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Intraspecific variation through ontogeny in late Cretaceous ammonites. (American Museum novitates, no. 3922)

Show simple item record Klein, Carina. Landman, Neil H. 2019-02-27T20:53:38Z 2019-02-27T20:53:38Z 2019-03-04
dc.description 25 pages : illustrations (some color), color map ; 26 cm. en_US
dc.description.abstract This project assesses intraspecific variation through the ontogeny of the ammonite Scaphites whitfieldi Cobban, 1951, from the Upper Cretaceous of the U.S. Western Interior. Our sample consists of 103 dorsoventral cross sections from nine localities that represent two lithofacies (shale and siltstone). We measured four shell parameters (ww/dm, ww/wh, uw/dm, and WER) to describe the ontogenetic changes in shell morphology. We investigated the variation at three growth stages: immediately after hatching (dm = 1 mm), the neanoconch (dm = 4 mm), and the submature stage (defined as at or near the base of the mature hooklike body chamber). In general, the shell becomes more discoidal through ontogeny with a narrower umbilicus and a more compressed whorl section. The results of the univariate analysis indicate that the variation is statistically significantly higher in the neanoconch than in either the hatchling or submature stage. This pattern is also apparent in the multivariate analysis in which the disparity increases markedly from the hatchling to the neanoconch and then decreases again at the submature stage. These results are consistent with the hypothesis that the neanoconch represents a transition in the life history of the animal to a more demersal mode of life followed by a canalization of morphology toward maturity. However, because the neanic transition occurs over a range of sizes, it is possible that some individuals may have already undergone these changes at dm = 4 mm, whereas others may not have, thus inflating the degree of variation. To resolve this issue in the future, it is critical to examine each ontogenetic trajectory individually to pinpoint the exact size at which the morphological changes occur. We also compared the values of ww/dm, ww/wh, uw/dm, and WER of the three growth stages for the sample from siltstone versus the sample from shale. The comparison reveals that the specimens from siltstone occupy lower regions of the morphospace, implying that these specimens are generally more compressed than those from shale. This difference may be related to selection pressures for improved hydrodynamic efficiency in the higher energy environment represented by siltstone. en_US
dc.language.iso en_US en_US
dc.publisher American Museum of Natural History. en_US
dc.relation.ispartofseries American Museum novitates;no.3922.
dc.subject Scaphites whitfieldi -- Variation. en_US
dc.subject Scaphites whitfieldi -- Development. en_US
dc.subject Ontogeny. en_US
dc.subject Ammonoidea. en_US
dc.subject Great Plains. en_US
dc.subject West (U.S.) en_US
dc.title Intraspecific variation through ontogeny in late Cretaceous ammonites. (American Museum novitates, no. 3922) en_US
dc.title.alternative Intraspecific variation in late Cretaceous ammonites. en_US

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  • American Museum Novitates
    Novitates (Latin for "new acquaintances"), published continuously and numbered consecutively since 1921, are short papers that contain descriptions of new forms and reports in zoology, paleontology, and geology. New numbers are published at irregular intervals.

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