Origin of the Ceratitida(Ammonoidea) inferred from the early internal shell features.

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The early internal shell features in 40 species of the Goniatitida, Prolecanitida and Ceratitida are described on the basis of well-preserved specimens from the Carboniferous and the Permian of North America, England, Siberia and Urals. Seven morphotypes were recognized in the species examined by differences of the caecum shape (bottle-shaped, gourd-shaped, subelliptical, or elliptical), the proseptum length on the dorsal side (long or short), position of the second septum (close to proseptum or not) and initial position of the siphuncle (ventral, subcentral, or central). Paraceltites elegans, the oldest known representative of the Ceratitida, has a long proseptum on the dorsal side, a relatively small ammonitella angle, the second septum does not appear in close vicinity to proseptum, and the siphuncle is ventral. These features are essentially the same as those of the prolecanitid Daraelites elegans. This fact supports the hypothesis that the Ceratitida evolved from the Prolecanitida, probably Daraelites.
日本古生物学会の論文