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.

中文翻译：

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晚白垩世菊石个体发育的种内变异

摘要 该项目通过 1951 年来自美国西部内陆上白垩纪的菊石 Scaphites whitfieldi Cobban 的个体发育来评估种内变异。我们的样本由代表两种岩相（页岩和粉砂岩）的九个地点的 103 个背腹横截面组成。我们测量了四个壳参数（ww/dm、ww/wh、uw/dm 和 WER）来描述壳形态的个体发育变化。我们研究了三个生长阶段的变化：孵化后立即（dm = 1 mm）、neanoconch（dm = 4 mm）和亚成熟阶段（定义为成熟钩状体室底部或附近）。一般来说，外壳通过个体发育变得更加圆盘状，脐部更窄，螺纹部分更压缩。单变量分析的结果表明，新海螺的变异在统计学上显着高于孵化阶段或未成熟阶段。这种模式在多元分析中也很明显，其中差异从孵化到新海螺显着增加，然后在亚成熟阶段再次减小。这些结果与以下假设一致，即新甲壳虫代表了动物生活史向更底层生活模式的转变，随后是形态学向成熟的运河化。然而，由于 neanic 过渡发生在一系列尺寸上，一些人可能已经在 dm = 4 毫米处经历了这些变化，而其他人可能没有，从而扩大了变化的程度。为了以后解决这个问题，单独检查每个个体发育轨迹以查明发生形态变化的确切大小至关重要。我们还比较了粉砂岩样品与页岩样品的三个生长阶段的 ww/dm、ww/wh、uw/dm 和 WER 值。比较表明，粉砂岩标本占据形态空间的较低区域，这意味着这些标本通常比页岩标本更受压缩。这种差异可能与在以粉砂岩为代表的高能环境中提高水动力效率的选择压力有关。粉砂岩样品与页岩样品的三个生长阶段的 WER 和 WER。比较表明，粉砂岩标本占据形态空间的较低区域，这意味着这些标本通常比页岩标本更受压缩。这种差异可能与在以粉砂岩为代表的高能环境中提高水动力效率的选择压力有关。粉砂岩样品与页岩样品的三个生长阶段的 WER 和 WER。比较表明，粉砂岩标本占据形态空间的较低区域，这意味着这些标本通常比页岩标本更受压缩。这种差异可能与在以粉砂岩为代表的高能环境中提高水动力效率的选择压力有关。