Agnostoid Trilobite Research Articles

Cyclostratigraphy of late Cambrian marine sedimentary records and reconstruction of Earth-Moon system parameters

The Cambrian period is an important era in geological history, marked by numerous major geological and biological events. To better understand these events, it is important to establish a high-precision chronostratigraphic framework. An astronomical time scale can be created by the identified Milankovitch cycles in well-preserved stratigraphic successions. Duibian B section in Jiangshan (Zhejiang Province, China) is the global stratotype section and point (GSSP) for the Jiangshanian Stage, and this section provides well-preserved records covering the upper Paibian to lower Jiangshanian stages. In this study, we use high-resolution magnetic susceptibility record to conduct cyclostratigraphic analysis of the Duibian B section. Our results indicate that the 2.44 m, 0.84–0.73 m, 0.20–0.19 m, and 0.12–0.09 m sedimentary cycles may represent long-eccentricity, short-eccentricity, obliquity, and precession cycles, respectively. Using the stable 405-kyr eccentricity cycles, we constructed an ∼5.96 Myr floating astronomical time scale to calibrate agnostoid trilobite zones. In addition, we reconstructed the precession rate k from geological records, and our result provides an insight into the Earth-Moon distance and length of day in the late Cambrian (Paibian–Jiangshanian, ∼494.2 Ma). It enriches our understanding of the Earth-Moon evolutionary history in deep time.

Trilobites from the Al Rose Formation (Lower Ordovician, Inyo Mountains, California)—faunas marginal to the Great Basin

Abstract The Lower Ordovician (Floian) Al Rose Formation from the Inyo Mountains, California, is a deeper-water, graptolitic equivalent of the well-known and richly fossiliferous successions described from Utah and Nevada. It is considered to have been originally marginal to the Laurentian paleocontinent. It has yielded a low-diversity trilobite fauna that differs strikingly from contemporary faunas to the east in its abundance of raphiophorid, nileid, olenid, and agnostoid trilobites, resembling that of the Nileid Biofacies known from scattered locations marginal to Laurentia. Two new trilobite species are described: Globampyx sexsegmentatus (Raphiophoridae) and Protopresbynileus divergens (Nileidae). Carolinites genacinaca Ross, 1951 is a link with the Great Basin. Other trilobites include the olenid Cloacaspis cf. C. ceryx anataphra Fortey, 1974, metagnostid Geragnostus cf. G. (Novoagnostus) longicollis Raymond, 1925, and pliomerid Hintzeia sp. UUID: https://zoobank.org/19a679ce-968d-4ce1-a590-f7f55bf9d62d

Some middle Cambrian agnostoids from the Precordillera Argentina.

Agnostoid trilobites are described from Middle Cambrian sediments in the Precordillera of Mendoza and San Juan provinces. The following taxa occur at Solitario Hill, Mendoza: Clavagnostus canotensis, Tomagnostella exsculpta, Agnostus exsulatus, Agnoslus ? sp., Oedorhachis australis, Ptychagnostus aculeatus and Ammagnostus sp. Taxa from the Ojos de Agua locality in San Juan comprise: Kormagnoslus seclusus, Peronopsis tenuis and Ballagnostus ? sp. There are the first agnostoids recorded from the San Juan area. Ptychagnostus aculeatus from Mendoza is here recorded for the first time in Argentina. A late Middle Cambrian age is assigned to the strata containing the assemblages of both Mendoza and San Juan.

The Cambrian Drumian carbon isotope excursion (DICE) in the Keping area of the northwestern Tarim Basin, NW China

The middle Cambrian is an important period for the decline of metazoans and the resurgence of microbialites. Based on the analysis of carbon and oxygen isotopes, detrital zircon UPb dating, and geochemical analysis, the middle Cambrian DICE (Drumian carbon isotope excursion) event was identified and its relationship with microbialites and sea-level changes was discussed in the Tarim Basin. The DICE event has a negative shift in δ13C from 0‰ to −3.2‰. About 15 m below the DICE, the youngest age of the detrital zircon was 504.5 Ma, which was consistent with the age of the DICE event. Stromatolites are abundant and agnostoid trilobite Ptychagnostus atavus is absent in the Keping area. The DICE event corresponds to the occurrence of hemispherical stromatolites in the shallow-water platform. Lithologic association and terrigenous clastic index show that the marine transgression and condensation layer occurred during the DICE event, which may be synchronized with organic matter oxidation when stratified water masses overturned during transgression.

Cyclostratigraphy of the global stratotype section and point (GSSP) of the basal Guzhangian Stage of the Cambrian Period

The Cambrian is the first period of the Phanerozoic during which numerous major geological and biological events occurred. Understanding these events requires a high-resolution time scale. The Luoyixi section in Guzhang (Hunan Province, China) is the global stratotype section and point (GSSP) for the Guzhangian Stage. Here, magnetic susceptibility and δ13C data were used to conduct cyclostratigraphic analyses covering the upper Drumian through lower Guzhangian stages. The results of the power spectral analyses and average spectral misfit analyses indicate that the 25–25.53 m, 6.25–6.91 m, 1.90–2.03 m, and 1.08–1.19 m sedimentary cycles may represent long-eccentricity, short-eccentricity, obliquity and precession cycles, respectively. A ~1400 kyr floating astronomical time scale was constructed using the stable 405-kyr eccentricity cycles to calibrate the agnostoid trilobite zones. The obtained fundamental obliquity period (30.7 ± 0.7 kyr) implies 370,180 ± 1220 km (vs. 384,000 km for present day) for the Earth–Moon distance and 21.58 ± 0.18 h (vs. 23.93 h for present day) for the length of day during the middle Cambrian (500 Ma).

Carbon-Isotope Excursions Recorded in the Cambrian System, South China: Implications for Mass Extinctions and Sea-Level Fluctuations

Cambrian carbonates with abundant fossils of agnostoid trilobites deposited on the southern slope (Jiangnan slope belt) of the Yangtze Platform and in the Jiangnan deepwater basin are well exposed in the Wangcun Section of western Hunan, South China, and in the Duibian A Section of western Zhejiang, southeastern China, respectively. To better understand the response of carbon- isotope excursions to depositional environment changes, mass extinctions and eustatic events, we collected 530 carbonate samples in fresh roadcut exposures of the two measured sections for analysis of carbon and oxygen isotopic compositions. Data of δ13C from the Wangcun Section, western Hunan, South China, demonstrate that the Cambrian carbon-isotope profile includes three remarkable positive excursions CPEwc-1, 2, 3 in the Upper Series 2, in the Lower and in the Middle Furongian Series. Three distinctive negative excursions CNEwc-1, 2, 3 were separately tested in the Lower Terreneuvian Series, Lower Series 3 and in the Upper Furongian Series. Similarly, in the corresponding horizons in the Duibian A Section, Zhejiang Province, southeastern China, three positive excursions CPEdb-1, 2, 3 and three negative excursions CNEdb-1, 2, 3 also have been discovered. We interpret these significant carbon-isotope excursions as being associated with enhanced biogenic productivity, mass extinctions and eustatic events.

Redescription of a Lotagnostus–Mendoparabolina faunule (Trilobita; late Furongian) from Quebrada San Isidro, Precordillera of Mendoza, Argentina

Lotagnostus is a biostratigraphically important late Furongian (late Cambrian) agnostoid trilobite whose interspecific and intraspecific variability is currently under discussion. In this context, type material of Lotagnostus atenuatus Rusconi, from an open-marine black limestone allochthonous block (La Cruz Olistolith) of Quebrada San Isidro, Precordillera of Mendoza, western Argentina, is revised herein. It includes several three-dimensionally preserved sclerites, allowing for an emended diagnosis of this species and comprehensive comparisons with other forms of Lotagnostus. Although to date L. atenuatus can be identified with confidence only at Quebrada San Isidro, it strongly resembles fragmentary material of Lotagnostus sp. indet., from late Furongian dysoxic upper slope facies of the Central Appalachians in eastern North America. The olenid Mendoparabolina brevicauda Rusconi, which is recorded in association with L. atenuatus, is also redescribed here. Mendoparabolina is closely allied with Wujiajiania Lu and Lin and Bienvillia Clark, differing by having a proportionately long and transversely bilobate pygidium with a posteriorly truncate axis and a postaxial median ridge.

CONODONTS FROM THE WA’ERGANG SECTION, CHINA, A POTENTIAL GSSP FOR THE UPPERMOST STAGE OF THE CAMBRIAN

Furongian (upper Cambrian) conodonts from the Shenjiawan Formation, Wa’ergang section, Taoyuan County, Hunan Province, South China are described. The Wa’ergang section has been proposed as a potential GSSP for the base of the uppermost stage of the Cambrian System, at the first appearance of the agnostoid trilobite Lotagnostus americanus . The Shenjiawan Formation consists of limestone with intercalations of marlstone and shale. The conodont yielding is low and the preservation is moderate. The conodont fauna includes proto-, para- and euconodonts. Conodont taxa characteristic of North China, South China and Korea occur together with cosmopolitan taxa. The biostratigraphic distribution of conodonts is directly correlated with the agnostoid trilobite biozones, with the base of the Proconodontus posterocostatus Zone very close to the base of the Lotagnostus americanus trilobite Zone.

The Auxiliary boundary Stratotype Section and Point (ASSP) of the Jiangshanian Stage (Cambrian: Furongian Series) in the Kyrshabakty section, Kazakhstan

The Kyrshabakty section in the Malyi Karatau Range, southern Kazakhstan, has been approved as the Auxiliary boundary Stratotype Section and Point (ASSP) for the base of the Jiangshanian Stage of the Furongian Series (Cambrian System). The first appearance datum (FAD) of the agnostoid trilobite Agnostotes orientalis coincides with the horizon marking the base of the Jiangshanian Stage, and the FAD of A. orientalis occurs at 259.0 m in the measured section, 61.2 m above the base of Unit 8 in the Zhumabai Formation. The boundary interval in the Kyshabakty section shows a relatively monotonous succession of carbonate deposits. The section contains an abundance of Cambrian fossils including diverse trilobites, brachiopods, molluscs, bradoriids, protoconodonts and paraconodonts.

A potential GSSP for the base of the uppermost Cambrian stage, coinciding with the first appearance of Lotagnostus americanus at Wa'ergang, Hunan, China

The first appearance datum (FAD) of the cosmopolitan agnostoid trilobite Lotagnostus americanus is a widely used marker for defining the base of the uppermost stage of the Cambrian System. Detailed field study on the L. americanus-bearing interval of the Wa'ergang section, Hunan, China, reveals the FAD of L. americanus to be 29.65 m above the base of the Shengjiawan Formation. This horizon is close to the lowest known occurrences of the intercontinentally distributed polymerid trilobites Hedinaspis regalis and Charchaqia norini. The section has strong potential as a candidate for defining a global standard stratotype-section and point marking the base of provisional Stage 10.

Trilobites Tremadocianos en Facies de Lutitas Oscuras del Miembro Alfarcito (Formación Santa Rosita), Quebrada de Moya, Jujuy, Argentina

TREMADOCIAN TRILOBITES FROM DARK SHALE FACIES OF THE ALFARCITO MEMBER (SANTA ROSITA FORMATION), QUEBRADA DE MOYA, JUJUY, ARGENTINA. The Quebrada de Moya constitutes a distinctive locality of the Santa Rosita Formation (uppermost Furongian—lowermost Ordovician) in the Quebrada de Humahuaca (Sierra de Tilcara, Cordillera Oriental of Jujuy, Argentina). The trilobites Leptoplastides marianus (Hoek), Parabolinella sp., Hapalopleura acantha Malanca and Brandán, and Asaphellus sp. are described from the dark shale facies of the upper part of the Alfarcito Member. A grainstone interstratified in the mudstones contains a low-diversity conodont assemblage of the Cordylodus angulatus Zone (lower Tremadocian). The Ordovician age of the section is confirmed, in addition, by the record of the graptolite Rhabdinopora flabelliformis ssp. in a sandstone underlying the studied levels. The dark shales accumulated by suspension fall-out during periods of maximum flooding, in a reducing, distal offshore marine environment. The trilobite fauna is largely dominated by L. marianus. The upper part of the Alfarcito Member does not show dark facies in other sectors of the Sierra de Tilcara. The sedimentologic characters of the section described, the low diversity of the conodont assemblage, and the scarcity of agnostoid trilobites may be indicative of a partial isolation of the basin.

Global Standard Stratotype-Section and Point (GSSP) for the Base of the Jiangshanian Stage (Cambrian: Furongian) at Duibian, Jiangshan, Zhejiang, Southeast China

Shanchi Peng, Loren E. Babcock, Jingxun Zuo, Xuejian Zhu, Huanling Lin, Xianfeng Yang, Yuping Qi, Gabriella Bagnoli, Longwu Wang. Episodes 2012;35:462-77. https://doi.org/10.18814/epiiugs/2012/v35i4/002

Recently Ratified and Proposed Cambrian Global Standard Stratotype‐section and Points

Abstract:This paper briefly summarizes an officially ratified Global Standard Stratotype‐section and Point (GSSP) and a proposed GSSP for global stages of the Cambrian System. The Luoyixi section near Luoyixi town, Guzhang, northwestern Hunan is ratified as the boundary stratotype for the base of the global Guzhangian Stage, which is the upmost stage of an unnamed series termed provisionally as Cambrian Series 3. The GSSP position lies 121.3 m above the base of the Huaqiao Formation in the section, coinciding with the first appearance of the cosmopolitan agnostoid trilobite Lejopyge leavigata. The Duibian B section at Duibian village, Jiangshan, western Zhejiang, is proposed as the boundary stratotype for the base of the proposed global Jiangshanian Stage that is the second stage of the Furongian Series. The proposed GSSP position lies 108.12 m above the base of the Huayansi Formation in the section, coinciding with the first appearance of the cosmopolitan agnostoid trilobite Agnostotes orientalius. This horizon is also with the first appearance of the cosmopolitan polymerid trilobite Irvingella angustilimbata.

The Cambrian Drumian carbon isotope excursion (DICE) in the Great Basin, western United States

The Global boundary Stratotype Section and Point (GSSP) of the Cambrian Series 3 (formerly the Middle Cambrian) Drumian Stage is defined by the first appearance datum (FAD) of the agnostoid trilobite Ptychagnostus atavus in the deep-water Wheeler Formation of the Drum Mountains in Utah, USA. In the Great Basin (USA), China and Scandinavia the FAD of P. atavus is associated with transgression and with a negative carbon isotope excursion named the Drumian Carbon isotope Excursion (DICE). However, the relationship between the FAD of P. atavus, sea-level changes, and the DICE is tentative, mainly due to the paucity of biostratigraphic data in shallow-water sections that limit correlation between deep- and shallow-water chemostratigraphic sections. An integrated sequence and chemostratigraphic study across a platform-to-basin transect in western Utah and southern Nevada indicates that the DICE is stratigraphically located above the FAD of P. atavus, associated with retrogradation of the shallow-water carbonate platform and with stratigraphic condensation in the basin. The results suggest that in future global correlations, the DICE should be restricted to the negative carbon isotope anomaly slightly post-dating the Drumian GSSP.

Geological significance of middle Cambrian trilobites from near Melba Flats, western Tasmania

In western Tasmania, near Melba Flats, a basalt within the Serpentine Hill Complex is overlain unconformably by Dundas Group sediments (Hodge Slate). About 32 m above the unconformity, a siltstone contains agnostoid and polymerid trilobites that indicate correlation with the middle Cambrian Ptychagnostus punctuosus Zone (Drumian Stage of unnamed Epoch 3 of the Cambrian). This provides an upper limit of about 505.2–504.5 Ma to the emplacement of ultramafic allochthons in western Tasmania. Other fossils present include dendroids, inarticulate brachiopods, hyolithids, helcionellids and sponges. The widespread agnostoid trilobite Ptychagnostus affinis (Brøgger 1878) and the holocephalinid trilobites Meneviella and Holocephalina are recorded for the first time in Tasmania.

Proposed GSSP for the base of Cambrian Stage 9, coinciding with the first appearance of Agnostotes orientalis, at Duibian, Zhejiang, China

Exposed in natural outcrops near the Duibian Village, Jiangshan County, Zhejiang Province, China, the Duibian B section is proposed as the boundary stratotype for the base of an unnamed stage termed provisionally Cambrian Stage 9. The proposed position of the GSSP is 108.12 m above the base of the Huayansi Formation, at a horizon coinciding with the first appearance of the cosmopolitan agnostoid trilobite Agnostotes orientalis. This horizon coincides also with the first appearance of the cosmopolitan polymerid trilobite Irvingella angustilimbata. The section fulfills all the requirements for a GSSP, and the horizon can be constrained not only by the primary stratigraphic marker (A. orientalis) but also with secondary biostratigraphic, chemostratigraphic, and sequence-stratigraphic correlation tools. The first appearance of A. orientalis is one of the most readily recognizable levels in Cambrian, and can be correlated with precision to all paleocontinents.

Sequence stratigraphy, cyclic facies, and lagerstätten in the Middle Cambrian Wheeler and Marjum Formations, Great Basin, Utah

Recurrent taphofacies, including conservation lagerstätten, are identified within a spectrum of facies in fourth-order sequences in the Middle Cambrian Wheeler, and Marjum formations of the Drum Mountains and House Range, west-central Utah. These sequences are 3–20 m thick and commence with sharply-based compact, oncolitic, oolitic, or pelletal pack- and grainstones with sharply-defined, corroded and mineralized upper contacts that record drowning discontinuities and early transgressive systems tracts (TSTs). Overlying intervals of calcareous shale and thin-bedded wacke- to packstones with abundant, disarticulated polymerid and agnostoid trilobites represent late TSTs. These are commonly followed by lavender-gray mudstones rich in sponge spicules and comminuted fossil debris that reflect condensed maximum flooding zones. The overlying early highstand (HST) intervals of black, fissile shales are typically barren except for indistinct, circular carbonized algae, but in rare instances, include soft-bodied animal remains. A combination of lower dysoxic–anoxic conditions, with a fluctuating oxycline, and relatively rapid episodic influx of fine-grained detrital sediment favored repeated burial and preservation of abundant organic detritus and rarely soft-bodied animals. Interbedded dark gray, shales include abundant articulated agnostoid trilobites and diminutive polymerids (e.g., Jenkinsonia). Overlying platy, calcareous bedding planes covered with articulated bodies and molts of the polymerid Elrathia indicate rapid blanketing of undisturbed seafloors by calcareous mud layers. These beds grade upward successively into interbedded, sparsely fossiliferous platy to flaggy shale and thin, pale gray weathering calcisiltites, and burrow-mottled to nodular limestones, recording late HST to falling stage (FSST) carbonate shedding. Thin calcisiltites include fossil debris and articulated larger polymerid trilobites and the eocrinoid Gogia, preserved by obrutionary deposits. The repeated recurrence of these patterns provides the rudiments of a predictive model that not only explains the differing modes of preservation but may also aid in prospecting for new lagerstätten.

The Global Boundary Stratotype Section and Point (GSSP) of the Guzhangian Stage (Cambrian) in the Wuling Mountains, Northwestern Hunan, China

The Global boundary Stratotype Section and Point (GSSP) for the base of the Guzhangian Stage (Cambrian Series 3) is defined at the base of a limestone (calcisiltite) layer 121.3 m above the base of the Huaqiao Formation in the Louyixi section along the Youshui River (Fengtan Reservoir), about 4 km northwest of Luoyixi (4 km southeast of Wangcun), in northwestern Hunan, China. The GSSP is exposed in a road cut at a position of 28°43.20' N and 109°57.88' E. The GSSP level contains the lowest occurrence of the cosmopolitan agnostoid trilobite Lejopyge laevigata (base of the L. laevigata Zone). Secondary global markers near the base of the stage include the appearance of either L. calva or L. armata just below the base of the stage, the appearance of conodonts associated with the base of the Laiwugnathus laiwuensis Zone, and the transgressive phase of a small eustatic event. Faunal turnovers close to the base of the Guzhangian Stage are recognized as near the base of the Boomerangian Stage in Australia, the base of the Aldanaspis Zone (polymerid trilobites) in Siberia, and the base of the Paradoxides forchhammeri Zone in western Avalonia. The horizon corresponding to the first appearance of L. laevigata is near the peak of a rather long negative δ 13 C excursion of up to 0.58 ‰.

Conodonts from the Global Stratotype section for the base of the Guzhangian Stage (Cambrian) at Luoyixi, Hunan, South China

The base of the Guzhangian Stage (Cambrian) has been ratified recently at the lowest occurrence of the cosmopolitan agnostoid trilobite Lejopyge laevigata (base of the Lejopyge laevigata Zone) in the Huaqiao Formation in the Luoyixi section, Guzhang County, Hunan Province, China. Sampling of the Luoyixi section in the boundary interval reveals 16 protoconodont and paraconodont taxa. The conodont succession can be correlated with the Gapparodus bisulcatus- Westergaardodina brevidens and the Shandongodus priscus- Hunanognathus tricuspidatus zones of South China and with the Laiwugnathus laiwuensis and the Shandongodus priscus zones of North China. Laiwugnathus laiwuensis occurs immediately below the lowest occurrence of Lejopyge laevigata. The first occurrence of Shandongodus priscus is 14.95 m above the base of the Lejopyge laevigata Zone. A significant change in the conodont associations occurs in the interval corresponding to the uppermost Lejopyge armata trilobite Zone. This interval is characterized by an increase in diversity with the appearance of the paracondont genera Furnishina and Laiwugnathus. Laiwugnathus laiwuensis, which occurs immediately below the lowest occurrence of Lejopyge laevigata, has a short stratigraphic record and allows confident correlation with North China.

Upper Cambrian polymerid trilobites from the Machari Formation, Yongwol, Korea

This article deals with the systematic description of polymerid trilobites from the Upper Cambrian Eochuangia hana, Agnostotes orientalis, and Pseudoyuepingia asaphoides Zones of the Machari Formation, Konggiri section, Yongwol, Korea. Agnostoid trilobites from the same locality were reported in 2004. Described are 20 polymerid trilobite taxa, including two new genera ( Joshuaspis and Macharia) and four new species ( Metachangshania brachypyga, Proceratopyge promisca, Joshuaspis parvus, and Macharia pengi). While agnostoid trilobite assemblages were used for correlation with distant regions such as South China, Australia, Kazakhstan, Siberia, and Laurentia, these polymerid trilobite assemblages provide valuable information for correlation of the Machari Formation with the Furongian (Upper Cambrian) strata of North China.