Hou et al.—Lower Cambrian trilobite ontogeny southern China
Table 1. Morphometric data summarizing the size changes during the ontogenetic series of Zhangshania typica described in this paper. The units are in millimeters (mm).
Exoskeleton Length
M10 M11 M12 M13 H1 H2 H3 H4
M2 M3 M4 M5 M6 M7 M8 M9
0.96–1.22 1.14–1.26 1.28–1.70 1.52–1.90 1.64–2.12 2.26–2.37 2.14–2.57 2.35–2.85 2.84–3.05 2.84–3.87 –
3.72–4.55 4.53–6.55 5.26–11.28 8.96–27.06 32.51–85.00
Cranidium Length
1.50–1.93 1.73–2.26 1.84–3.67 2.87–7.50 8.69–22.00
0.63–0.72 0.71–0.81 0.70–0.93 0.86–1.07 0.83–1.05 1.08–1.17 1.01–1.22 1.06–1.28 1.22–1.31 1.19–1.55 1.55
Width
1.30–1.63 1.52–1.90 1.47–3.11 2.49–7.28 8.19–23.00
0.50–0.61 0.59–0.68 0.69–0.80 0.76–0.87 0.84–0.90 0.89–1.00 0.95–1.10 1.01–1.12 1.13–1.25 1.21–1.34 1.28
Length
0.12–0.23 0.23–0.43 0.34–1.34 0.95–3.73 5.49–10.82
0.10–0.17 0.09–0.15 0.11–0.16 0.12–0.17 0.07–0.22 0.15–0.17 0.09–0.22 0.11–0.22 0.12–0.18 0.10–0.32 0.14
Pygidium Width
0.23–0.46 0.50–0.67 0.53–1.67 1.10–5.50 6.26–17.52
0.21–0.37 0.21–0.28 0.26–0.35 0.26–0.38 0.18–0.45 0.32–0.40 0.24–0.42 0.27–0.39 0.33–0.40 0.32–0.69 0.28
0.09–0.68 0.28–6.24 6.79–13.03
Pygidial spine Length
95
Holaspid period.—60 specimens ranging in size from 4.53 to 85.0mm long (Figs. 4.5, 4.6, 5.1–5.6, 6.14–6.17; Supplementary Data 3.6–3.9; Table 1), all of which have fourteen thoracic segments. They are divided into a series of stages based on the number of segments expressed in the pygidium. Stage 1 (H1).—Represented by three specimens (Figs. 4.5, 6.14; Supplementary Data 3.6, 3.7; Table 1). Length varies from 4.53 to 6.55mm. Glabella conical, convex, extending forward to anterior border furrow, rounded in front. Eye ridge long (exs.), convex, bifurcated, fused with frontal glabellar lobe and front of glabellar lobe 3, extending posterolaterally and gradually becoming short (exs.). Occipital ring slightly curved rearward adaxially, with small medial node weakly expressed. Fixigena gently convex, slightly narrower than half glabellar width. Librigena wide (tr.). Genal spine extending up to T2, base located opposite posterior corner of cranidium. Lateral border slightly convex, same width (tr.) as anterior border length (sag.). Thoracic axis convex, each segment with small central node. Pygidial axis convex, divided into four segments. Pleural lobe triangular, extending posteriorly into single pair of short pygidial spines. Two pairs of pleural furrows weakly impressed. Stage 2 (H2).—Represented by 18 articulated specimens (Figs. 4.6, 6.15; Supplementary Data 3.8, 3.9; Table 1). Length varies between 5.26 and 11.28mm. Preglabellar field short. Fixigena narrow and flat, less than half glabellar width. Anterior branches of facial suture markedly anterolaterally divergent. Posterior branches of facial suture extend strongly outward and then curve backward. Posterior area of fixigena short (sag.) and wide (tr.). Librigena wide, one and half times width of fixigena. Genal spine slender, extending posteriorly up to T3. Pleural spines increasing in length from the first to fifth pleural spines, after which spine lengths decrease gradually backwards. Pygi- dium of moderate size. Axis convex, divided into five segments. Pleural lobe triangular with pleurae sloping downward, with two pleural furrows weakly impressed extending posterolaterally. Pygidial spine extends backward beyond pygidial margin. Border narrow. Stage 3 (H3).—Represented by 32 articulated specimens (Figs. 5.1–5.4, 6.16; Table 1). Length varies between 8.96 and 27.06mm. Antennae slightly longer than cephalic length, tapering distally, consist of at least 28 articles.
Articles range from width (tr.) slightly longer than length (exs.) at base of antennae to width almost half of article length terminally. Proximal six articles, each ornamented with pair of sharp spines on anterior corners. More distal articles with single sharp, adaxially directed spine. Cephalon semi-circular in outline. Cranidium sub-trapzoidal, convex. Glabella conical, convex, extending forward almost to anterior border furrow, rounded in front. Three transglabellar furrows weakly impres- sed, shallowest axially: S1 and S2 extend backward slightly adaxially; S3 shallow, transverse. Anterior lobe of hypostomal median body elliptical, distinctly convex. Preglabellar field short (sag.) but distinct. Eye ridge slightly convex, bifurcated, fused anteriorly with frontal glabellar lobe and front of glabellar lobe 3, tapering posterolaterally to fuse with palpebral lobe. Palpebral lobe small, two fifths of cephalic length, convex, located opposite of glabellar lobes 2 and 3. Axial furrow wide (tr.) and deep, extending forward into anterior border furrow. Anterior border of constant length (sag.), curved forward slightly. Anterior border furrow shallow. Anterior branches of facial suture diverging lateral strongly and then sweeping adaxially forward, posterior branches of facial suture diverging laterally, then sweeping backward to posterior margin. Fixigena narrow, flat, one third of glabellar width. Librigena flat, wider (tr.) than twice width of fixigena. Genal spine long, extending backward to T4 or T5. Lateral border narrow (tr.), slightly convex, widening posteriorly. Lateral border furrow shallow. Occipital furrow shallow, curved posteriorly slightly. Occipital ring curves posteriorly slightly adaxially, with small axial medial node. Thoracic pleural lobe subrectangular, slightly wider (tr.) than axial width (tr.), curving slightly posterior abaxially. Pleural furrow long (exs.) and deep, extending posterolaterally. Thoracic pleural spine continues curve of pleural furrow, slightly increasing in length from anterior to posterior. Pygidium large, sub-trapzoidal. Axis convex, divided into six segments by three firmly impressed anterior axial furrows, followed by two weakly impressed furrows. Terminal axial piece large, with posterior margin curved inward defining a pair of rounded lateral lobes. Postaxial region short in early part of this phase, becoming gradually longer ontogenetically, up to one seventh of pygidial length (sag.). Pleural lobe triangular, with even slope toward margin. Three pairs of pleural furrows
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