Tae-Yoon S. Park

A stem-group cnidarian described from the mid-Cambrian of China and its significance for cnidarian evolution

Palaeontological data of extinct groups often sheds light on the evolutionary sequences leading to extant groups, but has failed to resolve the basal metazoan phylogeny including the origin of the Cnidaria. Here we report the occurrence of a stem-group cnidarian, Cambroctoconus orientalis gen. et sp. nov., from the mid-Cambrian of China, which is a colonial organism with calcareous octagonal conical cup-shaped skeletons. It bears cnidarian features including longitudinal septa arranged in octoradial symmetry and colonial occurrence, but lacks a jelly-like mesenchyme. Such morphological characteristics suggest that the colonial occurrence with polyps of octoradial symmetry is the plesiomorphic condition of the Cnidaria and appeared earlier than the jelly-like mesenchyme during the course of evolution.

Post-embryonic development of the Furongian (late Cambrian) trilobite Tsinania canens: implications for life mode and phylogeny

SUMMARY The current concept of the order Asaphida was proposed to accommodate some Cambrian and Ordovician trilobite clades that are characterized by the possession of a ventral median suture. The family Tsinaniidae was recently suggested to be a member of the order Asaphida on the basis of its close morphological similarity to Asaphidae. Postembryonic development of the tsinaniid trilobite, Tsinania canens, from the Furongian (late Cambrian) Hwajeol Formation of Korea, reveals that this trilobite had an adult‐like protaspis. Notable morphological changes with growth comprise the effacement of dorsal furrows, sudden degeneration of pygidial spines, regression of genal spines, and loss of a triangular rostral plate to form a ventral median suture. Programmed cell death may be responsible for degenerating the pygidial and genal spines during ontogeny. Morphological changes with growth, such as the loss of pygidial spines, modification of pleural tips, and effacement of dorsal furrows, suggest that T. canens changed its life mode during ontogeny from benthic crawling to infaunal. The protaspid morphology and the immature morphology of T. canens retaining genal and pygidial spines suggest that tsinaniids bear a close affinity to leiostegioids of the order Corynexochida. Accordingly, development of a ventral median suture in T. canens demonstrates that the ventral median suture could have evolved polyphyletically, and thus the current concept of the order Asaphida needs to be revised.

Ontogeny of the Furongian (late Cambrian) remopleuridioid trilobite Haniwa quadrata Kobayashi, 1933 from Korea: implications for trilobite taxonomy

The monophyly of the trilobite Order Asaphida has been challenged. The Superfamily Remopleuridioidea was included in the Order Asaphida, based on the ventral median suture and highly bulbous protaspis of the late Furongian–Tremadocian representatives of the group. The remopleuridioid, Haniwa quadrata Kobayashi, 1933 from the Furongian (late Cambrian) Hwajeol Formation of Korea, represents a primitive morphology of the Remopleuridioidea. This trilobite does not have a typical globular morphology of asaphoid protaspis, and the free cheeks remained yoked together during the whole of the development. This supports the previous proposition that the Superfamily Remopleuridioidea should be excluded from the Order Asaphida. In addition, the evolution of a highly globular protaspis of the Ordovician remopleuridioid trilobites from the less bulbous protaspis corroborates the possibility of multiple evolutions of a highly globular protaspis. It can be argued that the possession of a highly globular protaspis does not guarantee the membership of the Order Asaphida, and thus the concept of the Order Asaphida should be emended.

Patterns of allozyme diversity in several selected rare species in Korea and implications for conservation

Because some endemic plants appear to be adapted to a narrow setof environmental conditions with limited genetic diversity, an analysis ofpopulation genetic structure is necessary to fully evaluate the impact of rarityon genetic variation. Listed as endangered species in Korea, only fewpopulations of Abeliophyllum distichum, Leonticemicrorhyncha, Bupleurum euphorbioides, andBerchemia berchmiaefolia were found. A reduced level ofgenetic variation in B. berchemiaefolia is consistent withthe occurrence of a genetic bottleneck and inbreeding. Leonticemicrorhyncha differed dramatically from other taxa in its observed level of geneticvariation, probably due to its predominant selfing. The level ofallozyme variation maintained by A. distichum was high forendemic species. Compared to species with similar traits, A.distichum maintained a relatively higher genetic diversity, probably dueto floral heteromorphism and preferred outcrossing. Bupleurumeuphorbioides maintained a higher genetic diversity due to outcrossing,but at the individual locus, deficiency of heterozygosity prevailed. Probablyinbreeding between local neighborhoods was frequent because A.distichum and B. euphorbioides were pollinatedby small fly species which might be less effective as a pollen dispersal, andtheir visits were extremely scarce and controlled by the weather conditions.Since much of the species-to-species variation in genetic diversity is due tothe specific ecological and evolutionary history of a species, any managementplan developed should be based on historical changes in the population size anddistribution to better predict the amounts and patterns of genetic diversity.

The Okcheon Supergroup in the Lake Chungju area, Korea: Neoproterozoic volcanic and glaciogenic sedimentary successions in a rift basin

The Okcheon Belt in southern Korea is an NE-SW trending fold-and-thrust belt consisting of two sedimentary basins of different origins: namely, the Chungcheong Basin and the Taebaeksan Basin. The Chungcheong Basin was a Neoproterozoic rift basin belonging to the South China Craton, while the Taebaeksan Basin was a Paleozoic shallow marine to non-marine sedimentary basin fringing the Sino-Korean Craton. These two basins merged to form the Okcheon Belt in the early Triassic by the collision of Sino-Korean and South China cratons and their boundary is currently demarcated by the South Korean Tectonic Line. The Okcheon Supergroup is herein refined to include the Neoproterozoic volcanic and glaciogenic sedimentary successions deposited in the Chungcheong Basin and is divided into the two groups: the Chungju Group consists of the Gyemyeongsan Formation, Hyangsanni Dolomite, and Daehyangsan Quartzite and the Suanbo Group is proposed to include the Munjuri, Hwanggangni, Myeongori, and Gounni formations in ascending order. The Myeongori Formation is emended to comprise the Geumgang Limestone and the Seochangni members. This lithostratigraphic scheme is correlatable with that of the Nanhua Basin in South China, suggesting that the Chungcheong Basin was an eastward extension of the Nanhua Basin during the Neoproterozoic. The geological structure of the Okcheon Supergroup in the Lake Chungju area is characterized by a number of isoclinal to tight, frequently overturned, anticlines and synclines. No major thrust faults were recognized within the study area, except the constraining bend of the South Korean Tectonic Line. Three deformational phases are empirically differentiated: D1 deformation most strongly affected the rocks of the Okcheon Supergroup; D2 deformation was produced by the collision between the Sino-Korean and South China cratons; and D3 deformation is represented by normal to strike-slip faults. D1, D2 and D3 deformational phases are referred to the Okcheon (mid-Paleozoic), Songnim (Triassic) and/or Daebo (Jurassic) orogenies, and post-Jurassic events, respectively. The Chungcheong Basin was initiated as a part of an intracratonic rift basin (Nanhua Basin) within the South China Craton in association with early Neoproterozoic break-up event of the supercontinent Rodinia. The bimodal volcanic succession of the Gyemyeongsan Formation corresponds to the initial rift episode of the Chungcheong Basin, and was succeeded by shallow marine Hyangsanni Dolomite and Daehyangsan Quartzite. The second phase of rifting at ∼750 Ma accumulated a thick bimodal volcanic succession of the Munjuri Formation which is overlain by the diamictites of the Hwanggangni Formation representing the Cryogenian global glacial event, snowball Earth. The immediately-succeeding cap carbonate, Geumgang Limestone Member of the Myeongori Formation, recorded the deglaciation event. The Seochangni Member of the Myeongori Formation is characterized by dark gray slate/phyllite facies indicating a poorly-oxygenated basin during the Ediacaran. No stratigraphic unit overlying the Gounni Formation, the youngest Neoproterozoic formation of the Okcheon Supergroup, occurs in the Lake Chungju area, and thus little is known on the Paleozoic tectonic evolution of the Chungcheong Basin. It is inferred that the medium-pressure type regional metamorphism and the predominance of ductile deformation of the Okcheon Supergroup can be attributed to the mid-Paleozoic Okcheon Orogeny which would have been in line with the Wuyun Orogeny of South China. The South China and Sino-Korean cratons should have been drifted away from the Gondwana sometime during the mid-Paleozoic and collided to form the East Asian continent at ∼250 Ma.

Ontogeny and Ventral Median Suture of the Ptychaspidid Trilobite Asioptychaspis subglobosa (Sun, 1924) from the Furongian (upper Cambrian) Hwajeol Formation, Korea

Abstract The order Asaphida has been characterized by the possession of a globular protaspis and a ventral median suture. The superfamily Dikelocephaloidea was included in the Asaphida, although there has been no reliable ontogenetic information for this superfamily, and thus whether dikelocephaloids had a protaspis of globular shape has remained unclear. The ontogenetic study of the dikelocephaloid trilobite, Asioptychaspis subglobosa, from Korea reveals that the protaspis of A. subglobosa is not of globular morphology but of general benthic adult-like morphology, heralding that the Dikelocephaloidea may not be closely related to other trilobite groups within the Order Asaphida. This protaspid morphology is clearly distinguished from the highly globular protaspis of the Remopleuridioidea, which has been considered the sister group to the Dikelocephaloidea. In addition, the ontogenetic development of the free cheeks shows that A. subglobosa possessed anteriorly yoked free cheeks during the early phase of development but formed a ventral median suture by splitting the yoked cheeks in the later phase of ontogeny. This contrasts with the previously suggested mode of ventral median suture formation of the Order Asaphida. This alternative mode of ventral median suture formation of A. subglobosa, along with non-asaphoid protaspid morphology, warrants removal of the Dikelocephaloidea from the current Order Asaphida. This study further demonstrates that the ventral median suture did not evolve only once in the evolutionary history of trilobites. With the addition of the mode here documented for Asioptychaspis, the ventral median suture is now known to have developed independently and in a different fashion at least three times in the history of Cambrian trilobites.

Two middle Cambrian trilobite genera, Cyclolorenzella Kobayashi, 1960 and Jiulongshania gen. nov., from Korea and China

Cyclolorenzella is an important member of the middle Cambrian trilobite faunas in China and Korea. Morphometric analysis based on well-preserved new material assignable to Cyclolorenzella from China and Korea reveals that most of the species assigned previously to Cyclolorenzella in China are morphologically distinct from the genotype. Accordingly, the new genus Jiulongshania, with J. acalle (Walcott, 1905) as type species, is proposed to accommodate the following species: J. acalle, J. regularis (Walcott, 1906), J. rotundata (Resser & Endo in Endo & Resser, 1937), J. longispina (Wittke & Zhu in Zhu & Wittke, 1989), J. acuta (Duan in Duan et al., 2005) and J. longa sp. nov. Species tentatively transferred to Jiulongshania are J.? subcylindrica (Chu, 1959), J.? yentaiensis (Chu, 1959), J.? humilis (Zhang in Qiu et al., 1983), and J.? latisulcata (Zhang in Qiu et al., 1983). This taxonomic revision results in only two species remaining in Cyclolorenzella, the type species C. quadrata (Kobayashi, 1935) and C. convexa (Resser & Endo in Endo & Resser, 1937). Jiulongshania has a relatively long stratigraphic range from the Damesella–Yabeia Zone to the Blackwelderia Zone of China, whereas Cyclolorenzella is restricted to the stratigraphically younger Drepanura Zone in China and Korea.

Late Middle Cambrian (Cambrian Series 3) Trilobite Faunas from the Lowermost Part of the Sesong Formation, Korea and Their Correlation with North China

Abstract The Sesong Formation is a member of the Taebaek Group, Korea, which extends from late Cambrian Series 3 to middle Furongian in age. Recent studies on the trilobites of the Sesong Formation have contributed significantly to the revision of the biostratigraphy. However, trilobites in the lower part of the formation, which may include the “Stephanocare Zone”, have remained essentially overlooked since the establishment of the biozone, making it difficult to correlate with the equivalent biozones of North China. Here we report trilobite faunas from the lower part of the Sesong Formation in two different sections, the Seokgaejae and the Jikdong sections, which yield two species of Jiulongshania among other species. Species of Jiulongshania have been known to occur successively in North China, so are useful for detailed correlation. Specimens of Stephanocare richthofeni are fragmentary and rarely occur in association with Jiulongshania regularis, while Jiulongshania species occur throughout the studied intervals. Accordingly, it is reasonable to extend the previously established Jiulongshania Zone of the uppermost part of the underlying Daegi Formation into the lower part of the Sesong Formation. By doing so, the Jiulongshania Zone is correlated with the Blackwelderia Zone of North China with confidence. The lowermost part of the Sesong Formation in the Jikdong section yields a fauna including J. regularis, which implies that the boundary between the Daegi and Sesong formations is diachronous within the Taebaeksan Basin. The Daegi/Sesong formation boundary in Korea is comparable to the Zhangxia/Gushan boundary in North China in that it displays an abrupt change from a carbonate-dominant facies to a shale-dominant facies. The correlation employing the Jiulongshania species indicates that the facies shift occurred significantly earlier in Shandong, North China than in the Taebaeksan Basin, Korea.

Middle Furongian (late Cambrian) polymerid trilobites from the upper part of the Sesong Formation, Taebaeksan Basin, Korea

The upper part of the Sesong Formation of the Taebaek Group, Taebaeksan Basin, Korea, is known to contain the Kaolishania Zone which can be equated to the Kaolishania Zone of North China. Silicified trilobite sclerites were recovered from six horizons of the upper part of the Sesong Formation at the Sagundari section, which include a total of thirteen polymerid species: i.e., Shirakiella elongata Kobayashi, 1935, Shirakiella sp. 1, Taishania? sp. 1, Acanthometopus sp. 1, Pagodia sp. cf. P. spina Qian, 1994, Kaolishania granulosa Kobayashi, 1933, Gumunsoia triangularis gen. et sp. nov., Gumunsoia sp. 1, kaolishaniid genus and species indeterminate 1, Elaphraella? taebaeksanensis Park and Choi, 2012, Elaphraella microforma Lu and Qian, 1983, Elaphraella nodus (Qian, 1994), and Lingyuanaspis sp. The occurrence of Kaolishania suggests that the studied interval may be part of the Kaolishania Zone. However, Kaolishania occurs only from the lowermost horizon, and the full stratigraphic range of Kaolishania is unknown at present. Accordingly, the studied interval is provisionally called the Kaolishania fauna. The uppermost interval of this Kaolishania fauna produces Acanthometopus sp. 1, and thus can be correlated with the Acanthometopus Zone in Northeast China, which is underlain by the Kaolishania Zone.

Post-embryonic development of the Early Ordovician (ca. 480 Ma) trilobite Apatokephalus latilimbatus Peng, 1990 and the evolution of metamorphosis.

In many marine invertebrates metamorphosis entails a shift from a free‐swimming larva to a benthic juvenile or adult. However, how the metamorphosis‐entailing “indirect development” in arthropods arose from direct‐developing ancestor is poorly understood. Trilobites left a rich fossil record, and some trilobite lineages had a metamorphosis‐undergoing early developmental stage, termed the “asaphoid protaspis”‐stage, providing a good opportunity to elucidate the rise of indirect development. Among others, the Ordovician representatives of Remopleuridioidea are known to possess a highly bulbous “asaphoid protaspis,” while the Furongian (Late Cambrian) remopleuridioidean genus Haniwa did not possess it. Here we show the post‐embryonic development of the remopleuridioidean trilobite, Apatokephalus latilimbatus, from the Tremadocian (485.4 Ma–477.7 Ma) Dongjeom Formation, Korea. The post‐embryonic development of A. latilimbatus contains a free‐swimming “commutavi protaspis” (a term replacing “asaphoid protaspis”). Interestingly, the earlier protaspid stage shows more similar morphology and size to the meraspis than the commutavi protaspid stage does. This indicates that the commutavi protaspid stage was intercalated into the ancestral direct development as a specialized stage for a better dispersal, and thus the “commutavi protaspis” of A. latilimbatus represents the initial phase of the evolution of indirect development. The duration of the free‐swimming phase became longer in more derived remoplueridioidean trilobites, implying that the intercalated free‐swimming strategy became emphasized during subsequent evolution. The morphological gap between the commutavi protaspis and the subsequent earliest meraspis provides a convincing case for the “selective independence” of developmental stages, explaining the various morphologies of commutavi protaspides in many trilobite lineages.