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Journal of Paleontology 91(1):86–99
extend posterolaterally. One pair of pygidial spines showing positive allometry through this phase, extending from anterior margin of pleural lobe, posterolaterally (exs.) and curved inward slightly. Postaxial emargination present. Stage 4 (H4).—Represented by seven specimens (Figs. 5.5, 5.6, 6.17; Table 1). Length varies between 32.51 and 85.0mm. Glabella extending strongly forward to anterior border furrow. Preglabellar field absent. Genal spine long, extending up to T6. Hypostome shield-shaped. Hypostomal anterior border short (sag.), narrow (tr.), curved ventrally. Anterior wing extending posterolaterally. Lateral border narrow (tr.). Anterior lobe of hypostomal median body elliptical, distinctly convex. Posterior lobe of hypostomal median body slightly convex. Posterior border narrow, rounded and convex. Thoracic pleural lobe spines increase gradually in length from anterior to posterior. Pygidium large. Axis distinctly convex, divided into seven segments. Terminal axial piece large, convex, with paired lobes distinct. Single large pygidial spine pair, extends posteriorly. Postaxial region long (sag.) about one sixth of pygidial length (sag.).
Summary of morphological variation of ontogeny.—The length of the exoskeleton increases from 0.55mm in the smallest pro- taspis up to 85.0mm in the largest holaspis. The main mor- phological changes include: (1) cranidium circular in protaspis, becoming gradually trapzoidal towards M5 and then sub- trapzoidal in H3 and thereafter; (2) glabella rod-like, reaching anterior margin in protaspis, becoming gradually cylindrical and then conical in H1 onwards (frontal area is present in H2 but ultimately disappearing in H4); (3) fixigena slightly convex, 1.5 times wider than glabella in protaspis, gradually diminishing in width until one third of glabellar width in H3 onwards; (4) eye ridge convex, fused with frontal glabellar lobe and extending posteriorly in meraspis, but in M13 beginning to separate from the glabella, and bifurcating in H1 and tapering posterolaterally;
(5) librigenae gradually increasing in width until twice width of fixigena in H3 onwards; (6) genal spine appears on librigenae in M4, with positive allometry throughout rest of meraspid and early holaspid phases, reaching up to the sixth thoracic segment in H4; (7) posterior border of librigena migrates posteriorly from an initially advanced position, anterior of the genal angle, to a position in which the adaxial base of the spine is opposite to the posterior corner of the cranidium, or even behind to it; (8) from M2 until M9 the position of the longest pleural spine moves posteriorly from T1 to T5, but from M10 all spines become relatively short and their size distinction diminishes (in the holaspid phase 3, a gradual increase in the relative spine length of more posterior spines becomes evident); and (9) the sub-rectangular pygidium remains relatively small until the middle of the H2 stage, after which it becomes large and sub-trapezoidal, with the single pair of pygidial spines markedly increasing in length ontogenetically, ultimately reaching up to 13.03mm long.
Growth rate.—Based on data from successive instars (Fig. 7) from the interval meraspid 2 to meraspid 13 (based on a suc- cessive series of instars each with a minimum sample size of at least one specimen) the length of the cranidium showed an average per-molt growth rate of 1.08 (AGR of Fusco et al.,
Figure 7. Bivariate scatter diagrams of cranidial growth in Zhangshania typica:(1), pooled data for the meraspid and holaspid phases; (2), enlargement of the meraspid portion in Figure 7.1.
2012), and an index of conformity to Dyar’s rule of 0.85 (IDC of Fusco et al., 2012). The growth rate is at the low end of those reported for trilobites (Fusco et al., 2012, table 1), but similar to that seen among some other articulated specimens, such as A. koninckii, preserved in mudstones. The IDC value is similarly a little lower than that known for other meraspid cephala (Fusco et al., 2012, table 2).
Discussion
Antennae.—The antennae of Z. typica are like those of some other redlichiids in that they extend well beyond the cephalon and exceed it in length. In Z. typica paired spines are inserted into the anterior corners of the six most proximal articles exposed; one of the pair faces adaxially, the other faces abaxi- ally. Conversely, on more distal articles only a single spine is evident, directed adaxially from their inner, distal side. This is different from the condition in Eoredlichia intermedia, in which eight or ten basal articles bear single spines that face adaxially (Hou et al., 2009). A wider review of antennal structures in and among trilobites will be presented elsewhere.
Trunk development.—The almost complete series permits study of the progressive development of trunk segmentation of Z. typica (Fig. 8). From this series, it is evident that one thoracic segment was released from the anterior margin of the meraspid pygidium as one new segment was added subterminally, indi- cating that the meraspid pygidium experienced an extended equilibrium phase (Simpson et al., 2005). As individuals entered the holaspid phase, the number of segments expressed in the pygidium continued to increase. The net result is that initially the holaspid pygidium had one more segment than the meraspid pygidium. After the H1 phase, the holaspid pygidium continued
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