Nature and significance of intraspecific variation in the early Cambrian oryctocephalid trilobiteOryctocephalites palmeriSundberg and McCollum, 1997

Mark Webster,Frederick A Sundberg

doi:10.1017/jpa.2019.85

Mark Webster, Frederick A Sundberg

Open Access

https://doi.org/10.1017/jpa.2019.85

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Abstract

AbstractOryctocephalid 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 palmeriSundberg and McCollum, 1997 andOryctocephalitessp. 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 inO. palmeri. However, that species exhibits considerable variation in cranidial shape and pygidial spine arrangement and number. Cranidial shape variation withinO. palmeriis approximately one-half of the among-species disparity within the genus. Comparison of cranidial shape between noncompacted and compacted samples reveals that compaction causes significant change in mean shape and an increase in shape variation; such changes are interpretable in terms of observed fracture patterns. Nontaphonomic variation is partitioned into ontogenetic and nonallometric components. Those components share similar structure with each other and with interspecific disparity, suggesting that ontogenetic shape change might be an important source of variation available for selection. This highlights the importance of ontogenetic and taphonomic sources of variation with respect to species delimitation, morphospace occupation, and investigation of evolutionary patterns and processes.

Highlights

Oryctocephalid trilobites play a major role in intercontinental biostratigraphic correlation in Stage 4 and the succeeding Wuliuan Stage of the Cambrian (Sundberg et al, 2016; Zhao et al, 2019; Fig. 1)
A is excluded from consideration, all other specimens from the Combined Metals Member form a single continuum of data (Fig. 12) that represents O. palmeri sensu stricto
(2) PC1 versus PC3. (3–5) Thin-plate splines depicting shape variation described by each principal components (PCs), shown as shape deformation of consensus configuration toward positive value along that axis

Summary

Introduction

Oryctocephalid trilobites play a major role in intercontinental biostratigraphic correlation in Stage 4 and the succeeding Wuliuan Stage of the Cambrian (Sundberg et al, 2016; Zhao et al, 2019; Fig. 1). This places a premium on robust species delimitation. Specimens from many localities have experienced tectonic deformation as a result of collisional events at continental margins (e.g., Whittington, 1995; Jell and Hughes, 1997; Yuan et al, 2002; Peng et al, 2009), introducing even more taphonomic overprint on the preserved phenotype

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