MORPHOMETRICS AND QUANTIFICATION OF INTRASPECIFIC VARIATION THROUGH ONTOGENY IN THE LATE CRETACEOUS NAUTILOID EUTREPHOCERAS USING LINEAR- AND GEOMETRIC MORPHOMETRIC APPROACHES

Eutrephoceras, a group of nautiloids which survived the K/Pg mass extinction event, were relatively abundant in the Late Cretaceous Western Interior Seaway. Although Eutrephoceras has been frequently documented in a number of studies, detailed quantitative research on their morphology and intraspecific variation through ontogeny is still scarce. In this study, 30 specimens of Eutrephoceras were collected in the upper Campanian of South Dakota (Baculites compressus zone) and the lower Maastrichtian of Montana (Baculites baculus–Baculites grandis zones) to examine morphological changes through ontogeny and to quantify intraspecific variation. All specimens were ground to the median plain to produce cross sections for measurements. To quantify the morphology, both linear- and geometric morphometric approaches were employed. Linear measurements enabled us to calculate the whorl expansion rate (WER), which characterizes the morphology through ontogeny. The number of septa was also counted every 180 degrees to yield the septal spacing index (SSI). These indices were used to quantify intraspecific variation through ontogeny. In addition, geometric morphometry was applied for the first time to quantify intraspecific variation of externally shelled cephalopods. Our preliminary data reveal two differing patterns in WER, each of which seems to correspond to a certain embryonic size range, i. e., all specimens from South Dakota and some specimens from Montana with a smaller embryonic size tend to show a lower whorl expansion rate. However, there is a possible mixing of specimens from two ammonite zones (B. baculus and B. grandis) in Montana, which may have resulted in the two patterns of WER. The septal spacing in specimens with a smaller embryonic size is smaller in early ontogeny, while it becomes constant in all specimens at later ontogenetic stages. The variation in WER through ontogeny calculated with the linear- and geometric morphometric approaches demonstrate that higher variation occurs in early ontogeny, which appears to correspond to the point of hatching. The variation shows a decreasing trend in later ontogeny. This pattern appears similar to that of the modern nautiloid Nautilus pompilius.