The causes of the Great Ordovician Biodiversification Event (GOBE) remain a topic of debate. Recent studies indicate that global and regional oceanic oxygenation was a plausible trigger of the GOBE. ...The Ce anomaly of rare earth and yttrium (REY) elements in shallow marine carbonates is thought to be a powerful indicator of marine redox conditions of local waters. In this study, we analyzed REY, major and trace element compositions, and C and O isotopes of the Lower Ordovician microbial-dominant reefs on the Yangtze Platform, South China. We evaluate which carbonate components within the reefs most reliably record the seawater REY composition. The elemental correlations show that the REY elements of oncoids, stromatoporoids, sponges and carbonate infillings were affected by clastic influence, diagenetic processes, and the vital effects of carbonate secreting organisms; whereas the microbialites REY were partially modified by synsedimentary terrigenous and little diagenetic modification. Such introduction of synsedimentary terrigenous REY increased ΣREY and flattened the REY patterns of the microbialites, but the redox signals (Ce anomalies) of input source of the water column might still preserved in microbialites formed in situ. Although the magnitude of Ce anomalies is small, variation might still reflect authentic redox history of primary local seawater. In the middle Tremadocian, microbialites have positive Ce anomalies which were coeval to positive δ13Ccarb shifts. The positive δ13Ccarb shifts were documented from other sections of central Yangtze Platform, North American and Argentina, which might indicate global anoxic event. In the late Tremadocian to early Floian, microbialites have more visible negative Ce anomalies, which might indicate regional oceanic oxygenation event. The negative Ce anomalies provide direct local-geochemical evidence of oceanic oxygenation in the late Tremadocian-early Floian on the Yangtze Platform of South China, which might facilitate the first acme of biodiversification of local faunas predating the main pulse of the GOBE on the other palaeoplates.
•We investigated the REY sources of various carbonate components in microbial reefs.•Visual network of geochemical correlation was applied to evaluate the REY sources.•The Lower Ordovician microbialites were mainly affected by terrestrial inputs.•The Ce anomalies of microbialites indicate ocean oxidation in early Floian.•The oxidation might trigger the first biodiversification on the Yangtze Platform.
Introduction
The Early Ordovician Fezouata Shale Formation (485–475Ma, Morocco) is a critical source of evidence for the unfolding Great Ordovician Biodiversification Event (GOBE), the largest ...radiation in animal diversity during the Paleozoic. The Fezouata Shale preserves abundant remains of ancient marine organisms, including hundreds of specimens of radiodonts, a diverse and globally distributed group of stem lineage arthropods that first appeared as raptorial predators during the Cambrian Explosion.
Methods
In this work, we study 121 radiodont frontal appendages from the Fezouata Shale. Frontal appendages are the most commonly preserved body parts of radiodonts, and their well-preserved anatomical characters are crucial for describing taxonomic diversity at the species level, while also providing essential data on mode of life, paleoecology, and feeding behaviour.
Results
Our data allow for a systematic review of suspension-feeding Hurdiidae radiodonts from Fezouata. The genus
Pseudoangustidontus
is recognised as a radiodont and ascribed to Hurdiidae, and a new second species of this genus is identified,
Pseudoangustidontus izdigua
sp. nov.
Aegirocassis benmoulai
is also reviewed and its diagnosis amended with new details of differentiated endites in this appendage. The morphological similarity between both genera allows us to erect Aegirocassisinae subfam. nov., which groups together the suspension-feeding hurdiids of the Fezouata Shale.
Discussion
Suspension-feeding radiodont appendages are more abundant than those of sediment sifting or raptorial radiodonts, with the Fezouata Shale showing the highest diversity of suspension-feeding radiodonts in the history of the group. This dominance and diversity of frontal filter-feeding appendages follows the “Ordovician Plankton Revolution”, which started in the upper Cambrian and saw a huge radiation in plankton diversity.
•Bacterially-dominated microbial environment with enhanced methane cycling.•Aromatic carotenoids reveal anoxic and episodically euxinic conditions.•Below detectable limits of the C30 marine algal ...biomarker extends the C30 hiatus.•Nutrient supply contributes to natural selection of marine invertebrates.
One of the most dramatic episodes of sustained diversification of marine ecosystems in Earth history took place during the Early to Middle Ordovician Period. Changes in climate, oceanographic conditions, and trophic structure are hypothesised to have been major drivers of these biotic events, but relatively little is known about the composition and stability of marine microbial communities controlling biogeochemical cycles at the base of the food chain. This study examines well-preserved, carbonate-rich strata spanning the Tremadocian through Upper Dapingian stages from the Oslobreen Group in Spitsbergen, Norway. Abundant bacterial lipid markers (elevated hopane/sterane ratios, average = 4.8; maximum of 13.1), detection of Chlorobi markers in organic-rich strata, and bulk nitrogen isotopes (δ15Ntotal) averaging 0 to −1‰ for the open marine facies, suggest episodes of water column redox-stratification and that primary production was likely limited by fixed nitrogen availability in the photic zone. Near absence of the C30 sterane marine algal biomarker, 24-n-propylcholestane (24-npc), in most samples supports and extends the previously observed hiatus of 24-npc in Early Paleozoic (Late Cambrian to Early Silurian) marine environments. Very high abundances of 3β-methylhopanes (average = 9.9%; maximum of 16.8%), extends this biomarker characteristic to Early Ordovician strata for the first time and may reflect enhanced and sustained marine methane cycling during this interval of fluctuating climatic and low sulfate marine conditions. Olenid trilobite fossils are prominent in strata deposited during an interval of marine transgression with biomarker evidence for episodic euxinia/anoxia extending into the photic zone of the water column.
Sub-metre-scale patch reefs composed primarily of stromatoporoids and bryozoans are reported from the Duwibong Formation (upper Middle Ordovician), Taebaeksan Basin, Korea, in the eastern margin of ...the Sino-Korean Block. The reef framework is constructed of alternating thin laminae of the primitive labechiid stromatoporoid Cystostroma, the bryozoan Nicholsonella, subordinate Solenopora and minor siliceous sponges. Alternating laminae of stromatoporoids and bryozoans are largely responsible for the formation of a globular framework composed of columnar, branching, bulbous to irregular masses. Solenopora sporadically occurring as tiny patches or thin laminae attached to the stromatoporoid–bryozoan framework is considered to be subordinate encrusters. Siliceous sponges occur within the stromatoporoid–bryozoan framework and within the growth framework and bored cavities, interpreted as subordinate encrusters and cryptic dwellers. The compact globular framework of the Duwibong stromatoporoid–bryozoan consortium represents a new type of Ordovician skeletal bioconstruction, but with a certain structural similarity to Lower Ordovician bryozoan reefs in the South China Block. Together with coeval labechiid reefs occurring near the current study area, the Duwibong reefs suggest that incursion of the primitive stromatoporoids into the earliest bryozoan reefs resulted in the dominance of reef-building stromatoporoids in peri-Gondwana in contrast to coeval reefs in Laurentia, which commonly contain tabulate corals.
•The oldest stromatoporoid–bryozoan reefs are documented from Middle Ordovician of Korea.•The reefs resemble the earliest bryozoan reefs in structure and shape.•The reefs differ in biologic makeup from coeval Laurentian examples.•The pathway of earliest skeletal reef evolution may have been different among regions.
Post-mortem feeding activities along or within the interiors of shallowly buried shells can be preserved by sediment infilling and be visible on internal moulds of shelly fossils. Almost no such ...activities were recorded in three lowermost Ordovician formations (Třenice, Mílina and Klabava; Tremadocian to Dapingian) of the Prague Basin in the Barrandian area (Czech Republic). In contrast, a distinct increase occurs in the Šárka Formation (early and middle Darriwilian). Šárka feeding traces are common in shells preserved within siliceous nodules. They are associated with almost all taxonomic groups composing a rich and diversified fossil assemblage within that formation; but there is no evidence of similar traces produced by feeding on soft-bodied fauna. Arachnostega, Pilichnus and Palaeophycus are the most abundant ichnogera observed. Their trace makers selectively oriented on decaying soft tissues. Palaeophycus producers apparently preferred an easily accessible and nourishing food that was easily consumed. These traces typically intersect a digestive tract, especially the gut. The Arachnostega and Pilichnus traces are oriented in a manner suggesting systematic feeding. These tracemakers spent more time in a carcass, reaching lower nutrient tissues, including various membranes, that degraded more slowly and were more difficult to access. The preferential distribution of feeding traces in each shell reflects the topology of the soft tissues serving as a food. All studied feeding traces were found in nodules, never in shales of the Šárka Formation. This reflects a taphonomical bias influencing occurrences of these traces. Therefore, a lack of the feeding traces in the units underlying the Šárka Formation could be the result of taphonomic differences related to depositional conditions and diagenetic processes. Nevertheless, the abundance of those traces in the Šárka Formation illustrates the prosperity of this feeding behavior during the Darriwilian. An increase in the abundance, diversity and complexity of communities that fed within the internal ecospace of shells while utilizing feeding strategies developed during previous periods is considered to be a primary effect related to increased ecosystem complexity during the Great Ordovician Biodiversification Event.
•Feeding traces are common in shells of most taxa in Darriwilian nodules in Barrandian.•Preservation of feeding traces inside shells is taphonomically biased.•Feeding behavior on Middle Ordovician shells reflects structured food preferences.•Feeding traces can be helpful in reconstruction of soft tissues in fed organisms.•GOBE increased complexity and efficiency of feeding in internal ecospace of shells.
Communities of organisms that encrusted hardgrounds and skeletal material developed and became widespread during the Great Ordovician Biodiversification Event, and this community structure was a ...well-established part of the marine ecosystem by the Late Ordovician. Here we describe and investigate an example of skeletal encrustation from the Bardstown Member of the Drakes Formation (Upper Ordovician, Katian, Richmondian) near Mount Washington, Kentucky. This occurrence exhibits three dense coral beds near its base which primarily contain the problematic colonial organism Tetradium, the colonial rugose coral Cyathophylloides, and the solitary rugose corals Grewingkia and Streptelasma, as well as several species of stromatoporoids, bryozoans, brachiopods, cephalopods, and trilobites. In one locality, the corals in the uppermost bed are fragmented, reoriented, and encrusted, demonstrating reworking and taphonomic feedback of the skeletons. Tetradium skeletons created hard-substrate clasts that were exploited by a community of sclerobionts including stromatoporoids, bryozoans, and edrioasteroid echinoderms. The skeletons of Cyathophylloides, abundant in the underlying coral beds, are in life orientation and host a lower diversity of encrusters. Corals at other exposures of the Bardstown Member are not significantly reworked and do not host encrusters, suggesting localization of submarine erosion. Here, we describe the stratigraphy, paleoecology, and comparative taphonomy of the Bardstown coral beds at two localities to assess: 1) the burial and taphonomic feedback of the coral skeletons; 2) the community structure and evolution, attachment strategies, and substrate preferences of the encrusting community; and 3) the spatial patterns of submarine erosion and disturbance at the Bardstown coral bed horizons.
•A reworked and encrusted Ordovician coral bed is described from central Kentucky.•Sclerobionts occur only where coral skeletons have been significantly reworked.•Taphonomic feedback promoted the recruitment of several clades of sclerobionts.•Stratigraphic comparisons reveal localized skeletal reworking.•Ordovician echinoderms display preferences for smooth substrate surface textures.
The rare Middle Ordovician brachiopod genus Ukoa has remained unassigned to any existing family. The original description by Öpik in 1932 was based primarily on the external morphology; all later ...revisions of the genus also lack information regarding the interior of the dorsal valve, as well as information on the shell ultrastructure. As these are critical morphological features for designating a placement within the superfamily Plectambonitoidea – or indeed the order Strophomenida – not even a higher-level taxonomical determination has hitherto been attempted for Ukoa. The current study has examined new material, as well as museum specimens, that allowed for an analysis of the shell ultrastructure confirming the genus placement within the plectambonitoids. Moreover, as dorsal cardinalia are preserved in one of the new specimens, we further place Ukoa within the family Taffiidae Schuchert and Cooper, 1931, assigned to a new subfamily, the Ukoainae n. subfam. This corresponds well with the stratigraphic range of the Taffiidae, and as such, Ukoa, therefore represents yet another short-ranged plectambonitoid genus that originated during the Middle Ordovician Darriwilian Stage. As this coincides with both the onset of the main phase of the Great Ordovician Biodiversification Event (GOBE) and suddenly cooling climate, the current study also reviews the early diversification of the Plectambonitoidea within that context. Based on a specific focus on Baltica, we suggest that the unusual morphology that characterizes the plectambonitoids could be a particularly successful functional morphological adaptation to the unfolding Darriwilian ice age. As sea level dropped – possibly in the same order of magnitude as that experienced during the Quaternary glaciations – increased siliciclastic input washed onto the epicontinental seas globally providing muddy substrates, and thus new ecospace for the plectambonitoids to exploit and utilize during the early Darriwilian sea-level lowstands. The millennial glacioeustatic oscillations further accelerated plectambonitoid diversity through multiple allopatric speciation events, contributing to the overall accumulation of biodiversity that characterises the GOBE.
Display omitted
•New material confirms that the Baltic genus Ukoa is a plectambonitoid brachiopod.•A new subfamily, the Ukoainae, is erected within the family Taffiidae.•Ukoa is an exponent for the rapid mid-Ordovician rise of the Plectambonitoidea.•Plectambonitoids adapted to a climate-induced increase in muddy substrates.•This adaptive response represents a mega-scale example on functional morphology.
A large colonial rugose coral was recently discovered as float partially buried beneath several inches of sediment in the Kope Formation of northern Kentucky. Corals are unreported in the Kope ...assemblage, not appearing until much later during the C4 and C5 sequences during the Richmondian Invasion, a period in which the area was inundated with invasive taxa migrating from the midcontinent. The slab contains the remains of several other species that are diagnostic of the Kope Formation, such as the brachiopods Sowerbyella rugosa and Zygospira cincinnatiensis. The large colonial rugose coral is composed of corallites ranging in diameter from 3 to 5 mm containing 10–14 twisting major septa with crenulated walls, which is consistent with Cyathophylloides cf. C. burksae.
New taxa or new occurrences in a well-sampled formation, such as the Kope Formation, are unexpected especially for large skeletonized taxa, and highlights the rarity of coral within the assemblage. The Richmondian Invasion is linked to a transgressive event that increased connectivity between marine biogeographic provinces, disrupted ecosystem structure as well as acted as a significant evolutionary driver during the late Ordovician in the Cincinnatian faunal assemblages. Isolated dispersal events may have brought planktonic larva onto the Cincinnati Arch much earlier during transgression in the C1 sequence. The failure of the coral to become established might be linked to the low abundance of larva entering the region or non-ideal environmental conditions. The discovery of this coral in the Kope Formation further indicates the occurrence of rare and ephemeral dispersal events that have recently been reported within brachiopods.
•Colonial coral are rare, but occur within the Ordovician Kope Formation.•Their presences highlight multiple faunal invasions prior to the Richmondian.•The colony size suggests low larval abundance resulted in the failed invasion.•Paleobiogeography can clarify the factors that determine the fate of an invasion.
During the Early-Middle Ordovician, the Yangtze Platform in South China was drowned, as indicated by the replacement of pure grey grainstones and packstones of the Hunghuayuan Formation by purple ...red, argillaceous limestone of the Zitai Formation and correlative units, which are mixed siliciclastic and carbonate deposits. The Xiangshuidong Section, located in Songzi City, southwestern Hubei Province, was selected to study the process in detail. Cluster analysis of point-count groups of the Hunghuayuan and the Zitai formations, indicates that the former contains six microfacies: MF-1 peloidal grainstone, MF-2 thrombolitic grainstone, MF-3 oolitic grainstone, MF-4 bioclastic grainstone, MF-5 lithoclastic grainstone, MF-6 interreef bioclastic grain-packstone, indicating a shelf setting before drowning, while the latter contains seven microfacies: MF-7 bioclastic grainstone, MF-8 bioclastic grain- to packstone, MF-9 bioclastic packstone, MF-10 bioclastic wackestone, MF-11 argillaceous bioclastic wackestone, MF-12 open-marine bioclastic wackestone, MF-13 calcimudstone, indicating the ramp setting during and after the drowning. The drowning was gradual and fluctuating, starting as a marginal shoal, through the inner ramp, and becoming a stable ramp setting. This drowning is mainly attributed to a global sea-level rise, and an increase of terrigenous supply that resulted from local tectonic movement (i.e. the initiation of collision between the South China and Cathaysia blocks). After drowning, red offshore marine facies were well developed along the platform margin, indicating an oxic sea bottom environment during the first acme of GOBE in South China. The changes in environments associated with this transition may have provided new evolutionary opportunities that contributed to the Middle Ordovician diversification.
•Point-counting and cluster analysis were applied to divide carbonate microfacies.•We investigated the process of the Early-Middle Ordovician Yangtze Platform drowning.•The interaction of drowning event and the GOBE, as well as their controlling factors
The Cambrian and Ordovician radiations were the most important biodiversification events in the history of animal life, yet they were separated by intervals of recurrent anoxic sedimentation and ...regional biomere extinction events in the Late Cambrian and Early Ordovician. Recent work linked biomere extinction events to positive carbon isotopic excursions interpreted to represent enhanced organic matter burial under anoxic conditions. This led to the hypothesis that an increase in oceanic oxygenation following the last of the biomere extinctions paved the way for the rapid diversification of the Ordovician Radiation. Here we test the hypothesis of increasing ocean oxygenation using Th/U preserved in carbonate rocks from the Lower–Middle Ordovician succession near Ibex, Utah, USA. The global seawater uranium inventory is expected to vary inversely with the amount of deep-water anoxic deposition due to uranium sequestration in black shales. Therefore in shallow water carbonates higher Th/U ratios may reflect increased global ocean anoxia. Our data show that the highest Th/U values are coincident with the heaviest carbon isotopic values reported from the base of this succession, which is latest Tremadocian in age. Th/U then decreases and remains low through the Floian up through the contact with the Kanosh Formation, a shale-dominated unit with interspersed limestone units. The geochemistry of the carbonates within the Kanosh Formation likely represents local rather than global conditions in the marine environment. We interpret the overall decrease in Th/U ratios throughout the Tremadocian to Floian interval to represent an increase in global seawater uranium concentration. These data are consistent with the hypothesis that an increase in ocean oxygenation occurred prior to the major pulses of diversification associated with the Ordovician Radiation.
•Th/U can be used as a global redox proxy in carbonate rocks.•The Ordovician Radiation was likely delayed due to biomere extinctions.•An increase in oxygen follows the last Early Ordovician biomere extinction.•Increased oxygen likely suppressed further biomere extinction events.
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