Hou et al.—Lower Cambrian trilobite ontogeny southern China
(sag.), located opposite glabellar lobes 2 and 3. Anterior bran- ches of facial suture divergent anterolaterally. Posterior bran- ches of facial suture divergent posterolaterally. Lateral border narrow (tr.), slightly convex. Lateral border furrow and poster- ior border furrow shallow. Posterior border slightly convex, becoming gradually shorter (exs.) towards occipital ring. Librigena narrow (tr.). Trunk axis convex, each thoracic seg- ment with small axial node. Pleural lobe extending transversely, slightly narrower (tr.) than axis. Thoracic pleural spine equiva- lent length on both segments, extending backward well beyond pygidium. Pleural furrow deep, extending posterolaterally. Pygidium small, sub-rectangular in outline. Axis convex, tapering backward, divided into three distinct segments, anterior two with small axial node. Pleural spines appear on each of anterior two segments, first longer than second, both extending well beyond pygidium. Pleural furrows and interpleural furrows weakly impressed. Degree 3 (M3).—Represented by four articulated specimens (Figs. 3.3, 6.3; Supplementary Data 1.5, 1.6; Table 1). Length varies between 1.14 and 1.26mm. Eye ridge long (exs.), convex, length (exs.) same as palpebral lobe width (tr.), fused with frontal glabellar lobe and extending posteriorly to fuse with palpebral lobe. Thoracic pleural spines extend beyond posterior margin of pygidium, anterior two of equivalent length and longer than third spine. Pygidial border narrow. Degree 4 (M4).—Represented by nine articulated specimens (Figs. 3.4, 6.4; Supplementary Data 2.1–2.3; Table 1). Length varies between 1.28 and 1.70mm. Genal spine at posterior of small librigena, not extending posterolaterally beyond occipital furrow, advanced. Lateral border narrow (tr.), slightly convex, width (tr.) same as anterior border length (sag.). Lateral border furrow shallow, wide (tr.). Thoracic pleural spines long, anterior two of equivalent length, then decreasing in length from third to fourth. Degree 5 (M5).—Represented by seven articulated specimens (Figs. 3.5, 6.5; Supplementary Data 2.4; Table 1). Length varies between 1.52 and 1.90mm. Cranidium trapezoid. Thoracic pleural spines long, all except first pleural spine extending beyond posterior margin of pygidium. Anterior three thoracic pleural spines of nearly equivalent length, then decreasing in length from fourth to fifth spine. Degree 6 (M6).—Represented by seven articulated specimens (Figs. 3.6, 6.6; Supplementary Data 2.5, 2.6; Table 1). Length varies between 1.64 and 2.12mm. Thoracic pleural spines long, all extending beyond posterior margin of pygidium, except for first and second pleural spines. Anterior four thoracic pleural spines of nearly equivalent length, then decreasing in length from fifth to sixth. Pygidium with three pairs of pleural spines present, decreasing in size rearwards. Degree 7 (M7).—Represented by three complete specimens (Figs. 3.7, 6.7; Supplementary Data 2.7, 2.8; Table 1).
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Length varies between 2.26 and 2.37mm. Axial furrow extending slightly beyond S3. First thoracic pleural spines shorter than the following three, which are of equivalent length, after which length gradually decreases posteriorly.
Degree 8 (M8).—Represented by eight articulated specimens (Figs. 3.8, 6.8; Supplementary Data 2.9, 2.10; Table 1). Length varies between 2.14 and 2.57mm. Cephalon sub- circular in outline. Librigena half fixigenal width (tr.). Fixigena convex slightly, slightly wider than half glabellar width. Relative length of thoracic pleural spines increases rearward in first five thoracic segments and thereafter decreases.
Degree 9 (M9).—Represented by six complete specimens (Figs. 3.9, 6.9; Supplementary Data 2.11, 2.12; Table 1). Length varies between 2.35 and 2.85mm. Relative length of long thoracic pleural spines increases rearward in first five thoracic segments and thereafter decreases.
Degree 10 (M10).—Represented by four articulated specimens (Figs. 4.1, 6.10; Supplementary Data 3.1; Table 1). Length varies between 2.84 and 3.05mm. Relative length of pleural spines increases rearward in first five thoracic segments, decreasing thereafter, with the posterior tips of the rearward trunk spines all extending posteriorly to roughly the same extent, forming an ontogenetically transient fringe of spines of approximately equal posterior extent.
Degree 11 (M11).—Represented by three articulated specimens (Figs. 4.2, 6.11; Supplementary Data 3.2; Table 1). Length varies between 2.84 and 3.87mm. Genal spine extends posterolaterally almost to posterior margin of cephalon. Pleural spine lengths approximately equal throughout much of thorax, but longest in mid-thorax. Posterior of trunk extends farther rearward than tips of longest thoracic pleural spines.
Degree 12 (M12).—Represented by two articulated specimens (Figs. 4.3, 6.12; Supplementary Data 3.3; Table 1). Fixigena slightly convex, half glabellar width. Librigena of medium-size, same width (tr.) as fixigena. Genal spine extends poster- olaterally up to T1. Pleural spines lengths approximately equal throughout much of thorax, longest in mid-thorax. Pygidial pleural spine seemingly absent on third pygidial segment.
Degree 13 (M13).—Represented by four complete specimens (Figs. 4.4, 6.13; Supplementary Data 3.4, 3.5; Table 1). Length varies between 3.72 and 4.55mm. Axial furrow deep, extending forward to merge with anterior border furrow. Librigena large, slightly wider than fixigena. Genal spine protruding strongly posterolaterally up to T2. Pygidial axis convex, divided into three indistinct segments, with pleural spine only showing on 14th trunk segment. Two pairs of pleural lobe furrows and one pair of interpleural lobe furrows present, extending posterolaterally. Posterior border narrow. Posterior margin rounded.
Figure 5. Late holaspides of Zhangshania typica from the Hongjingshao Formation, in Kunming, Yunnan Province, southern China: (1), Holaspid stage 3, YKLP 12264; (2), Holaspid stage 3, YKLP 12265; (3), enlargement and accompanied drawing of left antenna of the specimen in Figure 5.2: the yellow arrows indicate the upper boundaries of corresponding segments that are marked by yellow numbers; (4), enlargement of the proximal part of the antenna in figure 5.3; the paired yellow arrowheads indicate two spines on both sides of the associated article, except where a single arrowhead denotes one spine on the right side distally; white points represent the boundaries between articles; (5), Holaspid stage 4, YKLP 12266; (6), a single hypostome, YKLP 12267. Scale bars are (1, 3, 4, 6) 1.0mm; (2, 5) 5.0mm.
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