Abstract: The morphology and ontogeny of the eodiscid trilobite Tsunyidiscus acutus Sun is described on the basis of numerous calcified specimens collected from the Lower Cambrian Shuijingtuo Formation in Yichang and Changyang, Hubei Province, South China. An ontogenetic series is established based on the articulated material including the previously unknown protaspides and meraspides (degrees 0 and 1). The material, revealing some prominent morphological changes such as the number of pygidial axial rings, genal spines retained throughout ontogeny and pygidial pleurae from furrowed to unfurrowed, enables a discussion on the trunk segmentation schedule, indicating that somitogenesis and tagmosis occurred independently during the ontogenetic development of T. acutus.
E odiscoid trilobites from the Lower Cambrian Shuijingtuo Formation in Yichang and Zigui of the Yangtze Gorge area, western Hubei, were reviewed by Lin et al. (2004). They include only three valid species belonging to two genera and two families: Tsunyidiscus acutus (Sun, 1983) and Tsunyidiscus yanjiazhaiensis S. Zhang et al. in Yin and Li, 1978 within the family Tsunyidiscidae S. Zhang in Zhang et al., 1980, and Sinodiscus changyangensis S. Zhang inZhou et al., 1977 within the family Calodiscidae Kobayashi, 1943.
The discovery of new material provides an opportunity to describe ontogenetic sequence of the eodiscoid trilobite T. acutus from the lower Cambrian of South China. Compared with the ontogeny of T. longquanensis (Zhang and Clarkson 1993), the segmentation schedule of T. acutus is investigated, ascribing its developmental mode of trunk segmentation to the protarthrous development (Hughes et al. 2006).
Material, occurrence and terminology
Entirely preserved specimens comprising 102 protaspides, two meraspides and 15 holaspides have been collected in black shales from two localities in the lower part of the Shuijingtuo Formation in Yichang and Changyang in the Hubei Province, South China: Wangjiaping, 19 km north-west of Yichang, and Dingjiaping, 5.5 km north-west of Changyang (Text-fig. 1). Additionally, several thousand disarticulated exuviae comprising cranidia/cephala and pygidia of T. acutus, preserved as external and internal moulds, were obtained. Owing to the secondary diagenetic alteration of many specimens, only unfragmented, well-preserved specimens were investigated, including 76 disarticulated cranidia/cephala and 199 pygidia. The specimens described and figured are housed in the collection of the Geology Department of Northwest University, Xian, China (NWUJT 20201–20214).
The morphological terms and abbreviations used in this paper largely follow Whittington and Kelly (1997), Edgecombe et al. (1988) and Lee and Chatterton (1996, 1997). Abbreviations used in the descriptions include the following: excl., excluding; exs., exsagittal; incl., including; L, length; LA, frontal glabellar lobe; sag., sagittal; T, tergite; tr., transverse; W, width; WM, maximum width; M0–M2 represent meraspid degree 0–2, respectively.
Description of ontogeny
One hundred and two protaspid specimens, 0.29–0.36 mm long and 0.27–0.34 mm wide, are investigated (Text-figs 2A–C, 6A). Shield subovate in outline, gently to moderately convex transversely and longitudinally. Anterior margin slightly curved forward; anterior border of subequal width (sag., exs.); anterior border furrow shallow. Axis tapering forward from occipital ring, defined by shallow axial furrow, extending nearly throughout the length of the shield, with frontal lobe close to anterior border furrow; divided into five axial segments, anterior three as protoglabellar lobes (L1–L3), fourth as occipital ring (LO); occipital spine not preserved; protoglabellar furrows shallow, slightly curved backward. One lobe behind the occipital ring, downsloping backward and extending to posterior margin. Two elliptical lobes strongly protuberant, located at anterior portion of pleural field, sagittal length c. 37–40 per cent of shield length, upsloping from axial furrow and lateral border furrow, respectively, with anterior margin close to anterior border furrow and posterior margin located opposite to L1. A pair of bacculae posterior to lobes, situated opposite SO or LO, with a pair of tubercles located posteriorly. Posterior margin concave medially, W-shaped in outline.
Two complete meraspid specimens were collected, i.e. degrees 0 and 1, preserved as internal and external moulds, respectively (Text-figs 2D, E, 6B, C).
Degree 0 meraspis (M0). Shield oval in outline, 0.98 mm long (sag.), of moderate convexity (Text-figs 2D, 6B). Cranidium subtrapezoidal in outline, sagittal length 0.53 mm, 54 per cent length (sag.) of the body, c. 1.5 times wider than long. Anterior margin concave in front of glabella, somewhat M-shaped in outline. Anterior border narrow, defined by shallow anterior border furrow. Preglabellar field extremely narrow (sag.). Glabella, defined by shallow axial furrow, tapering evenly forward to LA, sagittal length c. 70 per cent of cranidial length; frontal lobe acute with anterior margin close to anterior border furrow, WM (tr.) of LA 62 per cent width of L1. One elliptical fixigenal lobe can be observed, located in the anterior portion of the right side of the fixigena, strongly protuberant; sagittal length 56 per cent of cranidial length, upsloping from axial furrow and palpebral lobe, respectively, with anterior margin close to anterior border furrow. Proparian facial suture can be observed in the right side of the cranidium, anterior sections diverging anterolaterally, with γ opposite LA; posterior sections diverging posterolaterally, with ε probably opposite L2. Posterior margin straight to genal angle. Fixigenal spine minute can be observed in the left side of the cranidium.
Pygidium semi-elliptical in outline, 86 per cent width (tr.) of the cranidium, with posterior margin concave medially, W-shaped in outline; sagittal length 74 per cent width (tr.). Axis poorly preserved, with posterior portion absent; probably divided into five rings according to the furrowed pleural field. Four pairs of interpleural furrows shallow and faint, directed posterolaterally and reaching border furrow, dividing pleural field into four pairs of pleural ribs and a terminal band. Four pairs of nodes on pleural ribs abaxially, less prominent posteriorly. Lateral border narrow (tr.), broadening posteromedially, reaching maximum width (exs.) lateral to the notch.
Degree 1 meraspis (M1). Shield subelliptical in outline, 0.91 mm long (Text-figs 3E, 6C). Cephalon semicircular in outline, sagittal length 0.5 mm, 55 per cent length (sag.) of the body, c. 1.3 times wider than long. Anterior margin slightly curved laterally; anterior border moderately flat, slightly widening (exs.) abaxially. Preglabellar field narrow (sag.), subequal in width to preocular fixigenal field. Glabella with two transglabellar furrows weakly impressed, sagittal length (incl. LO) 84 per cent of cranidial length. Occipital ring moderately convex; occipital spine not preserved but might present. Two elliptical fixigenal lobes less prominent, sagittal length c. 50 per cent of cranidial length, with posterior margin located opposite S1. Bacculae minute, opposite SO. Eye ridge, observed in the left side of the fixigena, extremely narrow, extending from anterior portion of LA and curved posterolaterally. Palpebral lobe crescentic, with anterior and posterior tip situated opposite S2 and S1, respectively. Facial suture can be observed in the left side of the cranidium. Librigena in the left side, short (exs.) and narrow (tr.), up to 0.24 mm in length.
Thorax with one segment, without axial spine; pleurae of equal width laterally (exs.), with a pair of nodes located abaxially and anterior to pleural furrow; pleural spines absent.
Pygidium semicircular in outline, 37 per cent length (sag.) of the body; c. 1.7 times wider than long. Axis protuberant, with posterior portion absent, probably bears four rings according to three pairs of nodes on the pleural ribs. Three pairs of interpleural furrows probably present, dividing pleural field into three pairs of pleural ribs and a terminal band; three nodes observed in the left side of the pleural ribs abaxially, minute posteriorly.
Fifteen complete holaspid specimens were collected, 2.2–6.6 mm long, most lacking librigenae (Text-figs 3A–C, 4A–F, 6D, E). In addition, length and width are measured on 76 cranidia/cephala and 199 pygidia (Text-fig. 5). Shield convex and oval in outline. Cranidium subtrapezoidal in outline, 1.13–1.25 times wider than long (excl. occipital ring). Anterior border wide and convex, equal in length (sag., exs.) abaxially; anterior border furrow shallow and wide. Preglabellar field depressed, narrower (sag.) than anterior border; preocular fixigenal field downsloping forward. Glabella, defined by deep and narrow axial furrows, rising above fixigena, tapering evenly forward, sagittal length (incl. LO) 72–76 per cent of cranidial length; LA pointed with anterior margin a short distance from anterior border, WM (tr.) of LA 51–62 per cent width of LO; two transglabellar furrows slightly curved posteriorly. Occipital ring with posterior margin curved backward, SO shallow and wide, occipital spine not preserved but might present. Fixigenal field strongly protuberant, considerably raised above the level of cranidial border, slightly beneath that of glabella. Eye ridge narrow and faint, extending from LA then curved posterolaterally and forming a continuation of palpebral lobe. Anterior sections of facial sutures diverging anterolaterally, with γ opposite to LA; posterior sections diverging posterolaterally, with ε opposite to S1 or L1. Posterior margin straight to genal angle; posterior border narrow (exs.) adaxially, broadening laterally, reaching WM (exs.) at genal angle; posterior border furrow deep, extending laterally to genal angle, then anterolaterally conjoined with lateral border furrow. Genal spine minute, tapering posterolaterally.
The thorax has three segments. Axial rings convex; axial spines might present at T2 and T3. Pleural spines of T1 pointing posterolaterally, of T2 directed laterally, of T3 pointing anterolaterally. The pleurae of T1 wider (exs.) than those of T2 and T3. Lateral lobes present near the axial rings, anterior to pleural furrow, less expanded from T1 to T3.
Pygidium semicircular in outline, 31–34 per cent length (sag., excl. articulating half-ring) of the body; c. 1.60–1.72 times wider than long. Axis narrow, defined by deep and narrow axial furrows, rising above pleural region; divided into four axial rings, tapering evenly backward, with posterior tip rounded and not reaching posterior border furrow; WM (tr.) 22–26 per cent pygidial width. Pleural field convex and unfurrowed, covered with granules. Anterior border slightly widening outward to fulcrum, then more broadening posterolaterally. Lateral and posterior border narrow, separated from pleural field by border furrow.
Two stages can be distinguished in the complete holaspides according to the two size clusters. The stage 1 is represented by eight complete specimens ranging from 2.2 to 4.6 mm in length, of which three were figured (Text-figs 3A–C, 6D); the stage 2 is represented by seven articulated specimens, 5.1–6.6 mm long, of which six were figured (Text-figs 4A–F, 6E). The morphological changes between the two stages are very subtle including: longer (sag.) preglabellar field; lateral lobes in the each side of the thoracic axial rings more expanded and prominent; a decrease in the L/W ratio of cranidia; an increase in the L/W ratio of pygidium. Furthermore, the addition of a new segment in the pygidium can be observed in some holaspides of stage 2 (Text-fig. 4A, C, D).
Summary of morphological variation during ontogeny
The length of the exoskeleton increases from 0.29 mm in the protaspis to 6.6 mm in the largest holaspis. During ontogeny, the anterior cranidial border, preglabellar field and preocular fixigenal field become wider (sag.); the axial furrow becomes deeper and better defined, as well as prominent border furrows on both cranidia and pygidia. Anterior cranidial margin is slightly curved in protaspides and concave medially in M0 and becomes curved anteriorly in later ontogeny. Posterior pygidial margin is concave medially in protaspides and M0 and becomes curved posteriorly in M1 and holaspides. Eye ridge and palpebral lob become more distinct in holaspides. The two elliptical fixigenal lobes from strongly protuberant in protaspides and M0 to contracted in M1 are absent in holaspides, in which the fixigena is convex. The bacculae, observed in protaspides and M1, might be present in M0 accordingly and absent in holaspides. The facial sutures, not clear in protaspides, are apparently proparian in meraspid and holaspid period. Pygidial fulcrum appeared in holaspides.
Zhang and Clarkson (1993) studied the ontogeny of T. longquanensis in detail, in which the morphology and developmental trends during all growth stages were described based on excellently preserved phosphatized specimens isolated from a single limestone lens. The material comprises five protaspides and numerous disarticulated cranidia, pygidia and thoracic segments of all subsequent growth stages. There are a number of similarities in the ontogenetic development between T. acutus and T. longquanensis. Note that the nodes in the pygidial pleurae of the M0 and M1 of T. acutus (Text-fig. 2D, E) are also observed in T. longquanensis defining the positions of pleural ribs of pygidia (Text-fig. 3A–D), which are considered to develop into lateral pleural lobes when the segment bearing them is released to form the thoracic segments. The lateral lobes in each side of the holaspid thoracic axial rings of T. acutus are also observed in those of T. longquanensis, and they might be independent of the nodes in the meraspid pleural ribs. The eye ridge and palpebral lobe, not observed in protaspides of the two species, probably are undeveloped in this period. In M0, eye ridge and palpebral lobe are not observed in T. acutus but might present, while they are present in T. longquanensis, extending laterally from each side of the frontal glabellar lobe and curved between the anterior border and the fixigenal lobe (Text-fig. 3A–D). In addition, some differences during ontogeny can be recognized: the genal spines are retained throughout the ontogeny of T. acutus, while they disappear and finally form a rounded genal angle in the holaspides of T. longquanensis. The pygidial pleurae change from furrowed in M0 and M1 to unfurrowed in holaspides of T. acutus, and similar to T. yanjiazhaiensis (Zhang et al. in Yin and Li, 1978), they are still visible in holaspides of T. longquanensis. Additionally, the numbers in the addition of the pygidial segments are different through ontogeny: T. longquanensis has five and six pygidial segments in M2 (1993, pl. 1, fig. 9) and six and seven pygidial segments in holaspides (1993, pl. 1, figs. 8, 10), which were speculated only on the disarticulated pygidia, while T. acutus possesses observed four and five pygidial segments in the complete holaspides (Text-figs 3A–C, 4A, C, D), respectively.
Trunk segmentation schedule
The ontogenetic series allows a study on the trunk segmentation schedule of T. acutus (Text-fig. 7). Although only one protopygidial segment can be observed in the collected protaspides, the length (sag.) of the protopygidial axis and a pair of tubercles posterior to the bacculae (Text-fig. 2A, B) imply that the protaspid pygidia might comprise two segments. It is similar to those of T. longquanensis in which ‘there were no transverse furrows visible on the protopygidial axis, but its length would suggest that two rings were present’. Consequently, we favour defining the onset of the meraspid period with two (or three ?) pygidial segments. Owing to the observed five and four pygidial segments in the M0 and M1, respectively (Text-figs 2D, E), it is speculated that the number of postcephalic segments of M0 probably increases from two to five comprising four growth forms (M01–M04), and in each form, a new segment was added. As McNamara et al. (2003, 2006) have demonstrated that the two different mechanisms, somitogenesis and tagmosis, occurred independently during the ontogenetic development of the postcephalic region. The successive addition of segments into the pygidium of M0 indicates that only somitogenesis occurred during this period. Between M0 and M1, one segment was released from the anterior of the pygidium to become the thorax, so there is no change in the number of the postcephalic segments as the boundary between the thorax and pygidium migrated posteriorly (M1), indicating that only tagmosis occurred at this point and each moulting event does not always bring the addition of segment in T. acutus. Similar to M0, more than one moult might occur in M1 with a segment added into the pygidium. The M2 is unknown and probably like that of the preceding phase, one further segment was liberated into the thorax between M1 and M2, with one segment added into the pygidium afterwards. After the addition of the third segment into the thorax, still only four segments exist in the holaspid pygidia (H1). Additionally, some holaspides assigned to stage 2 (Text-fig. 4A, C, D) show the addition of a segment into the pygidia (H2). The addition of a new pygidial segment did not stop during the holaspid phase indicates that the somitogenesis continues after the tagmosis becomes fixed in the holaspides of T. acutus.
The trilobite growth and trunk segmentation has been discussed extensively in recent years based on the increasing data on ontogeny and developmental schedules of various species (e.g. Fusco et al. 2004; Hughes et al. 2006; Hughes 2007; McNamara et al. 2003, 2006). Hughes et al. (2006) proposed an additional and complementary ontogenetic scheme based on the generation of new trunk segments which include anamorphic and epimorphic phase and examined the developmental mode of trunk segmentation of 35 trilobite species assigned to various developmental modes. The addition of a new segment into the pygidium in the holaspides of T. acutus indicates that the start of holaspid phase preceded the onset of the epimorphic phase. Additionally, the segmentation of T. acutus showing multiple moults per meraspid degree could be attributed to the protarthrous development.
Acknowledgements. We would like to thank Liu Wei, Lei Ming, and Jia Zhixin for help in collecting samples, Gong Hujun for help with the scanning electron micrographs, and Peng Shanchi and Nigel C. Hughes for helpful advice. Financial supports by the Natural Science Foundation of China (NSFC, Grants: 40872004, 40925005 and 40830208) and the Major Basic Research Project of the Ministry of Science and Technology of China (Grant: 2006CB806400) are greatly acknowledged.