Ammonoid Diversity Research Articles

Morphological Diversity of Carboniferous Ammonoids of Central Asia and Kazakhstan

Morphological Diversity of Carboniferous Ammonoids of Central Asia and Kazakhstan

The Basque coastal sections of the K/T boundary – A key to understanding “mass extinction” in the fossil record

 lnvestigations at the K/T boundary section al Zumaya, northern Spain, underline the importance of this section in (1) establishing a Maastrichtian ammonite zonation and (2) in understanding "mass extinctions" in the fossil record. "Mass extinction" at the K/T boundary can be restricted to two groups of oceanic surface plankton. Ammonites and inoceramids exhibit a gradual decline through the late Cretaceous, as probably also do the Temperate belemnites and the bulk of involved vertebrates. Most of these groups disappear a long time before the boundary. Compared with the available environmental factors, the course of ammonite diversity through time perfectly parallels the course of global sea level changes. Times of high extinction rates (e.g. the era and system boundaries) coincide with times of maximum regressions, while the subsequent evolutionary radiations match with the new sea level rise. There is no need for any cosmic catastrophy to explain the decline of most invertebrate and vertebrate groups. Coeval with the short-time extinction of oceanic surface plankton at the boundary itself, the following environmental and depositional changes were observed: minor (positive as well as negative) changes in temperature, boundary clay sedimentation, enrichment of iridium and noble metals, occurrence of sanidium spheres and shocked quartz, and changes in carbon and strontium isotopes. Ali these events were used to support the Alvarez hypothesis of a cosmic impact at the boundary. But they are much easier explained by increasing explosive volvanicity which, on the other hand, might correlate also with the time-equivalent magnetic reversals.

Ontogeny of highly variable ceratitid ammonoids from the Anisian (Middle Triassic).

Ammonoids reached their greatest diversity during the Triassic period. In the early Middle Triassic (Anisian) stage, ammonoid diversity was dominated by representatives of the family Ceratitidae. High taxonomic diversity can, however, be decoupled from their morphologic disparity. Due to its high phenotypic variability, the high diversity of ceratitids of the Anisian of Nevada was initially assumed to be caused by artificial over-splitting. This study aims to contribute data to settle this issue by applying geometric morphometrics methods, using landmarks and semi-landmarks, in the study of ontogenetic cross-sections of ammonoids for the first time. The results reveal that alterations in ontogenetic trajectories, linked to heterochronic processes, lead to the morphologic diversification of the species studied herein. Our knowledge, based on these ontogenetic changes, challenge the traditional treatment of species using solely adult characters for their distinction. This study furthermore demonstrates that the high diversity of the Anisian ammonoid assemblages of Nevada based on the traditional nomenclatoric approach is regarded to be reasonably accurate.

Advanced gonioclymeniid ammonoids from Central Europe

Advanced gonioclymeniids comprise two families, Gonioclymeniidae and Sphenocly-meniidae. These are the phylogenetic end-members of the order Clymeniida, and are usually rarely represented in the Late Famennian ammonoid assemblages of Central Europe. The two families include the latest Famennian genera Finiclymenia, Medioclymenia and Sphenoclymenia. Most of their species are poorly known and some are revised here. We describe the new species Finiclymenia langei, Finiclymenia czarnockii and Medioclymenia media to complement the knowledge of Famennian ammonoid diversity.

Ion microprobe–measured stable isotope evidence for ammonite habitat and life mode during early ontogeny

AbstractAmmonites have disparate adult morphologies indicative of diverse ecological niches, but ammonite hatchlings are small (~1 mm diameter), which raises questions about the similarity of egg incubation and hatchling life mode in ammonites. ModernNautilusis sometimes used as a model organism for understanding ammonites, but despite their outward similarities, the groups are only distantly related. Trends in ammonite diversity and extinction vulnerability in the fossil record contrast starkly with those of nautilids, and embryonic shells from Late Cretaceous ammonites are two orders of magnitude smaller than nautilid embryonic shells. To investigate possible environmental changes experienced by ammonite hatchlings, we used secondary ion mass spectrometry to analyze the oxygen and carbon isotope composition of the embryonic shells and early postembryonic whorls of five juveniles ofHoploscaphites comprimusobtained from a single concretion in the Fox Hills Formation of South Dakota. Co-occurring bivalves and diagenetic calcite were also analyzed to provide a benthic baseline for comparison. The oxygen isotope ratios of embryonic shells are more like those of benthic bivalves, suggesting that ammonite eggs were laid on the bottom. Ammonite shell immediately after hatching has more negative δ18O, suggesting movement to more shallow water that is potentially warmer and/or fresher. After approximately one whorl of postembryonic growth, the values of δ18O become more positive in three of the five individuals, suggesting that these animals transitioned to a more demersal mode of life. Two other individuals transition to even lower δ18O values that could suggest movement to nearshore brackish water. These data suggest that ammonites, like many modern coleoids, may have spawned at different times of the year. Because scaphites were one of the short-term Cretaceous–Paleogene extinction survivors, it is possible that this characteristic allowed them to develop a broader geographic range and, consequently, a greater resistance to extinction.

Examining the Ladinian crisis in light of the current knowledge of the Triassic biodiversity changes

Several mass extinctions are known from the history of life, but knowledge about “minor” biotic crises remains incomplete. The Ladinian is the second stage of the Middle Triassic epoch. Available reconstructions of the diversity dynamics of fossil organisms across the entire Triassic allow the proposed Ladinian biotic crisis to be investigated. Total biodiversity curves (both the conventional curve and that based on sampling standardization) fail to identify the crisis, but this may be explained by low resolution data or “noise”. In fact, the global generic diversity of ammonoids, brachiopods, and tetrapods declined significantly (by 1.1–2.5 times) during the Ladinian, as did the species diversity of conodonts. Bivalves radiated, but their origination rate also decreased during the Ladinian. Land plants were apparently unaffected. Based on the representative regional record for the Northwestern Caucasus, the number of macroinvertebrate genera dropped by ~7.5 times, and brachiopods almost totally disappeared; foraminifers were also affected significantly. Thus, the proposed biotic crisis appears to have been selective. It appears the Ladinian biotic crisis was a relatively long-term event, and its magnitude is comparable to that of other “minor” mass extinctions, e.g., those of the Silurian, Early Jurassic, or end-Cenomanian. There are several possible explanations for what caused the Ladinian biotic crisis. It is possible that global climate cooling was responsible, although it is unclear why the palaeobiogeographical differentiation was not favourable for a biotic radiation. A eustatic explanation for the drop in diversity is unlikely. Instead, the crisis was mostly likely caused by unstable recovery of the Earth's biota after the end-Permian catastrophe.

Mid-Carboniferous ammonoids from the Shannon Basin, western Ireland: identification of crushed material

In the Carboniferous Shannon Basin, Western Ireland, the majority of ammonoids spanning the Serpukhovian-Bashkirian Stages (E1–R2 biozonal indices) are preserved as flattened, partial 2D impressions, for which identification is problematic. Ammonoid ornament changes through successive developmental stages of the animal, but the changes of ornament with ontogeny have not been studied for many of the taxa that occur in the Shannon Basin. An approach involving the ontogenetic study of all available 3D material from concretions and museum collections can be used to improve the confidence of identification of flattened material. This is described here for the following Bashkirian ammonoid species: Phillipsoceras circumplicatile, Vallites henkei and Reticuloceras subreticulatum. Systematic ontogenetic changes in ornament are described here for these taxa for the first time. This represents a first step towards advancing 2D ammonoid taxonomy, which will improve biostratigraphic resolution, with implications for large-scale ammonoid diversity and evolutionary studies.

Morphological variability in response to palaeoenvironmental change – a case study on Cretaceous ammonites

Changes in ammonite morphology during the earliest Early Aptian (Cretaceous) of an epicontinental sea in northern Germany were investigated based on new and rich material of Deshayesites (Deshayesitidae). This is a globally distributed genus and one of the most important ammonites of the Cretaceous with respect to biostratigraphy and abundance. The purpose of our study was to describe changes in morphology over few hundred thousand years and to discuss their relationships to major palaeoenvironmental perturbations. Our material is derived from four different horizons, and the studied stratigraphical interval includes a sea-level change, a period of warming and an oxygen depletion event. We observe variable patterns in the occurrence of different morphological groups through time, probably indicating that morphotypes were individually impacted by environmental change. These morphological groups, unequivocally attributed to Deshayesites, cannot be fit into the existing species classification system. The greatest morphological disparity in Deshayesites is multi-causal. It may be the result of: (1) an invasion of two morphogroups new to the habitat, which thus immediately face considerably greater competition in an environment just recovered from low-oxygen conditions in parts of the water column during an early interval within Oceanic Anoxic Event 1a (OAE 1a); or, (2) it may be due to a rise in sea level with a simultaneous invasion of competitors of Tethyan origin. This sheds new light on the current concept for high morphological variability within ammonite species and poses challenges to our current ideas about ammonite diversity and the use of well-established index-species in supraregional correlations.

Ammonoid diversification in the Middle Triassic: Examples from the Tethys (Eastern Lombardy, Balaton Highland) and the Pacific (Nevada)

The diversity dynamics of the Anisian ammonoids is analyzed in terms of generic richness and turnover rates in one North American (Nevada) and two western Tethyan (Eastern Lombardy, Balaton Highland) regions. Two pulses of diversification are outlined: one in the middle Anisian (Pelsonian) and another near the end of the late Anisian (late Illyrian). The Pelsonian global diversification is interpreted as an effect of global sea-level rise. In the early late Anisian the ammonoid generic richness definitely decreased both in the western Tethys and in Nevada. The latest Anisian peak of ammonoid diversity was low in Nevada, which is explained by the uniform local sedimentary environment and the absence of major global changes. In the western Tethys the late Illyrian diversity peak was very prominent: ammonoid generic richness, turnover and proportion of originations were very high. This explosive peak is interpreted in terms of major changes of two regional environmental factors: coeval volcanic activity and ...

Evolution and extinction of Maastrichtian (Late Cretaceous) cephalopods from the López de Bertodano Formation, Seymour Island, Antarctica

One of the most expanded records to contain the final fortunes of ammonoid cephalopods is within the López de Bertodano Formation of Seymour Island, James Ross Basin, Antarctica. Located at ~65° South now, and during the Cretaceous, this sequence is the highest southern latitude onshore outcrop containing the Cretaceous-Paleogene (K–Pg) transition. We present comprehensive new biostratigraphic range data for 14 ammonite and one nautiloid species based on the collection of >700 macrofossils from high-resolution sampling of parallel sedimentary sections, dated Maastrichtian to earliest Danian in age, across southern Seymour Island. We find evidence for only a single, abrupt pulse of cephalopod extinction at the end of the Cretaceous when the final seven ammonite species disappeared, consistent with most evidence globally. In the lead up to the K–Pg extinction in the James Ross Basin, starting during the Campanian, ammonite diversity decreased overall, but the number of endemic taxa belonging to the family Kossmaticeratidae actually increased. This pattern continued into the Maastrichtian and may be facies controlled, linked to changes in sea level and seawater temperature. During the early Maastrichtian, ammonite diversity dropped significantly with only two species recorded from the basal López de Bertodano Formation on Seymour Island. The subsequent diversification of endemic taxa and reappearance of long-ranging, widespread species into the basin resulted in an increase in ammonite diversity and abundance during the mid-Maastrichtian. This was coincident with an apparent period of warming temperatures and sea level rise interpreted from palynology and sedimentology, perhaps reflecting a high latitude expression of the Mid-Maastrichtian Event. Late Maastrichtian diversity levels remained stable despite reported climatic and environmental variation. Ammonite diversity patterns during the Maastrichtian parallel those of microfossil species such as nannofossil and planktonic foraminifera, suggesting that dynamic climatic and environmental changes affected many planktonic and nektonic organisms during the latest Cretaceous. However, we suggest that these perturbations had a minimal effect on overall diversity prior to the catastrophic extinction event at the K–Pg boundary.

Palaeoenvironments and palaeoceanography changes across the Jurassic/Cretaceous boundary in the Arctic realm: case study of the Nordvik section (north Siberia, Russia)

The Jurassic/Cretaceous transition was accompanied by significant changes in palaeoceanography and palaeoenvironments in the Tethyan Realm, but outside the Tethys such data are very scarce. Here we present results of a study of the most complete section in the Panboreal Superrealm, the Nordvik section. Belemnite δ 18 O data show an irregular decrease from values reaching up to +1.6‰ in the Middle Oxfordian and from +0.8 to −1.7‰ in the basal Ryazanian, indicating a prolonged warming. The biodiversity changes were strongly related to sea-level oscillations, showing a relatively low belemnite and high ammonite diversity during sea-level rise, accompanied by a decrease of the macrobenthos taxonomical richness. The most prominent sea-level rise is marked by the occurrence of open sea ammonites with Pacific affinities. Peak abundances of spores and prasinophytes correlate with a negative excursion in organic carbon δ 13 C near the J/K boundary and could reflect blooms of green algae caused by disturbance of the marine ecosystem. Keywords: Biodiversity; stable isotopes; J/K boundary; Arctic Realm; palaeoceanography (Published: 25 March 2014) Citation: Polar Research 2014, 33 , 19714, http://dx.doi.org/10.3402/polar.v33.19714

Biogeographic evolution of Permian ammonoids

The distribution of Permian ammonoids within the five major biogeographic provinces (Arctic, Uralian, American, Tethyan, and Australian) is discussed. Changes in the taxonomic and biogeographic structure of assemblages according to stages in the evolution of Permian ammonoids are considered for each region. It is shown that the geographical distributions and taxonomic diversity of ammonoids changed continuously throughout the Late Permian.

Spatial heterogeneity of the Early–Middle Toarcian (Jurassic) ammonite diversity and basin geometry in the Northwestern Caucasus (southwestern Russia; northern Neo-Tethys)

Paleontological and geological information from the Northwestern Caucasus, a large region in southwestern Russia, allows to evaluate the spatial heterogeneity of the Early–Middle Toarcian ammonite diversity and its possible controls. The total number of ammonite species and genera is calculated for four time slices, which correspond to the Dactylioceras semicelatum, Harpoceras falciferum, Hildoceras bifrons, and Haugia variabilis zones, in each of the ten areas distinguished within the study region. These areas differ by the ammonite diversity, which indicates heterogeneity. The latter persisted through the entire Early–Middle Toarcian. This heterogeneity was relatively low in the beginning of the Toarcian, when the total regional diversity of ammonites was minimal. Long- and short-term landward shoreline shifts facilitated spatial distribution of ammonites and increase in their taxa number. Shallow-marine paleoenvironments tended to sustain a higher diversity than those deep-marine, and, therefore, changes in the basin depth were also responsible for the observed spatial heterogeneity of the ammonite diversity. Interestingly, a more or less similar diversity dynamics is established in the areas of the Northwestern Caucasus. The distribution of ammonites in the study region indicates that these invertebrates migrated there from the open sea(s) stretched along the northern margin of the Neo-Tethys Ocean. Results of the present analysis also imply that the mass extinction might have been responsible for the low diversity of ammonites (observed in the entire region and its particular areas) in the beginning of the Toarcian.

The Pliensbachian–Toarcian (Early Jurassic) extinction, a global multi-phased event

During the Pliensbachian–Toarcian interval of the Early Jurassic, there is a well-known second order marine extinction that is observable at the species and genus levels. Ammonite diversity data from successions throughout Europe and parts of the Arctic suggest that this extinction may have been multi-phased with diversity declining over six separate intervals. The main-phase of decline begins at the Pliensbachian–Toarcian boundary and extends into the Lower Toarcian, to a level that is correlative with the Tenuicostatum/Serpentinum Zone boundary. To date, only the main-phase of extinction has been demonstrated as being global in extent and affecting multiple taxonomic groups. This multi-phased extinction has been attributed to regional and global controlling mechanisms that are associated with the Volcanic Greenhouse Scenario which links the eruption of the Karoo–Ferrar large igneous province (LIP) to global warming and mass extinction.We compare stratigraphic ranges of ammonite and foraminiferal species in Pliensbachian–Toarcian successions of western North America to the record in Europe and parts of the Arctic in order to test the geographic extent of the multiple phases of extinction. Our results show six intervals of species level decline that correlate with those recognized in Europe: 1) middle of the Lower Pliensbachian (middle Whiteavesi–middle Freboldi Zones), 2) middle of the Upper Pliensbachian (upper Kunae–lower Carlottense Zone), 3) Pliensbachian/Toarcian boundary into the Lower Toarcian (upper Carlottense–middle Kanense Zones), 4) Middle Toarcian (upper Planulata–lower Crassicosta Zones), 5) upper Middle–lower Upper Toarcian (middle Crassicosta–Hillebrandti Zones) and 6) Upper Toarcian (lower Yakounensis Zone).Recognition of this multi-phased event in three separate ocean basins (paleo Pacific, paleo Arctic, and Tethys Oceans), in at least two taxonomic groups, greatly expands the known geographic extent of this multi-phased event and argues for a controlling mechanism that is global in its reach. In relation to the Volcanic Greenhouse Scenario, our study shows that four of the six pulses of extinction occur within the main-phase of Karoo magmatism. The decline in the Early Pliensbachian, previously thought to be separate from this event, occurs within error range of the onset of Karoo magmatism and the decline in the Late Toarcian coincides with the later stages of magmatism. These observations extend the known duration of this multi-phased extinction event to the Early Pliensbachian and support the Volcanic Greenhouse Scenario, specifically the eruption of the Karoo–Ferrar LIP, as a preeminent factor driving the multi-phased extinction of the Pliensbachian–Toarcian.

Palaeoenvironmental evolution of the Southern Alps across the Faraoni Level equivalent: new data from the Trento Plateau (Upper Hauterivian, Dolomites, N. Italy)

Abstract Lukeneder, A. and Grunert, P. 2013. Palaeoenvironmental evolution of the Southern Alps across the Faraoni Level equivalent: new data from the Trento Plateau (Upper Hauterivian, Dolomites, N. Italy). Acta Geologica Polonica, 63 (1), 89-104. Warszawa. New stratigraphic and palaeoenvironmental data are presented for the northeastern part of the Trento Plateau (Puez area, Southern Alps, Italy). The studied section corresponds to the upper Hauterivian Balearites balearis and “Pseudothurmannia ohmi” ammonite zones and normal palaeomagnetic chron upper M5. A c. 30-cm-thick bed is identified as the equivalent of the Faraoni Level, based on its position within the Pseudothurmannia mortilleti Subzone, the composition of its ammonite fauna and the peak of a minor positive trend in the δ 13 C bulk record. Microfacies and geochemical proxies compare well with those of the southeastern part of the Trento Plateau and indicate palaeoceanographic continuity along the eastern margin of the plateau. The abundances of radiolarians and nannoconids suggest a turnover in the trophic structure from eutrophic conditions around the Faraoni Level equivalent to oligotrophic conditions. Low organic matter and sulphur content and frequent bioturbation indicate a well-oxygenated environment. Ammonite diversity and life-habitat groups document the influence of sea level on the plateau: while epi- and mesopelagic ammonites occur commonly during sea-level highstands, all life-habitat groups become reduced during a sea-level lowstand in the Spathicrioceras seitzi and P. ohmi subzones. The Faraoni event is heralded by a faunal turnover expressed as the diversification of epipelagic ammonites. Palaeoenvironmental conditions along the eastern margin of the Trento Plateau during the Faraoni event contrast with those of the organic-rich black shales in the west. A re-evaluation of the depositional model based on the new results suggests a general water depth of 300-500 m for the plateau. The severe reduction of mesopelagic ammonites during the sea-level lowstand indicates a shallowing towards the epi-/mesopelagic boundary. During the Faraoni event, the eastern areas of the Trento Plateau were located at the upper limit of the oxygen-minimum layer and were thus only occasionally affected by oxygen depletion, whereas the western areas were located well within the upper part of the oxygen-depleted layer.

Ammonite Faunal Dynamics Across Bio-Events During the Mid-and Late Cretaceous Along the Russian Pacific Coast

The present paper focuses on the evolutionary dynamics of ammonites from sections along the Russian Pacific coast during the mid-and Late Cretaceous. Changes in ammonite diversity (i.e., disappearance [extinction or emigration], appearance [origination or immigration], and total number of species present) constitute the basis for the identification of the main bio-events. The regional diversity curve reflects all global mass extinctions, faunal turnovers, and radiations. In the case of the Pacific coastal regions, such bio-events (which are comparatively easily recognised and have been described in detail), rather than first or last appearance datums of index species, should be used for global correlation. This is because of the high degree of endemism and provinciality of Cretaceous macrofaunas from the Pacific region in general and of ammonites in particular.

Diversity dynamics of Toarcian–Aalenian (Jurassic) ammonites and transgressions/regressions in the Greater Caucasus Basin (Caucasian Sea, northern Neo-Tethys Ocean): An evidence of partial dependence

Possible dependence of ammonite diversity dynamics on shoreline shifts, i.e., transgressions and regressions, is yet to be understood. Data from the Greater Caucasus Basin, which was located on the northern active margin of the Neo-Tethys Ocean in the Mesozoic, permit to analyze the noted dependence at the Toarcian–Aalenian time interval. Distribution of 161 species and 40 genera in each regional ammonite-based biozone of this interval allows to evaluate the diversity dynamics of ammonites. The total number of both species and genera changed cyclically. The diversity maxima occurred in the H. bifrons, G. thouarsense, and L. murchisonae zones. The diversity minima occurred in the D. semicelatum, H. variabilis, P. aalensis, and G. concavum zones. Semi-quantitative analysis of facies distribution in 28 areas of the basin indicates a stepwise early Toarcian–middle Aalenian transgression, followed by a late Aalenian regression. The general trend towards species diversification through the study interval coincided with the noted transgression. This is an evidence of partial dependence, but no dependence between the generic diversity dynamics and shoreline shifts is visible. It is hypothesized that transgressions facilitated the growth of the ammonite diversity, but, when they stopped, the diversity was controlled by other factors. Changes in the spatial distribution of deep-marine palaeoenvironments might also affect the ammonite diversity dynamics, but no regional evidence of importance of oxygen depletion and seawater temperature fluctuations is found. The minimal ammonite diversity in the D. semicelatum Zone could be a consequence of the Early Toarcian mass extinction. In the Greater Caucasus Basin, the latter occurred at a peak of regression.

Quantitative biochronology of Devonian ammonoids from Morocco and proposals for a refined unitary association method

Monnet, C., Klug, C., Goudemand, N., De Baets, K. & Bucher, H. 2011: Quantitative biochronology of Devonian ammonoids from Morocco and proposals for a refined unitary association method. Lethaia, Vol. 44, pp. 469–489. Based on a rich dataset, the biostratigraphy of the late Emsian and the Eifelian (Early–Middle Devonian) ammonoids from the Moroccan Tafilalt is re-evaluated. We processed this dataset (comprising 53 species from 15 sections) with the unitary association method (UAM), by means of the UA-graph freeware. This led to the construction of a sequence of 17 UAs (maximal sets of actually or virtually coexisting taxa), which are grouped into 10 laterally reproducible association zones. This biostratigraphical subdivision of this interval is in some parts finer than the classically used empirical stratigraphical scheme and than a previous graphic correlation analysis. It enabled us to measure regional ammonoid diversity of this interval in detail. The UAM is a powerful biochronological method, which benefits from complementary tools to analyse conflicting inter-taxon stratigraphical relationships inherent to complex biostratigraphical datasets. In cases of under-constrained superpositional relationships between the taxa, the UAM can yield results, which are not parsimonious in terms of reconstructed virtual coexistences. We suggest several additions to complement the algorithmic steps of the method. The most important is the exhaustive or heuristic reconstruction of possible solutions resolving the observed biostratigraphical contradictions (conflicting inter-taxon superpositional relationships and cycles between maximal cliques) and the selection among the solutions of the most-parsimonious one(s) in terms of reconstructed virtual coexistences. Multiple equivalent results may then be processed with standard consensus techniques. Finally, the robustness of the results can be tested by bootstrapping methods to provide confidence estimates on the ranges and associations of studied taxa. □Ammonites, Anti-Atlas, biostratigraphy, correlation, zonation, diversity.

Ammonoid diversity and disparity track episodes of chaotic carbon cycling during the early Mesozoic

Episodes of mass extinction represent the largest events of biodiversity loss known in the geologic record, and may provide tests of biodiversity-ecosystem stability hypotheses. Here we present the first correlation between ammonoid diversity and disparity and ecosystem stability as represented by stable carbon isotopic records spanning the end-Permian through end-Triassic mass extinctions. Ammonoid generic richness from a single biogeographic realm shows that nearly all taxa disappeared coincident with major carbon isotopic shifts to lighter values. The intervals following these two major mass extinctions were characterized by multiple positive-negative couplets of carbon cycling and were composed of low-richness ammonoid faunas characterized by higher proportions of passively floating, non-swimming morphotypes than before or after. In contrast, richness was highest during intervals of stable carbon isotope values. We propose that these chaotic carbon episodes reflect the breakdown of functional redundancy in the ecosystem, and that the post-extinction carbon cycle did not stabilize until redundancy was restored.

Early and Middle Triassic trends in diversity, evenness, and size of foraminifers on a carbonate platform in south China: implications for tempo and mode of biotic recovery from the end-Permian mass extinction

Delayed biotic recovery from the end-Permian mass extinction has long been interpreted to result from environmental inhibition. Recently, evidence of more rapid recovery has begun to emerge, suggesting the role of environmental inhibition was previously overestimated. However, there have been few high-resolution taxonomic and ecological studies spanning the full Early and Middle Triassic recovery interval, leaving the precise pattern of recovery and underlying mechanisms poorly constrained. In this study, we document Early and Middle Triassic trends in taxonomic diversity, assemblage evenness, and size distribution of benthic foraminifers on an exceptionally exposed carbonate platform in south China. We observe gradual increases in all metrics through Early Triassic and earliest Middle Triassic time, with stable values reached early in the Anisian. There is little support in our data set for a substantial Early Triassic lag interval during the recovery of foraminifers or for a stepwise recovery pattern. The recovery pattern of foraminifers on the GBG corresponds well with available global data for this taxon and appears to parallel that of many benthic invertebrate clades. Early Triassic diversity increase in foraminifers was more gradual than in ammonoids and conodonts. However, foraminifers continued to increase in diversity, size, and evenness into Middle Triassic time, whereas diversity of ammonoids and conodonts declined. These contrasts suggest decoupling of recovery between benthic and pelagic environments; it is unclear whether these discrepancies reflect inherent contrasts in their evolutionary dynamics or the differential impact of Early Triassic ocean anoxia or associated environmental parameters on benthic ecosystems.