Gymnitidae is a family of Lower to Middle

Triassic The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago (Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...

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 with evolute, discoidal shells. Hyatt and Smith (1905, p. 114-115) included the Gymnitidae in the suborder Ceratitoidea, which later became the superfamily

Ceratitaceae Ceratitoidea, formerly Ceratitaceae, is an ammonite superfamily in order Ceratitida characterized in general by highly ornamented or tuberculate shells with ceratitic sutures that may become goniatitic or ammonitic in some offshoots. (Arkell ' ...

and included in it genera more primitive than '' Gymnites'' as well as the more advanced ''Gymnites''. Those being '' Xenaspis'', ''

Flemingites ''Flemingites'' is a genus of evolute ammonoids from the Lower Triassic with spiral ridges on the shell. Diagnosis The shell of ''Flemingites'' is evolute so that all whorls show externally. Whorls are robust, slightly embracing, usually a lit ...

'', and ''

Ophiceras ''Ophiceras'' is a genus of smooth, evolute ceratitid ammonites from the Early Triassic, with a rounded venter. Fossils of the genus have been found in Armenia, Azerbaijan, China, Greenland, and India.Xenodiscus ''Xenodiscus'' is an extinct ammonoid cephalopod genus and one of the earliest ceratites, found in the Upper Permian The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous ...

''. The more primitive ''Xenaspis'', ''Flemingites'', and ''Ophiceras'', found in Lower Triassic beds in western America have ceratitic sutures. The more developed ''Gymnites'' has deeply digitate ammonitic sutures. Arkell, et al., 1957, in the Treatise on Invertebrate Paleontology, on the other hand included the Gymnitidae in the

Pinacocerataceae Pinacoceratoidea, formerly Pinacocerataceae, are generally smooth, compressed, evolute to involute ammonoids from the Triassic, belonging to the Ceratitida, in which the suture is ammonitic, with adventitious and auxiliary elements. As presently ...

as the earlier and more primitive of its two families, combining ''Gymnites'' with coeval and more advanced forms. Genera included in the Gymnitidae sensu Arkell include '' Eogymnites'', '' Buddhaites'', '' Japanites'', and of course, ''Gemnites''. The genus '' Xiphogymnites'' was included by Tozer in 1981. Whether to place the Gymnitidae in the Ceratitaceae, as in Hyatt and Smith, combining ''Gymnites'' with its more primitive relatives, or in the Pinacocerataceae, as in Arkell et al., combining ''Gymnites'' with its coeval or more advanced relatives is a matter of perspective which does little to change the overall phylogeny.

References

* Arkell, et al., 1957. Mesozoic Ammonidea. Treatise on Invertebrate Paleontology, Part L, Ammonoidea. Geol Soc of America and Univ. Kansas Press. *Alpheus Hyatt and James Perrin Smith, 1905. The Triassic Cephalopod Genera of America. U.S. Geological Survey Professional Paper no. 40. (p. 114-115) *

James Perrin Smith James Perrin Smith (November 27, 1864 – January 1, 1931) was an American geologist and paleontologist. Smith was of English descent. T. M. Forster, one of his ancestors, was a surgeon in the Royal Navy and moved to Virginia in 1745. His paternal ...

, 1932. Lower Triassic Ammonoids of North America. U.S. Geological Survey Professional Paper no. 167 (p. 30) Ceratitida families Pinacocerataceae Early Triassic first appearances Middle Triassic extinctions {{Ceratitida-stub