Morphological assessment of the Cambrian trilobites Oryctocephalus indicus (Reed 1910) from China and Oryctocephalus ‘reticulatus’ (Lermontova 1940) from Siberia

Abstract

Oryctocephalus indicus is a widespread trilobite species described from the lower–middle Cambrian (provisional Cambrian Stage 5) of south China, North Korea, India, USA and possibly North Greenland. Principal component analysis (PCA) and thin‐plate splines (TPSs) based on a pooled sample of 359 specimens from four sections in the Guizhou Province, south China, demonstrated that the shape of this species is very constant, and only a small difference is detected in specimens from deeper part of the basin. Most of the variation is taphonomically controlled, and only the specimens from Sanwan Section show a modest natural difference due to a slightly different depositional environment. A comparison between the Chinese specimens and Oryctocephalus ‘reticulatus’ from the Molodo Section, Siberia, has been carried out. PCA and TPS show that O. ‘reticulatus occupies the same morphospace of O. indicus. However, the canonical variates analysis and the analysis of similarities detect small differences between the Siberian and Chinese populations, indicating a minor geographical variation caused by different environmental settings. The ontogenic variation is also described from early meraspids to holaspids. The most substantial changes take place during the first period of the early meraspid development, in which the glabella changes from a bell‐like shape to a cylindrical shape. The ocular ridge changes from poorly defined in early meraspid to well developed in late merapids. The results indicate that O. ‘reticulatus’ is a junior synonym of O. indicus, suggesting that the base O. indicus of Siberia can be correlated with the O. indicus Zone of south China.

Global Standard Stratotype-Section and Point (GSSP) for the conterminous base of the Miaolingian Series and Wuliuan Stage (Cambrian) at Balang, Jianhe, Guizhou, China

“…Yuan and Ng. (2014), Geyer (2015), Zhao et al (2015Zhao et al ( , 2017, Hughes (2016), Sundberg et al (2016), Peng et al (2017) and Esteve et al (2017).…”

The oryctocephalid trilobite zonation across the Cambrian Series 2-Series 3 boundary at Balang, South China: a reappraisal

“…Esteve et al . () also show that Oryctocephalus ‘ reticulatus ’ from the Molodo Section in Siberia is a junior synonym of O. indicus , suggesting that the appearance of O. indicus in Siberia can be correlated with the O. indicus Zone of South China. This taxon seems to demonstrate some geographical differences in sections from South China, but mainly these differences are due to different degrees of preservation (compaction and shearing), although some small morphological differences have been detected (e.g.…”

“…Preliminary comparisons, using morphometric techniques, between O. indicus from Wuliu‐Zengjiayan Section and O. reticulatus from Molodo River Section, showed minor differences in the shape of the cranidia, though again, both populations likely represent the same taxa (Esteve et al . ).…”

“…As such, recently, Esteve et al . () have considered O. ‘ reticulatus ’ as a junior synonym of O. indicus .…”

“…indicus under open nomenclature by Geyer & Peel (2011) due to the lack of pygidia. However, given the variability in the number of segments seen in the pygidia of O. indicus from the Kaili Formation, but the conservative cranidium shape from China and Siberia (Esteve et al 2017), the cranidium found in North Greenland could belong to O. indicus. If so, a straightforward correlation from Greenland and South China is possible.…”