Data from: Evolutionary process of extremely twisted heteromorph ammonites from the Upper Cretaceous in Japan

The heteromorph ammonite Pravitoceras sigmoidale with
extremely twisted retroversal hook and its probable
ancestors, Didymoceras
awajiense and D. morozumii, occur mainly in the
Upper Cretaceous Izumi and Sotoizumi groups in southwest Japan. Their
morphological characteristics were investigated in this study. Morphotypes
I–II were recognized in D. morozumii. Morphotypes III–V
were recognized in D. awajiense. Morphotypes VI–VIII
were recognized in P. sigmoidale. Moreover, all
morphotypes were recognized in both the dextral and sinistral forms of
these three species. Based on the morphological and stratigraphic
relationships, it is considered that the evolutionary process of these
species is divided into three steps: (1) shift from morphotypes I–II to
III; (2) increase in variation from morphotypes III to III–V; and (3)
shift from morphotypes III–V to VI–VIII. By comparing these morphotypes
based on the ‘growing tube model’ belonging to the moving frame analysis,
it is seen that there are no significant gaps of coiling patterns in the
evolutionary processes of this lineage. Meanwhile, morphotypes III–V
of D. awajiense have a larger morphological
variation than morphotypes I–II
of D. morozumii and morphotypes VI–VIII
of P. sigmoidale. These results, and the radiometric
ages from previous studies, suggest that D. awajiense,
with considerable variation between ancestor and descendant species,
existed in a short interval. It seems
that P. sigmoidale occurred after the
continuous but rapid and drastic morphological evolution
from D. morozumii via D. awajiense.