Laibl et al.—Post-embryonic development in Ellipsostrenua 92(6):1018–1027


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stages, although only one late protaspid stage was reported by Zhang and Pratt (1999). The late protaspides of all three taxa also share a down-sloping trunk with one pair of pleural spines. These spines are relatively short in Ellipsostrenua and Ichangia, but extremely long in Estaingia, which may reflect an adaptation to a planktonic mode of life, as suggested by Zhang and Pratt (1999). In general, the morphological changes between early and late protaspid stages in all three of these genera are minor, including only slight allometric modifications and the development of a trunk portion. Early meraspid cranidia (Fig. 7.1) of Ellipsostrenua,


Estaingia, and Ichangia have forwardly diverging anterior branches of the facial suture (and therefore a comparatively wide anterior cranidial margin). They also have prominent palpebral lobes, intergenal spines, and bacculae. Near the intergenal spine, the posterior border furrow of the cranidium is still curved forward as in the late protaspid stage, forming a distinct intergenal angle. Consequently, the posterior branch of the facial suture is always very short in early meraspid cranidia and remains comparatively short up to the holaspid period (cf., Zhang et al., 1980, pl. 76, figs. 4–8; Cederström et al., 2012, fig. 12; Dai and Zhang, 2012, fig. 3; Fig. 1.1 herein). Recent studies suggest that conservativeness of early


development is not always applicable (e.g., Poe, 2006) and that even the earliest stages of closely related taxa or within a particular clade can be morphologically distinct (Scholtz, 2005). Such early stage distinction has been reported also in some trilobites (e.g., Park and Kihm, 2015; Laibl et al., 2017). Nevertheless, the complex and uniform morphology in early stages of several genera of the Ellipsocephaloidea suggests developmental conservativeness within this group. The only known exception seems to be a modification of late protaspides of Estangia to a planktonic mode of life, resulting in slight allometric re-pattering of its morphology. Based on comparison with other contemporaneous


Cambrian trilobites, the following characters of ellipsocepha- loid trilobites are considered important for phylogenetic analyses: (1) conservative morphology of the early protaspid stage; (2) development of a trunk region with one pair of spines


cranidial width (mm)


Figure 7. Comparative development of Ellipsocephalidae and Estaingiidae: (1) morphological comparison of protaspid and early meraspid stages of Ellipsostrenua, Estaingia,and Ichangia, based on Zhang and Pratt (1999), Dai and Zhang (2012), Zhang and Clarkson (2012), and data presented herein; (2)size comparison of protaspid and early meraspid stages of Ellipsostrenua granulosa, E. gripi, Ichangia ichangensis,and Estaingia sinensis, circles represent minimal and maximal cranidial width of the protaspid and early meraspid stages, based on Kautsky (1945), Zhang and Pratt (1999), Dai and Zhang (2012), and data presented herein. Abbreviations: lst, limestone;M, meraspid stages; mud, mudstone; P1, early protaspid stage; P2, late protaspid stage; P2a, first late protaspid stage; P2b, second late protaspid stage; sh, shale.


the two former genera. Estaingia may also possess two morphologically similar late protaspid stages, designated by Dai and Zhang (2012) as the anaprotaspid and metaprotaspid


between the early and late protaspid stage; (3) only two pairs of fixigenal spines are present in the early protaspid stage (shared with some species of Paradoxididae; cf., Westergård, 1936; Laibl et al., 2017); (4) genal swellings are present (shared with members of Eodiscida; cf., Dai and Zhang, 2011; Zhang and Clarkson, 2012); and (5) prominent bacullae developed already in the early protaspid stage (shared with Eodiscida and Paradoxididae). An outgroup comparison is, however, necessary for resolving the polarity of these characters.


Variation in protaspid size of Ellipsocephalidae and Estaingiidae.—Although the earliest known protaspid stages in Ellipsostrenua, Estaingia,and Ichangia likely represent homo- logous stages in the development of these genera (see above), they show variability in their size (Fig. 7.2). The width of early protaspides of E. granulosa and E. gripi from Sweden (Baltica) is comparable and varies from 0.60–0.75mm. Similarly, the width of the smallest late protaspid specimens ranges from 0.69– 0.86mm. By contrast, the phosphatized protaspides from Henan (SouthChina plate) are considerably smaller. Thewidth of the early


South China


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