Placenticeras

Placenticeras placenta, copyright James St. John.

Belongs within: Hoplitaceae.

Placenticeras is a diverse genus of ammonites known from the Upper Albian to the Lower Campanian of the Cretaceous period (Wright et al. 1996).

Characters (from Wright et al. 1996): Populations showing great variability, ranging from compressed, involute forms with convergent, slightly convex sides, narrow, tabulate venter, no to weak, falcoid ribs, long umbilical bullae, and inner and more numerous outer ventrolateral tubercles to forms with similar inner whorls, but outer becoming more evolute, rectangular to square in section, with more prominent umbilical and ventrolateral tubercles, the inner of which, originally midlateral, may move outwards to form the ventrolateral margin while the outer may disappear. In any population macroconchs comprising larger, more compressed, and more feebly ornamented forms. Relative size of sutural elements varying slightly with time; early species tending to have 4th lateral lobe smaller than 5th.

<==Placenticeras Meek 1876 (see below for synonymy)WCH96
    |--*P. placenta (DeKay 1828)WCH96 [=Ammonites placentaWCH96; incl. A. placenta var. intercalarisW77]
    |--*Baghiceras’ ambai Chiplonkar & Ghare 1976WCH96
    |--*Mediasiceras’ beliakovae Il’in 1975WCH96
    |--*Beschtubeites’ beschtubensis Il’in 1975WCH96
    |--P. bidorsatum (Roemer 1841) [=Ammonites bidorsatus, *Diplacomoceras bidorsatum]WCH96
    |--*Placentoscaphites’ dangerfieldi Chiplonkar & Ghare 1977WCH96
    |--P. fritschi Grossouvre 1894 [=*Proplacenticeras fritschi]WCH96
    |--P. gouerdakensis Luppov 1963 [=*Turkmenites gouerdakensis]WCH96
    |--P. grossouvrei Semenov 1899 [=*Karamaites grossouvrei]WCH96
    |--P. guadaloupae (Roemer 1849) [=Ammonites guadaloupae; incl. *Stantonoceras pseudocostatum Johnson 1903]WCH96
    |--*Karamaiceras’ kolbajense Sokolov 1967WCH96
    |--*Kopetdagites’ kopetdagensis Il’in 1975WCH96
    |--*Gissarites’ kysylchense Il’in 1958WCH96
    |--P. mediasiaticumWCH96
    |--‘Ammonites’ milleri Hauer 1866 [=*Pseudoplacenticeras milleri]WCH96
    |--*Parastantonoceras’ murphyi Collignon 1965WCH96
    |--P. polyopsisWCH96
    |--P. sancarlosense Hyatt 1903 [=*Sancarlosia sancarlosensis]WCH96
    |--P. syrtaleK79
    |--‘Stantonoceras’ tagamense Il’in 1975 [=*Asiatostantonoceras tagamense]WCH96
    `--*Malwiceras’ variabilis Chiplonkar & Ghare 1976WCH96

Placenticeras Meek 1876 [=Placentocerus Meek 1871 (n. o.); incl. Asiatostantonoceras Il’in 1975, Baghiceras Chiplonkar & Ghare 1976, Beschtubeites Il’in 1975, Diplacomoceras Hyatt 1900, Diplacmoceras Hyatt 1903, Gissarites Il’in 1958, Karamaiceras Sokolov 1967, Karamaites Sokolov in Casey 1965, Kopetdagites Il’in 1975, Malwiceras Chiplonkar & Ghare 1976, Mediasiceras Il’in 1975, Parastantonoceras Collignon 1965, Placentoscaphites Chiplonkar & Ghare 1977, Proplacenticeras Spath 1926, Pseudoplacenticeras Spath 1926, Sancarlosia Chiplonkar & Ghare 1978, Stantonoceras Johnson 1903, Turkmenites Il’in 1975]WCH96

*Type species of generic name indicated

References

[K79] Kauffman, E. G. 1979. Cretaceous. In: Robison, R. A., & C. Teichert (eds) Treatise on Invertebrate Paleontology pt A. Introduction. Fossilisation (Taphonomy), Biogeography and Biostratigraphy pp. A418–A487. The Geological Society of America, Inc.: Boulder (Colorado), and The University of Kansas: Lawrence (Kansas).

[W77] White, C. A. 1877. Report upon the invertebrate fossils collected in portions of Nevada, Utah, Colorado, New Mexico, and Arizona, by parties of the expeditions of 1871, 1872, 1873, and 1874. U.S. Geographical Surveys West of the One Hundredth Meridian 4 (1): 1–219, pls 1–21.

[WCH96] Wright, C. W., J. H. Calloman & M. K. Howarth. 1996. Treatise on Invertebrate Paleontology pt L. Mollusca 4, revised vol. 4. Cretaceous Ammonoidea. The Geological Society of America, Inc.: Boulder (Colorado), and The University of Kansas: Lawrence (Kansas).

Leave a comment

Your email address will not be published. Required fields are marked *