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Journal of Paleontology 91(1):86–99
broader, integrated structure, presumably with an adaptive function related to the pygidium as a whole, rather than as an entity composed of individual segments. The condition in Z. typica thus in some ways presages the development of the highly tagmatized holaspid pygidia seen in several derived trilobite clades (Hughes, 2003; Zhu et al., 2013). The ontogenies of several other redlichiid trilobites have
been described in some detail (Kobayashi and Kato, 1951; Dai et al., 2008, 2014; Dai and Zhang 2012, 2013) and are broadly comparable to that of Z. typica. The only other redlichiid for which we have comparable detail for the development of segmentation is Eoredlichia intermediata, which has been reconstructed to show a similar equilibrium phase during known meraspid ontogeny. Given the thoracic segment-rich trunks of redlichiid species, the pattern of an extended equilibrium phase may be general for the meraspid phase in this group. However, the onset of the holaspid phase of E. intermediata is described as characterized by depletion of one segment, followed by molts in which the number of holaspid pygidial segments increased (Dai and Zhang, 2013). This pattern implies the temporary cessation of new trunk segment expression at the onset of the holaspid stage for one instar, followed by rejuvenation of segment generation in later instars prior to final cessation of segment generation at the onset of epimorphosis. An alternative explanation might be phenotypic variation in the number of trunk segments expressed in the holaspid pygidium of this species (see also Hou et al., 2015). Because no size data for the nine holaspid specimens were presented by Dai and Zhang (2013), it is not possible for us to evaluate this possibility further. Whatever the case, evidence for intermittent ana- morphic expression of new segments in trilobites is now quite strong.
Conclusions
The ontogenetic series shows marked but progressive changes and demonstrates that Parazhangshania sichuanensis is a junior synonym of Zhangshania typica. The protarthrous development of Z. typica (Fig. 8) comprised an extended pygidial equilibrium phase in the meraspid phase, but on entering the holaspid phase, the number of pygidial segments increased and accumulation took place, although apparently not with a new segment expressed at every molt. A comparable pattern of early holaspid growth also appears to characterize Eoredlichia intermediata (see Dai and Zhang, 2013). During the meraspid phase, the length of Z. typica’s cranidium increased by an average per-molt growth rate of 1.08 between successive instars. However, rates calculated for the holaspid stages H1-H2 and H2-H3 are much higher, and likely too great to suggest that each stage represents a single instar. The notably long holaspid pygidial spines must have had very high relative growth rates compared to some preceding them in the trunk. Individual segment identity was reduced within the holaspid pygidium. Trilobites preserved with soft tissues are rarely known. The antennae of Z. typica bear paired spines on at least six proximal articles and, moving dis- tally, the individual articles gradually become twice as long as broad. The discovery of Z. typica in the Hongjingshao Forma- tion of Kunming provides a basis for regional stratigraphic correlation across southern China, allowing definition of the
Yiliangella-Zhangshania biozone in the Yunnan area and ser- ving as a marker of the base of the Tsanglangpuan Stage of the traditional Chinese early Cambrian stadial system.
Acknowledgments
This study was supported by the National Natural Science Foundation of China (41472022, U1402232), Department of Science and Technology, Yunnan Province (2015HA045), US National Science Foundation EAR-1124303, Scholarship Award for Excellent Doctoral Student of Yunnan Province.We thank T. Hegna, an anonymous reviewer, and the editors for their careful work that much improved the manuscript.
Accessibility of supplemental data
Data available from the Dryad Digital Repository:
http://dx.doi. org/10.5061/dryad.7g1m9
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