2. Fossil locality and material

Material for this study was recovered from the Asioptychaspis subglobosa Zone of the Hwajeol Formation in the Sagundari section (37° 04′ 57.0″ N, 129° 01′ 03.4″ E) (see Park & Choi, Reference Park and Choi2010, for locality map, and Park & Kihm, Reference Park and Kihm2015a for stratigraphic occurrences). The geological settings of the Taebaeksan Basin were given in detail by Choi et al. (Reference Choi, Chough, Kwon, Lee, Woo, Kang, Lee, Lee, Sohn, Shinn and Lee2004). The Hwajeol Formation is an alternating succession of limestone and shale beds, with frequent intercalation of limestone conglomerate beds, and is interpreted to have been deposited in inner to outer ramp environments (Kwon et al. Reference Kwon, Chough, Choi and Lee2006). Sohn & Choi (Reference Sohn and Choi2007) recognized three trilobite faunas within the formation: in ascending order, the Asioptychaspis Zone, the Quadraticephalus Zone and the saukiid-dominated fauna. Subsequently, Park & Kihm (Reference Park and Kihm2015a) revised the biostratigraphy of the lowermost part of the Hwajeol Formation, documenting 14 polymerid species belonging to 14 genera. They renamed the Asioptychaspis Zone the Asioptychaspis subglobosa Zone, and correlated it with the Asioptychaspis–Tsinania Zone of North China, and the Rhaptagnostus clarki patulus – Caznaia squamosa-Hapsidocare lilyensis and Rhaptagnostus clarki prolatus – Caznaia sectatrix zones of Australia, which are Jiangshanian (middle Furongian) in age. Limestone–shale couplets from the horizon 2.5 m above the base of the formation (CHBd08 horizon of Park & Kihm, Reference Park and Kihm2015a, fig. 1) were digested by hydrochloric acid to collect silicified specimens of trilobites. The recovered specimens included Psuedokoldinioidia sp. cf. P. granulosa Endo, Reference Endo1944, Tsinania canens (Walcott, Reference Walcott1905), Gumunsoia sp. 2, ?Mansuyia sp. 1, Guangxiaspis sp. 1, Caznaia coreaensis Park & Kihm, 2015, ?Saukia aojii (Kobayashi, Reference Kobayashi1933), Asioptychaspis subglobosa (Sun, Reference Sun1924), Koldinioidia choii Park & Kihm, Reference Park and Kihm2015a, and Akoldinioidia latus Park & Kihm, Reference Park and Kihm2015a (see Park & Kihm Reference Park and Kihm2015a). Ontogenetic researches of Tsinania canens, Asioptychaspis subglobosa and Haniwa quadrata have been carried out on the basis of the silicified specimens from this horizon (Park & Choi, Reference Park and Choi2009, Reference Park and Choi2010, Reference Park and Choi2011b, respectively). For this study, eight protaspides, more than 150 cranidia, 48 free cheeks, and 88 post-protaspid pygidia of A. latus, and two protaspides, 72 cranidia, five free cheeks and 53 post-protaspid pygidia of K. choii were collected. Length and width of protaspides, post-protaspid cranidia, and pygidia were measured; for the post-protaspid cranidia of A. latus, only 120 well-preserved post-protaspid cranidia were measured. Forty-one thoracic segments of shumardiids were also collected. The protaspides, cranidia, free cheeks, and morphologically mature pygidia of the co-occurring two shumardiids in this study were relatively easily distinguished. The meraspid pygidia of the two trilobites are quite similar in morphology, but the size and subtle differences in morphology enable them to be assigned to two different species. Nevertheless, it is currently almost impossible to differentiate thoracic segments of the two species; only those with the axial ring sculptured with transversely arranged nodes may be assigned to Akoldinioidia latus. The unassigned shumardiid thoracic segments are illustrated collectively. All of the specimens illustrated in this study are deposited in the palaeontological collections of the Korea Polar Research Institute, prefixed with KOPRIF.

3. Ontogeny of Akoldinioidia latus

Designation of lectotype. Park & Kihm (Reference Park and Kihm2015a) did not designate the holotype of this species, hence KOPRIF1189 (Fig. 1y) is designated here as the lectotype of A. latus.

Protaspid phase. Eight protaspides have been collected, which likely represent a single instar. The protaspid exoskeleton is oval in outline, 0.33–0.46 mm long and 0.30–0.38 mm wide, with the surface weakly effaced. The overall morphology is non-adult-like, being ovoid. The axis is clavate and convex, marked by slightly incised narrow axial furrows; the maximum width of the axis is 0.40–0.47 of the exoskeletal width. Three pairs of marginal spines are present; the laterally directed anteriormost pair is located at c. 0.4 of the exoskeletal length; the second pair of marginal spines is located at c. 0.7 of the exoskeletal length, and is directed obliquely backward; and the posteriormost pair of marginal spines is directed rearward from the posterior margin of the exoskeleton. The weakly sinuous facial suture is recognizable in lateral views (Fig. 1e, l). A doublure is present in the posterior margin. A notch is indented dorsally in the central part of the doublure (Fig. 1h, i). A pair of weak apodemes is recognized on the ventral side of the glabellar front (Fig. 1j).

Development of post-protaspid cranidium. During metamorphosis from the protaspid phase to the meraspid phase the cranidium became less convex, the lateral spines disappeared and the moderately wide axis became a slender glabella.

The smallest morphologically immature post-protaspid cranidia (Fig. 1m, n) are 0.27–0.30 mm long and 0.35–0.41 mm wide, and semicircular in outline. Given the similar width of the smallest cranidia and the protaspid exoskeletons (Fig. 2), the smallest cranidia likely includes the earliest meraspid instar. The glabellar is parallel-sided or conical, defined by moderately incised axial furrows, and is c. 0.36 of the cranidial width at the base. The preglabellar area is short, less than 0.1 of the cranidial length. The lateral margin is smooth and rounded. The posterior border is hardly defined. The posterior margin is abaxially curved rearward.

Figure 2. Scatter plots of exoskeletal length vs width dimensions of protaspides and post-protaspid cranidia of Akoldinioidia latus Park & Kihm, Reference Park and Kihm2015a from the Hwajeol Formation, Taebaeksan Basin, Korea. The dotted rectangle in (a) denotes the portion which is magnified in (b).

During the subsequent development (Fig. 1o–r), the preglabellar area became longer (sag.); more than one-tenth of the cranidial length. The axial furrows became deeper except for the frontal part. The posterior border is defined by a shallowly incised, narrow and straight posterior border furrow.

Subsequently, the cranidial outline became more transverse; the glabella became more broad-based and less tapering forward; the posterior border furrows abaxially curved rearward, eventually attaining the holaspid morphology (Fig. 1y) which has been described by Park & Kihm (Reference Park and Kihm2015a).

Development of free cheek. Free cheeks remained yoked throughout the ontogeny. The smallest free cheeks (Fig. 3a–j) have a narrow genal field, and short, tumid genal spines. The frontal part is slightly raised. Then the genal field became larger, and the frontal part became elevated to form a dorsally arched cephalic doublure. Interestingly, the genal spines seem to be morphologically variable throughout the ontogeny; some have short and tumid genal spines (Fig. 3k, m, r, y and ac), while others have a long genal spine (Fig. 3n, p, aa and ag).

Figure 3. Free cheeks of Akoldinioidia latus Park & Kihm, Reference Park and Kihm2015a from the Hwajeol Formation, Taebaeksan Basin, Korea. (a, b) KOPRIF1220, dorsal (a) and lateral (b) views; (c, d) KOPRIF1221, dorsal (c) and lateral (d) views; (e, f) KOPRIF1222, dorsal (e) and lateral (f) views; (g, h) KOPRIF1223, dorsal (g) and lateral (h) views; (i, j) KOPRIF1224, dorsal (i) and lateral (j) views; (k, l) KOPRIF1225, dorsal (k) and lateral (l) views; (m) KOPRIF1226; (n, o) KOPRIF1227, dorsal (n) and lateral (o) views; (p, q) KOPRIF1228, dorsal (p) and posterior (q) views; (r) KOPRIF1229; (s,t) KOPRIF1230, dorsal (s) and lateral (t) views; (u, v) KOPRIF1231, dorsal (u) and lateral (v) views; (w, x) KOPRIF1232, dorsal (w) and lateral (x) views; (y, z) KOPRIF1233, dorsal (y) and lateral (z) views; (aa–ab) KOPRIF1234, dorsal (aa) and lateral (ab) views; (ac, ad) KOPRIF1235, dorsal (ac) and lateral (ad) views; (ae–af) KOPRIF1236, dorsal (ae) and lateral (af) views; (ag, ah) KOPRIF1237, dorsal (ag) and lateral (ah) views; (aiak) KOPRIF1238, dorsal (ai), lateral (aj) and postero-lateral (ak) views. Scale bar is 1 mm.

Development of post-protaspid pygidium. The morphologically immature pygidia of Akoldinioidia latus has been divided into eight developmental stages including the earliest holaspid stage by size and morphology. The third and fourth thoracic segments likely have macropleural spines, but the macropleural spines of the fourth segment are much larger those of the third segment. The position of the fourth segment might be helpful in defining the degree of disarticulated meraspid pygidia. However, determining the meraspid degree for the earliest developmental stages is difficult because the macropleural spines are not prominent and the preservation is not sufficient. The developmental stages prior to the meraspid degree 2 are collectively grouped into the early meraspid developmental stage (EDS). Most shumardiids are known to have possessed six thoracic segments in the holaspid phase, although some species, such as Acanthopleurella grindrodi Groom, Reference Groom1902 (see Fortey & Rushton, Reference Fortey and Rushton1980) and Koldinioidia orientalis (Mansuy, Reference Mansuy1916) (see Zhu & Peng, Reference Zhu and Peng2006), had four or five thoracic segments. One of the oldest shumardiid species, Elaphraella microforma, from the Kaolishania fauna of the upper part of the Sesong Formation, which immediately underlies the Asioptychaspis subglobosa Zone of the Hwajeol Formation, had six thoracic segments (Park & Choi Reference Park and Choi2012a). With only disarticulated pygidia at hand, however, it is hard to ascertain whether Akoldinioidia latus had six thoracic segments or not. The designation of the last three developmental stages of the morphologically immature pygidia of A. latus as the meraspid degree 4, the meraspid degree 5, and the holaspid phase, in this study is based on an assumption that A. latus had six thoracic segments in the holaspid phase. There is likely to be no release of new thoracic segments between some developmental stages: between the meraspid degree 2a and meraspid degree 2b, and between meraspid degree 3a and meraspid degree 3b. Such a phenomenon is not uncommon in trilobite developments (see Hughes, Minelli & Fusco, Reference Hughes, Minelli and Fusco2006; Dai, Zhang & Peng, Reference Dai, Zhang and Peng2014), and is frequently observed in minute trilobites, such as eodiscoids (Dai & Zhang, Reference Dai and Zhang2011, Reference Dai and Zhang2013; Dai, Zhang & Peng, Reference Dai, Zhang and Peng2016).

The early meraspid developmental stage pygidia (Fig. 4a–c) are 0.19–0.0.24 mm long and 0.30–0.36 mm wide, and characterized by having multiple segments before the macropleural segment destined to be the fourth thoracic segment. One of the three specimens is assignable to meraspid degree 1 (Fig. 4c), but it is unclear whether the others belong to meraspid degree 0 or 1. They are inverted-trapezoidal in outline, with the posterior margin weakly indented medially. The axial width is 0.26–0.30 of the pygidial width. The wide axial furrows are weakly incised. Three axial rings including the terminal piece can be recognized. The segment destined to be the third thoracic segment has small pleural spines. n = 3.

Figure 4. Post-protaspid pygidia of Akoldinioidia latus Park & Kihm, Reference Park and Kihm2015a from the Hwajeol Formation, Taebaeksan Basin, Korea. (a–c) Early meraspid developmental stage (EDS) pygidia: (a) KOPRIF1239; (b) KOPRIF1240; (c) KOPRIF1241. (d–g) Meraspid degree 2a pygidia: (d) KOPRIF1242; (e) KOPRIF1243; (f) KOPRIF1244; (g) KOPRIF1245; (h) KOPRIF1246, ventral view. (i–k) Meraspid degree 2b pygidia: (i) KOPRIF1247; (j) KOPRIF1248; (k) KOPRIF1249. (l–o) Meraspid degree 3a pygidia: (l) KOPRIF1250; (m) KOPRIF1251; (n) KOPRIF1252; (o) KOPRIF1253, ventral view. (p–u) Meraspid degree 3b pygidia: (p) KOPRIF1254; (q) KOPRIF1255; (r) KOPRIF1256; (s) KOPRIF1257; (t) KOPRIF1258; (u) KOPRIF1259. (v–y), Meraspid degree 4 pygidia: (v) KOPRIF1260; (w) KOPRIF1261; (x) KOPRIF1262; (y) KOPRIF1263. (z–ab) Meraspid degree 5 pygidia: (z) KOPRIF1264; (aa) KOPRIF1265; (ab) KOPRIF1266. (ac–ae) Early holaspid stage pygidia: (ac) KOPRIF1267; (ad) KOPRIF1268; (ae) KOPRIF1269. (af–aj) Pygidia of later holaspid stages: (af) KOPRIF1270; (ag) KOPRIF1271; (ah) KOPRIF1272, ventral view; (ai) KOPRIF1273; (aj) KOPRIF1194. Scale bar is 0.5 mm for (a–ae), 0.83 mm for (af–ai), and 1 mm for (aj).

The meraspid degree 2a pygidia (Fig. 4d–h) are 0.18–0.27 mm long and 0.36–0.42 mm wide. The segment destined to be the third thoracic segment forms the anteriormost segment. The pleural spines of this segment are relatively longer than that of the previous stage. They are inverted-trapezoidal to rectangular in outline excluding the macropleural spines. The posterior margin is weakly indented medially. The axial width is 0.26–0.33 of the pygidial width. Three or four axial rings including the terminal piece can be recognized. n = 7.

The meraspid degree 2b pygidia (Fig. 4i–k) are 0.21–0.33 mm long and 0.40–0.50 mm wide. They are morphologically similar to the previous stage, but are larger and longer, probably due to the proliferation of new segments at the rear end. The variation in the length of the macropleural spines in the segment destined to be the fourth thoracic segment became prominent from this stage. Four axial rings including the terminal piece can be recognized. n = 14.

The meraspid degree 3a pygidia (Fig. 4l–o) are 0.23–0.30 mm long and 0.40–0.51 mm wide. The segment destined to be the fourth thoracic segment which has large macropleural spines forms the anteriormost segment. They are inverted-trapezoidal in outline excluding the macropleural spines. The posterior margin weakly indented medially. The axial width is 0.31–0.33 of the pygidial width. Three or four axial rings including the terminal piece can be recognized. n = 22.

The meraspid degree 3b pygidia (Fig. 4p–u) are 0.26–0.41 mm long and 0.42–0.55 mm wide. They are morphologically similar to the previous stage, but are larger and longer. The posterior margin is generally rounded, probably due to the proliferation of new segments at rear end. Pleural furrows are recognized in a specimen which does not have a rounded posterior margin (Fig. 4p). Four axial rings including the terminal piece can be recognized. n = 25.

The meraspid degree 4 pygidia (Fig. 4v–y) are 0.24–0.36 mm long and 0.41–0.54 mm wide. Due to the release of the large macropleural spines-bearing segment, they have a simple morphology, and the overall size is slightly smaller than the previous stage (Fig. 5). The pygidia are sub-oval in outline. The posterior margin is weakly indented medially or rounded. The axial width is 0.27–0.33 of the pygidial width. Five axial rings including the terminal piece can be recognized. Pleural and inter-pleural furrows are weakly incised. The anterior margin of pleurae runs horizontally outward, and then abruptly curves obliquely rearward. n = 7.

Figure 5. The relationship between the developmental stages and the size of the post-protaspid pygidia of Akoldinioidia latus Park & Kihm, Reference Park and Kihm2015a from the Hwajeol Formation, Taebaeksan Basin, Korea. Large holaspid pygidia are excluded. (a) Scatter plots of pygidial length vs width. (b) The mean for each stage, and the standard deviation bars extending to horizontal and vertical sides of the mean. EDS: early meraspid developmental stage; M2a: meraspid degree 2a; M2b: meraspid degree 2b; M3a: meraspid degree 3a; M3b: meraspid degree 3b; M4: meraspid degree 4; M5: meraspid degree 5; EH: early holaspid stage pygidia.

The meraspid degree 5 pygidia (Fig. 4z–ab) are 0.28–0.40 mm long and 0.49–0.65 mm wide. They have a similar morphology to the previous stage, but are larger, with a flat posterior margin. The axial width is 0.30–0.33 of the pygidial width. Five axial rings including the terminal piece can be recognized. The anterior margin of pleurae runs horizontally outward, and then smoothly curves obliquely rearward. n = 5.

The early holaspid stage pygidia (Fig. 4ac–ae) are 0.31–0.36 mm long and 0.57–0.74 mm wide. They have a more transverse outline, compared to those of the previous stage. The axial width is 0.30–0.33 of the pygidial width. A rim-like ridge running sub-parallel to the posterior margin is present, which is reminiscent of the rim-like ridge of Elaphraella nodus from the underlying Kaolishania fauna (see Park & Choi, Reference Park and Choi2012a). Four or five axial rings can be recognized in front of the rim-like ridge. n = 3.

During the subsequent development, the number of axial rings increased, eventually with six axial rings including the terminal pieces. The description of the large holaspid pygidia has been given by Park & Kihm (Reference Park and Kihm2015a) and is not repeated here.

4. Ontogeny of Koldinioidia choii

Protaspid phase. Two protaspid exoskeletons are available. One is 0.48 mm long and 0.40 mm wide, and the other is 0.61 long and 0.48 mm wide. In spite of the different size, it is uncertain whether the two protaspides represent different developmental stages, since their morphology is almost identical.

The protaspid exoskeleton is oval in outline, with the surface moderately effaced. The overall morphology is non-adult-like, being convex and ovoid, like that of A. latus. The clavate axis is broad, marked by weakly incised wide axial furrows; the maximum width of the axis is c. 0.6 of the exoskeletal width. A narrow preglabellar area is visible in the anterior views (Fig. 6d). Three pairs of marginal spines are present; the anteriormost pair is located at c. 0.63 of the exoskeletal length, and is oblique-posteriorly and oblique-dorsally directed; the second pair of marginal spines is located at c. 0.85 of the exoskeletal length, and is directed more oblique-posteriorly and oblique-dorsally than the anteriormost pair; and the ventrally directed posteriormost pair of marginal spines is located at each flank of a dorsally indented notch. The weakly sinuous facial suture is visible in lateral views (Fig. 6i). A moderately thick doublure with the dorsally indented notch in the central part is present in the posterior margin (Fig. 6f–h).

Figure 6. Protaspides, post-protaspid cranidia, and free cheeks of Koldinioidia choii Park & Kihm, Reference Park and Kihm2015a from the Hwajeol Formation, Taebaeksan Basin, Korea. (a–j) Protaspides: (a–d) KOPRIF1274, dorsal (a), oblique lateral (b), posterior (c) and oblique anterior (d) views; (e–j) KOPRIF1275, dorsal (e), ventral (f), oblique ventral (g), postero-ventral (h), lateral (i) and anterior (j) views. (k–v) Post-protaspid cranidia: (k) KOPRIF1276; (l) KOPRIF1277; (m) KOPRIF1278; (n) KOPRIF1279; (o) KOPRIF1280; (p) KOPRIF1281; (q) KOPRIF1282; (r) KOPRIF1283; (s) KOPRIF1284; (t) KOPRIF1175; (u) KOPRIF1285; (v) KOPRIF1286. (w–af) Free cheeks: (w, x) KOPRIF1287, dorsal (w) and oblique lateral (x) views; (y, z) KOPRIF1288, dorsal (y) and lateral (z) views; (aa, ab) KOPRIF1289, dorsal (aa) and lateral (ab) views; (ac, ad) KOPRIF1178, dorsal (ac) and lateral (ad) views; (ae, af) KOPRIF1179, dorsal (ae) and lateral (af) views. The scale bar is 0.5 mm for (a–n), 0.69 mm for (o–s) and 0.79 mm for (t–af).

Development of post-protaspid cranidium. The width of the smallest post-protaspid cranidia is somewhat greater than the width of the larger protaspis (Fig. 7), which means the earliest post-protaspid cranidial instar might be missing in this study. It should be noted that during metamorphosis from the protaspid to the meraspid phase the broad axis of the protaspides became a less broad glabella in the post-protaspid cranidia, as in A. latus.

Figure 7. Scatter plots of exoskeletal length versus width dimensions of protaspides and post-protaspid cranidia of Koldinioidia choii Park & Kihm, Reference Park and Kihm2015a from the Hwajeol Formation, Taebaeksan Basin, Korea. The dotted rectangle in (a) denotes the portion which is magnified in (b).

The smallest morphologically immature post-protaspid cranidia (Fig. 6k, l) are 0.42–0.45 mm long and 0.62–0.67 mm wide, and semicircular in outline. The tapering-forward glabella is convex, defined by moderately incised axial furrows. The glabellar width at the base is c. 0.4 of the cranidial width; the frontal part of the glabella is effaced, so the glabellar front is not delineated. The lateral margin is smooth and rounded. The posterior border is defined by a shallow border furrow, and slightly widens abaxially. The posterior margin is abaxially curved rearward.

During the subsequent development (Fig. 6m, n), the axial furrows and the posterior border furrows tend to become shallower. With development (Fig. 6o–s), the frontal part of the axial furrows became effaced, and the glabellar width at the base became c. 0.46 of the cranidial width. A slight and gentle constriction in the middle part of axial furrows can be recognized in some specimens (Fig. 6q). Subsequently, the glabella became broader-based, eventually attaining the holaspid morphology (Fig. 6v) described by Park & Kihm (Reference Park and Kihm2015a).

Development of free cheek. Only five yoked free cheeks were collected for this species, the largest two of which have been illustrated and described by Park & Kihm (Reference Park and Kihm2015a) (Fig. 6ac, ae). The free cheeks of K. choii are easily distinguished from those of A. latus in lacking genal spines and having a relatively wider genal field. The smaller free cheeks are morphologically similar to the larger free cheeks, but the genal field is significantly small (Fig. 6w, y). With growth, the genal field expanded, as in the case of the free cheeks of A. latus.

Development of post-protaspid pygidium. The morphologically immature pygidia of Koldinioidia choii has also been divided into eight developmental stages, including the earliest holaspid stage, by size and morphology. The third and fourth thoracic segments seem to have macropleural spines, but that of the fourth segment is much larger than that of the third segment as in A. latus. The position of the fourth segment could also be used to define the meraspid degree of disarticulated pygidia, but the developmental stages prior to meraspid degree 2 are also assigned to the early meraspid developmental stage (EDS). A congeneric species, Koldinioidia orientalis (Mansuy, Reference Mansuy1916), had five thoracic segments in holaspides (see Zhu & Peng, Reference Zhu and Peng2006). Nevertheless, the last three developmental stages of the morphologically immature pygidia of K. choii are designated as meraspid degree 4, meraspid degree 5, and the holaspid phase as in A. latus, because the trunk segmental pattern is morphologically more similar to A. latus than to K. orientalis, which had no macropleural segment. Still, it should be noted that this is based on the assumption that this species also had six thoracic segments in the holaspid phase. As in A. latus, there was likely to be no addition of new thoracic segments between meraspid degrees 2a and 2b, and between meraspid degrees 3a and 3b.

The early meraspid developmental stage pygidium (Fig. 8a) is 0.19 mm long and 0.41 mm wide. Due to the ambiguity of the position of the macropleural segments, it is unclear whether this single specimen belongs to meraspid degree 0 or 1. It is rectangular in outline, excluding the posteriorly protruded spines. The axial width is c. 0.27 of the pygidial width. The wide axial furrows are weakly incised. Three axial rings including the terminal piece can be recognized. Narrow pleural furrows are moderately incised, and inter-pleural furrows are more deeply incised. n = 1.

Figure 8. Post-protaspid pygidia of Koldinioidia choii Park & Kihm, Reference Park and Kihm2015a from the Hwajeol Formation, Taebaeksan Basin, Korea. (a) Early meraspid developmental stage (EDS) pygidium, KOPRIF1290. (b–d) Meraspid degree 2a pygidia: (b) KOPRIF1291; (c) KOPRIF1292; (d) KOPRIF1293, ventral view. (e–j) Meraspid degree 2b pygidia: (e) KOPRIF1294; (f) KOPRIF1295; (g) KOPRIF1296; (h) KOPRIF1297; (i) KOPRIF1298; (j) KOPRIF1299. (k–q) Meraspid degree 3a pygidia: (k) KOPRIF1300; (l) KOPRIF1301; (m) KOPRIF1302; (n) KOPRIF1303; (o) KOPRIF1304; (p) KOPRIF1305; (q) KOPRIF1306. (r–w) Meraspid degree 3b pygidia: (r) KOPRIF1307; (s) KOPRIF1308; (t) KOPRIF1309; (u) KOPRIF1310; (v) KOPRIF1311; (w) KOPRIF1312. (x–aa), Meraspid degree 4 pygidia: (x) KOPRIF1313; (y) KOPRIF1314; (z) KOPRIF1315; (aa) KOPRIF1316. (ab, ac) Meraspid degree 5 pygidia: (ab) KOPRIF1317; (ac) KOPRIF1318. (ad–ag) Early holaspid stage pygidia: (ad) KOPRIF1319; (ae) KOPRIF1320; (af) KOPRIF1321; (ag) KOPRIF1322. (ah–ao) Pygidia of later holaspid stages: (ah) KOPRIF1323; (ai) KOPRIF1324; (aj) KOPRIF1325; (ak) KOPRIF1326; (al) KOPRIF1327; (am) KOPRIF1328; (an) KOPRIF1329; (ao) KOPRIF1330. Scale bar is 0.5 mm for (a–an), and 0.62 for (ao).

The meraspid degree 2a pygidia (Fig. 8b–d) are 0.24–0.31 mm long and 0.42–0.50 mm wide. The segment destined to be the third thoracic segment forms the anteriormost segment. They are inverted-trapezoidal to rectangular in outline, excluding the macropleural spines. The surface is more effaced than at the previous stage. The posterior margin is weakly indented medially. The axial width is 0.30–0.33 of the pygidial width. Three or four axial rings including the terminal piece can be recognized. n = 5.

The meraspid degree 2b pygidia (Fig. 8e–j) are 0.19–0.40 mm long and 0.46–0.54 mm wide. They are morphologically similar to the previous stage, but are larger, and the macropleural spines are thicker. There seems to be a variation in the pygidial length; some are short and transverse (Fig. 8e, g), while others are long (Fig. 8h, i). The axial width is 0.30–0.34 of the pygidial width. Three or four axial rings including the terminal piece can be recognized. n = 11.

The meraspid degree 3a pygidia (Fig. 8k–q) are 0.18–0.29 mm long and 0.40–0.66 mm wide. The segment destined to be the fourth thoracic segment which has large macropleural spines forms the anteriormost segment. They are rectangular to sub-oval in outline excluding the macropleural spines. The posterior margin is weakly indented medially. The axial width is 0.25–0.30 of the pygidial width. Three or four axial rings including the terminal piece can be recognized. The length of the macropleural spines is variable. n = 11.

The meraspid degree 3b pygidia (Fig. 8r–w) are 0.22–0.37 mm long and 0.56–0.68 mm wide. They are morphologically similar to the previous stage, but are larger and slightly longer, due to the proliferation of new segments at the rear end. The macropleural spines are relatively thicker than those of the previous stages. The axial width is 0.28–0.32 of the pygidial width. Shallow and wide pleural furrows are recognized in the anteriormost segment. Four axial rings including the terminal piece can be recognized. n = 9.

The meraspid degree 4 pygidia (Fig. 8x–aa) are 0.20–0.32 mm long and 0.41–0.59 mm wide. Due to the release of the large macropleural spines-bearing segment, the overall size is significantly smaller than at the previous stage (Fig. 9). The pygidya are sub-oval in outline. The axial width is 0.29–0.33 of the pygidial width. Four or five axial rings including the terminal piece can be recognized. Pleural and inter-pleural furrows are weakly incised. n = 7.

Figure 9. The relationship between the developmental stages and the size of the post-protaspid pygidia of Koldinioidia choii Park & Kihm, Reference Park and Kihm2015a from the Hwajeol Formation, Taebaeksan Basin, Korea. Large holaspid pygidia are excluded. (a) Scatter plots of pygidial length vs width. (b) The mean for each stage, and the standard deviation bars extending to horizontal and vertical sides of the mean. EDS: early meraspid developmental stage; M2a: meraspid degree 2a; M2b: meraspid degree 2b; M3a: meraspid degree 3a; M3b: meraspid degree 3b; M4: meraspid degree 4; M5: meraspid degree 5; EH: early holaspid stage pygidia.

The meraspid degree 5 pygidia (Fig. 8ab–ac) are 0.29–0.30 mm long and c. 0.61 mm wide. They have a similar morphology to the previous stage, but are larger, and can be distinguished by the relatively wide axis; the axial width is 0.34–0.35 of the pygidial width. Despite the poor preservation, it is likely that there are five axial rings including the terminal piece. n = 2.

The early holaspid stage pygidia (Fig. 8ad–ag) are 0.25–0.37 mm long and 0.50–0.65 mm wide. The axial width is 0.30–0.35 of the pygidial width. A rim-like ridge running sub-parallel to the posterior margin is present as in A. latus and Elaphraella nodus. Four axial rings including the terminal piece can be recognized in front of the rim-like ridge. The anterior margin of pleurae runs horizontally outward, then abruptly curves obliquely rearward. The posterior margin is rounded. n = 7.

During the subsequent development, the number of axial rings did not increase. The description of the large holaspid pygidia has been given by Park & Kihm (Reference Park and Kihm2015a) and is not repeated here.

5. Thoracic segments

The meraspid pygidial morphology and development of the two shumardiid species indicate that they likely had a similar morphological pattern for thoracic segments, so distinguishing disarticulated thoracic segments of the two shumardiids is extremely difficult. Therefore, thoracic segments of the two shumardiids are described collectively here. The morphology of the meraspid pygidia and the articulated trunk of the Furongian shumardiids, Elaphraella microforma (see Park & Choi Reference Park and Choi2012a) and Koldinioidia orientalis (see Zhu & Peng, Reference Zhu and Peng2006), helps to determine the relative position of disarticulated thoracic segments within the thorax. The thoracic segments could be divided into three groups: the first and second thoracic segments (Fig. 10a–g); the third and fourth macropleural thoracic segments (Fig. 10h–aq); and the fifth and sixth thoracic segments (Fig. 10ar–aw).

Figure 10. Thoracic segments of the two shumardiids, Akoldinioidia latus Park & Kihm, Reference Park and Kihm2015a and Koldinioidia choii Park & Kihm, Reference Park and Kihm2015a from the Hwajeol Formation, Taebaeksan Basin, Korea. (a–g) The first and second thoracic segments: (a) KOPRIF1331; (b, c) KOPRIF16035, dorsal (b) and anterior (c) views; (d) KOPRIF1332; (e–g) KOPRIF1333, dorsal (e), anterior (f) and lateral (g) views. (h–aq) the third and fourth thoracic segments: (h,i) KOPRIF1334, dorsal (h) and lateral (i) views; (j) KOPRIF1335; (k) KOPRIF1336; (l, m) KOPRIF1337, dorsal (l) and lateral (m) views; (n) KOPRIF1338, ventral view; (o) KOPRIF1339; (p) KOPRIF1340; (q) KOPRIF1341; (r) KOPRIF1342; (s, t) KOPRIF1343, dorsal (s) and lateral (t) views; (u) KOPRIF1344; (v) KOPRIF1345; (w) KOPRIF1346; (x) KOPRIF1347; (y, z) KOPRIF1348, ventral (y) and anterior (z) views; (aa, ab) KOPRIF1349, dorsal (aa) and lateral (ab) views; (ac) KOPRIF1350; (ad, ae) KOPRIF1351, dorsal (ad) and lateral (ae) views; (af, ag) KOPRIF1352, dorsal (af) and lateral (ag) views; (ah, ai) KOPRIF1353, dorsal (ah) and lateral (ai) views; (aj) KOPRIF1354; (ak, al) KOPRIF1355, dorsal (ak) and anterior (al) views; (am) KOPRIF1356 – the left macropleural spine is morphologically different from the right macropleural spine due to abnormality; (an–ap) KOPRIF1357, dorsal (an), posterior (ao) and lateral (ap) views; (aq) KOPRIF1358. (ar–aw) the fifth and sixth thoracic segments: (ar–at) KOPRIF1359, dorsal (ar), anterior (as) and lateral (at) views; (au) KOPRIF1360; (av, aw) KOPRIF1361, dorsal (av) and anterior (aw) views. Scale bar is 1 mm.

The first and second thoracic segments tend to have a wider axis than the others; the maximum width of the axis is 0.4–0.5 of the width of the thoracic segment. The pleurae are deflected rearward. Clearly incised pleural furrows are curved rearward. Two small nodes are sculptured on the axial ring of the largest specimen (Fig. 10b). The pleural region is convex. The anterior band is about twice as long (sag.) as the posterior band. The articulating facets are relatively narrow in some specimens (Fig. 10a, b), but wider in others (Fig. 10d, e). It is likely that those with narrow facets represent the first thoracic segment. Park & Choi (Reference Park and Choi2011b, fig. 8a, b) mistakenly assigned a thoracic segment (likely the first thoracic segment) of a shumardiid to Haniwa quadrata Kobayashi, Reference Kobayashi1933, which is re-illustrated herein (Fig. 10b, c).

The third and fourth thoracic segments are distinguished by the presence of macropleural spines. Given the morphology of meraspid degree 2, the macropleural spine of the fourth segment is significantly longer than that of the third segment, and the macropleural spine of the third segment of K. choii is likely to be shorter than that of A. latus. However, because it is unknown how the macropleural spines of the third thoracic segments developed after releasing from the meraspid pygidium, and the length of the macropleural spines of the fourth segments are rather varied in the meraspid pygidia, differentiation of the third and fourth thoracic segments is not attempted for now. The axis is c. one-third of the width of the thoracic segment. The pleural furrows are curved rearward and extend slightly into the pleural spine. The smaller specimens tend to have a straight macropleural spine (Fig. 10h–l). As they grow, the macropleural spines of some specimens became so curved that the endpoints are directed adaxially (Fig. 10q–s, v–af), while those of some other specimens are moderately curved and the endpoints are directed rearward (Fig. 10u, aj, am–aq), or are still straight (Fig. 10n, ah). The axial rings of the largest specimens at hand are sculptured with transversely arranged nodes which are reminiscent of those in the holaspid pygidia of A. latus (Fig. 10ak–aq).

The thoracic segments possibly belonging to the fifth and sixth segments are relatively rare (Fig. 10ar–aw). The pleural region of these specimens is slightly directed rearward with the axial width c. one-third or less than one-third of the width of the thoracic segment. One of the specimens has transversely arranged nodes on the axial ring and the anterior band (Fig. 10ar).

6. Ontogenetic comparison of two co-occurring shumardiids

The two shumardiids in this study have different cranidial and pygidial morphology from the protaspid phase onward, although the thoracic segments of the two shumardiids are not distinctive enough for differentiation. The protaspides of Koldinioidia choii are larger than those of Akoldinioidia latus. Comparing protaspid morphology requires careful consideration (Park & Choi, Reference Park and Choi2011c), and the size difference in protaspides in closely related taxa may have been due to the difference in the appearance timing of the first articulation, as in the case of the two closely related ptychaspidids, Asioptychaspis subglobosa (Sun, Reference Sun1924) and Quadraticephalus elongatus Kobayashi, Reference Kobayashi1935 (see Kihm, Park & Choi, Reference Kihm, Park and Choi2013). However, given the metamorphosis occurring at the protaspid/meraspid phase transition, and the significantly wider axial width in the protaspides of K. choii, the size difference of the two shumardiid protaspides does not seem to be related to the difference in appearance timing of the first articulation. The axial width in the protaspides became relatively less in the post-protaspid cranidia of both trilobites. The macropleural segments in meraspid pygidium tend to be thicker in K. choii. The pygidia of K. choii are wider than those of A. latus at the equivalent meraspid stage (Fig. 11). In the trunk segmentation schedule diagrams (Fig. 12), the two trilobites had a similar number of pygidial segments from meraspid degree 2a to meraspid degree 5. Interestingly, the number of trunk segments in A. latus kept increasing after entering the holaspid phase, until there were six axial rings including the terminal piece in the pygidium, while those of K. choii did not increase after meraspid degree 5 (Fig. 12). This indicates the trunk developmental mode of A. latus is protarthrous, in which the onset of the holaspid phase preceded the onset of the epimorphic phase, while that of K. choii is protomeric in which the onset of the epimorphic phase precedes the onset of the holaspid phase (see Hughes, Minelli & Fusco, Reference Hughes, Minelli and Fusco2006).

Figure 11. Comparison of the mean for each stage of pygidial development of two shumardiid trilobites in this study. Note that the pygidia of K. choii are wider than those of A. latus at equivalent meraspid stage.

Figure 12. Trunk segmentation schedule diagrams for the two shumardiids in this study. Where the exact number of axial rings in the pygidium is equivocal, the maximum number was taken to reconstruct the diagrams. Note that the trunk developmental mode of A. latus is protarthrous, in which the onset of the holaspid phase precedes the onset of the epimorphic phase, while that of K. choii is protomeric in which the onset of the epimorphic phase precedes the onset of the holaspid phase. EDS: early meraspid developmental stage; M2a: meraspid degree 2a; M2b: meraspid degree 2b; M3a: meraspid degree 3a; M3b: meraspid degree 3b; M4: meraspid degree 4; M5: meraspid degree 5; EH: early holaspid stage pygidia; LH: late holaspid stage pygidia.

Since no thoracic segment was released between M2a and M2b, and M3a and M3b in the two shumardiid trilobite developments, the growth rate between those developmental stages can be calculated. The size of the complicated morphology of the spine bearing meraspid pygidia is difficult to measure precisely, but the estimated size from simple pygidial length and pygidial width (see Supplementary Material at https://doi.org/10.1017/S0016756817000012) can be used for simplicity. It reveals that in A. latus, the growth rate between M2a and M2b is c. 1.26, and interestingly, that between M3a and M3b is also c. 1.26. This value seems rather high, but is still within the range of average growth rate (AGR) values for meraspid pygidium (1.03–1.33) suggested by Fusco et al. (Reference Fusco, Garland, Hunt and Hughes2012). However, in K. choii, the growth rate between M2a and M2b is c. 1.10, while that between M3a and M3b is c. 1.38, which beyond the range of AGR values given by Fusco et al. (Reference Fusco, Garland, Hunt and Hughes2012). This might indicate that the growth rate was rather high in K. choii, but the AGR value of K. choii could be much lower if the growth rates between all moults within the meraspid degree are taken into account.