Hou et al.—Lower Cambrian trilobite ontogeny southern China


trilobites varied in whether or not termination of trunk segment expression preceded termination of developing segment articu- lation (Hughes et al., 2006, table 1; Zhang and Clarkson, 2012). Such variation was also common among derived species. On the other hand, meraspid growth of early Cambrian polymerid trilobites had prolonged pygidial equilibrium phases (Simpson et al., 2005) and few, if any, such species had any trunk segment longer than their first segment in that body region. That condi- tion is different from that of many derived species, including A. koninckii, in which the longest segment was located posterior to the leading trunk segment. This difference is important both because it offers the possibility of surprisingly detailed insight into the nature of growth control in these ancient animals (e.g., Fusco et al., 2014, 2016), and because elevated growth rates of posterior segments were likely critical to the production of several derived trilobites body plans. Diligent collecting of ontogenetic series of articulated specimens of early Cambrian species thus offers rich promise for further insights into the controls of early arthropod evolution and development. Herein, we describe ontogenetic development of the nearly complete, articulated early Cambrian yiliangelline trilobite


87


Zhangshania typica Li and Zhang, 1990 in Li et al., 1990 as a first step in establishing it as a candidate for more extensive,


quantitative investigation when sample size permits. Geological setting and biostratigraphic correlation


Zhangshania typica characterizes a regional biozone in Sichuan Province of southern China (Li et al., 1990; Xiang et al., 1999). Our new report of its occurrence in Kunming permits refined cor- relation between Sichuan and eastern Yunnan province (Fig. 1). Li et al. (1990) divided the Yuxiansi Formation of Leshan in Sichuan into two members, the lower being the Zhangshan Member, where the species described herein was first reported (Fig. 1.1), and an upper unit called the Youfanggou Member. They recognized a Yiliangella-Zhangshania biozone in the upper part of the Zhangshan Member in Leshan that they cor- related with the Yunnanaspis-Yiliangella biozone of the Yunnan area (Zhou and Yuan, 1980), also known as the Yiliangella bio- zone, which is the earliest trilobite zone of the Tsanglangpuan Stage (Luo et al., 2008) based on Yiliangella’swidespread distribution in the Hongjingshao Formation. These correlations


Figure 1. Locality map and stratigraphic section: (1), sketch map showing the positions of two localities reported with Zhangshania typica;(2), location map of the studied fossil site (marked with a star) in the early Cambrian Hongjingshao Formation, Kunming, Yunnan Province; (3), lithological section through the trilobite-bearing strata of the exposed Hongjingshao Formation in Kunming. The upper mudstone represents the level from which the species described in this paper has been collected.


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