Hou et al.—Lower Cambrian trilobite ontogeny southern China


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phases until H4 was reached. This pattern of segment accretion, combined with the growth of the existing segments, and possibly also accelerated growth rates toward the posterior of the trunk (Fusco et al., 2014, 2016), may have ultimately permitted the holaspid pygidium to occupy a much larger proportion of the total exoskeletal area than it achieved in meraspis. Addition of segments into the pygidium during the early


part of the holaspid phase raises the question of whether the holaspid stages could be successive instars. To be so, the average per-molt growth rate between H1-H2 and between H2-H3 would have been 1.28 and 1.89, respectively (Fig. 7). Factors of the latter magnitude are not commensurate with those known from any trilobite (Fusco et al., 2012), and it seems unlikely that growth increment would have accelerated so markedly from the meraspid average of 1.08. Accordingly, the additional terminal trunk segments seem likely to have been expressed intermittently over a series of molts, and perhaps as many as 2 or 3 between H1-H2 if meraspid growth rates persisted into the holaspid phase. Such a change, from the regular expression of new trunk segments in each molt during much of the anamorphic phase to an apparently intermittent pattern of segment accretion toward the end of the anamorphic phase is unusual among hemianamorphic arthropods, as is the idea that molts accompanied by the expression of new segments (accumulation phase) alternate with those that do not (equili- brium phase). However, such alternation has been reported in other trilobites, for example during the meraspid growth of


Figure 8. Reconstruction of the trunk segmentation schedule of Zhangshania typica. Dotted lines represent inferred stages not found but anticipated based on the known sample. Note progressive rearward shift of the longest trunk spines through ontogeny.


to express additional segments until a form was reached with seven axial rings, which defines onset of the last stage of holaspid growth, holaspid stage 4. The trunk development was thus protarthrous, because onset of the holaspid phase sig- nificantly preceded onset of the epimorphic phase (Hughes et al., 2006). Relative to Redlichia and Eoredlichia, in which the


holaspid pygidium is a small structure throughout ontogeny, the holaspid pygidium of gigantopygids is notable both for its relatively large size and higher number of segments. This enlarged condition is even more marked in the Yinitidae, which also show fewer thoracic segments in the holaspid phase than in gigantopygids. The meraspid pygidium in Z. typica, in contrast, is a small structure throughout that growth phase. Accordingly, the transition from the meraspid to holaspid phases was accompanied by a marked change in pygidial growth mode in this species. During the extended equilibrium phase (Simpson et al., 2005) the meraspid pygidium showed limited size increase because expression of new segments near the rear of the structure was matched by release of segments from its anterior into the thorax. The molt transitioning into the holaspid phase, however, may have expressed more than a single segment (Fig. 8), and the pygidium thereafter apparently intermittently accumulated additional segments during the early


Shumardia (Conophyrs) salopiensis (see Hughes et al., 2006, fig. 4C) or possibly in Hunanocephalus ovalis (see Dai et al., 2014; but see also Hou et al., 2015). Another possibility is that there is some intraspecific phenotypic variation in the number of holaspid pygidial segments expressed in Z. typica. However, such phenotypic variation cannot explain the absence of small holaspid pygidia with 7 segments, the smallest of which occurs well above the size threshold of the meraspid-holaspid transition. Anotable feature of later holaspid Z. typica is the extremely


long pygidial spines that cannot be directly associated with any particular pygidial segment. Because these spines only appear late in meraspid ontogeny, in order to reach such large size their growth rate must have been very high relative to those that preceded them in the trunk. One factor partly responsible for this could be an anteriorly declining growth gradient operating throughout the trunk, such that posterior segments grew at marked higher growth rates. Such gradients are known to have operated in trilobites (Fusco et al., 2014, 2016), and might explain the apparent rearward migration of the position of the longest pleural spine during meraspid ontogeny, as the growth rates of anterior segments were superseded by those behind them. It will be interesting to test for the presence of such a growth gradient in a biometric study of choice specimens. The cessation of segment release in the holaspid pygidium


resulted in a structure in which the development of segmental architecture related to articulation, necessary in the meraspid pygidium, was not essential to its function. A consequence of this is that features of the holaspid pygidium, such as spines, that almost certainly originated related to a particular segment, could lose their segment-specific identity and become part of a


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