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Morphology and ontogeny of Hunanocephalus ovalis (trilobite) from the Cambrian of South China
Gondwana Research 25 (2014) 991–998
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Morphology and ontogeny of Hunanocephalus ovalis (trilobite) from the Cambrian of South China Tao Dai a, b, Xingliang Zhang a,⁎, Shanchi Peng b a b
State Key Laboratory for Continental Dynamics and Department of Geology, Northwest University, Xian 710069, PR China State Key Laboratory of Palaeobiology and Stratigraphy (Nanjing Institute of Geology and Palaeontology, CAS), Nanjing, Jiangsu 210008, PR China
a r t i c l e
i n f o
Article history: Received 31 August 2012 Received in revised form 15 March 2013 Accepted 1 May 2013 Available online 13 May 2013 Keywords: Trilobita Corynexochida Ontogeny Lower Cambrian South China
a b s t r a c t The juvenile morphology and ontogeny of the Cheiruroidid trilobite Hunanocephalus ovalis Lee, 1963 from the lower Cambrian Shuijingtuo Formation in Hubei Province, South China is presented. The new material comprises a relatively complete meraspid ontogenetic series (degree 0 to 10), which reveals more details on their morphological changes such as the contraction and disappearance of the pronounced posteromedial notch in the pygidium and the addition of the trunk segments, which are all documented for the first time and can also be used as developmental markers defining their ontogenetic phases. The trunk segmentation schedule of H. ovalis is also discussed, which is similar to the other early Cambrian oryctocephalid trilobites, i.e. as the boundary between the thorax and pygidium migrated posteriorly there is no change in the trunk segment number; the processes of liberation of the thoracic segment and segment insertion into the pygidium are separated from one another, implying that the control of trunk exoskeletal segment appearance and articulation might be decoupled in these trilobites. © 2013 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
1. Introduction The corynexochid trilobites spanned a long time ranging from the early Cambrian to the middle Devonian, but little is known of their complete ontogenetic sequences, with a small quantity of species having been published (e.g. Albertella limbata and Fieldaspis quadriangulatus (Hu, 1985a); Bathyuriscus fimbriatus (Rasetti, 1967; Robison, 1967); Bathyuriscus? sp. (Fortey and Chatterton, 1988); Chilometopus artus and Corynexochus felix (Suvorova, 1964); Corynexochus minor (Walcott, 1916); Dentaloscutellum hudsoni and Scutellum calvum (Chatterton, 1971); Fuchoia fecunda (Öpik, 1982); Fuchoia chiai (Lu and Qian, 1983); Glossopleura boccar (Hu, 1985b); Mendospidella digesta (Poulsen, 1958); Ogygopsis klotzi (Walcott, 1916; McNamara and Rudkin, 1984); Ptarmigania aurita (Hu, 1971) and Thoracocare minuta (Robison and Campbell, 1974)). However, all the materials published are those from the periods posterior to the Series 2 of the Cambrian and most of them were principally based upon the descriptions and illustrations of incomplete ontogenetic sequences. As for those from the Series 2 of the Cambrian, to our knowledge, none but McNamara et al. (2003, 2006) gave a detailed ontogenetic investigation on four genera of the oryctocephalids from the Balang Formation in eastern Guizhou Province, southwest China, i.e. Arthricocephalus Bergeron, 1899, Balangia Chien, 1961, Changaspis Lee in Qian, 1961 and Duyunaspis Chang and Chien in Zhou et al., 1977.
⁎ Corresponding author. Tel.: +86 02988303200; fax: +86 02988363659. E-mail address: [email protected] (X. Zhang).
The early Cambrian Shuijingtuo Formation (Zhang, 1953), exposed at Changyang County in western Hubei Province, South China (Dai and Zhang, 2011, text-fig. 1) contains a rich fauna of trilobites, dominated by two genera of eodiscinids: Tsunyidiscus (Zhang, 1966) and Sinodiscus Zhang in Lu et al. (1974a), and three genera of polymerids: Estaingia Pocock, 1964, Metaredlichia Lu, 1950 and Hunanocephalus Lee, 1963. Among these, the ontogenetic development of several species has been described in detail elsewhere, e.g. Tsunyidiscus acutus (Dai and Zhang, 2011), Estaingia sinensis (Dai and Zhang, 2012a), Metaredlichia cylindrica (Dai and Zhang, 2012b) and Sinodiscus changyangensis (Dai and Zhang, 2013a). As for Hunanocephalus, Lee in Egorova et al. (1963) first described the adult specimens of Hunanocephalus ovalis Lee, 1963 based on the material that comprises a few articulated exoskeletons and cranidia from the lower Cambrian Shuijingtuo Formation in Rongxi, Xiushan, Sichuan Province, China, characterized by the broad axis, trapeziform glabella, three pairs of glabellar furrows and 12 or 13 thoracic segments. The present paper deals with the first known ontogenetic series of H. ovalis Lee, 1963 from the lower Cambrian of South China. Besides displaying new details on morphology and morphogenesis during ontogeny, the juvenile instars of H. ovalis offer a unique opportunity for understanding the exceptional schedule on the trunk development of the Cheiruroidid trilobite, i.e. the release of the thoracic segment was not associated with the addition of any new segment, and the addition of new segment does not correspond with each molting event. By comparison with the ontogenetic sequence of the other early Cambrian oryctocephalids (McNamara et al., 2003, 2006), the unique trunk segmentation schedule may be shared with these early occurring
1342-937X/$ – see front matter © 2013 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.gr.2013.05.004
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corynexochid trilobites, which played a significant role in controlling the number of their adult thoracic segments. 2. Locality and material Fieldwork during 2008–2011 resulted in a collection of some thousand disarticulated and articulated sclerites left by juvenile instars of H. ovalis. All the material were collected from the carbonaceous black shales of the lower part of the Shuijingtuo Formation (Stage 3, Series 2 of the Cambrian) at the Dingjiaping section, about 5.5 km northwest of Changyang County, Hubei Province, South China (for stratigraphy see Dai and Zhang, 2011, text-fig. 1). This formation is overlain by the yellowish-green, silty-shale and -mudstone of the Shipai Formation, but the base of the formation is not exposed in the section. Despite the large number of specimens, most are essentially uninformative due to fragmentation or diagenetic deformation. Among these, 128 complete meraspid specimens have thus been selected for detailed study, of which all dimensions were measured as straight-line distances and the measurements of the sagittal length are made from the anterior cranidial margin to the posterior pygidial margin. The described and figured material from this study is housed in the collection of the Geology Department of Northwest University, Xian, China (NWULXHN 20201–20223). 3. Systematic paleontology Terminology. — Morphological terms and abbreviations used in this paper largely follow Whittington and Kelly (1997). In addition, some abbreviations are used in the description below: excl., excluding; exs., exsagittal; incl., including; L, length; LA, frontal glabellar lobe; sag., sagittal; T, tergite; tr., transverse; W, width; M0–M9, meraspid degree 0 to 9 respectively. Order Corynexochida Kobayashi, 1935 Suborder Corynexochina Kobayashi, 1935 Family Cheiruroididae Zhang, 1963 Genus Hunanocephalus Lee, 1963
Remarks. — Zhou in Lu et al. (1974b) proposed the subgenus Hunanocephalus (Duotingia), within which Zhou placed H. duotingensis Chow in Lu et al. (1974b). However, comparing this subgenus with the genus Hunanocephalus, there is no character sufficiently different to warrant their emplacement in a different subgenus. Consequently, it should be regarded as a junior synonym of Hunanocephalus. In addition, on account of the co-occurrence with T. acutus and S. changyangensis, Hunanocephalus could be assigned to Tsunyidiscus–Sinodiscus Zone (Chiungchussuan). Type species. — H. ovalis Lee in Egorova et al. (1963) from the lower Cambrian Shuijingtuo Formation in Rongxi, Xiushan, Sichuan Province, China. H. ovalis Lee, 1963 H. ovalis Lee, Egorova et al. (1963), p. 14, pl. 1, figs. 1–7. H. ovalis Lee, Lu et al. (1963), p. 26, pl. 1, fig. 4. H. ovalis Lee, Zhou et al. (1977), p. 128, pl. 41, fig. 1. H. ovalis Lee, Zhang et al. (1980), p. 280, pl. 94, fig. 12. 3.1. Meraspid ontogeny One hundred and twenty-eight articulated specimens assigned to meraspid phases of H. ovalis Lee, 1963 are investigated and their sizes range from 0.61 to 6.18 mm in length (Figs. 1–4). The morphological changes during this period (M0–M10) were very subtle. In addition to the morphological changes of some structures as developmental markers, such as the contraction and disappearance of the pronounced posteromedial notch in the pygidium, the ontogenetic tracing of each stage in meraspid period is mainly based upon the number of thoracic and pygidial segments. Degree 0 (M0). — Exoskeleton sub-elliptical in outline, 0.61–0.64 mm in length, represented by two articulated specimens (Figs. 1A, 5A). Cranidium trapeziform in outline, of low convexity. Anterior margin slightly curved forward; anterior border indistinct. Glabella wide and of low convexity, three (?) pairs of glabellar furrows weakly impressed. LO moderately convex and short (sag.), with posterior margin curved
Fig. 1. Complete meraspides of Hunanocephalus ovalis Lee, 1963 from the Shuijingtuo Formation, in Dingjiaping, Changyang County, Hubei Province, South China. A, Degree 0, NWULXHN 20201; ×67. B, Degree 1, NWULXHN 20202; ×50. C–D, Degree 2, NWULXHN 20203–20204; ×35, ×45. E–F, Degree 3, NWULXHN 20205–20206; ×30, ×32.
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backward, occipital furrow shallow. Facial suture indistinct. Fixigena wide (tr.), subequal width (tr.) of glabella. Posterior border furrow shallow. Pygidium small, semi-elliptical in outline, with posterior margin not preserved. Axis wide (tr.), tapering evenly backward, with posterior tip not preserved; divided into three (four?) segments, defined by posteriorly curved axial ring furrows. Axial rings lacking spines, successively shorter (sag.) and narrower (tr.) posteriorly. Pleural field of low convexity. Pleural furrow indistinct. A pair of interpleural furrows differentiating the anteriormost pygidial segment from the succeeding ones can be observed anteriorly, defining the posterior edge of the anterior segment that would be released into the thorax as the first thoracic segment. Degree 1 (M1). — Exoskeleton 0.92 mm in length, represented by a single articulated specimen (Figs. 1B, 5B). Cranidium sub-trapezoidal in outline, moderately convex. Anterior margin moderately straight;
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Anterior border narrow (sag., exs.). Glabella sub-trapezoidal in outline, narrower (tr.) in the middle portion, slightly expanded anteriorly; three transglabellar furrows weakly impressed. Facial suture opisthoparian, anterior branch slightly convergent forward; posterior branch of facial suture extending posterolaterally. Eye ridge and palpebral lobe indistinct. Posterior margin straight laterally and then curved anterolaterally to lateral margin. Posterior border extremely narrow (exs.); posterior border furrow shallow, extending anterolaterally. Thorax with one segment. Axial ring wide and of low convexity, weakly defined by shallow axial furrows, approximately 1.5 times wider than pleurae. Pleurae moderately flat, of equal width (exs.) laterally, with pleural spine short and obtuse, pleural furrow shallow. Pygidium small, semi-elliptical in outline, with posterior margin concave medially, W-shaped in outline. Axis wide (tr.), divided into four segments, tapering evenly backward. Pleural field of low convexity. Pleural furrow indistinct.
Fig. 2. Complete meraspides of Hunanocephalus ovalis Lee, 1963 from the Shuijingtuo Formation, in Dingjiaping, Changyang County, Hubei Province, South China. A–C, Degree 4, NWULXHN 20207–20209; ×30, ×29, ×28. D–F, Degree 5, NWULXHN 20210–20212; ×28, ×24, ×24. G–I, Degree 6, NWULXHN 20213–20215; ×20, ×23, ×24.
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Fig. 3. Complete meraspides of Hunanocephalus ovalis Lee, 1963 from the Shuijingtuo Formation, in Dingjiaping, Changyang County, Hubei Province, South China. A–B, Degree 7, NWULXHN 20216–20217; ×17, ×20. C–E, Degree 8, NWULXHN 20218–20220; ×15, ×31, ×18. F, Degree 9, NWULXHN 20221; ×19.
Degree 2 (M2). — Exoskeleton 0.94–1.39 mm in length, represented by seven articulated specimens (Figs. 1C–D, 5C). Thorax with two segments. Axis of low convexity, weakly defined by shallow axial furrows; axial ring of T2 narrower (tr.) than that of T1. Pleurae moderately flat, gently shorter (exs.) and narrower (tr.) from T1 to T2. Pygidium small, semi-elliptical in outline, with anterior margin extending posterolaterally
Fig. 4. Complete meraspides of Hunanocephalus ovalis Lee, 1963 from the Shuijingtuo Formation, in Dingjiaping, Changyang County, Hubei Province, South China. A–B, Degree 10, NWULXHN 20221–20223; ×17, ×25.
and posterior margin concave medially, W-shaped in outline. Axis convex, divided into three (Figs. 1C, 5C1) and four segments (Figs. 1D, 5C2). Pleural field of low convexity, poorly segmented. Degree 3–10 (M3–10). — Exoskeletons range from 1.26 to 6.18 mm in length (Figs. 1E–F, 2–4, 5D–K). In addition to the extra thoracic segments and some striking variations that can be regarded as developmental markers defining their ontogenetic phases, morphological changes among these phases were very subtle, consisting most obviously of the disappearance of the pronounced posteromedial notch in the pygidium. Of the collected material, two stages can also be recognized in M5, M6, M7 and M8 on the basis of the pygidial segment number, respectively. Degree 3 (M3). — Exoskeleton 1.26–1.77 mm in length, represented by 14 articulated specimens (Figs. 1E–F, 5D). Pygidium with four axial segments; posterior margin slightly curved inward. Degree 4 (M4). — Exoskeleton 1.55–2.02 mm in length, represented by 19 articulated specimens (Figs. 2A–C, 5E). Degree 5 (M5). — Exoskeleton 1.84–2.83 mm in length, represented by 33 articulated specimens (Figs. 2D–F, 5F). Pygidial notch contracted, slightly curved inward. Two stages can be recognized, with three (Figs. 2D, 5F1) and four segments (Figs. 2E–F, 5F2) in the pygidial axis, respectively. Degree 6 (M6). — Exoskeleton 2.05–2.96 mm in length, represented by 13 articulated specimens (Figs. 2G–I, 5G). Two stages can be recognized, with three (Figs. 2G, 5G1) and four segments (Figs. 2H–I, 5G2) in the pygidial axis, respectively. Pygidial notch disappeared completely, with posterior margin rounded posteriorly. Degree 7 (M7). — Exoskeleton 2.52–3.55 mm in length, represented by 12 articulated specimens (Figs. 3A–B, 5H). Two stages can be
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Fig. 5. Reconstructions in dorsal view of ontogenetic series of Hunanocephalus ovalis Lee, 1963. A, meraspid degree 0; ×50. B, meraspid degree 1; ×50. C, meraspid degree 2; ×48. C1, stage 1; C2, stage 2. D, meraspid degree 3; ×45. E, meraspid degree 4; ×45. F, meraspid degree 5; ×38. F1, stage 1; F2, stage 2. G, meraspid degree 6; ×36. G1, stage 1; G2, stage 2. H, meraspid degree 7; ×28. H1, stage 1; H2, stage 2. I, meraspid degree 8; ×25. I1, stage 1; I2, stage 2. J, meraspid degree 9; ×23. K, meraspid degree 10; ×14.
recognized, with three (Figs. 3B, 5H1) and four segments (Figs. 3A, 5H2) in the pygidial axis, respectively. Degree 8 (M8). — Exoskeleton 3.10–4.68 mm in length, represented by 21 articulated specimens (Figs. 3C–E, 5I). Two stages can be recognized, with three (Figs. 3C, 5I1) and four segments (Figs. 3D–E, 5I2) in the pygidial axis, respectively. Degree 9 (M9). — Exoskeleton 3.57 mm in length, represented by a single articulated specimen (Figs. 3F, 5J). Pygidium with three (or four?) axial segments. Degree 10 (M10). — Exoskeleton oval in outline, 4.91–6.18 mm in length, of moderate convexity, represented by five articulated specimens (Figs. 4, 5K). Cranidium sub-trapezoidal in outline, moderately convex. Anterior margin moderately straight or slightly curved forward; anterior border narrow and upturned, of uniform width (sag., exs.) laterally; anterior border furrow shallow. Glabella large, defined by
deep axial furrow, sub-trapezoidal in outline, wide and of moderate convexity, maximum width opposite L1, narrower in the middle portion, slightly expanded anteriorly; L1 slightly wider than L2, LA expanded anteriorly and slight rounded in front, with anterior margin reaching anterior border. Three pairs of glabellar furrows weakly impressed, seemingly not extending axial furrows; S1 posteriorly convergent at about 90°; S2 shorter, slightly convergent backward; S3 short and groove-like, anteriorly convergent at about 120°. Occipital ring wider (tr.) and narrower (sag.) than L1, with posterior margin rounded, lacking an occipital spine or node; occipital furrow deep, slightly curved backward. Eye ridge weakly defined, of moderate width and convexity, extending laterally from LA at midpoint, and then gently curved posterolaterally to palpebral lobe, with posterior tip opposite L2. Facial suture opisthoparian, anterior branches convergent forward, with γ opposite S3; posterior branches extending posterolaterally, with
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ε opposite S1. Posterior margin straight laterally, and then extending anterolaterally to genal spine base. Posterior border narrow (exs.) and convex, slightly expanding abaxially; posterior border furrow extremely deep, extending laterally. Fixigenal field moderately protuberant, with a maximum width in posterolateral border, of equal width with glabella. Librigena absent. Thorax with 10 segments. Axial rings of moderate convexity, well defined by deep axial furrows, gently tapering backward; axial width slightly wider (exs.) than pleural width. Pleurae gently convex, terminated in short and blunt pleural spines; pleural furrow distinct. Pleurae with equal width (tr.) in T1–T3, with maximum width (tr.) in T4–T5, then gently shorter (sag.) and narrower (tr.) posteriorly from T6 to T10. Pygidium small, sub-elliptical in outline, with posterior margin rounded. Axis wide (tr.), tapering evenly backward, with posterior tip rounded, not reaching posterior border furrow; divided into two axial rings plus a minute terminus, defined by shallow axial ring furrows. Axial rings lacking spines, successively narrower (tr.) and shorter (sag.) posteriorly. Pleural field gently convex, defined by shallow border furrow. Anterior and posterior border narrow. Interpleural furrow indistinct.
are recognized in M1, M3 and M4, and three and four are recognized in M2, M5, M6, M7 and M8 respectively, thus, it can be concluded that there are probably at least two instars at each meraspid degree, one with three and the other with four axial segments in the pygidium. For instance, accompanied by the liberation of the anteriormost pygidial segment as the first thoracic segment, the M1 commences without the addition of any new segments into the pygidium, at this point, accordingly, there is no change in the number of the post-cephalic segments as the boundary between the thorax and pygidium migrated posteriorly (M11), indicating that only tagmosis (McNamara et al., 2003, 2006) occurred at this point (shedding phase) and each molting event does not always bring the addition of a new segment in H. ovalis. At a later stage (M12), subsequently, a new segment was added into the posterior tip of the pygidium (Accumulation phase). The pygidial segment number possibly remained fixed at three and four throughout meraspid period, suggesting that somitogenesis and tagmosis might occur independently during the meraspid ontogenetic development in the post-cephalic region of H. ovalis.
3.2. Summary of morphological variation during ontogeny The length of the exoskeleton increases from 0.61 mm in the smallest meraspid degree 0 to 6.18 mm in the largest meraspid degree 10 of H. ovalis. The shape change seems to be progressive with few pronounced morphological modifications between each meraspid degree, particularly during the late meraspid phases. As to the overall ontogenetic trends, main morphological changes which took place are summed up as follows: (1) Crandium sub-trapezoidal in outline, with a gradual decrease in the length/width ratio until meraspid degree 10, becomes proportionally small on account of the continual addition of segments in the trunk region. (2) Glabella remains essentially sub-trapezoidal in outline, largely restricted to a proportional widening throughout ontogeny, with anterior margin reaching anterior cranidial border. (3) Three pairs of glabellar furrows, indistinct in early meraspid periods, become more incised in later ontogeny, probably not extending to axial furrow. S1 groove-like, extending posteromedially; S2 shorter than S1, slightly convergent backward; S3 short and pit-like, slightly convergent anteriorly. It is noteworthy that three pairs of glabellar furrows are all actually discontinuous, although seemingly being transverse in dorsal view of some specimens owing to the secondary diagenetic deformation. (4) Facial sutures, indistinct in early meraspides, are opisthoparian throughout ontogeny; anterior branches convergent between β and α, and posterior branches extending posterolaterally. (5) The pygidial notch can still be observed in M5 though extremely indistinct (Fig. 2D–F), but not in later phases and accordingly, is considered to disappear entirely in M6. (6) Eye ridge and palpebral lobe short and poorly preserved through all ontogenetic phases, extending laterally from LA at midpoint and then extending posterolaterally. (7) Fixigena becomes slightly wider (tr.) by degrees. (8) Librigena preserved only in a few articulated specimens (Fig. 3C, E), extremely narrow (tr.) and short (exs.). Librigenal spines extremely tiny in early meraspides and become slightly longer in later ontogeny, extending posterolaterally. (9) Axial and posterior cranidial border furrow become deeper. 4. Trunk segmentation schedule The relatively complete meraspid ontogenetic sequence of H. ovalis permits analysis of the trunk segmentation pattern with respect to their dorsal exoskeletons (Fig. 6). Throughout ontogeny there was a steady increase in the number of post-cephalic segments. Accompanied by the release of the anterior pygidial segment into the thorax, the boundary between the thorax and pygidium was displaced posteriorly by a single segment. Except that the number of axial segments in the pygidia of M0 and M9 is unclear (probably three), four pygidial segments
Fig. 6. Reconstruction of the trunk segmentation schedule of Hunanocephalus ovalis Lee, 1963. Dotted lines represent hypothetical growth forms; open triangles mark the liberation of thoracic segments; solid triangles mark the inserting of additional segments into the pygidium; white, dark gray, and gray colors represent a cephalon, thoracic segment and pygidium, respectively.
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Trilobite trunk segmentation has been discussed extensively in recent years based on the increasing knowledge on the development of various trilobite lineages (e.g. Hughes, 2003a,b; Fusco et al., 2004; Hughes et al., 2006; Hughes, 2007; Dai and Zhang, 2011, 2013a,b). The developmental mode of trunk segmentation of the post-Cambrian corynexochid trilobites has been listed and summed up in a few species by Hughes et al. (2006), i.e. Dolichometopidae: O. klotzi (Rominger, 1887) by Walcott (1916) and McNamara and Rudkin (1984), and B. fimbriatus Robison, 1967 by Robison (1967); Styginidae: D. hudsoni Chatterton, 1971 and S. calvum Chatterton, 1971 by Chatterton (1971), which were classified to protarthrous and hypoprotomeric development pattern, respectively. As for those from the Series 2 of the Cambrian, McNamara et al. (2003, 2006) described the ontogeny and heterochrony of four genera of early Cambrian oryctocephalid trilobites from the Balang Formation in eastern Guizhou Province, southwest China. In their study, the ontogenetic series of some taxa showed that the increase in segment number does not correspond to a one-to-one increase at each molting event. As McNamara et al. (2006) argued, for instance, in Duyunaspis duyunensis, ‘two segments were added between degree 0 and 1, but none was added between degree 1 and 2. Again two were added between M2 and M3 … (text-fig. 9)’. Such a seemingly ‘irregular somitogenesis’ (if each molting event resulted in one further thoracic segment being added into the thorax) explained by McNamara et al. (2006) was most likely due to the absence of some growth stages in each meraspid degree (Fig. 7A). Given the five pygidial segments in the degrees 0, 2, 5 and 6, and six in the degrees 1, 3 and 4, similarly, there might be two stages in each meraspid degree of D. duyunensis, with five pygidial segments in the first and six in the second, and the pygidial segment number remained fixed at five and six throughout meraspid ontogeny (the same is true of Balangia balangensis) (Fig. 7). Consequently, it is advocated that D. duyunensis and B. balangensis may process similar trunk segmentation schedule with H. ovalis, i.e. in each molting event, the addition and release of pygidial segments are separated from one another. In other words, the processes of liberation of each thoracic segment and addition of new segments into the post-thoracic region might be controlled by separate genomic programs. The realization that tagmosis and somitogenesis may each progress independently during ontogeny of these early Cambrian corynexochid
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trilobites provides a useful tool in attempts to draw an ontogenetic comparison between them. When comparing equivalent meraspid degrees between B. balangensis and D. duyunensis, for instance, McNamara et al. (2006) found that it is easy in degree 0 and 3, but not in degree 1 on account for the discrepancy in their pygidial segment number. Since both species have two stages in each meraspid degree as discussed above, it is unsuitable to compare the first instar of degree 1 (M11) of B. balangensis (with five segments in the pygidium) with the second instar of degree 1 (M12) of D. duyunensis (with six segments in the pygidium). Additionally, the holaspis of B. balangensis has seven segments, in having one more segment than the meraspid degree 4 of D. duyunensis, and might belong to a late holaspid stage (Fig. 7B). It demonstrates that in the holaspides of B. balangensis the exoskeletons increased not only in size, but also in segment number; new trunk segments were all inserted into the pygidium, but none was released into the thorax. In D. duyunensis, by contraries, tagmosis becomes fixed until the thorax processed seven segments. Accordingly, in B. balangensis (Fig. 7B), the early holaspid stages (H1 and H2), rather than the later one (H3), can be used to compare with the meraspid degree 4 of D. duyunensis (Fig. 7A). As far as known, trilobites constructed their thorax during the meraspid period through successive release of segments from the anterior portion of the transitory pygidium (Stubblefield, 1926; Fortey and Owens, 1991; Chatterton and Speyer, 1997), which is evidently unique to the Trilobita and their close relatives (Cotton and Braddy, 2004; Simpson et al., 2005). The progressive addition of exoskeletal segments near the terminus of the trunk region is deemed to be homologous with the postembryonic appearance of skeletal segments in some extant arthropods (Størmer, 1942; Hessler, 1962; Hu, 1971), and some characteristics of the trilobite trunk segmentation were probably shared with those living arthropods. As the case stands, the rates of release of anterior segments and appearance of new segments during the meraspid phase might be incoordinate within a large amount of trilobite species, however, such decoupling of segment appearance and liberation has been explored detailedly only in a limited taxa to date (e.g. McNamara et al., 2003, 2006; Simpson et al., 2005; Dai and Zhang, 2011, 2013a). The ontogenetic study of H. ovalis in this article presents new evidence to support and confirm this development pattern.
Fig. 7. Reconstructions of the trunk segmentation schedule of Duyunaspis duyunensis Zhang and Qian, in Zhou et al. (1977) (A) and B. balangensis Qian (1961) (B) based on the descriptions from McNamara et al. (2006). Dotted lines represent hypothetical growth forms which are not presented in McNamara et al. (2006). Marks are the same as in Fig. 6.
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The exchange of segments between thorax and pygidium in trilobite trunk region is remarkable because the pygidium has commonly been considered to represent a separate tagma and is equated to the abdomen of other arthropods (Cisne et al., 1980). It represents a highly unusual growth pattern among arthropods and is thus of interest with regard to the evolution of arthropod body patterning. Consequently, investigating more ontogenetic rules and trunk development of the early Cambrian trilobites is significant not only for understanding the whole extinct clade, but also provides insights into the evolution of arthropod body plan as a whole (Hughes, 2003a,b; Minelli et al., 2003). Acknowledgments We are grateful to Shigenori Maruyama and Yukio Isozaki for organizing this special issue. Financial supports by the Major Basic Research Project of the Ministry of Science and Technology of China (Grants: 2013CB835002 and 2013CB837100), the Natural Science Foundation of China (NSFC, Grants: 40925005, 41272036 and 41221001), and the “Sanqin Scholarship” project of the Shaanxi Authority are greatly acknowledged. References Bergeron, J.N., 1899. Étude de quelques trilobites de Chine. Bulletin de la Société Géologique du France 27 (Series 3), 499–516. Chatterton, B.D.E., 1971. Taxonomy and ontogeny of Siluro-Devonian trilobites from near Yass, New South Wales. Palaeontographica Abteilung 137, 1–108. Chatterton, B.D.E., Speyer, S.E., 1997. Ontogeny. In: Kaesler, R.L. (Ed.), Treatise on Invertebrate Paleontology. Part O, Revised, Trilobita, Introduction, Order Agnostina, Order Redlichiida. Geological Society of America and University of Kansas, Boulder, Colorado and Lawrence, Kansas, pp. 173–247 (xxiv + 530 pp.). Cisne, J.L., Chandlee, G.O., Rabe, B.D., Cohen, J.A., 1980. Geographic variation and episodic evolution in an Ordovician trilobite. Science 209, 925–927. Cotton, T.J., Braddy, S.J., 2004. The phylogeny of arachnomorph arthropods and the origin of the Chelicerata. Transactions of the Royal Society of Edinburgh: Earth and Environmental Science 94, 169–193. Dai, T., Zhang, X.L., 2011. Ontogeny of the eodiscoid trilobite Tsunyidiscus acutus from the lower Cambrian of South China. Palaeontology 54, 1279–1288. Dai, T., Zhang, X.L., 2012a. Ontogeny of the trilobite Estaingia sinensis (Zhang) from the Lower Cambrian of South China. Bulletin of Geosciences 87, 151–158. Dai, T., Zhang, X.L., 2012b. Ontogeny of the redlichiid trilobite Metaredlichia cylindrica from the lower Cambrian (Cambrian stage 3) of South China. Journal of Paleontology 86, 647–652. Dai, T., Zhang, X.L., 2013a. Morphology and ontogeny of the eodiscoid trilobite Sinodiscus changyangensis from the lower Cambrian of South China. Palaeontology 56, 411–420. Dai, T., Zhang, X.L., 2013b. Ontogeny of the redlichiid trilobite Eoredlichia intermedia from the Chengjiang Lagerstätte, lower Cambrian, southwest China. Lethaia 46, 262–273. Egorova, L.I., Xiang, L.W., Lee, S.J., Nan, R.S., Guo, Z.M., 1963. The Cambrian trilobite faunas of Guizhou and western Hunan. Special Paper, Institute of Geology and Mineral Resources (Beijing), Series B, Stratigraphy and Palaeontology, 3, pp. 1–117 (In Chinese). Fortey, R.A., Chatterton, B.D.E., 1988. Classification of the trilobite suborder Asaphina. Palaeontology 31, 165–222. Fortey, R.A., Owens, R.M., 1991. A trilobite fauna from the highest Shineton Shales in Shropshire, and the correlation of the latest Tremadoc. Geological Magazine 128, 437–464. Fusco, G., Hughes, N.C., Webster, M., Minelli, A., 2004. Exploring developmental modes in a fossil arthropod: growth and trunk segmentation of the trilobite Aulacopleura konincki. American Naturalist 163, 167–183. Hessler, R.R., 1962. Secondary segmentation of the trilobite thorax. Journal of Paleontology 36, 1305–1312. Hu, C.H., 1971. Ontogeny and sexual dimorphism of Lower Paleozoic Trilobita. Palaeontographica Americana 44, 1–155. Hu, C.H., 1985a. Ontogenies of two Middle Cambrian corynexochid trilobites from the Canadian Rocky Mountains. Transactions and Proceedings of the Palaeontological Society of Japan (New Series) 138, 138–147. Hu, C.H., 1985b. Ontogenetic development of Cambrian trilobites from British Columbia and Alberta. Canada (Part I). Journal of the Taiwan Museum 38, 121–158. Hughes, N.C., 2003a. Trilobite body patterning and the evolution of arthropod tagmosis. Bioessays 25, 386–395.
Hughes, N.C., 2003b. Trilobite tagmosis and body patterning from morphological and developmental perspectives. Integrative and Comparative Biology 41, 185–206. Hughes, N.C., 2007. The evolution of trilobite body patterning. Annual Review of Earth and Planetary Sciences 35, 401–434. Hughes, N.C., Minelli, A., Fusco, G., 2006. The ontogeny of trilobite segmentation: a comparative approach. Paleobiology 32, 602–627. Kobayashi, T., 1935. The Cambro-Ordovician formations and faunas of South Chosen. Palaeontology. Part 3: Cambrian faunas of South chosen with a special study on the Cambrian trilobite genera and families. Journal of the Faculty of Science, Imperial University of Tokyo, Section II 4, 49–344. Lu, Y.H., Qian, Y.Y., 1983. Cambro-Ordovician trilobites from eastern Guizhou. Palaeontologia Cathayana 1, 1–106. Lu, Y.H., Qian, Y.Y., Zhu, Z.L., 1963. Trilobita. Science Press, Beijing (186 pp. [In Chinese]). Lu, Y.H., Zhang, W.T., Qian, Y.Y., Zhu, Z.L., Lin, H.L., Zhou, Z.Y., Qian, Y., Zhang, S.G., Wu, H.J., 1974a. Cambrian trilobites. In: Nanjing Institute of Geology, Palaeontology, Academic Sinica (Eds.), Handbook of Stratigraphy and Palaeontology, Southwest China. Science Press, Beijing, pp. 82–107 (454 pp. [In Chinese]). Lu, Y.H., Zhu, Z.L., Qian, Y.Y., Zhou, Z.Y., Yuan, K.X., 1974b. Bio-environmental control hypothesis and its application to Cambrian biostratigraphy and palaeozoogeography. Memoir of the Nanjing Institute of Geology and Palaeontology, Academia Sinica 5, 27–110 (In Chinese). McNamara, K.J., Rudkin, D.M., 1984. Techniques of trilobite exuviation. Lethaia 17, 153–173. McNamara, K.J., Yu, F., ZHOU, Z.Y., 2003. Ontogeny and heterochrony in the oryctocephalid trilobite Arthricocephalus from the Early Cambrian of China. Special Papers in Palaeontology 70, 103–126. McNamara, K.J., Yu, F., Zhou, Z.Y., 2006. Ontogeny and heterochrony in the Early Cambrian oryctocephalid trilobites Changasips, Duyunaspis and Balangia from China. Palaeontology 49, 1–19. Minelli, A., Fusco, G., Hughes, N.C., 2003. Tagmata and segment specification in trilobites. In: Lane, P.D., Siveter, Derek J., Fortey, R.A. (Eds.), Trilobites and Their Relatives: Special Paper in Palaeontology, 70, pp. 31–43. Öpik, A.A., 1982. Dolichometopid trilobites of Queensland, Northern Territory, and New South Wales. Bureau of Mineral Resources, Geology and Geophysics of Australia, Bulletin 175, 1–85. Poulsen, V., 1958. Contribution to the palaeontology of the Lower Cambrian Wulff River Formation. Meddelelser om Grønland 162, 1–25. Qian, Y.Y., 1961. Cambrian trilobites from Sandu and Duyun, southern Guizhou. Acta Palaeontologica Sinica 9, 91–109. Rasetti, F., 1967. Lower and Middle Cambrian trilobites faunas from the Taconic sequence of New York. Miscellaneous Collections of the Smithsonian Institution, 152, pp. 1–111. Robison, R.A., 1967. Ontogeny of Bathyuriscus fimbriatus and its bearing on the affinities of corynexochid trilobites. Journal of Paleontology 41, 213–221. Robison, R.A., Campbell, D.P., 1974. A Cambrian corynexochid trilobite with only two thoracic segmerûs. Lethaia 7, 273–282. Simpson, A.G., Hughes, N.C., Kopaska-Merkel, D.C., Ludvigsen, R., 2005. Development of the caudal exoskeleton of the pliomerid trilobite Hintzeia plicamarginis new species. Evolution & Development 7, 528–541. Størmer, L., 1942. Studies on trilobite morphology, Part II. The larval development, the segmentation and the sutures, and their bearing on trilobite classification. Norsk Geologisk Tidsskrift 21, 49–164. Stubblefield, C.J., 1926. Notes on the development of a trilobite, Shumardia pusilla (Sars). Journal of the Linnean Society, Zoology 36, 345–372. Suvorova, N.P., 1964. The trilobites Corynexochidea and their historical development. Proceedings of the Palaeontological Institute, Moscow, 103, pp. 1–319 (In Russian). Walcott, C.D., 1916. Cambrian geology and paleontology III, no. 5: Cambrian trilobites. Smithsonian Miscellaneous Collections, 64, pp. 303–457. Whittington, H.B., Kelly, S.R.A., 1997. Morphological terms applied to Trilobita. In: Kaesler, R.L. (Ed.), Treatise on Invertebrate Paleontology, Part O, Arthropoda 1, Trilobita, Revised. Volume 1: Introduction, Order Agnostida, Order Redlichiida. Geological Society of America and University of Kansas, Boulder, Colorado and Lawrence, Kansas, pp. 313–329 (xxiv + 530 pp.). Zhang, W.T., 1953. Some Lower Cambrian trilobites from western Hupei. Acta Palaeontologica Sinica 1, 121–149. Zhang, W.T., 1963. A classification of the Lower and Middle Cambrian trilobites from North and northwestern China, with description of new families and new genera. Acta Palaeontologica Sinica 11, 447–491 (In Chinese, English summary). Zhang, W.T., 1966. On the classification of Redlichiacea, with description of new families and new genera. Acta Palaeontologica Sinica 14, 135–184 (In Chinese, English summary). Zhang, W.T., Lu, Y.H., Zhu, Z.L., Qian, Y.Y., Lin, H.L., Zhou, Z.Y., Zhang, S.G., Yuan, J.L., 1980. Cambrian trilobite faunas of southwestern China. Palaeontologica Sinica 16, 1–497 (In Chinese, English summary). Zhou, T.M., Liu, Y.R., Meng, X.S., Sun, Z.H., 1977. Trilobita. In: Geological Bureau of Henan Province (Ed.), Palaeontological Atlas of Central and South China. Guizhou (1). Geological Publishing House, Beijing, pp. 107–266 (420 pp. [In Chinese]).
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Morphology and ontogeny of Hunanocephalus ovalis (trilobite) from the Cambrian of South China Tao Dai a, b, Xingliang Zhang a,⁎, Shanchi Peng b a b
State Key Laboratory for Continental Dynamics and Department of Geology, Northwest University, Xian 710069, PR China State Key Laboratory of Palaeobiology and Stratigraphy (Nanjing Institute of Geology and Palaeontology, CAS), Nanjing, Jiangsu 210008, PR China
a r t i c l e
i n f o
Article history: Received 31 August 2012 Received in revised form 15 March 2013 Accepted 1 May 2013 Available online 13 May 2013 Keywords: Trilobita Corynexochida Ontogeny Lower Cambrian South China
a b s t r a c t The juvenile morphology and ontogeny of the Cheiruroidid trilobite Hunanocephalus ovalis Lee, 1963 from the lower Cambrian Shuijingtuo Formation in Hubei Province, South China is presented. The new material comprises a relatively complete meraspid ontogenetic series (degree 0 to 10), which reveals more details on their morphological changes such as the contraction and disappearance of the pronounced posteromedial notch in the pygidium and the addition of the trunk segments, which are all documented for the first time and can also be used as developmental markers defining their ontogenetic phases. The trunk segmentation schedule of H. ovalis is also discussed, which is similar to the other early Cambrian oryctocephalid trilobites, i.e. as the boundary between the thorax and pygidium migrated posteriorly there is no change in the trunk segment number; the processes of liberation of the thoracic segment and segment insertion into the pygidium are separated from one another, implying that the control of trunk exoskeletal segment appearance and articulation might be decoupled in these trilobites. © 2013 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
1. Introduction The corynexochid trilobites spanned a long time ranging from the early Cambrian to the middle Devonian, but little is known of their complete ontogenetic sequences, with a small quantity of species having been published (e.g. Albertella limbata and Fieldaspis quadriangulatus (Hu, 1985a); Bathyuriscus fimbriatus (Rasetti, 1967; Robison, 1967); Bathyuriscus? sp. (Fortey and Chatterton, 1988); Chilometopus artus and Corynexochus felix (Suvorova, 1964); Corynexochus minor (Walcott, 1916); Dentaloscutellum hudsoni and Scutellum calvum (Chatterton, 1971); Fuchoia fecunda (Öpik, 1982); Fuchoia chiai (Lu and Qian, 1983); Glossopleura boccar (Hu, 1985b); Mendospidella digesta (Poulsen, 1958); Ogygopsis klotzi (Walcott, 1916; McNamara and Rudkin, 1984); Ptarmigania aurita (Hu, 1971) and Thoracocare minuta (Robison and Campbell, 1974)). However, all the materials published are those from the periods posterior to the Series 2 of the Cambrian and most of them were principally based upon the descriptions and illustrations of incomplete ontogenetic sequences. As for those from the Series 2 of the Cambrian, to our knowledge, none but McNamara et al. (2003, 2006) gave a detailed ontogenetic investigation on four genera of the oryctocephalids from the Balang Formation in eastern Guizhou Province, southwest China, i.e. Arthricocephalus Bergeron, 1899, Balangia Chien, 1961, Changaspis Lee in Qian, 1961 and Duyunaspis Chang and Chien in Zhou et al., 1977.
⁎ Corresponding author. Tel.: +86 02988303200; fax: +86 02988363659. E-mail address: [email protected] (X. Zhang).
The early Cambrian Shuijingtuo Formation (Zhang, 1953), exposed at Changyang County in western Hubei Province, South China (Dai and Zhang, 2011, text-fig. 1) contains a rich fauna of trilobites, dominated by two genera of eodiscinids: Tsunyidiscus (Zhang, 1966) and Sinodiscus Zhang in Lu et al. (1974a), and three genera of polymerids: Estaingia Pocock, 1964, Metaredlichia Lu, 1950 and Hunanocephalus Lee, 1963. Among these, the ontogenetic development of several species has been described in detail elsewhere, e.g. Tsunyidiscus acutus (Dai and Zhang, 2011), Estaingia sinensis (Dai and Zhang, 2012a), Metaredlichia cylindrica (Dai and Zhang, 2012b) and Sinodiscus changyangensis (Dai and Zhang, 2013a). As for Hunanocephalus, Lee in Egorova et al. (1963) first described the adult specimens of Hunanocephalus ovalis Lee, 1963 based on the material that comprises a few articulated exoskeletons and cranidia from the lower Cambrian Shuijingtuo Formation in Rongxi, Xiushan, Sichuan Province, China, characterized by the broad axis, trapeziform glabella, three pairs of glabellar furrows and 12 or 13 thoracic segments. The present paper deals with the first known ontogenetic series of H. ovalis Lee, 1963 from the lower Cambrian of South China. Besides displaying new details on morphology and morphogenesis during ontogeny, the juvenile instars of H. ovalis offer a unique opportunity for understanding the exceptional schedule on the trunk development of the Cheiruroidid trilobite, i.e. the release of the thoracic segment was not associated with the addition of any new segment, and the addition of new segment does not correspond with each molting event. By comparison with the ontogenetic sequence of the other early Cambrian oryctocephalids (McNamara et al., 2003, 2006), the unique trunk segmentation schedule may be shared with these early occurring
1342-937X/$ – see front matter © 2013 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.gr.2013.05.004
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corynexochid trilobites, which played a significant role in controlling the number of their adult thoracic segments. 2. Locality and material Fieldwork during 2008–2011 resulted in a collection of some thousand disarticulated and articulated sclerites left by juvenile instars of H. ovalis. All the material were collected from the carbonaceous black shales of the lower part of the Shuijingtuo Formation (Stage 3, Series 2 of the Cambrian) at the Dingjiaping section, about 5.5 km northwest of Changyang County, Hubei Province, South China (for stratigraphy see Dai and Zhang, 2011, text-fig. 1). This formation is overlain by the yellowish-green, silty-shale and -mudstone of the Shipai Formation, but the base of the formation is not exposed in the section. Despite the large number of specimens, most are essentially uninformative due to fragmentation or diagenetic deformation. Among these, 128 complete meraspid specimens have thus been selected for detailed study, of which all dimensions were measured as straight-line distances and the measurements of the sagittal length are made from the anterior cranidial margin to the posterior pygidial margin. The described and figured material from this study is housed in the collection of the Geology Department of Northwest University, Xian, China (NWULXHN 20201–20223). 3. Systematic paleontology Terminology. — Morphological terms and abbreviations used in this paper largely follow Whittington and Kelly (1997). In addition, some abbreviations are used in the description below: excl., excluding; exs., exsagittal; incl., including; L, length; LA, frontal glabellar lobe; sag., sagittal; T, tergite; tr., transverse; W, width; M0–M9, meraspid degree 0 to 9 respectively. Order Corynexochida Kobayashi, 1935 Suborder Corynexochina Kobayashi, 1935 Family Cheiruroididae Zhang, 1963 Genus Hunanocephalus Lee, 1963
Remarks. — Zhou in Lu et al. (1974b) proposed the subgenus Hunanocephalus (Duotingia), within which Zhou placed H. duotingensis Chow in Lu et al. (1974b). However, comparing this subgenus with the genus Hunanocephalus, there is no character sufficiently different to warrant their emplacement in a different subgenus. Consequently, it should be regarded as a junior synonym of Hunanocephalus. In addition, on account of the co-occurrence with T. acutus and S. changyangensis, Hunanocephalus could be assigned to Tsunyidiscus–Sinodiscus Zone (Chiungchussuan). Type species. — H. ovalis Lee in Egorova et al. (1963) from the lower Cambrian Shuijingtuo Formation in Rongxi, Xiushan, Sichuan Province, China. H. ovalis Lee, 1963 H. ovalis Lee, Egorova et al. (1963), p. 14, pl. 1, figs. 1–7. H. ovalis Lee, Lu et al. (1963), p. 26, pl. 1, fig. 4. H. ovalis Lee, Zhou et al. (1977), p. 128, pl. 41, fig. 1. H. ovalis Lee, Zhang et al. (1980), p. 280, pl. 94, fig. 12. 3.1. Meraspid ontogeny One hundred and twenty-eight articulated specimens assigned to meraspid phases of H. ovalis Lee, 1963 are investigated and their sizes range from 0.61 to 6.18 mm in length (Figs. 1–4). The morphological changes during this period (M0–M10) were very subtle. In addition to the morphological changes of some structures as developmental markers, such as the contraction and disappearance of the pronounced posteromedial notch in the pygidium, the ontogenetic tracing of each stage in meraspid period is mainly based upon the number of thoracic and pygidial segments. Degree 0 (M0). — Exoskeleton sub-elliptical in outline, 0.61–0.64 mm in length, represented by two articulated specimens (Figs. 1A, 5A). Cranidium trapeziform in outline, of low convexity. Anterior margin slightly curved forward; anterior border indistinct. Glabella wide and of low convexity, three (?) pairs of glabellar furrows weakly impressed. LO moderately convex and short (sag.), with posterior margin curved
Fig. 1. Complete meraspides of Hunanocephalus ovalis Lee, 1963 from the Shuijingtuo Formation, in Dingjiaping, Changyang County, Hubei Province, South China. A, Degree 0, NWULXHN 20201; ×67. B, Degree 1, NWULXHN 20202; ×50. C–D, Degree 2, NWULXHN 20203–20204; ×35, ×45. E–F, Degree 3, NWULXHN 20205–20206; ×30, ×32.
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backward, occipital furrow shallow. Facial suture indistinct. Fixigena wide (tr.), subequal width (tr.) of glabella. Posterior border furrow shallow. Pygidium small, semi-elliptical in outline, with posterior margin not preserved. Axis wide (tr.), tapering evenly backward, with posterior tip not preserved; divided into three (four?) segments, defined by posteriorly curved axial ring furrows. Axial rings lacking spines, successively shorter (sag.) and narrower (tr.) posteriorly. Pleural field of low convexity. Pleural furrow indistinct. A pair of interpleural furrows differentiating the anteriormost pygidial segment from the succeeding ones can be observed anteriorly, defining the posterior edge of the anterior segment that would be released into the thorax as the first thoracic segment. Degree 1 (M1). — Exoskeleton 0.92 mm in length, represented by a single articulated specimen (Figs. 1B, 5B). Cranidium sub-trapezoidal in outline, moderately convex. Anterior margin moderately straight;
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Anterior border narrow (sag., exs.). Glabella sub-trapezoidal in outline, narrower (tr.) in the middle portion, slightly expanded anteriorly; three transglabellar furrows weakly impressed. Facial suture opisthoparian, anterior branch slightly convergent forward; posterior branch of facial suture extending posterolaterally. Eye ridge and palpebral lobe indistinct. Posterior margin straight laterally and then curved anterolaterally to lateral margin. Posterior border extremely narrow (exs.); posterior border furrow shallow, extending anterolaterally. Thorax with one segment. Axial ring wide and of low convexity, weakly defined by shallow axial furrows, approximately 1.5 times wider than pleurae. Pleurae moderately flat, of equal width (exs.) laterally, with pleural spine short and obtuse, pleural furrow shallow. Pygidium small, semi-elliptical in outline, with posterior margin concave medially, W-shaped in outline. Axis wide (tr.), divided into four segments, tapering evenly backward. Pleural field of low convexity. Pleural furrow indistinct.
Fig. 2. Complete meraspides of Hunanocephalus ovalis Lee, 1963 from the Shuijingtuo Formation, in Dingjiaping, Changyang County, Hubei Province, South China. A–C, Degree 4, NWULXHN 20207–20209; ×30, ×29, ×28. D–F, Degree 5, NWULXHN 20210–20212; ×28, ×24, ×24. G–I, Degree 6, NWULXHN 20213–20215; ×20, ×23, ×24.
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Fig. 3. Complete meraspides of Hunanocephalus ovalis Lee, 1963 from the Shuijingtuo Formation, in Dingjiaping, Changyang County, Hubei Province, South China. A–B, Degree 7, NWULXHN 20216–20217; ×17, ×20. C–E, Degree 8, NWULXHN 20218–20220; ×15, ×31, ×18. F, Degree 9, NWULXHN 20221; ×19.
Degree 2 (M2). — Exoskeleton 0.94–1.39 mm in length, represented by seven articulated specimens (Figs. 1C–D, 5C). Thorax with two segments. Axis of low convexity, weakly defined by shallow axial furrows; axial ring of T2 narrower (tr.) than that of T1. Pleurae moderately flat, gently shorter (exs.) and narrower (tr.) from T1 to T2. Pygidium small, semi-elliptical in outline, with anterior margin extending posterolaterally
Fig. 4. Complete meraspides of Hunanocephalus ovalis Lee, 1963 from the Shuijingtuo Formation, in Dingjiaping, Changyang County, Hubei Province, South China. A–B, Degree 10, NWULXHN 20221–20223; ×17, ×25.
and posterior margin concave medially, W-shaped in outline. Axis convex, divided into three (Figs. 1C, 5C1) and four segments (Figs. 1D, 5C2). Pleural field of low convexity, poorly segmented. Degree 3–10 (M3–10). — Exoskeletons range from 1.26 to 6.18 mm in length (Figs. 1E–F, 2–4, 5D–K). In addition to the extra thoracic segments and some striking variations that can be regarded as developmental markers defining their ontogenetic phases, morphological changes among these phases were very subtle, consisting most obviously of the disappearance of the pronounced posteromedial notch in the pygidium. Of the collected material, two stages can also be recognized in M5, M6, M7 and M8 on the basis of the pygidial segment number, respectively. Degree 3 (M3). — Exoskeleton 1.26–1.77 mm in length, represented by 14 articulated specimens (Figs. 1E–F, 5D). Pygidium with four axial segments; posterior margin slightly curved inward. Degree 4 (M4). — Exoskeleton 1.55–2.02 mm in length, represented by 19 articulated specimens (Figs. 2A–C, 5E). Degree 5 (M5). — Exoskeleton 1.84–2.83 mm in length, represented by 33 articulated specimens (Figs. 2D–F, 5F). Pygidial notch contracted, slightly curved inward. Two stages can be recognized, with three (Figs. 2D, 5F1) and four segments (Figs. 2E–F, 5F2) in the pygidial axis, respectively. Degree 6 (M6). — Exoskeleton 2.05–2.96 mm in length, represented by 13 articulated specimens (Figs. 2G–I, 5G). Two stages can be recognized, with three (Figs. 2G, 5G1) and four segments (Figs. 2H–I, 5G2) in the pygidial axis, respectively. Pygidial notch disappeared completely, with posterior margin rounded posteriorly. Degree 7 (M7). — Exoskeleton 2.52–3.55 mm in length, represented by 12 articulated specimens (Figs. 3A–B, 5H). Two stages can be
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Fig. 5. Reconstructions in dorsal view of ontogenetic series of Hunanocephalus ovalis Lee, 1963. A, meraspid degree 0; ×50. B, meraspid degree 1; ×50. C, meraspid degree 2; ×48. C1, stage 1; C2, stage 2. D, meraspid degree 3; ×45. E, meraspid degree 4; ×45. F, meraspid degree 5; ×38. F1, stage 1; F2, stage 2. G, meraspid degree 6; ×36. G1, stage 1; G2, stage 2. H, meraspid degree 7; ×28. H1, stage 1; H2, stage 2. I, meraspid degree 8; ×25. I1, stage 1; I2, stage 2. J, meraspid degree 9; ×23. K, meraspid degree 10; ×14.
recognized, with three (Figs. 3B, 5H1) and four segments (Figs. 3A, 5H2) in the pygidial axis, respectively. Degree 8 (M8). — Exoskeleton 3.10–4.68 mm in length, represented by 21 articulated specimens (Figs. 3C–E, 5I). Two stages can be recognized, with three (Figs. 3C, 5I1) and four segments (Figs. 3D–E, 5I2) in the pygidial axis, respectively. Degree 9 (M9). — Exoskeleton 3.57 mm in length, represented by a single articulated specimen (Figs. 3F, 5J). Pygidium with three (or four?) axial segments. Degree 10 (M10). — Exoskeleton oval in outline, 4.91–6.18 mm in length, of moderate convexity, represented by five articulated specimens (Figs. 4, 5K). Cranidium sub-trapezoidal in outline, moderately convex. Anterior margin moderately straight or slightly curved forward; anterior border narrow and upturned, of uniform width (sag., exs.) laterally; anterior border furrow shallow. Glabella large, defined by
deep axial furrow, sub-trapezoidal in outline, wide and of moderate convexity, maximum width opposite L1, narrower in the middle portion, slightly expanded anteriorly; L1 slightly wider than L2, LA expanded anteriorly and slight rounded in front, with anterior margin reaching anterior border. Three pairs of glabellar furrows weakly impressed, seemingly not extending axial furrows; S1 posteriorly convergent at about 90°; S2 shorter, slightly convergent backward; S3 short and groove-like, anteriorly convergent at about 120°. Occipital ring wider (tr.) and narrower (sag.) than L1, with posterior margin rounded, lacking an occipital spine or node; occipital furrow deep, slightly curved backward. Eye ridge weakly defined, of moderate width and convexity, extending laterally from LA at midpoint, and then gently curved posterolaterally to palpebral lobe, with posterior tip opposite L2. Facial suture opisthoparian, anterior branches convergent forward, with γ opposite S3; posterior branches extending posterolaterally, with
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ε opposite S1. Posterior margin straight laterally, and then extending anterolaterally to genal spine base. Posterior border narrow (exs.) and convex, slightly expanding abaxially; posterior border furrow extremely deep, extending laterally. Fixigenal field moderately protuberant, with a maximum width in posterolateral border, of equal width with glabella. Librigena absent. Thorax with 10 segments. Axial rings of moderate convexity, well defined by deep axial furrows, gently tapering backward; axial width slightly wider (exs.) than pleural width. Pleurae gently convex, terminated in short and blunt pleural spines; pleural furrow distinct. Pleurae with equal width (tr.) in T1–T3, with maximum width (tr.) in T4–T5, then gently shorter (sag.) and narrower (tr.) posteriorly from T6 to T10. Pygidium small, sub-elliptical in outline, with posterior margin rounded. Axis wide (tr.), tapering evenly backward, with posterior tip rounded, not reaching posterior border furrow; divided into two axial rings plus a minute terminus, defined by shallow axial ring furrows. Axial rings lacking spines, successively narrower (tr.) and shorter (sag.) posteriorly. Pleural field gently convex, defined by shallow border furrow. Anterior and posterior border narrow. Interpleural furrow indistinct.
are recognized in M1, M3 and M4, and three and four are recognized in M2, M5, M6, M7 and M8 respectively, thus, it can be concluded that there are probably at least two instars at each meraspid degree, one with three and the other with four axial segments in the pygidium. For instance, accompanied by the liberation of the anteriormost pygidial segment as the first thoracic segment, the M1 commences without the addition of any new segments into the pygidium, at this point, accordingly, there is no change in the number of the post-cephalic segments as the boundary between the thorax and pygidium migrated posteriorly (M11), indicating that only tagmosis (McNamara et al., 2003, 2006) occurred at this point (shedding phase) and each molting event does not always bring the addition of a new segment in H. ovalis. At a later stage (M12), subsequently, a new segment was added into the posterior tip of the pygidium (Accumulation phase). The pygidial segment number possibly remained fixed at three and four throughout meraspid period, suggesting that somitogenesis and tagmosis might occur independently during the meraspid ontogenetic development in the post-cephalic region of H. ovalis.
3.2. Summary of morphological variation during ontogeny The length of the exoskeleton increases from 0.61 mm in the smallest meraspid degree 0 to 6.18 mm in the largest meraspid degree 10 of H. ovalis. The shape change seems to be progressive with few pronounced morphological modifications between each meraspid degree, particularly during the late meraspid phases. As to the overall ontogenetic trends, main morphological changes which took place are summed up as follows: (1) Crandium sub-trapezoidal in outline, with a gradual decrease in the length/width ratio until meraspid degree 10, becomes proportionally small on account of the continual addition of segments in the trunk region. (2) Glabella remains essentially sub-trapezoidal in outline, largely restricted to a proportional widening throughout ontogeny, with anterior margin reaching anterior cranidial border. (3) Three pairs of glabellar furrows, indistinct in early meraspid periods, become more incised in later ontogeny, probably not extending to axial furrow. S1 groove-like, extending posteromedially; S2 shorter than S1, slightly convergent backward; S3 short and pit-like, slightly convergent anteriorly. It is noteworthy that three pairs of glabellar furrows are all actually discontinuous, although seemingly being transverse in dorsal view of some specimens owing to the secondary diagenetic deformation. (4) Facial sutures, indistinct in early meraspides, are opisthoparian throughout ontogeny; anterior branches convergent between β and α, and posterior branches extending posterolaterally. (5) The pygidial notch can still be observed in M5 though extremely indistinct (Fig. 2D–F), but not in later phases and accordingly, is considered to disappear entirely in M6. (6) Eye ridge and palpebral lobe short and poorly preserved through all ontogenetic phases, extending laterally from LA at midpoint and then extending posterolaterally. (7) Fixigena becomes slightly wider (tr.) by degrees. (8) Librigena preserved only in a few articulated specimens (Fig. 3C, E), extremely narrow (tr.) and short (exs.). Librigenal spines extremely tiny in early meraspides and become slightly longer in later ontogeny, extending posterolaterally. (9) Axial and posterior cranidial border furrow become deeper. 4. Trunk segmentation schedule The relatively complete meraspid ontogenetic sequence of H. ovalis permits analysis of the trunk segmentation pattern with respect to their dorsal exoskeletons (Fig. 6). Throughout ontogeny there was a steady increase in the number of post-cephalic segments. Accompanied by the release of the anterior pygidial segment into the thorax, the boundary between the thorax and pygidium was displaced posteriorly by a single segment. Except that the number of axial segments in the pygidia of M0 and M9 is unclear (probably three), four pygidial segments
Fig. 6. Reconstruction of the trunk segmentation schedule of Hunanocephalus ovalis Lee, 1963. Dotted lines represent hypothetical growth forms; open triangles mark the liberation of thoracic segments; solid triangles mark the inserting of additional segments into the pygidium; white, dark gray, and gray colors represent a cephalon, thoracic segment and pygidium, respectively.
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Trilobite trunk segmentation has been discussed extensively in recent years based on the increasing knowledge on the development of various trilobite lineages (e.g. Hughes, 2003a,b; Fusco et al., 2004; Hughes et al., 2006; Hughes, 2007; Dai and Zhang, 2011, 2013a,b). The developmental mode of trunk segmentation of the post-Cambrian corynexochid trilobites has been listed and summed up in a few species by Hughes et al. (2006), i.e. Dolichometopidae: O. klotzi (Rominger, 1887) by Walcott (1916) and McNamara and Rudkin (1984), and B. fimbriatus Robison, 1967 by Robison (1967); Styginidae: D. hudsoni Chatterton, 1971 and S. calvum Chatterton, 1971 by Chatterton (1971), which were classified to protarthrous and hypoprotomeric development pattern, respectively. As for those from the Series 2 of the Cambrian, McNamara et al. (2003, 2006) described the ontogeny and heterochrony of four genera of early Cambrian oryctocephalid trilobites from the Balang Formation in eastern Guizhou Province, southwest China. In their study, the ontogenetic series of some taxa showed that the increase in segment number does not correspond to a one-to-one increase at each molting event. As McNamara et al. (2006) argued, for instance, in Duyunaspis duyunensis, ‘two segments were added between degree 0 and 1, but none was added between degree 1 and 2. Again two were added between M2 and M3 … (text-fig. 9)’. Such a seemingly ‘irregular somitogenesis’ (if each molting event resulted in one further thoracic segment being added into the thorax) explained by McNamara et al. (2006) was most likely due to the absence of some growth stages in each meraspid degree (Fig. 7A). Given the five pygidial segments in the degrees 0, 2, 5 and 6, and six in the degrees 1, 3 and 4, similarly, there might be two stages in each meraspid degree of D. duyunensis, with five pygidial segments in the first and six in the second, and the pygidial segment number remained fixed at five and six throughout meraspid ontogeny (the same is true of Balangia balangensis) (Fig. 7). Consequently, it is advocated that D. duyunensis and B. balangensis may process similar trunk segmentation schedule with H. ovalis, i.e. in each molting event, the addition and release of pygidial segments are separated from one another. In other words, the processes of liberation of each thoracic segment and addition of new segments into the post-thoracic region might be controlled by separate genomic programs. The realization that tagmosis and somitogenesis may each progress independently during ontogeny of these early Cambrian corynexochid
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trilobites provides a useful tool in attempts to draw an ontogenetic comparison between them. When comparing equivalent meraspid degrees between B. balangensis and D. duyunensis, for instance, McNamara et al. (2006) found that it is easy in degree 0 and 3, but not in degree 1 on account for the discrepancy in their pygidial segment number. Since both species have two stages in each meraspid degree as discussed above, it is unsuitable to compare the first instar of degree 1 (M11) of B. balangensis (with five segments in the pygidium) with the second instar of degree 1 (M12) of D. duyunensis (with six segments in the pygidium). Additionally, the holaspis of B. balangensis has seven segments, in having one more segment than the meraspid degree 4 of D. duyunensis, and might belong to a late holaspid stage (Fig. 7B). It demonstrates that in the holaspides of B. balangensis the exoskeletons increased not only in size, but also in segment number; new trunk segments were all inserted into the pygidium, but none was released into the thorax. In D. duyunensis, by contraries, tagmosis becomes fixed until the thorax processed seven segments. Accordingly, in B. balangensis (Fig. 7B), the early holaspid stages (H1 and H2), rather than the later one (H3), can be used to compare with the meraspid degree 4 of D. duyunensis (Fig. 7A). As far as known, trilobites constructed their thorax during the meraspid period through successive release of segments from the anterior portion of the transitory pygidium (Stubblefield, 1926; Fortey and Owens, 1991; Chatterton and Speyer, 1997), which is evidently unique to the Trilobita and their close relatives (Cotton and Braddy, 2004; Simpson et al., 2005). The progressive addition of exoskeletal segments near the terminus of the trunk region is deemed to be homologous with the postembryonic appearance of skeletal segments in some extant arthropods (Størmer, 1942; Hessler, 1962; Hu, 1971), and some characteristics of the trilobite trunk segmentation were probably shared with those living arthropods. As the case stands, the rates of release of anterior segments and appearance of new segments during the meraspid phase might be incoordinate within a large amount of trilobite species, however, such decoupling of segment appearance and liberation has been explored detailedly only in a limited taxa to date (e.g. McNamara et al., 2003, 2006; Simpson et al., 2005; Dai and Zhang, 2011, 2013a). The ontogenetic study of H. ovalis in this article presents new evidence to support and confirm this development pattern.
Fig. 7. Reconstructions of the trunk segmentation schedule of Duyunaspis duyunensis Zhang and Qian, in Zhou et al. (1977) (A) and B. balangensis Qian (1961) (B) based on the descriptions from McNamara et al. (2006). Dotted lines represent hypothetical growth forms which are not presented in McNamara et al. (2006). Marks are the same as in Fig. 6.
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The exchange of segments between thorax and pygidium in trilobite trunk region is remarkable because the pygidium has commonly been considered to represent a separate tagma and is equated to the abdomen of other arthropods (Cisne et al., 1980). It represents a highly unusual growth pattern among arthropods and is thus of interest with regard to the evolution of arthropod body patterning. Consequently, investigating more ontogenetic rules and trunk development of the early Cambrian trilobites is significant not only for understanding the whole extinct clade, but also provides insights into the evolution of arthropod body plan as a whole (Hughes, 2003a,b; Minelli et al., 2003). Acknowledgments We are grateful to Shigenori Maruyama and Yukio Isozaki for organizing this special issue. Financial supports by the Major Basic Research Project of the Ministry of Science and Technology of China (Grants: 2013CB835002 and 2013CB837100), the Natural Science Foundation of China (NSFC, Grants: 40925005, 41272036 and 41221001), and the “Sanqin Scholarship” project of the Shaanxi Authority are greatly acknowledged. References Bergeron, J.N., 1899. Étude de quelques trilobites de Chine. Bulletin de la Société Géologique du France 27 (Series 3), 499–516. Chatterton, B.D.E., 1971. Taxonomy and ontogeny of Siluro-Devonian trilobites from near Yass, New South Wales. Palaeontographica Abteilung 137, 1–108. Chatterton, B.D.E., Speyer, S.E., 1997. Ontogeny. In: Kaesler, R.L. (Ed.), Treatise on Invertebrate Paleontology. Part O, Revised, Trilobita, Introduction, Order Agnostina, Order Redlichiida. Geological Society of America and University of Kansas, Boulder, Colorado and Lawrence, Kansas, pp. 173–247 (xxiv + 530 pp.). Cisne, J.L., Chandlee, G.O., Rabe, B.D., Cohen, J.A., 1980. Geographic variation and episodic evolution in an Ordovician trilobite. Science 209, 925–927. Cotton, T.J., Braddy, S.J., 2004. The phylogeny of arachnomorph arthropods and the origin of the Chelicerata. Transactions of the Royal Society of Edinburgh: Earth and Environmental Science 94, 169–193. Dai, T., Zhang, X.L., 2011. Ontogeny of the eodiscoid trilobite Tsunyidiscus acutus from the lower Cambrian of South China. Palaeontology 54, 1279–1288. Dai, T., Zhang, X.L., 2012a. Ontogeny of the trilobite Estaingia sinensis (Zhang) from the Lower Cambrian of South China. Bulletin of Geosciences 87, 151–158. Dai, T., Zhang, X.L., 2012b. Ontogeny of the redlichiid trilobite Metaredlichia cylindrica from the lower Cambrian (Cambrian stage 3) of South China. Journal of Paleontology 86, 647–652. Dai, T., Zhang, X.L., 2013a. Morphology and ontogeny of the eodiscoid trilobite Sinodiscus changyangensis from the lower Cambrian of South China. Palaeontology 56, 411–420. Dai, T., Zhang, X.L., 2013b. Ontogeny of the redlichiid trilobite Eoredlichia intermedia from the Chengjiang Lagerstätte, lower Cambrian, southwest China. Lethaia 46, 262–273. Egorova, L.I., Xiang, L.W., Lee, S.J., Nan, R.S., Guo, Z.M., 1963. The Cambrian trilobite faunas of Guizhou and western Hunan. Special Paper, Institute of Geology and Mineral Resources (Beijing), Series B, Stratigraphy and Palaeontology, 3, pp. 1–117 (In Chinese). Fortey, R.A., Chatterton, B.D.E., 1988. Classification of the trilobite suborder Asaphina. Palaeontology 31, 165–222. Fortey, R.A., Owens, R.M., 1991. A trilobite fauna from the highest Shineton Shales in Shropshire, and the correlation of the latest Tremadoc. Geological Magazine 128, 437–464. Fusco, G., Hughes, N.C., Webster, M., Minelli, A., 2004. Exploring developmental modes in a fossil arthropod: growth and trunk segmentation of the trilobite Aulacopleura konincki. American Naturalist 163, 167–183. Hessler, R.R., 1962. Secondary segmentation of the trilobite thorax. Journal of Paleontology 36, 1305–1312. Hu, C.H., 1971. Ontogeny and sexual dimorphism of Lower Paleozoic Trilobita. Palaeontographica Americana 44, 1–155. Hu, C.H., 1985a. Ontogenies of two Middle Cambrian corynexochid trilobites from the Canadian Rocky Mountains. Transactions and Proceedings of the Palaeontological Society of Japan (New Series) 138, 138–147. Hu, C.H., 1985b. Ontogenetic development of Cambrian trilobites from British Columbia and Alberta. Canada (Part I). Journal of the Taiwan Museum 38, 121–158. Hughes, N.C., 2003a. Trilobite body patterning and the evolution of arthropod tagmosis. Bioessays 25, 386–395.
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