Intraspecific variation of trunk segmentation in the oryctocephalid trilobiteDuyunaspis duyunensisfrom the Cambrian (Stage 4, Series 2) of South China

Abstract

A detailed exploration of growth and trunk segmentation of the oryctocephalid trilobite, Duyunaspis duyunensis Chang & Chien in Zhou et al. 1977, from the lower Cambrian (Stage 4, Series 2) Balang Formation in western Hunan Province, South China, is presented. Because of the excellent preservation, the complete post‐protaspid ontogenetic series from merapsid degree 0 to the holaspid phase is described. The ontogenetic series reveals new information on morphological changes such as the migration of the posterior branch of the facial sutures (from proparian to opisthoparian) and contraction of the posteromedial notch in the pygidium. The abundance of articulated specimens available from a narrow stratigraphical interval makes this material singularly useful for studying the morphogenesis and post‐embryonic growth of D. duyunensis in comparison with other oryctocephalids. Strong evidence that multiple numbers of pygidial segment are recognized in each meraspid degree as well as in the holaspid period showed unusual intraspecific variability in the rate of trunk segmentation, providing insights into how Cambrian subisopygous trilobites controlled their body patterning, including size, shape and trunk segment number in both thorax and caudal plate during growth.

Developmental bias in the fossil record

“…Patterns of phenotypic variation, and chiefly variability, uncovered by fine-scale studies of fossil morphology (e.g., Dai, Zhang, Peng, & Yao, 2017;Esteve, 2012;Neubauer, Harzhauser, & Kroh, 2013) ought to be considered within the framework of PLE. Patterns of phenotypic variation, and chiefly variability, uncovered by fine-scale studies of fossil morphology (e.g., Dai, Zhang, Peng, & Yao, 2017;Esteve, 2012;Neubauer, Harzhauser, & Kroh, 2013) ought to be considered within the framework of PLE.…”

Enrolment and trunk segmentation of a Cambrian eodiscoid trilobite

“…Different trilobite groups have their own morphotypes. In the process of their development, they must have kept a balanced rate of segment addition (from the pygidium) and liberation (into the thorax) within a reasonable and controllable range, if they were to achieve a perfect enrolment by maintaining an equilibrium in the length and size ratio of each sclerite to the whole body (Dai et al 2017).…”

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

“…However, the figured specimens of Oryctocephalus indicus (Esteve et al, 2017, fig. 14A, C) appear to show an anterior cranidial border extending continuously around the front of the glabella, so the reconstructions might be inaccurate in that regard. Differences in the ontogenetic dynamics of trunk segmentation also exist between taxa (McNamara et al, 2003(McNamara et al, , 2006Dai et al, 2017;Du et al, 2018 Material.-Silicified cranidium USNM 488937, figured by Sundberg and McCollum (1997, fig. 12.11).…”