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(1)Geosciences Journal Vol. 9, No. 2, p. 215 − 222, June 2005. Heterochrony of the Late Cambrian olenid trilobites from the Machari Formation, Yeongwol, Korea: implications for biostratigraphy and intercontinental correlation Hyun-A Hwang Duck K. Choi*. } School of Earth and Environmental Sciences, Seoul National University, Seoul 151-747, Korea. ABSTRACT: This study deals with the heterochrony of two Late Cambrian olenid trilobites, Olenus asiaticus Kobayashi, 1944 from the Glyptagnostus reticulatus Zone and Hancrania brevilimbata Kobayashi, 1962 from the Hancrania brevilimbata Zone, recovered from the Machari Formation in Yeongwol, Korea. Comparison of ontogenetic developments of Olenus asiaticus and Hancrania brevilimbata led to consider that the two species are phylogenetically related: viz., Hancrania brevilimbata is a probable descendant of Olenus asiaticus. The ontogenetic trajectories of the two species are closely comparable in the protaspid and early meraspid stages, but deviate in the late meraspid and holaspid stages. The morphological changes with growth in the Olenus asiaticus-Hancrania brevilimbata lineage may have resulted from combined heterochronic processes: i.e., the reduction of preglabellar field in the descendant is attained by paedomorphic progenesis, whereas the expansion of glabella is a consequence of peramorphic acceleration. Of these, the progressive reduction of preglabellar field has been known to occur in other parts of the world and thus is assumed to be useful in improving the biostratigraphic correlation among widely separated paleogeographic provinces, such as Baltica and Avalonia. The Glyptagnostus reticulatus Zone of the Paleozoic Gondwana can be correlated with the Olenus gibbosus Subzone of Avalonia and Baltica, while the Hancrania brevilimbata Zone of Korea with the O. attenuatus and O. dentatus subzones of Baltica and the lower part of the O. cataractes Subzone of Avalonia. Key words: Late Cambrian, trilobites, heterochrony, Korea, correlation. 1. INTRODUCTION The Machari Formation has been well known to yield abundant and diverse trilobites of Middle to Late Cambrian age (Kobayashi, 1962). The trilobite faunas of the Machari Formation have been extensively studied and provided a basis for establishment of refined biostratigraphic zonation: namely, the Tonkinella, Lejopyge armata, Glyptagnostus stolidotus, Glyptagnostus reticulatus, Proceratopyge tenuis, Hancrania brevilimbata, Eugonocare longifrons, Eochuangia hana, Agnostotes orientalis and Pseudoyuepingia asaphoides zones in ascending order (Lee and Choi, 1994, 1995, 1996; Choi and Lee, 1995; Hong et al., 2003a, 2003b; Choi et al., 2004). Most of these paleontological studies have been focused on the biostratigraphy and systematic description of trilobites, whereas little attention has been *Corresponding author: [email protected]. paid to their paleobiologic aspects. Lee et al. (2001) tried to explore a phylogenetic significance of Eugonocare (Pseudoeugonocare) bispinatum (Kobayashi, 1962), based on an aberrant pygidium with two mal-formed posterolateral spines. In addition, ontogenetic developments of two Upper Cambrian olenid trilobites, Olenus asiaticus Kobayashi, 1944 and Hancrania brevilimbata Kobayashi, 1962, were examined by Lee and Choi (1999) and Hwang et al. (2000), respectively. Olenus asiaticus is an important member of the Glyptagnostus reticulatus Zone, while Hancrania brevilimbata Kobayashi, 1962 occurs in the stratigraphically younger Hancrania brevilimbata Zone of the Machari Formation (Lee and Choi, 1994, 1995). The ontogenetic developments of Olenus asiaticus and Hancrania brevilimbata are closely similar to each other in the protaspid and early meraspid stages, but display morphological deviation in the late meraspid and holaspid stages. Based on this observation, the two species are considered phylogenetically related: i.e., Hancrania brevilimbata may be a descendant of Olenus asiaticus. The holaspid morphological contrasts between the two species are presumed to be related to heterochronic processes. Accordingly, this study aims to investigate the possible heterochronic processes acting on the Olenus asiaticus–Hancrania brevilimbata lineage and to explore their significance in stratigraphic and intercontinental correlation. 2. MATERIAL AND METHODS Specimens of Olenus asiaticus were obtained from the Glyptagnostus reticulatus Zone of the Machari Formation in three measured sections, namely the Machari, Deoku and Bundeokchi-A sections (Lee and Choi, 1999), whereas Hancrania brevilimbata was recovered exclusively from the Hancrania brevilimbata Zone in the Bundeokchi-B section in Yeongwol area, Korea (Hwang et al., 2000). The Machari Formation is composed largely of laminated black shale and shale-parted limestone with intercalation of thin grainstone beds, thin-bedded dolomitic limestone and lime breccia. The formation is overlain conformably by the Upper Cambrian Wagok Formation to the west, while it is in thrust contact with the Permo-Carboniferous Pyeongan Super-.

(2) 216. Hyun-A Hwang and Duck K. Choi. group to the east (Lee and Choi, 1994; Choi, 1998). The Glyptagnostus reticulatus Zone has been recognized in the 44–46 m interval above the base of the Machari section, 58–61 m interval above the base of the Deoku section and 2–5 m interval above the base of the Bundeokchi-A section, respectively. It comprises six trilobite species, such as Glyptagnostus reticulatus, Aspidagnostus stictus, Innitagnostus innitens, Peratagnostus obsoletus, Olenus asiaticus and Proceratopyge sp. (Lee and Choi, 1994, 1997). The Hancrania brevilimbata Zone was documented from the Bundeokchi-B section, which is located approximately 250 m northwest of the Bundeokchi-A section and covers about 10-m-thick sequence of the Machari Formation. In the Bundeokchi-B section, trilobites occur abundantly within the interval 2.5–8 m above the base of the measured section, and include seven species; i.e., Hancrania brevilimbata, Proceratopyge elongata, Stigmatoa coreanica, Innitagnostus inexpectans, Peratagnostus obsoletus, Phalagnostus minor and Pseudagnostus idalis (Lee and Choi, 1995). A total of 75 specimens of Olenus asiaticus and 79 specimens of Hancrania brevilimbata are employed for the morphometric measurements. Specimens were observed under the stereomicroscope which is linked to the video camera and frame grabber to a microcomputer. The image analysis software was used to analyze the captured images of trilobites and measurement for the parameters was made in mm-scale down to three decimal places. All of the specimens are deposited in the Paleontological Collections of Seoul National University (SNUP), Korea. 3. ONTOGENTIC CHARACTERISTICS OF OLENUS ASIATICUS AND HANCRANIA BREVILIMBATA. forwards in the later meraspides; (2) preglabellar field appears in group 3 and becomes progressively longer in later stages; and (3) ocular structure is differentiated into transverse eye ridges and crescentic palpebral lobes in later meraspid stages. The holaspid cranidia are distinct in having subquadrate glabella with three pairs of glabellar furrows, smooth genal fields and upturned anterior cranidial border. The ontogenetic stages of Hancrania brevilimbata are divided into two protaspid stages and five meraspid groups. The early protaspides have circular dorsal shields with five axial rings, whereas the late protaspides are characterized by subelliptical outline with protopygidium bearing two to three axial segments. Some characteristic morphological changes with growth include: (1) cranidia are semi-elliptical in the early meraspid, but have subtrapezoidal outline with differentiated eye ridges, palpebral lobes and tubercles on fixigena in the late meraspid stages; (2) in the holaspid period, the preglabellar field expands considerably and the glabella broadens markedly; and (3) large tubercles appear on the fixigena in the early meraspid, become prominent in the late meraspid, but are replaced by fine tubercles in the holaspid period. It is noteworthy that Hancrania brevilimbata is closely similar to Olenus asiaticus in sharing a number of morphological features in the juvenile stages, but they display morphological disparity in the later growth stages. In the holaspid period, Hancrania brevilimbata clearly deviates from Olenus asiaticus in possessing appreciably shorter preglabellar field, broader (tr.) glabella and larger palpebral lobes among others. 4. HETEROCHRONY. The ontogenetic developments of Olenus asiaticus Kobayashi, 1944 and Hancrania brevilimbata Kobayashi, 1962 were described in detail by Lee and Choi (1999) and Hwang et al. (2000), respectively, which are summarized below. The ontogenetic stages of Olenus asiaticus are differentiated into two protaspid stages and six meraspid groups (Lee and Choi, 1999). The early protaspides are represented by circular dorsal shields with faintly segmented axis, whereas the late protaspides have five axial rings, eye ridges and a differentiated protopygidium. Morphological changes from late protaspid to early meraspid stages are not particularly radical. The meraspid group 1 (presumably meraspid degree 0) is distinguished from the late protaspides in appearance of posterior cranidial border with intergenal spines, prominent eye-ridges, separation of occipital ring with a medial node from the rest of glabella and larger transitory pygidium. Group 2 differs from group 1 in having parallel-sided (rather than rearward-tapering) glabella and less-prominent intergenal spines. The morphological changes in the successive meraspid stages are gradual, but some notable features are listed as follows: (1) the glabella tapers. The importance of morphological changes with growth has been extensively explored in the past (Gould, 1977, 1980; Alberch, 1980, 1982; McNamara, 1978, 1982, 1986a, 1986b; McKinney, 1986; McKinney and McNamara, 1991). In particular, McNamara (1986b) reviewed in depth the role of heterochrony in the evolution of Cambrian trilobites. The evolutionary lineages of Olenus were previously described by Kaufmann (1933), McNamara (1986b), and Clarkson et al. (1998). McNamara (1986b) also noticed the pattern of progressive reduction of frontal area in Olenus lineage along the paedomorphocline and interpreted that the paedomorphic process was progenesis. The most notable morphological contrast between Olenus asiaticus and Hancrania brevilimbata with growth is also observed in the preglabellar field. In Olenus asiaticus the preglabellar field appears in the middle stages of the meraspid period and becomes progressively longer with growth, whereas in Hancrania brevilimbata the glabella remains in contact with the anterior border throughout much of the meraspid stages and the preglabellar field is.

(3) Heterochrony of the Late Cambrian olenid trilobites from the Machari Formation, Yeongwol, Korea. 217. Fig. 1. Comparison of the ontogenetic developments of two Upper Cambrian olenid trilobites, Olenus asiaticus Kobayashi, 1944 and Hancrania brevilimbata Kobayashi, 1962, from the Machari Formation of Yeongwol, Korea. The symbols m1 to m6 represent the stages (or groups) in the meraspid period, recognized by Lee and Choi (1999) and Hwang et al. (2000), respectively. The letter H indicates the holaspid period.. only recognizable in the latest meraspid and holaspid stages (Fig. 1). When the ratios of preglabellar field length to cranidial length are plotted against the cranidial length (Fig. 2A), it is evident that Olenus asiaticus has significantly higher ratios than Hancrania brevilimbata. Morphological comparison between Olenus asiaticus and Hancrania brevilimbata clearly demonstrates that the holaspid morphology of the descendant (H. brevilimbata) reflects late meraspid morphology of the ancestor (O. asiaticus) with reduced frontal area in the descendant species. The holaspides of Hancrania brevilimbata are also generally smaller than those of Olenus asiaticus, indicating earlier onset of maturity in the descendant (Fig. 2B). The immature morphology (i.e., reduced preglabellar field), smaller size and earlier onset of maturity of the descendant (cf., McNamara, 1986a) suggest that paedomorphic progenesis may be involved in the Olenus asiaticus-Hancrania brevilimbata lineage. An additional morphological change, though less remarkable, between Olenus asiaticus and Hancrania brevilimbata with growth is appreciated in the glabella: i.e., the glabella of Hancrania brevilimbata is comparatively broader than that of Olenus asiaticus. To analyze quantitatively this morphological feature, the ratios of glabellar width to glabellar. length are plotted against the cranidial length, which shows that Hancrania brevilimbata has slightly higher ratios than Olenus asiaticus (Fig. 3A). This suggests that the glabella broadens more rapidly with growth in descendant (H. brevilimbata) than in ancestor (O. asiaticus). The rapid expansion of glabella, smaller size and earlier onset of maturity of the descendant (H. brevilimbata) can be considered to represent peramorphic acceleration (cf., McNamara, 1986a) (Fig. 3B). In short, the ontogenetic morphological changes observed in the Olenus asiaticus-Hancrania brevilimbata lineage are presumably combined products of both paedomorphic and peramorphic processes, which must have been ubiquitous in the evolution of Cambrian trilobites (McNamara, 1986b). 5. OCCURRENCES OF OLENUS SPECIES Since the genus Olenus were established in 1827 by Dalman, a total of 24 species have been assigned or transferred to Olenus: namely, O. alpha Henningsmoen, 1957; O. altaicus Ivshin, 1962; O. apoxysomatus Jell et al., 1991; O. asiaticus Kobayashi, 1944; O. attenuatus (Boeck, 1838); O. austriacus Yang in Zhou et al., 1977; O. cataractes Salter, 1864; O. delicatus Öpik, 1963; O. dentatus Westergård, 1922; O. gibbo-.

(4) 218. Hyun-A Hwang and Duck K. Choi. Fig. 2. (A) Bivariate plot of the ratios of preglabellar field length to cranidial length against cranidial length for the collections of Olenus asiaticus and Hancrania brevilimbata. (B) Diagrammatic illustration of Figure 2A to show changes in morphology and size with growth.. sus (Wahlenberg, 1821); O. guizhouensis Lu and Chien in Yin and Li, 1978; O. henningsmoeni Ahlgren and Ahlberg, 1996; O. micrurus Salter, 1849; O. mundus Lake, 1908; O. ogilviei Öpik, 1963; O. proximus Lazarenko, 1966; O. rotundatus Westergård, 1922; O. scanicus Westergård, 1922; O. sinensis Lu, 1964; O. solitarius (Westergård, 1922); O. transversus Westergård, 1922; O. truncatus (Brünnich, 1781); O. veles Rushton, 1983; and O. wahlenbergi Westergård, 1922. Stratigraphic occurrences of Olenus were particularly well documented in Avalonia and Baltica: eleven species are known from Avalonia (Lake, 1908; Rushton, 1983) and Baltica (Westergård, 1922; Henningsmoen, 1957), respectively. Species of Olenus reported from other continents include six species from Asia (Kobayashi, 1944, 1962; Ivshin, 1962; Lazarenko, 1966; Lu and Lin, 1989; Yang in Zhou et al., 1977; Yang, 1978; Peng, 1992; Lee and Choi, 1994) and three species from Australia (Öpik, 1963; Jell et al., 1991). In Avalonia, Rushton (1983) carefully documented the stratigraphic occurrences of Olenus in England and Wales, which formed a basis for subdivision of the Olenus-Homagnostus Zone into four subzones: i.e., Olenus gibbosus, O. truncatus, O. wahlenbergi, and O. cataractes subzones in ascending order. Olenus gibbosus marks the oldest member of the genus in Avalonia and is succeeded by O. austriacus and O. transversus, all of which occur within the O. gibbosus Subzone. The Olenus truncatus Subzone is represented by the nominal species, while the Olenus wahlenbergi Subzone comprises Olenus wahlenbergi and O. veles. Olenus mun-. Fig. 3. (A) Bivariate plot of the ratios of glabellar width to glabellar length against cranidial length for the collections of Olenus asiaticus and Hancrania brevilimbata. (B) Diagrammatic illustration of Figure 3A to show changes in morphology and size with growth.. dus was only known from the lower part of the O. cataractes Subzone, whereas O. cataractes, Olenus cf. dentatus and Olenus cf. solitarius occur successively in the O. cataractes Subzone of central England. O. micrurus was also reported from the O. cataractes Subzone of Wales (Lake, 1908). The stratigraphic distribution of Olenus in Baltica was compiled by Henningsmoen (1957) and was supplemented by recent contributions (Clarkson and Taylor, 1995; Clarkson et al., 1998; Ahlgren and Ahlberg, 1996): from oldest to youngest, Olenus alpha from the Agnostus pisiformis Zone; Olenus gibbosus from Norway (Westergård, 1922; Størmer, 1942; Henningsmoen, 1957; Hu, 1971) and O. henningsmoeni from Sweden (Ahlgren and Ahlberg, 1996) from the O. gibbosus Subzone; Olenus transversus and O. truncatus from the O. truncatus Subzone; Olenus wahlenbergi from the O. wahlenbergi Subzone (Westergård, 1922; Henningsmoen, 1957; Clarkson et al., 1995, 1998); O. attenuatus from the O. attenuatus Subzone of Norway (Westergård, 1922; Henningsmoen, 1957); O. dentatus from the O. dentatus Subzone of Norway (Henningsmoen, 1957); O. scanicus and O. rotundatus from the O. scanicus Subzone of Norway (Westergård, 1922; Henningsmoen, 1957); and O. solitarius from the Parabolina brevispina Zone of Norway (Westergård, 1922). In Asia, six species were assigned to Olenus. The oldest occurrence of Olenus is represented by O. asiaticus reported from the Glyptagnostus reticulatus Zone of the Machari.

(5) Heterochrony of the Late Cambrian olenid trilobites from the Machari Formation, Yeongwol, Korea. Formation, Korea (Kobayashi, 1944, 1962; Lee and Choi, 1994). The nearly contemporaneous Olenus austriacus was reported from the Chuangia-Prochuangia Zone of the Huaqiao Formation, Hunan, China (Yang in Zhou et al., 1977; Yang, 1978) and Glyptagnostus reticulatus - Chuangia wulingensis Zone of Cili-Taoyaun area, Hunan, China (Peng, 1992). Olenus sinensis occurs in the much younger Lotagnostus punctatus Zone of Zhejiang, China (Lu, 1964; Lu et al., 1974; Lu and Lin, 1989) and O. guizhouensis was reported from the Upper Cambrian of Guizhou province, China (Lu and Qian, 1983). In addition, Olenus proximus and O. altaicus were described from Siberia (Lazarenko,. 219. 1966) and Kazakhstan (Ivshin, 1962), respectively, but their stratigraphic information is unclear. In Australia, three species of Olenus were reported: O. ogilviei from the Glyptagnostus reticulatus Zone, O. delicatus from the Irvingella tropica Zone of Queensland (Öpik, 1963), and O. apoxysomatus from the putative Iverian of Tasmania (Jell et al., 1991). 6. DISCUSSION The progressive reduction of preglabellar field observed in this study is assumed to have phylogenetic significance. Fig. 4. Comparison of the paedomorphoclinal trait (i.e., progressive reduction of preglabellar field) observed in the Olenus-lineages among three different paleogeographic provinces in the Cambrian times..

(6) 220. Hyun-A Hwang and Duck K. Choi. and thus is attempted to compare with well-documented lineages of Olenus from Baltica and Avalonia (Fig. 4). The ratio of preglabellar field to cranidial length of Olenus species measured from available literature ranges from 0.063 to 0.292. Hancrania brevilimbata is included in the analysis on the basis of its close morphological similarity with Olenus, although the synonymization of the two genera has been deferred due to necessity of more rigorous morphological analysis for olenid trilobites (Hwang et al., 2000). In Avalonia, progressive reduction of preglabellar field with time is clearly demonstrated (Fig. 4A), indicating a probable paedomorphoclinal trait. Olenus gibbosus and O. austriacus have generally long preglabellar field, whereas O. transversus and O. truncatus from the O. truncatus Subzone retain relatively reduced preglabellar field. Still younger forms, such as O. cataractes, O. micrurus and O. solitarius from the O. cataractes Subzone, have very short preglabellar field. On the other hand, in Baltica apparent reduction of preglabellar field is appreciated in the Olenus gibbosus - O. transversus - O. truncatus - O. wahlenbergi - O. attenuatus - O. dentatus lineage (cf., Kaufmann, 1933), while younger forms, such as Olenus scanicus and O. rotundatus, have longer preglabellar field than O. attenuatus and O. dentatus. Olenus and H. obesus. Olenus Homagnostus obesus. Iverian Idamean Mindyallan. Taoyuanian. Waergangian Youshuian. Bundeokchian. Deokuan. Middle Cambrian. Upper Cambrian. Gonggirian. Table 1. Correlation of the Upper Cambrian biostratigraphy of the Machari Formation of Yeongwol with those of South China, Australia, Avalonia, and Baltica. Regional stages for Yeongwol are adopted from Kim et al. (2004), while the biostratigraphic information for other areas is modified from Geyer and Shergold (2000). KOREA SOUTH CHINA AUSTRALIA BALTICA AVALONIA Hapsidocare Eolotagnostus lilyensis decorus Leptoplastus Leptoplastus P. tertia – Kaolishaniella P. quarta P. secunda – Parabolina Rhaptagnostus Parabolina spinulosa P. glabella spinulosa ciliensis – Onchonotellus cf. Pseudoyuepingia Wentsuia iota – Parabolina kuruktagensis asaphoides Rhapt. apsis Parabolina brevilspina brevilspina Agnostotes orientalis Agnostotes clatava – Irvingella Irvingella tropica Eochuangia hana angustilimbata Olenus scanicus Eugonocare longifrons Corynexochus Stigmatoa diloma Olenus cataractes plumula – Sinoproceratopyge cf. O. dentatus Hancrania Erixanium sentum kiangshanensis brevilimbata Innitagnostus O. attenuatus Olenus wahlenbergi inexpectans – O. wahlenbergi Proceratopyge Proceratopyge Proceratopyge tenuis cryptica O. truncatus O. truncatus protracta Glyptagnostus Glyptagnostus Glyptagnostus O. gibbosus O. gibbosus reticulatus reticulatus reticulatus Glyptagnostus Glyptagnostus Glyptagnostus Agnostus pisiformis Agnostus pisiformis stolidotus stolidotus stolidotus Linguagnostus Acmarhachis reconditus quasivespa. (Fig. 4B). A similar phenomenon has also been noticed in Asia (Fig. 4C): Olenus asiaticus from Korea and O. austriacus from China, both from the Glyptagnostus reticulatus Zone, have relatively long preglabellar field; Hancrania brevilimbata has short preglabellar field comparable to O. attenuatus or O. dentatus in Baltica; and Olenus sinensis from much younger Lotagnostus punctatus Zone of China has very long preglabellar field. Although the present analysis is rather simple, the result seems to be useful in supplementing the biostratigraphic correlation in that the Glyptagnostus reticulatus Zone established elsewhere can be confidently correlated with the Olenus gibbosus Subzone of Avalonia and Baltica. In corollary, the Hancrania brevilimbata Zone can best be correlatable with the O. attenuatus and O. dentatus subzones and the lower part of O. cataractes Subzone in Baltica and Avalonia (Table 1). ACKNOWLEDGEMENTS: We are grateful to D.-J. Lee (Andong National University) and D.C. Lee (Daejon Health Sciences College) for their constructive reviews of the manuscript. This study was supported by the Korea Research Foundation Grant (KRF-2002-070C00087). This is a contribution to the BK21 Project..

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