Radiolarian Fauna Research Articles

Radiolarians from Cambrian Series 2 Niujiaohe Formation in southern Jiangxi, South China

The origin and early evolution of radiolarian fossils have always provoked great interest among paleontologists. Yet, little is known about the earliest morphological and evolutionary characteristics of radiolarians largely due to the lack of documentation of early Cambrian radiolarian fossils. Herein, we conduct a thorough examination of the basinal Niujiaohe Formation in Chongyi County of southern Jiangxi and report two types of radiolarians: a large number of spherical radiolarians and a minor occurrence of ellipsoidal radiolarian fossils with bipolar rodded spines. The lengths of the spines on ellipsoid radiolarian fossils vary, and typically, the main spines are slightly offset from the polar axis, resulting in an asymmetrical appearance in some specimens. Through systematic taxonomic examination, this radiolarian fossil form is tentatively identified as Beothuka sp. of the spherical polycystine group. The genus Beothuka is commonly found in the Early Ordovician and has not yet been reported in the early Cambrian. Combined with previous reports, four species of radiolarians in three genera are identified from the Niujiaohe Formation in South China, including Paraantygopora porosa, Paraantygopora sp., spherical radiolarian, and Beothuka sp. Overall, the radiolarian fossils in the Niujiaohe Formation are characterized by low diversity and small size, in contrast to the relatively more diverse fauna found in the contemporary shallow-water Shuijingtuo Formation. Therefore, we speculate that radiolarians may have initially originated in shallow water areas and gradually expanded to deep water. This study further enriches the fossil record of the Cambrian Series 2 radiolarians. Combined with relevant data from South China, the radiolarian fauna began to exhibit diversity and complex morphology, indicating the presence of various, advanced spherical-skeletons in early radiolarians, and radiolarians from the early Cambrian appeared to display multiple evolutionary trends.

A review of the radiolarian faunas in the Bangong-Nujiang Suture Zone: Implications for the tectonic evolution of the Mesotethys Ocean

A review of the radiolarian faunas in the Bangong-Nujiang Suture Zone: Implications for the tectonic evolution of the Mesotethys Ocean

Albaillellarian (radiolaria) assemblages across the Guadalupian–Lopingian boundary (Permian) and their environmental implications

It remains unclear how radiolarian lineages adapted to the changing environments through the Guadalupian–Lopingian boundary (G-LB) interval. In this study, a well-preserved radiolarian fauna was obtained from silty cherts and siltstones deposited throughout the G-LB of the Sanpaoling, Yutouling, and Xiaqianling sections, located in Qinzhou City, Guangxi, China. Two genera of Albaillellaria, including ten species of Albaillella and two Neoalbaillella species/morphotypes, are presented in this study. This assemblage is correlated with the Albaillella cavitata Interval Zone. Based on the material obtained near the G-LB, the taxonomic value of pores on the test of Albaillella species is here evaluated. This research documents a relatively significant morphological variation of the Albaillellarian lineages and an evolutionary reversal in Albaillella through the G-LB. Based on the combination of the faunal and evolutionary response in the studied area, we suggest that the G-LB witnessed rapidly changing environmental conditions, imposing adaptive pressure on radiolarians, at least on the deep-dwelling Albaillellarians.

A revision of the Carnian/Norian boundary radiolarian assemblages: Insight from the biostratigraphic record of the Norian GSSP, Pizzo Mondello section (Sicily, Italy)

The Triassic was a critical period for a significant increase in the generic diversity of radiolarians after the disappearance of >80% of radiolarian genera during the Permian-Triassic mass extinction. In particular, the Upper Triassic radiolarian fauna is rich and has been documented in several world regions, such as North America, Western Europe, the Mediterranean area, Japan, the Philippines, Timor, and Far East Russia. In this study, a well-preserved and high-diversity fauna from a continuous pelagic-hemipelagic section deposited in western Tethys (Sicily, Italy) provides one of the most detailed radiolarian records of the Carnian-Norian boundary interval, which can be correlated worldwide, from high to low-latitude, from both sides of the Pangea and both Hemispheres. The 99 species belonging to 50 genera from this time interval have been collected from the Pizzo Mondello section (Sicanian Basin, Western Sicily), the selected GSSP for the base of the Norian stage, and then compared to the radiolarian associations documented in other coeval regions. Based on the occurrences of characteristic radiolarian species, we defined and proposed a new radiolarian biozonation across the Carnian-Norian boundary, which consists of five new assemblages (PM assemblage 1–5) that are correlated and calibrated to the already well-defined biozonation of bivalves, ammonoids, and conodonts.

Middle Triassic and middle Permian radiolarians from the Kamitaki Complex in the Sasayama area, Hyogo Prefecture, Southwest Japan: Evidence for Triassic plate subduction along the eastern margin of Paleo-Asia

The Kamitaki Complex, situated in the Sasayama area in Southwest Japan, has long been presumed to be a Permian subduction-related accretionary complex based on correlations from previous studies. However, because of the lack of fossil evidence, the exact age of the complex remained uncertain for a long time period. To address this gap in knowledge, a geological survey and microfossil mapping were conducted in the Kamitaki Complex to determine its age and geological context.A geological survey revealed that the Kamitaki Complex mainly consists of clastic rocks, and a mixture of sandstone, basalt, and chert blocks within the mudstones. The Kamitaki Complex is tectonically intercalated into the Lopingian (Late Permian) accretionary complex of the Ultra-Tamba Terrane and Late Triassic accretionary complex of the Tamba Terrane. The lithological and structural characteristics of the Kamitaki Complex confirm that it is an accretionary complex. Microfossil mapping yielded depositional ages, with radiolarian fauna such as Eptingium nakasekoi, Pseudostylosphaera japonica, Cryptostephanidium japonicum, and Oertlispongus cf. diacanthus identified in mudstones suggesting an Anisian (early Middle Triassic) age. In contrast, radiolarian fauna found in cherts, including Pseudoalbaillella? aff. longicornis, and Follicucullus cf. porrectus, indicate an early Capitanian (late Guadalupian, middle Permian) depositional age. These findings suggest that the Kamitaki Complex records a trenchward migration of the oceanic plate in a pelagic environment from the early Capitanian and an accretion at the trench during the Anisian period.Conventionally, the plate boundary between the Panthalassa and Paleo-Asia during the latest Permian to Middle Triassic was of the transform type, primarily because no subduction-related accretionary complexes from this period have been identified in the Japanese Islands. However, the discovery of Kamitaki Complex, an Anisian accretionary complex, provides evidence of Middle Triassic subduction activity along the eastern margin of Paleo-Asia. According to the internal structure and age polarity of the Ultra-Tamba and Tamba terranes, an accretionary complex developed over a prolonged period (approximately 120 million years) in a tectonic setting that persisted along the eastern margin of Paleo-Asia from the late Guadalupian to the earliest Cretaceous period.

Miocene radiolarian assemblages from the submarine Vityaz Ridge, Northwest Pacific: Biostratigraphy and paleoceanography

This study presents the first data on radiolarian fauna from Miocene deposits of the submarine Vityaz Ridge (SVR) and paraxial zone of the Kuril-Kamchatka Trench. Twenty-two dredge samples were studied, and 214 radiolarian taxa were identified. Taxonomic composition allowed their assignment to Miocene assemblage zones, including Lipmanella japonica conica-Gondwanaria dogieli, Pentactinosphaera hokurikuensis, Dendrospyris sakaii, Eucyrtidium inflatum Subzone a, Lychnocanoma magnacornuta, and Lychnocanoma parallelipes zones. These radiolarian assemblages correlate with studied sequences of many deep-sea cores in the northern Pacific and some sections of onshore Japan. As a result, we designed a biostratigraphic scheme of Miocene radiolarians for the SVR and reconstructed the environmental conditions in this area. In particular, two Miocene climatic optima that were previously established in the northern Pacific were identified in the Middle and Upper Miocene sediments of the southern plateau and Middle Miocene sediments of the northern plateau of the SVR.

Late Jurassic to early Cretaceous radiolarian faunas from Changku, Gyangze, southern Tibet: Implications for the evolution of the Neo-Tethys

Radiolarians found in siliceous sediments within the Yarlung-Tsangpo Suture Zone in southern Tibet contain valuable geochronological and tectonic information for the evolution of the Neo-Tethys. Radiolarians, as the main microfossils contained in radiolarian cherts, are commonly used to determine the age of the chert. The study of radiolarian biostratigraphy is crucial in interpreting the tectonic nature of associated oceanic terranes, which can aid in restoring the evolution of the Neo-Tethys. A large number of poorly to well-preserved radiolarian fossils were recovered from a chert block near Changku, Gyangze, in southern Tibet. A total of 57 species attributed to 36 genera have been identified from eight productive samples. Four radiolarian assemblages have been identified by comparison with established radiolarian zones of the Neo-Tethyan realm and have been precisely dated to the late Kimmeridgian to earliest Hauterivian. The origin of the exotic blocks in the Zongzhuo Formation has been subject to different interpretations, and the chert block near Changku should be of tectonic origin.

Uppermost Darriwilian radiolarians from the lower part of Wulalike Formation, Northwest Ordos Basin, North China

A well-preserved radiolarian assemblage is described from argillaceous limestone of the Wulalike Formation in the Northwest Ordos Basin, North China. A total of nine species of seven genera are described, including Inanigutta gansuensis, I. quadrispinosa, I. complanata, Inanibigutta pinglianensis, Oriundogutta bella, Triplococcus acanthicus, Haplentactinia juncta, Syntagentactinia pupila n. sp., and Kalimnasphaera maculosa. Three-dimensional X-ray micro-computed tomography is used to make a detailed examination of the new species. This is the first documentation of radiolarian fossils from the Wulalike Formation, which contributes to better understanding of the features of Ordovician radiolarian faunas and facilitate stratigraphic correlation and subdivision in the Ordos Basin.

Constraints on the expanse of Greater India in the Early Cretaceous from radiolarians

Defining the original size of Greater India is a critical parameter for reconstructing the kinematics and timing of the India–Asia collision. Early Cretaceous paleontology-based paleoceanographic and paleogeographic reconstruction for the northern coast of East Gondwanaland provides independent constraint for the extent of Greater India. In this study, newly discovered Early Cretaceous radiolarian faunas from the Tethys Himalaya were objectively compared with non-Tethyan austral faunas from the Argo Abyssal Plain (AAP) and the typical Tethyan fauna from Italy using the Hayashi's Quantification Method Type III. We found that (1) The occurrence of austral cold-water radiolarian species at the western coast of Australia and northern coast of Greater India is attributable to northward intrusion of circumantarctic cold water via a seaway newly opened during the breakup of East Gondwanaland from ∼136 Ma; (2) Gyangze was under shallow warm-water conditions with typical Tethyan radiolarian fauna (Cluster 3) until the late Valanginian when cold-water species (Cluster 1) arrived from the south. A water mass boundary shift from Tethyan species dominant to austral cold-water species dominant in Gyangze indicates its lower paleolatitude than Ocean Drilling Program (ODP) Leg 123, Site 765 in the AAP; (3) The same austral cold-water radiolarian-bearing intervals from the late Berriasian to Valanginian indicate that Kangma in southern Tibet was proximal to ODP Site 765; (4) Different radiolarian faunas in Italy–Zhongba, Kangma, and the AAP reveal complex water conditions under different degrees of effect by Tethyan warm water and austral cold water at different water depths during the Barremian to Aptian. This radiolarian biogeography is highly consistent with the paleomagnetic reconstruction and indicates that eastern Greater India extended to the Exmouth Plateau (>2400 km) during the Early Cretaceous.

The central Qiangtang Metamorphic Belt in northern Tibet is an in-situ Paleo-Tethys Ocean: Evidence from newly discovered Late Devonian radiolarians

The paleogeographic position of the North Qiangtang Block, as well as the origin of the Central Qiangtang Metamorphic Belt (CQMB) have subjected to considerable debate that hampers the understanding of the early evolution of the Paleo-Tethys Ocean. This study reports a new radiolarian fauna of a Famennian age (Late Devonian) from the ophiolitic mélange south of Gangtang Co, northern Tibet, including Callela parvispinosa Won, Entactinia foveolata Nazarov, and Plenoentactinia pinguis Won. The discovery of Devonian radiolarians in the CQMB strongly supports the model that the Longmu Co–Shuanghu suture zone represents the main branch of the Paleo-Tethys Ocean. A correlation of the Late Devonian radiolarian in Tethys realm reveals that the Longmu Co–Shuanghu suture zone was connected to the Changning–Menglian suture zone in western Yunnan, the Chiang Mai–Inthanon and Chanthaburi suture zones in Thailand, and the Bentong–Raub suture zone in Malay Peninsula. The synchronous advent of Late Devonian radiolarians suggests that the Paleo-Tethys Ocean may have opened during that time.

No dramatic changes observed in subtropical radiolarian plankton assemblages during the Middle Eocene Climatic Optimum (MECO); evidence from the North Atlantic ODP Site 1051

The Middle Eocene Climatic Optimum (MECO; ca. 40 Ma) was a prominent global warming event that lasted 400 kyr and was characterized by a 4–6 °C rise in high latitude surface and deep-water temperatures. As the radiolarian plankton response to this warming event is practically unknown, whole assemblage quantitative analyses were undertaken on well-preserved radiolarian assemblages from ODP Site 1051 (western North Atlantic). Although radiolarians have apparently benefited at this site from increase in oceanic fertility induced by the MECO, this event does not appear to have had a severe impact on subtropical radiolarian fauna. No prominent faunal turnover was found in the studied interval, suggesting that subtropical radiolarians are relatively resilient to transient warming events. Likewise, variations in radiolarian assemblage composition establish that ocean warming induced only a weak ecological response. One of the most striking faunal changes associated with the MECO is the clear increase in radiolarian diversity (taxic richness), as a result of the northward migration of warm tropical radiolarian species. Similarly, several typical middle Eocene tropical species are found to be more abundant in the warmest interval. In addition to these poleward migrations, we identified three radiolarian clusters composed of warm-water or cool-water species, as well as two abundant artostrobiid species which may represent nutrient opportunists.

Late Cretaceous radiolarians from the Noni Formation, West Timor, Indonesia

Late Cretaceous (Cenomanian–Turonian) radiolarian fauna was recovered from a single chert sample of the Noni Formation in the Miomaffo District of West Timor, which is generally viewed as part of the allochthonous ‘Banda Terrane’. This fauna is characterized by the presence of Cryptamphorella conara, Diacantocapsa euganea, Dictyomitra formosa, Dictyomitra montisserei, Guttacapsa biacuta, Stichomitra communis, Patelula verteroensis, Pessagnobrachia fabianii, Praeconocaryomma lipmanae and others. The character of the radiolarian fauna shows a close resemblance to those in South Sulawesi and is very different from age-equivalent radiolarian faunas in the ‘autochthonous’ southern foldbelts of West Timor (Kolbano) and Timor Leste (Viqueque). This report on the Late Cretaceous radiolarians in Timor attempts to identify the fauna, to clarify its age, and to indicate the paleogeographic origin of radiolarian-bearing chert of the Noni Formation.

Late Triassic (Late Early to Early Middle Norian) and Late Triassic or Early Jurassic Radiolarians from Limestone in the Tha Sao Area, Kanchanaburi Province, Western Thailand: Low-Latitude Fauna in the Eastern Tethys

Moderately-preserved Late Triassic (late early to early middle Norian) and Late Triassic or Early Jurassic radiolarians have been identified from two sections of limestone and dolomitic limestone, respectively in the Tha Sao area, Kanchanaburi Province in western Thailand. Previously these limestones have been assigned as the Permian because of the similarity of lithology without any fossil evidence. Section 1 is a succession of about 18 m in thickness, consisting of calcareous mudstone, micritic limestone, and dolomitic limestone, in ascending order. Section 2 is about 30 m in thickness and consisting of dolomitic limestone. Middle micritic limestone of about 10 m thick of Section 1 yielded diversified radiolarians. Dolomitic limestone of Section 2 yielded rare radiolarians from two levels. We identified radiolarians from five levels of micritic limestone and two levels of dolomitic limestone and totally identified 53 species belonging to 39 genera including two species of which generic position is uncertain and one new species Colum tekini Sashida and Ito sp. nov. At the Late Triassic time, the Tethys Ocean between Sibumasu and Indochina blocks almost closed due to the collision of these two continental blocks. Upper Triassic radiolarian-bearing limestones have been deposited in the eastern to western Tethyan areas within 35° of both hemispheres. The present radiolarian fauna is representative of low latitude Tethyan fauna. The faunal similarity between the eastern and western Tethys indicates the uniformity of the establishment of the low latitude fauna by the Late Triassic.

Progress in understanding middle Eocene nassellarian (Radiolaria, Polycystinea) diversity; new insights from the western equatorial Atlantic Ocean

AbstractMiddle Eocene deep-sea sediment sequences cored at Ocean Drilling Program Site 1260 (Leg 207; equatorial Atlantic Ocean) yielded diverse and abundant radiolarian faunas that are conducive to biostratigraphic and palaeoceanographic research, as well as to the study of radiolarian diversity dynamics during this epoch of significant climate changes. However, many species present in these sediments still have not been formally described and are therefore neglected in most biodiversity surveys. In an effort to improve the taxonomic resolution of middle Eocene radiolarians, 15 new species of nassellarians are described and illustrated. The species are: Cymaetron? dilatatus n. sp., Eucyrtidium levisaltatrix n. sp. (Eucyrtidiidae), Siphocampe pollen n. sp., Spirocyrtis? renaudiei n. sp. (Artostrobiidae), Pterocyrtidium eep n. sp. (Rhopalosyringiidae), Petalospyris cometa n. sp., Petalospyris castanea n. sp. (Cephalospyrididae), Velicucullus armatus n. sp. (Theophormididae), Lychnocanium nimrodi n. sp. (Lithochytrididae), Aphetocyrtis zamenhofi n. sp., Aphetocyrtis? columboi n. sp., Aphetocyrtis? spheniscus n. sp. (Lophocyrtiidae), Albatrossidium regis n. sp., Albatrossidium annikasanfilippoae n. sp., and Phormocyrtis lazari n. sp. (Pterocorythidae). Stratigraphic range data are provided for each new species, as well as the orbitally tuned ages for their first and last occurrences. In addition to these new species, we also illustrate and document the stratigraphic distribution of four species described in early radiolarian studies and rarely reported since.UUID: http://zoobank.org/b5993e6b-659b-4220-80c2-c6ddccc13793

Changhsingian (Lopingian, Permian) radiolarian paleobiogeography on and around the Yangtze Platform

We report a Changhsingian radiolarian assemblage from the Rencunping section of the Dalong Formation, in the northern margin of the Yangtze Platform, South China. The assemblage is composed of 37 species belonging to 20 genera, including two new species (Copicyntroides rencunpingensis n. sp. and Copicyntroides subplanus n. sp.). Together with the well documented radiolarian faunas of the Dalong Formation around the Yangtze Platform, we applied Sørensen Coefficient, an incidence-based measures of relative similarity, to examine the similarity of species associations among the sections of the Dalong Formation, including those at Rencunping, Shangsi, Hushan, Pingdingshan, Xinmin and Dongpan. We find that 165 species have been reported in the entire Dalong Formation of all sections, with variable paleobiogeographic setting from section to section within the same formation. The Sørensen Coefficient showed a high similarity between the Rencunping and Pingdingshan sections and low similarity in the Xinmin or Dongpan sections. This is interpreted as due to water mass separation between the northern and southern sides of the Yangtze Platform, probably by the physical barrier of the shallow carbonate platform in the region. We reviewed taxonomic groups and depositional conditions in each section, and determined “low latitudinal common species” or “cosmopolitan” species including Entactinia itsukaichiensis, Albaillella angusta, A. excelsa, A. protolevis, A. triangularis, Ishigaum obesum, I. trifustis, Trilonche crassispinosa, Triplanospongos musashiensis and “Hegleria-shaped” radiolarian. The radiolarian fauna of Rencunping was concluded as a typical fauna in the Dalong Formation, and its higher similarity to faunas from Japanese Islands and Southeast Asia indicates close paleogeographic positions and similar paleogeographic environments in these areas during the Changhsingian.

VALANGINIAN RADIOLARIANS OF NE SARDINIA (ITALY) IN THE FRAME OF THE WEISSERT EVENT

This article presents the very rich radiolarian fauna from the S’Ozzastru section of the Siniscola Marl crop-ping out in the northern part of Mt Albo massif (NE Sardinia, Italy). The formation was ascribed to the lower part of the upper Valanginian on the basis of the ammonite fauna indicating the Verrucosum Zone and the nannofossil con-tent suggesting the Zone NK3. Due to the scarcity of pantanelliids and especially of the absence of Cecrops septem-poratus, radiolarian marker of the upper Valanginian, radiolarians can only indicate a Valanginian age. Investigations for integrated litho-, bio-, and chemostratigraphy allowed the identification of the Valanginian Weissert Oceanic An-oxic Event (OAE), testified by a positive carbon isotope excursion (CIE) with values up to 2.98 ‰. Radiolarian study includes not only a determination of the species for biostratigraphic interpretations, but also a revision of the system-atics of the radiolarians occurring in the section. One new family (Crococapsidae), 3 new genera (Apiaromitra, Mi-crosandwichia and Ocellicapsa) and 43 new species are erected. The fauna has some elements in common with the Valanginian faunas from the Mediterranean area and many more with those from Japan, proving a wide circulation between the western and eastern Tethys.

Provenance analysis of Cenozoic conglomerates: Insights from Lower Cretaceous radiolarians in chert clasts from the Xigaze area, southern Tibet

Conglomerates, exposed on either side of the Yarlung Tsangpo suture zone (YTSZ) in southern Tibet, have attracted wide attention in elucidating uplift and erosion histories of the Himalayan-Tibetan orogen. However, the provenance of these conglomerates remains controversial. Although radiolarian-bearing chert clasts within these conglomerates have received little focus, identification of the radiolarian assemblages they contain could shed light on the provenance of these sedimentary units. We present the first report of Upper Jurassic to Lower Cretaceous (upper Kimmeridgian–lower Barremian) radiolarian assemblages recovered from chert clasts within the Liuqu and Gangrinboche conglomerates in the Xigaze area. To extract radiolarian fossils from independent clasts in the conglomerates, a detailed and efficient experimental process is illustrated. The assemblages are well correlated to those in the YTSZ and Tethyan Himalaya, showing typical Tethyan characteristics. The lithology of chert clasts and ages of constituent radiolarian faunas suggest that the chert clasts in Liuqu and Gangrinboche conglomerates were derived from the Bainang terrane. This interpretation implies that Early Cretaceous accretionary complexes in the YTSZ had been exposed and eroded before deposited as clasts in the Cenozoic Liuqu and Gangrinboche conglomerates.

Paleobiogeographic significance of Tethyan Lower Cretaceous Cecrops septemporatus Zone radiolarian faunas

The Cecrops septemporatus Zone is widely applied in correlation of Valanginian to lower Hauterivian successions, but paleobiogeographic distributions of radiolarian fauna from this zone remain ambiguous. To address this issue this study compares radiolarian faunas from the Vispi Quarry section, Umbria–Marche Basin, central Italy with the Guigong section, southern Tibet. Similarity between these C. septemporatus Zone faunas suggests a close marine connection may have existed between western and eastern Tethys during Early Cretaceous time. Lithologies, depositional setting, associated fossil records, and paleomagnetic data from contemporaneous radiolarian occurrences in different areas globally are also assessed. The paleobiogeographic distribution of the C. septemporatus Zone fauna was mainly restricted to low latitudes in the Tethyan and Pacific regions, but also shows a significant migration from low latitudes in the Tethyan Realm to mid latitudes in the Boreal and Austral realms. Climatic and oceanographic conditions during the Weissert Event may have played important roles in the development of the paleobiogeographic distribution patterns observed for the C. septemporatus Zone faunas.

Astronomical calibration of late middle Eocene radiolarian bioevents from ODP Site 1260 (equatorial Atlantic, Leg 207) and refinement of the global tropical radiolarian biozonation

Abstract. The middle Eocene sedimentary sequence drilled at Ocean Drilling Program Site 1260 (Leg 207), Demerara Rise, western equatorial Atlantic, yielded a rich and diverse radiolarian fauna. The exceptionally expanded and complete sedimentary record of this site, as well as the existence of an orbital chronological framework, allowed us to study a number of radiolarian bioevents with a very fine temporal resolution. We compiled a well-resolved succession of 71 radiolarian bioevents (first occurrences, last occurrences, and evolutionary transitions) and provided calibrations to the geomagnetic polarity timescale and astronomical timescale. Comparison of the radiolarian successions at ODP Site 1260A with the northwestern Atlantic IODP Site U1403 and the IODP Sites U1331, U1332, and 1333, situated in eastern equatorial Pacific, allowed the demonstration of the synchroneity of primary radiolarian bioevents that underpin the middle Eocene zonal scheme. Several secondary bioevents were also found to be synchronous between the two oceans and were therefore used to define seven new subzones for the low-latitudinal middle Eocene sequences: Siphocampe ? amygdala interval subzone (RP13a), Coccolarnacium periphaenoides interval subzone (RP13b), Artostrobus quadriporus interval subzone (RP14a), Sethochytris triconiscus interval subzone (RP14b), Podocyrtis (Podocyrtopsis) apeza interval subzone (RP14c), Artobotrys biaurita interval subzone (RP15a), and Thyrsocyrtis (Pentalocorys) krooni interval subzone (RP15b). This refined radiolarian biozonation has significantly improved stratigraphic resolution and age control for the late middle Eocene interval (an average of two subzones per 1.5 million years). A substantial diachronism was also found in 20 secondary radiolarian bioevents between the two oceans. The majority of radiolarian species appear to have evolved first in the equatorial Atlantic Ocean and subsequently in the equatorial Pacific. However, the reasons for this pattern of diachroneity currently remain uncertain and would require a greater sampling coverage to be elucidated.

Middle Jurassic radiolarians and chert geochemistry, Dajiweng ophiolite, SW Tibet: Implications for Neotethyan Ocean evolution

Well-preserved Middle Jurassic (upper Bajocian to lower Bathonian) radiolarian assemblages from bedded cherts constrain the age of cherts associated with the Dajiweng ophiolite in the northern sub-belt of the western section of the Yarlung Tsangpo suture zone (YTSZ). Sixty-one species belonging to 39 genera are recognized and illustrated. They represent the most diverse Middle Jurassic radiolarian fauna yet documented in this ophiolite and support correlation with UA zones 4–5 of Baumgartner et al. (1995). They are thus, the oldest known radiolarian-bearing strata in the northern sub-belt of the western YTSZ. The fauna exhibits a close degree of similarity to the other coeval assemblages previously reported from middle and eastern segments of the YTSZ, reveal widespread development of Middle Jurassic or even earlier oceanic crust within the Neotethyan Ocean. The geochemistry of these bedded cherts, in particular ratios of Al/(Al + Fe + Mn), Ce/Ce*, and LaN/YbN indicate they accumulated in a transitional environment between a continental margin and an oceanic basin.