoda
Subclass:	†Ammonoidea
Order:	†Ammonitida
Suborder:	†Lytoceratina
Family:	†Lytoceratidae Newmayr, 1875
Subfamilies
See text

Surface ornament may consist of various combinations of straight or crinkled growth lines, flares, constrictions, and, more rarely, plications. Sutures are highly complex and moss-like, but with few major elements. Lateral lobes are widely splayed and blunt, or with obliquely deflected end. The external, ventral, lobe is short.

The Lytoceratinae have a worldwide distribution and a stratigraphic range extending from the middle Lower Jurassic (Pliensbachian) to the early Upper Cretaceous (Cenomanian).

Subfamilies

The Lytoceratidae has been divided into four subfamilies, as follows.

• LytoceratinaeNeumayr 1875(Pliensbachian)
  • AmmonoceratitesBowditch 1822
  • ArgonauticerasAnderson 1938
  • CarinolytocerasWiedmann 1962
  • EulytocerasSpath 1927
  • HemilytocerasSpath 1927
  • LytocerasSuess 1865
  • MetalytocerasSpath 1927
  • PictetiaUhlig 1883
  • ProtetragonitesHyatt 1900
  • PterolytocerasSpath 1927

Lytoceratids with whorls that bear growth lines or lamellar flares, or both, and in which there are only two lateral lobes in the external suture, on either side, and the dorsal lobe is cruciform (cross like).

• Megalytoceratinae(Toarcian–Bajocian)

Planulate lytoceratids in which whorls and sutures tend to lose lytoceratid character and resemble those of the perisphinctidae.

• Villaniinae(Callovian)

Plantulate lytoceratids with sutures like those in the Lytoceratinae, but without the dorsal lobe being cruciform.

• AlocolytoceratinaeSpath 1927(Toarcian–Bajocian)
  • Alocolytoceras
  • Audaxlytoceras
  • Derolytoceras
  • Lobolytoceras
  • Pachylytoceras
  • Pleurolytoceras

Lytoceratids with many deep constrictions resulting and capricorns in middle whorls. Outer whorls become more smooth and involute. Capricorn: a shell encircled by blunt, wide spaced ribs separated by subequal rounded interspaces, resembling a goat's horn.

References

• Arkell, W.J.; Kummel, B.; Wright, C.W. (1957).Mesozoic Ammonoidea.Treatise on Invertebrate Paleontology, Part L, Mollusca 4. Lawrence, Kansas: Geological Society of America and University of Kansas Press.