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Miaolingian
Miaolingian
Miaolingian
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Miaolingian
~506.5 – ~497 Ma
A map of Earth as it appeared 505 million years ago during the Miaolingian Series, Wuliuan Stage
Chronology
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Orsten Fauna
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SSF diversification, first brachiopods & archaeocyatha
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Dresbachian extinction
Subdivision of the Cambrian according to the ICS, as of 2024.[1]
Vertical axis scale: Millions of years ago
Etymology
Name formalityFormal
Name ratifiedJune 25, 2018[2]
Former name(s)Cambrian Series 3
Usage information
Celestial bodyEarth
Regional usageGlobal (ICS)
Time scale(s) usedICS Time Scale
Definition
Chronological unitEpoch
Stratigraphic unitSeries
Time span formalityFormal
Lower boundary definitionFAD of the Trilobite Oryctocephalus indicus.
Lower boundary GSSPWuliu-Zengjiayan, Guizhou, China
26°44.843′N 108°24.830′E / 26.747383°N 108.413833°E / 26.747383; 108.413833
Lower GSSP ratifiedJune 25, 2018[2]
Upper boundary definitionFAD of the Trilobite Glyptagnostus reticulatus
Upper boundary GSSPPaibi section, Paibi, Hunan, China
28°23.37′N 109°31.54′E / 28.38950°N 109.52567°E / 28.38950; 109.52567
Upper GSSP ratified2003[3][4]

The Miaolingian is the third Series of the Cambrian Period, and was formally named in 2018.[2] It lasted from about 506.5 to 497 million years ago and is divided in ascending order into 3 stages: the Wuliuan, Drumian, and Guzhangian. The Miaolingian is preceded by the unnamed Cambrian Series 2 and succeeded by the Furongian series.[5]

It is named after the Miaoling Mountains in southeastern Guizhou Province, China.[6]

Definition

[edit]

A number of proposals for fossils and type sections were made before it was formally ratified in 2018. The most promising fossil markers were seen to be the respective first appearances of either trilobite species Ovatoryctocara granulata or Oryctocephalus indicus,[7] which both have an age close to 506.5 million years ago.[5] After some deliberation, the FAD of Oryctocephalus indicus was chosen to be the lower boundary marker, and the GSSP was placed in the Kaili Formation, Wuliu-Zengjiayan, Guizhou, China.[6][8]

The Miaolingian-Furongian boundary has the same definition as the Paibian Stage. It is defined as the first appearance of Glyptagnostus reticulatus around 497 million years ago.[5]

Subdivision

[edit]

The Miaolingian is subdivided into the following stages:[5]

Epoch / Series Age / Stage Age lower boundary (mya)
Furongian
Stage 10 491
Jiangshanian 494.2
Paibian 497
Miaolingian
Guzhangian 500.5
Drumian 504.5
Wuliuan 506.5
Series 2
Stage 4 514.5
Stage 3 521

The Ordian stage, which is use in Australian chronostratigraphical scale, was originally supposed to be the lowest stage of the Miaolingian, but may belong to upper Series 2. As of 2024, the base of the Ordian is not defined yet.[9]

Major events

[edit]

At the Cambrian Series 2–Miaolingian boundary, the first major trilobite extinction, known as the Olenellid Biomere boundary, occurred. In particular, trilobites of the families Ollenellidae and Redlichiidae have been extinct in Laurentia and South China, respectively.[10] The first O. indicus appear after this global extinction, and in areas where O. indicus fossils are absent, the Series 2–Miaolingian boundary is determined by chemostratigraphic data.[11]

Paleontology

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Benthic graptolites have reached a wide distribution in the Miaolingian. Encrusting colonies of the Rhabdopleuridae and erect growing branching colonies of the Dithecodendridae families evolved already at the beginning of the Miaolingian. The most common Wuliuan graptolite genus is Sphenoecium, whose robust colonies were found all over the world.[12]

References

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[edit]
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Miaolingian

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The Miaolingian Series is the third chronostratigraphic series of the Period in the , spanning approximately 509 to 497 million years ago and representing a key interval of early marine diversification. It is formally defined as the successor to and precedes the Series, with its base established at the first appearance datum (FAD) of the Oryctocephalus indicus. Named after the Miaoling Mountains in southeastern Province, , the series' Global Stratotype Section and Point (GSSP) is located in the Wuliu-Zengjiayan section of the Kaili Formation, where the boundary occurs 52.8 meters above the formation's base. Subdivided into three stages—the lowermost Wuliuan Stage (base at ~509 Ma), the Drumian Stage (~504.5–500.5 Ma), and the uppermost Guzhangian Stage (~500.5–497 Ma)—the Miaolingian is characterized by siliciclastic and carbonate deposits worldwide, reflecting transgressive events and eustatic sea-level changes. The lower boundary is further corroborated by secondary markers, including a negative carbon isotope excursion (the Redlichiid-Olenellid Extinction Carbon Isotope Excursion, or ROECE), the diversification of acanthomorphic acritarchs, and the onset of a major transgressive pulse. Biologically, this series documents accelerated evolutionary rates among trilobites and the emergence of diverse soft-bodied faunas, such as the exceptionally preserved Kaili Biota at the GSSP site, which includes trilobites, sponges, and early echinoderms. Globally, Miaolingian strata in regions like Laurentia (e.g., the Wheeler and Marjum Formations) and Baltoscandia reveal asynchronous trilobite turnovers and reef development dominated by microbial carbonates, underscoring the series' role in the post-Cambrian Explosion stabilization of marine ecosystems.

Definition and Stratigraphy

Naming and Ratification

The Miaolingian Series derives its name from the Miaoling Mountains (Miaoling Shan) in southeastern Province, , a region renowned for its well-exposed Cambrian stratigraphic sections that served as a primary reference for defining this unit. The term "Miaolingian" honors these mountains, which are located near the Miao ethnic minority's traditional homeland and host key lithological and biostratigraphic sequences essential to Cambrian . Prior to its formal adoption, the interval corresponding to the Miaolingian was designated informally as Series 3, a provisional label used since the early 2000s to denote the approximate "Middle " in global correlations, pending the establishment of named series. The International Subcommission on Stratigraphy (ISCS), a specialized body under the International Commission on (ICS), initiated the formal naming process as part of a broader effort to standardize subdivisions into four series and ten stages. This involved extensive voting and review within the ISCS, where members proposed and refined nomenclature to align with International Stratigraphic Guide principles, emphasizing type localities and international consensus. The proposal for the Miaolingian name emerged from collaborative research led by Chinese geologists, with contributions from international experts in and chemostratigraphy, facilitated through projects like IGCP 668 of the International Geoscience Programme. Following ISCS endorsement, the ICS forwarded the recommendation to the (IUGS) Executive Committee in March 2018. The IUGS ratified the Miaolingian Series in June 2018, officially replacing the provisional Series 3 and integrating it into the ICS International Chronostratigraphic Chart. This ratification marked a key milestone in standardization, ensuring consistent global usage.

Boundaries and Duration

The Miaolingian Series, the third series of the Period, is stratigraphically defined by its lower boundary at the datum (FAD) of the Oryctocephalus indicus. This marker coincides with the base of the Wuliuan Stage and is ratified as the Global Stratotype Section and Point (GSSP) in the Kaili Formation at the Wuliu-Zengjiayan section near Balang Village, Jianhe County, Province, . The boundary occurs at the base of a silty layer approximately 52.8 m above the formation's base. The upper boundary of the Miaolingian is defined by the FAD of the agnostoid trilobite Glyptagnostus reticulatus, which marks the base of the Paibian Stage and the overlying Series. This boundary is established at the GSSP in the Huaqiao Formation, Paibi section, northwestern Hunan Province, , at 396 m above the formation base. According to the Geologic Time Scale 2020, the Miaolingian spans from approximately 509 Ma to 497 Ma, yielding a total duration of about 12 million years; it immediately succeeds and precedes the Series. Global correlation of these boundaries faces challenges stemming from regional variations in fossil preservation and distribution, as well as uncertainties in methods such as U-Pb zircon geochronology. For instance, while the O. indicus FAD provides a robust biostratigraphic signal across and , discrepancies in its precise timing arise from asynchronous extinctions of underlying redlichiid trilobites and limited ash beds for direct dating. Similarly, the G. reticulatus horizon is globally recognizable but can exhibit slight offsets due to facies-dependent preservation in shallow-marine versus deeper-water settings. These factors underscore the need for integrated chemostratigraphic and multiproxy approaches to refine correlations.

Subdivisions

Stage Overview

The Miaolingian Series, the third series of the Period, is subdivided into three chronostratigraphic : the Wuliuan at the base, followed by the Drumian and the Guzhangian at the top. These stages collectively span approximately 12 million years, from roughly 509 Ma to 497 Ma, based on integrated radioisotopic dating and biostratigraphic correlations. The progression across the series reflects evolutionary and environmental developments in early marine ecosystems, with each stage marked by distinct biotic and sedimentary signatures. The Wuliuan Stage, the lowermost division of the Miaolingian, extends from approximately 509 Ma to 504.5 Ma and represents the initial phase of faunal recovery following the dynamic conditions of Series 2. This stage is defined by the first appearance of the trilobite Oryctocephalus indicus and features the establishment of diverse polymerid assemblages amid transgressive sea-level changes and negative carbon excursions. Its Global Stratotype Section and Point (GSSP) is located at the Wuliu-Zengjiayan section in Guizhou Province, China. The Drumian Stage occupies the middle portion of the Miaolingian, lasting from about 504.5 Ma to 500.5 Ma, and is characterized by relatively stable marine conditions that supported expansive shallow-water platforms and consistent oxygenation levels across paleocontinents. Key biostratigraphic markers include the agnostoid Ptychagnostus atavus, enabling precise global correlations, while sedimentary records indicate minimal tectonic disruption during this interval. The stage's GSSP is situated at the Stratotype Ridge section in the Drum Mountains of , USA. The Guzhangian Stage forms the upper part of the Miaolingian, spanning roughly 500.5 Ma to 497 Ma, and signals a transitional phase toward the faunas and depositional styles of the overlying Series. It is distinguished by the proliferation of Lejopyge laevigata and other trilobites, alongside increasing agnostoid influence in , amid subtle shifts in global sea levels. The GSSP for this stage is at the Louyixi section in Hunan Province, . Regionally, equivalents to the lower Miaolingian include the Ordian and Templetonian stages in Australia, where faunal assemblages show partial overlap with the Wuliuan, particularly in the Georgina and Warburton basins. Across the series, there is a notable faunal progression from the dominance of polymerid trilobites in the Wuliuan to greater agnostoid contributions by the Guzhangian, reflecting broader patterns of diversification and correlation tools.

Global Stratotype Sections and Points

The Global Stratotype Section and Point (GSSP) for the base of the Miaolingian Series, which coincides with the base of the Wuliuan Stage, is located in the Wuliu-Zengjiayan section of the Kaili Formation near Balang Village, Jianhe County, Province, , at coordinates 26°44.843′N, 108°24.830′E. This boundary is defined by the first appearance datum (FAD) of the Oryctocephalus indicus at the base of a silty layer 52.8 m above the base of the Kaili Formation. The section was selected due to its continuous sedimentation through the boundary interval, abundant and well-preserved fossils for biostratigraphic correlation, and supporting chemostratigraphic data from a negative carbon excursion (ROECE). The GSSP for the Drumian Stage is situated at Stratotype Ridge in the Drum Mountains, Millard County, western , , at coordinates 39°30.705′N, 112°59.489′W, within the Wheeler Formation. It is defined by the FAD of the agnostoid Ptychagnostus atavus at the base of a dark-gray thinly laminated (calcisiltite) layer, 62 m above the base of the formation. This site was chosen for its uninterrupted sedimentary record, high fossil abundance enabling global correlation, and auxiliary chemostratigraphic markers such as the Drumian carbon isotope excursion (DICE) and an onset of strontium isotope shift. The GSSP for the Guzhangian Stage occurs in the Louyixi section on the south bank of the Youshui River (Fengtan Reservoir), approximately 4 km northwest of Luoyixi village, Guzhang County, northwestern Province, , at coordinates 28°43.20′N, 109°57.88′E, in the Huaqiao Formation. The boundary is marked by the FAD of the agnostoid Lejopyge laevigata at the base of a (calcisiltite) layer, 121.3 m above the formation base. Selection criteria emphasized the section's continuous deposition, diverse and numerous fossils for precise , and chemostratigraphic reinforcement from a negative δ¹³C peak near the boundary. These GSSPs facilitate international correlation of the Miaolingian Series by integrating primary biostratigraphic markers with secondary geochemical and lithological signals, ensuring reliable identification across diverse global sections.

Paleoenvironment

Paleogeography and

During the Miaolingian Series (approximately 509–497 Ma), the global paleogeography was characterized by the continued dispersal of continental fragments following the breakup of the supercontinent, with major landmasses positioned in distinct latitudinal belts. , encompassing much of present-day , was situated near the , spanning low latitudes and serving as a stable ic core with surrounding passive margins. , the large southern supercontinent assembled from fragments including , , , , and , occupied high southern latitudes, extending from polar regions toward the and dominating the . , comprising and parts of eastern Europe, and , a separate , were drifting apart in the ; rotated counterclockwise from higher latitudes toward the , while lay just south of the , separated from by the widening . These configurations resulted in a world with extensive shallow epicontinental seas over and , contrasted by more restricted margins along . Tectonic activity during this interval was dominated by rifting and processes that shaped ocean basins and continental margins. Along the margins of , early rifting initiated the development of passive margins, particularly on its eastern and western flanks, as the expanded through between and the approaching Baltica-Gondwana assembly. Similarly, rifting occurred along the northern margins of , detaching peri-Gondwanan terranes such as and promoting the formation of new passive margins and back-arc basins. zones were active around these peri-Gondwanan terranes, particularly during the Cadomian , where oceanic subducted beneath the northern Gondwanan margin, generating volcanic arcs and influencing supply through of uplifted orogenic belts. These systems contributed to the assembly and stabilization of while providing detrital sediments to adjacent basins. Key fossil-bearing regions, such as those in the , occupied strategic positions relative to these major paleocontinents. The , a peri-n block, was positioned along the northern margin of East near the equator, effectively situated between to the northwest and the core of to the south, within the proto-Tethys region. This location facilitated connections via equatorial seaways, influencing the distribution of Miaolingian sedimentary sequences and fossil assemblages in sites like the Kaili Biota. Such positioning underscores the role of as a transitional zone in the global paleogeography.

Lithology and Sedimentary Settings

The Miaolingian Series is characterized by a predominance of rocks, including limestones and , deposited in shallow epicontinental seas across many paleocontinents, reflecting warm, tropical to subtropical conditions conducive to carbonate precipitation. Siliciclastic sediments, such as mudstones and shales, occur more commonly in deeper-water or marginal basin settings, where terrigenous input was higher due to proximity to landmasses. These lithologies indicate a mix of shallow-marine platforms, where stable shelves supported extensive carbonate accumulation, and basinal slopes with finer-grained deposits. In tropical regions like Laurentia, oolitic and bioclastic limestones dominate, formed in high-energy shoal and lagoonal environments on carbonate ramps and platforms; for example, the Bonanza King Formation features oolitic grainstones and packstones interbedded with mudstones, deposited above storm-wave base. Microbial-dominated reefs, including thrombolites and stromatolites constructed by cyanobacteria such as Girvanella, are prominent in these settings, often associated with ooid banks on the North China Platform during the Zhangxia Formation. In contrast, South China and Baltica exhibit more mudstones and shales; the Kaili Formation in South China consists of silty mudstones, calcareous shales, and thin limestones in open-shelf to slope environments, while the Alum Shale Formation in Baltica comprises dark, bituminous shales with subordinate limestones in deeper offshore settings. Lithological variations occur across Miaolingian stages, with the Drumian showing expanded carbonate platforms and oolitic shoals in regions like , transitioning from mixed clastics to dominantly oolitic limestones in intertidal-subtidal zones. The Guzhangian features increased silty shales and mudstones, as seen in the upper Kaili Formation and Alum Shale, reflecting deeper-water incursions and reduced carbonate productivity in basinal areas. These shifts highlight the influence of eustatic sea-level changes on depositional patterns, with slope deposits preserving finer siliciclastics alongside platform-derived carbonates.

Paleontology

Key Fossil Groups

During the Miaolingian Series, trilobites were the dominant metazoan fossils, with polymerids emerging as the primary group following the extinction of olenellids at the base of the series. Polymerids, characterized by their advanced thoracic segmentation and diverse cephalic morphologies, included prominent families such as the paradoxidids and oryctocephalids. Paradoxidids, like Acadoparadoxides and Eccaparadoxides, featured elongated, cylindrical exoskeletons adapted for benthic crawling in shallow marine environments, marking a key innovation in polymerid diversification. Oryctocephalids, exemplified by Oryctocephalus indicus, displayed primitive pygidial spines (typically two to three) and granular ornamentation on the cranidium, representing an early post-extinction radiation of polymerids in slope and shelf settings. The olenellid extinction, part of the broader Hawke Bay event, cleared ecological space for these polymerids, which spanned the full Miaolingian duration from Wuliuan to Guzhangian stages. Agnostoids, a subclass of trilobites with reduced body sizes and blind morphology suited for pelagic or infaunal lifestyles, increased in abundance toward the upper Miaolingian boundary. These arthropods, often preserved in offshore shales, exhibited cephalic shields with effaced sutures and pygidia mirroring the cephalon, adaptations for a potentially nektobenthic . Key genera included Glyptagnostus, such as G. reticulatus, which featured reticulate surface ornamentation and became more prevalent in Drumian to Guzhangian assemblages, aiding in global correlations. Their rising diversity reflected ecological shifts toward deeper-water niches, with over 39 taxa recorded in some Miaolingian successions. Benthic , colonial pterobranchs related to modern hemichordates, formed encrusting colonies on hard substrates in Miaolingian seafloors. The Sphenoecium was widespread, with robust, branching tubaria featuring a creeping basal layer and erect, widening thecae constructed via fusellar increments, indicating filter-feeding habits in shallow to mid-shelf habitats. These , often fragmented but globally distributed from to , represented an early diversification of benthic suspension feeders. Small shelly fossils (SSFs) persisted into the Miaolingian, with hyoliths and brachiopods comprising significant components of benthic assemblages in platforms. Hyoliths, such as indeterminate hyolithids, possessed conical shells with subtriangular to sub-circular cross-sections and opercula for sealing, functioning as probable deposit or suspension feeders in lagoonal settings. Brachiopods, including Micromitra modesta and Eoobolus sp., featured shells with laminar microstructures of compacted grains, enabling attachment to substrates via pedicles in shallow, oolitic environments. These groups, preserved as calcitic molds in limestones, highlighted the continued role of shelly in post-Series 2 recovery. Radiodontans, large nektonic predators, included notable Miaolingian representatives like Anomalocaris in the Wuliuan Stage. This genus, from assemblages in the Kaili Formation and , bore raptorial frontal appendages with short endites for grasping prey, a triradial oral cone with teeth, and compound eyes with up to 16,000 lenses for visual hunting. As apex predators up to 1 meter long, radiodontans like Anomalocaris canadensis drove evolutionary pressures on contemporaneous biotas through top-down control. Early echinoderms diversified in the Miaolingian, with stem-group forms exhibiting transitions from bilateral to pentaradial . Key groups included eocrinoids (e.g., Ubaghsicystis), featuring globular thecae with erect brachioles for suspension feeding and basal stereom as skeletal precursors. Cinctans and edrioasteroids displayed flattened, discoidal thecae with ambulacra for substrate crawling or attachment, innovations in water-vascular systems that foreshadowed crown-group echinoderms. These fossils, often disarticulated plates from shelf deposits, numbered dozens of genera and underscored rapid morphological experimentation. Microbial reefs, constructed primarily by calcimicrobes like Renalcis and Epiphyton, formed patch-reefs in subtidal settings during the Miaolingian. These structures, up to 4 cm thick with clotted (thromboid) and laminated (stromatoid) fabrics, dominated back-barrier and open-shelf environments, binding shelly debris into frameworks. In regions like the Mountains, they supported early metazoan encrusters, reflecting stable carbonate platforms amid tectonic activity. Trace fossils, indicative of infaunal activity, included the Cruziana ichnofacies in nearshore siliciclastics and carbonates. This facies featured trilobite-generated horizontal burrows with scratch marks, such as Cruziana spp., signaling grazing behaviors in shallow, high-energy zones. Complementary forms like Thalassinoides bacae, branched burrows with thick linings for microbial farming, occurred in restricted marine settings, evidencing ecosystem engineering by arthropods.

Biostratigraphy and Diversity Patterns

Biostratigraphy of the Miaolingian Series relies heavily on trilobite assemblages, particularly polymerid and agnostoid forms, to establish precise chronostratigraphic correlations within its three stages: the Wuliuan, Drumian, and Guzhangian. The base of the Wuliuan Stage, marking the onset of the Miaolingian, is defined by the first appearance datum (FAD) of the oryctocephalid trilobite Oryctocephalus indicus, which serves as a reliable marker due to its intercontinental distribution across Laurentia, South China, and Siberia. In the Drumian Stage, the Ptychagnostus atavus Zone provides the primary biostratigraphic framework, with the stage base coinciding with the FAD of the agnostoid Ptychagnostus atavus, enabling subdivision into subzones based on associated polymerid trilobites like Tomagnostus fissus and Hypagnostus parvifrons. The Guzhangian Stage begins at the FAD of the agnostoid Lejopyge laevigata, which defines the Lejopyge laevigata Zone and facilitates correlation through its cosmopolitan occurrence in shallow to deep-water facies. Global correlation across the Miaolingian is facilitated by agnostoid trilobites, which exhibit wide dispersal in open-marine environments due to their planktonic or pseudoplanktonic lifestyles, allowing intercontinental matching between paleocontinents such as , , , and . These taxa, including species of Ptychagnostus and Lejopyge, provide short stratigraphic ranges that align biozones across disparate lithofacies, supplementing polymerid trilobite zonations that are more facies-restricted. Diversity patterns in the Miaolingian reflect a recovery phase following earlier Cambrian extinctions, with marine taxon richness showing an overall increase through the series. In the Wuliuan Stage, post-extinction recovery is evident in the gradual buildup of and associated faunal diversity, transitioning from low post-Series 2 levels to moderate expansion amid stabilizing marine conditions. The Drumian Stage marks a peak in , driven by a threefold radiation in species richness and broader metazoan groups, including echinoderms and brachiopods, as ecospace utilization intensified in expanding shallow-shelf habitats. By the Guzhangian Stage, diversity stabilizes at elevated levels, with sustained high genus counts but reduced rates of origination compared to the Drumian, setting the stage for later turnovers. Regional variations in Miaolingian diversity highlight paleogeographic influences, with notable marine taxon richness in —exemplified by diverse assemblages in formations like the Wheeler Shale.

Major Events

Biological Extinctions and Diversifications

The Olenellid Biomere marked a significant faunal turnover at the Series 2–Miaolingian boundary, approximately 508 Ma (with estimates ranging 506–509 Ma), involving the regional disappearance of olenellid s across and parts of . In , this event eliminated the dominant olenellid s, which had defined earlier biomeres, while in , a parallel decline affected related redlichiid s, though olenellids were less prominent there. This has been linked to transgressive sea-level changes that altered shallow marine habitats, promoting shifts from nearshore sands to deeper offshore shales and facilitating the replacement of provincial faunas. The boundary itself is defined by the first appearance datum of the Oryctocephalus indicus. During the Drumian Stage of the Miaolingian, paradoxidid trilobites underwent a notable diversification, signifying a broader radiation of polymerid trilobites that became dominant in many paleocontinents. This expansion involved the immigration and proliferation of paradoxidids, such as species in the genera and , which adapted to a range of subtidal environments and contributed to increased morphological disparity within the group. The radiation reflected ecological opportunities following earlier turnovers, with paradoxidids exemplifying the shift toward more cosmopolitan polymerid assemblages that characterized mid-Miaolingian marine ecosystems. Trilobite extinctions during this interval were asynchronous across paleocontinents, with Laurentian events lagging those in by approximately 3 million years. In and West , the first appearance of paradoxidid trilobites preceded the olenellid extinction in , indicating diachronous faunal replacements driven by regional environmental gradients rather than a single global perturbation. This temporal offset highlights the role of paleogeographic isolation in modulating biotic responses to shared marine changes. The Miaolingian also witnessed the early diversification of chelicerates and pancrustaceans, inferred from an increase in complex trace fossils attributable to activity in marginal marine settings. Tracks and burrows, such as those resembling Monomorphichnus and shallow traces, suggest the emergence of these clades, with chelicerates showing initial radiation in nearshore habitats and pancrustaceans contributing to expanded benthic locomotion patterns. These ichnofossils indicate that both groups began exploiting diverse substrates, laying the groundwork for their later dominance in ecosystems.

Geochemical and Climatic Signals

The Drumian Carbon Isotope Excursion (DICE) represents a significant negative shift in δ¹³C values of approximately -2.5‰, occurring around 504–506 Ma near the base of the Drumian Stage in the Miaolingian Series. This excursion is globally recorded in carbonate successions and coincides with a major transgressive event, suggesting perturbations in the global carbon cycle. The negative δ¹³C signature indicates an influx of ¹³C-depleted carbon sources, potentially from intensified continental weathering or the remobilization of buried organic matter, and is associated with evidence of expanded anoxic conditions in shallow marine settings. Geochemical proxies from the Miaolingian reveal a predominantly warm, ice-free with elevated global levels, consistent with a greenhouse Earth state. Oxygen isotope (δ¹⁸O) analyses yield seawater temperatures around 30°C, indicating stable tropical conditions across paleocontinents like and . These high temperatures reflect the absence of polar ice caps and limited latitudinal thermal gradients during this interval. Minor eustatic -level fluctuations, on the order of tens of meters, are evident in stratigraphic records and are attributed to tectonic rifting along continental margins, which modulated basin subsidence and ocean basin volume. Phosphorite deposits in Miaolingian shelf environments further highlight dynamic nutrient fluxes, driven by of nutrient-rich deep waters onto continental margins. These authigenic , often interbedded with carbonates, signal enhanced oceanic productivity and cycling in shallow seas, particularly in regions like the Georgina Basin of and equivalents in . Such features underscore a nutrient-replete marine realm, with upwelling likely intensified by the prevailing high sea levels and tectonic reconfiguration. The and associated signals briefly align with environmental stresses that influenced faunal transitions at stage boundaries.

References

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