Nature and significance of intraspecific variation in the early Cambrian oryctocephalid trilobite Oryctocephalites palmeri Sundberg and McCollum, 1997

Oryctocephalid trilobites are seldom abundant and often tectonically deformed, creating problems for robust species delimitation and compromising their utility in biostratigraphic and evolutionary studies. By studying more than 140 specimens recovered from the upper portion of the Combined Metals Member (Pioche Formation, Nevada; Cambrian Stage 4, Series 2), we exploit a rare opportunity to explore how morphological variation among oryctocephalid specimens is partitioned into intraspecific variation versus interspecific disparity. Qualitative and quantitative analyses reveal that two species are represented: Oryctocephalites palmeri Sundberg and McCollum, 1997 and Oryctocephalites sp. A, the latter known from a single cranidium stratigraphically below all occurrences of the former. In contrast to the conclusions of a previous study, there is no evidence of cranidial dimorphism in O. palmeri. However, that species exhibits considerable variation in cranidial shape and pygidial spine arra...

掘头虫类三叶虫（Oryctocephalid trilobites）通常数量稀少且多经构造变形，这给可靠的物种界定带来诸多难题，同时削弱其在生物地层学与演化研究中的应用价值。本次研究对采自美国内华达州皮奥奇组（寒武系第2统第4阶）联合金属段上部的140余件标本开展分析，借此难得的契机探讨掘头虫类三叶虫的形态变异如何划分为种内变异与种间差异。定性与定量分析结果显示，本次研究涉及两个物种：帕尔默掘头虫（Oryctocephalites palmeri Sundberg & McCollum, 1997）以及掘头虫未定种A（Oryctocephalites sp. A），后者仅在所有帕尔默掘头虫产出层位之下的地层中以单件头甲标本被发现。与既往研究的结论不同，目前未发现帕尔默掘头虫存在头甲二态性的证据。不过该物种的头甲形态与尾刺排列……存在显著变异。