Moulting in the Cambrian oryctocephalid trilobiteArthricocephalites xinzhaiheensisfrom Guizhou Province, South China

Abstract

It is important to understand moulting behaviours in trilobites through different growth stages. Most studies have focused on patterns of moulting in trilobites based on a few exuviae. The present study is based on 177 disarticulated specimens of Arthricocephalites xinzhaiheensis from the Balang Formation (Cambrian Series 2, Stage 4, Guizhou Province, South China). The abundance of disarticulated assemblages allows the distinction between carcasses and exuviae to be made with confidence and enables a detailed exploration of moulting behaviours. About 96.6% of exuviae of both meraspides and holaspides of Ar. xinzhaiheensis show a disarticulated cranidium characterizing Henningsmoen's configuration. The trilobites commonly used the cephalic sutures and cephalothoracic joint for moulting, involving lateral rotation or inversion of the cranidium to create a wide exuvial gape. Only one specimen displays the Somersault configuration characterized by an overturned lower cephalic unit and the data also show that trilobites employing the cephalic sutures to create an anterior exuvial gape are rare in holaspides. Though some specimens are similar to Salter's configuration with an inverted cephalon with respect to the thoracopygon, the specimens show closed cephalic sutures and thus dissociated or inverted cephala are likely to represent disturbed carcasses. Several other species of oryctocephalid trilobites display Henningsmoen's configuration indicating that exuviation by disarticulated cranidium to enlarge the exuvial gape was prevalent in the family from Cambrian Series 2 Stage 4 to the succeeding Wuliuan Stage of the Miaolingian Series.

Saetaspongia sponges from the Cambrian (Stage 4) Balang Formation of Guizhou, China

“…No isotopic dating is available for the Balang Formation, therefore its age is regarded as Stage 4 according to a series of trilobite-based correlations of Cambrian strata between South China and other continents (Zhou et al, 1979, 1980; Zhou and Yuan, 1980; Peng and Babcock, 2001; Yuan et al, 2001, 2006, 2011; Qin et al, 2010; Yan et al, 2014; Yuan and Ng, 2014; Shen et al, 2016; Y.F. Wang et al, 2020), corresponding to the Qiandongian early Duyunian of the Cambrian chronostratigraphic system of China in slope facies (Peng and Babcock, 2001).

Figure 1.( 1 ) Map showing the distribution of the Balang Formation (gray areas) in Guizhou Province, the location of the fossil locality (star); and ( 2 ) the lithological column of the Lazizhai section.

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“…Many fossil assemblages have been discovered from the middle to upper part of the Balang Formation at various localities (Peng et al, 2005, 2012a, b; Peng, 2009; Da et al, 2011; Ma et al, 2011; Liu and Lei, 2013; Zhao et al, 2015; Y.F. Wang et al, 2020), and the Balang Biota comprises at least 80 species of 63 genera (Zhao et al, 2007; Peng et al, 2010a, 2012c, 2016; Sun et al, 2015, 2017; Liu, 2018; Liu et al, 2018a; Wen, 2019; Wen et al, 2019). In Guizhou, collections were conducted mainly in three localities: Xiangshuidong Village in Songtao County; Tongren City and Jiaobang and Lazizhai villages in Jianhe County; and Kaili City.…”

“…), sponges (e.g., Jianhella obconica Wang and Peng in Wang et al, 2019; Leptomitus teretiusculus Chen, Hou, and Lu, 1989; Hamptonia jianhensis Wang and Peng in Q.J. Wang et al, 2020), worms ( Wronascolex geyiensis Peng et al, 2016), brachiopods ( Diandongia cirularis Peng et al, 2010b; Lingulellotreta malongensis [Rong, 1974]), echinoderms ( Guizhoueocrinus yui Zhao, Parsley, and Peng, 2007), mollusks, hyoliths ( Haplophrentis reesei Babcock and Robison, 1988; Ambrolinevitus maximus Jiang, 1982; ? Galicornus seeneus Val'kov, 1975; Linevitus guizhouensis Sun et al, 2017; Meitanovitus guanyindongensis Qian, 1978), trilobites ( Redlichia [ Pteroredlichia ] chinensis [Walcott, 1905] Chang, 1966; Probowmania [ Probowmania ] balangensis Yuan and Zhao in Yuan et al, 1997; Arthricocephalus chauveaui Bergeron, 1899; Arthricocephalites xinzhaiheensis Chien and Lin in Lu et al, 1974; A .…”

Ontogenetic moulting behavior of the Cambrian oryctocephalid trilobiteArthricocephalites xinzhaiheensis

“…In a recent paper we studied mouting in Ar. xinzhaiheensis and found two kinds of moult configurations (Wang et al, 2020). A concentration of well-preserved disarticulated configurations of the early Cambrian oryctocephalid trilobite Ar.…”

“…A concentration of well-preserved disarticulated configurations of the early Cambrian oryctocephalid trilobite Ar. xinzhaiheensis provides an unparalleled opportunity to allow the distinction between carcasses and exuviae to be made with confidence (Wang et al, 2020), and then explore moulting behavior during development.…”