An analysis of the impacts of Cretaceous oceanic anoxic events on global molluscan diversity dynamics

In October 1983, the Subcommission on Cre­taceous Stratigraphy arranged a symposium on Cretaceous Stage Boundaries in Copenhagen to fulfil the Subcommission's primary responsibility: the defining of stage boundaries...

The stratigraphic record of the Frasnian-Famennian mass extinction in the eastern United States indicates that a reversal in the magnitude of extinction and speciation rates was the driving mechanism forcing the decline in species diversity in this region. Extinction rates increased sharply during the deposition of the West Falls Group in New York State, and remained high for the remainder of the Frasnian, a period of approximately 4.5 Myr. Sharp reductions in species diversity, however, only occurred during deposition of the Java Group at the very end of the Frasnian. The period of diversity loss was much more abrupt and short term than the period elevated extinction rates, and was more a function of a drop in speciation rate than a direct function of an increase in extinction rate.

C-isotope stratigraphy and paleoenvironmental changes across OAE2 (mid-Cretaceous) from shallow-water platform carbonates of southern Mexico

Marine Cretaceous rocks of Berriasian to Aptian age are restricted to northwestern Alberta and north­eastern British Columbia where many stage and substage boundaries may be drawn only tentatively on the basis of limited molluscan faunas. A complete marine Albian succession is similarly restricted, although the middle and upper divisions of the stage are much more widespread. The complete Albian succession of the Peace River district contains a refined and well-documented sequence of ammonite and foraminiferal zones and would be a most suitable continental standard. The Albian-Cenomanian (Lower-Upper Cretaceous) boundary has been reliably established in the continuous foraminifer- and mollusc-bearing sequences of the northwestern plains, but eastward it becomes enclosed in a hiatus. The same is true of successive stage boundaries from the Cenomanian­Turonian to the Santonian-Campanian. The bases for establishment of these boundaries, therefore, can be fully considered only on the western flank of the basin. The Campanian-Maastrichtian boundary may be precisely drawn in western Saskatchewan and traced westward to the Rocky Mountain front. Zones based on ammonites and inoceramid bivalves form· the cornerstone of the biostratigraphy and chronostratigraphy of the southern Interior Plains and hold the key to the stage boundaries. Zones based on assemblages of benthonic foraminifers, rarely of planktonic foraminifers, supplement the molluscan zones. The foraminiferal zones are less reliable and less useful, however, because some benthonic assemblages are weakly diachronous, most foraminiferal zones span several molluscan zones, and many stage boundaries fall within individual foraminifer zones.

The evolution of biodiversity among the Southwest European Neogene rodent (Mammalia, Rodentia) communities: pattern and process of diversification and extinction

Estimation of the ages of period and stage boundaries of the Geologic Time Scale (GTS) has a long history that commenced over a century ago with the pioneering work of Arthur Holmes. Frequencies, precision and accuracy of radiogenic isotope age determinations used for time scale construction continue to increase steadily. Later stage boundary age estimates are accompanied by error bars based on 2-sigma age dating errors with incorporation of stratigraphic uncertainty. Most GTS2004 and GTS2012 results involved spline-curve fitting. In GTS2012, Milankovitch-type orbital climate cyclicity was used to tune the Neogene geologic time scale while seafloor spreading was combined with sedimentary cycle scaling to construct the Paleogene time scale, and it also contributed to the construction of the Cretaceous and Jurassic time scales. Geomathematical procedures continue to be refined for the next GTS which is in Gradstein et al. (2020). In this study smoothing splines are used to construct Devonian and Late Jurassic - Early Cretaceous time scales. This methodology and its results are described and some estimates are refined by incorporating Milankovitch cycle durations.

The fossil record shows that the vast majority of all species that ever existed are extinct and that most lineages go through an expansion and decline in diversity. However, macroevolutionary analyses based upon molecular phylogenies have difficulty inferring extinction dynamics, raising questions about whether the neontological record can contribute to an understanding of the decline phenomenon. Two recently developed diversification methods for molecular phylogenies (RPANDA and BAMM) incorporate models that theoretically have the capacity to capture decline dynamics by allowing extinction to be higher than speciation. However, the performance of these frameworks over a wide range of decline scenarios has not been studied. Here, we investigate the behavior of these methods under decline scenarios caused by decreasing speciation and increasing extinction through time on simulated trees at fixed intervals over diversity trajectories with expansion and decline phases. We also compared method performance over a comprehensive data set of 214 empirical trees. Our results show that both methods perform equally well when varying speciation rates control decline. When decline was only caused by an increase in extinction rates both methods wrongly assign the variation in net diversification to a drop in speciation, even though the positive gamma values of those trees would suggest otherwise. We also found a tendency for RPANDA to favor increasing extinction and BAMM to favor decreasing speciation as the most common cause of decline in empirical trees. Overall our results shed light on the limitations of both methods, encouraging researchers to carefully interpret the results from diversification studies.

Estimating how the number of species in a given group varied in the deep past is of key interest to evolutionary biologists. However, current phylogenetic approaches for obtaining such estimates have limitations, such as providing unrealistic diversity estimates at the origin of the group. Here, we develop a robust probabilistic approach for estimating diversity through time curves and uncertainty around these estimates from phylogenetic data. We show with simulations that under various realistic scenarios of diversification, this approach performs better than previously proposed approaches. We also characterize the effect of tree size and undersampling on the performance of the approach. We apply our method to understand patterns of species diversity in anurans (frogs and toads). We find that Archaeobatrachia-a species-poor group of old frog clades often found in temperate regions-formerly had much higher diversity and net diversification rate, but the group declined in diversity as younger, nested clades diversified. This diversity decline seems to be linked to a decline in speciation rate rather than an increase in extinction rate. Our approach, implemented in the R package RPANDA, should be useful for evolutionary biologists interested in understanding how past diversity dynamics have shaped present-day diversity. It could also be useful in other contexts, such as for analyzing clade-clade competitive effects or the effect of species richness on phenotypic divergence.

Summary Current models estimating impact of habitat loss on biodiversity in the face of global climate change usually project only percentages of species ‘committed to extinction’ on an uncertain time‐scale. Here, we show that this limitation can be overcome using an empirically derived ‘background extinction rate–area’ curve to estimate natural rates and project future rates of freshwater fish extinction following variations in river drainage area resulting from global climate change. Based on future climatic projections, we quantify future active drainage basin area losses and combine them with the extinction rate–area curve to estimate the future change in extinction rate for each river basin. We then project the number of extinct species in each river basin using a global data base of freshwater fish species richness. The median projected extinction rate owing to climate change conditions is c. 7% higher than the median background extinction rate. A closer look at the pattern reveals great geographical variations highlighting an amplification of aridity by 2090 and subsequent increase in extinction rates in presently semi‐arid and Mediterranean regions. Among the 10% most‐impacted drainage basins, water availability loss will increase background extinction rates by 18·2 times (median value). Projected numbers of extinct species by 2090 show that only 20 river basins among the 1010 analysed would experience fish species extinctions attributable to water availability loss from climate change. Predicted numbers of extinct species for these rivers range from 1 to 5. Synthesis and applications. Our results strongly contrast with previous alarming predictions of huge surface‐dependent climate change–driven extinctions for riverine fishes and other taxonomic groups. Furthermore, based on well‐documented fish extinctions from Central and North American drainages over the last century, we also show that recent extinction rates are, on average, 130 times greater than our projected extinction rates from climate change. This last result implies that current anthropogenic threats generate extinction rates in rivers far greater than the ones expected from future water availability loss. We thus argue that conservation actions should be preferentially focused on reducing the impacts of present‐day anthropogenic drivers of riverine fish extinctions.

Microfossil Assemblages and the Cenomanian-Turonian (late Cretaceous) Oceanic Anoxic Event

Abstract. The principal palynological proxy for the Cenomanian–Turonian Stage boundary, the top of consistent/common Litosphaeridium siphoniphorum (a dinoflagellate cyst), occurs in Greenhorn Bed 73 at the international stratotype section, west of Pueblo, Colorado, USA. This datum occurs in the same position, as indicated by planktonic foraminifera (a few beds higher than the range top of R. cushmani), ammonites (upper part of the S. gracile/M. geslinianum Zone) and geochemistry (immediately below maximum δ13C values), at Pueblo (Western Interior Basin) and localities in southern England (Wessex–Paris Basin) and northern Germany (Lower Saxony Basin). Of over 100 dinoflagellate cyst taxa recorded from Pueblo and a correlative section at Lulworth, southern England, possibly as few as six do not range into the Turonian. In the uppermost Cenomanian – lowermost Turonian succession at Pueblo, there are no consistent absences of any common taxa (with four exceptions) and there is no evidence for a collapse in cyst-forming dinoflagellate populations during the Cenomanian–Turonian boundary mass extinction interval/‘oceanic anoxic event’. However, the composition of palynological assemblages from the Upper Cenomanian appears to reflect palaeoenvironmental stress and/or an increase in the supply of land-derived and relatively nearshore palynomorphs.

A perturbation in the carbon-isotope record at the time of the Pliensbachian–Toarcian boundary (~ 184 Ma) in the Early Jurassic is reported, based on new data from Yorkshire, England. Two sharp δ13Corgnegative excursions, each with a magnitude of ~ −2.5 ‰ and reaching minimum values of −28.5 ‰, are recorded in the bulk organic-matter record in sediments of latest Pliensbachian to earliest Toarcian age. A similar pattern of negative carbon-isotope excursions has been observed at the stage boundary in the SW European section at Peniche, Portugal in δ13Ccarbonate, δ13Cwoodand δ13Cbrachiopodrecords. The isotopic excursion is of interest when considering the genesis and development of the later Toarcian Oceanic Anoxic Event (OAE), as well as the second-order global extinction event that spans the stage boundary. Furthermore, the isotope excursion potentially provides a chemostratigraphic marker for recognition of the stage boundary, which is currently achieved on the basis of different ammonite faunas in the NW European and Tethyan realms.

The Ocean Anoxic Event 2 (OAE2) about 93.5 million years ago was marked by high atmospheric CO2 concentration, rapid global warming and marine anoxia and euxinia. The event lasted for about 440,000 years and led to habitat loss and mass extinction. The marine anoxia is thought to be linked to enhanced biological productivity, but it is unclear what triggered the increased production and what allowed the subsequent rapid climate recovery. Here we use lithium isotope measurements from carbonates spanning the interval including OAE2 to assess the role of silicate weathering. We find the lightest values of the Li isotope ratio (δ7Li) during OAE2, indicating high levels of weathering—and therefore atmospheric CO2 removal—which we attribute to an enhanced hydrological cycle. We use a geochemical model to simulate the evolution of δ7Li and the Ca, Sr and Os isotope tracers. Our simulations suggest a scenario in which the eruption of a large igneous province led to high atmospheric CO2 concentrations and rapid global warming, which initiated OAE2. The simulated warming was accompanied by a roughly 200,000 year pulse of accelerated weathering of mafic silicate rocks, which removed CO2 from the atmosphere. The weathering also delivered nutrients to the oceans that stimulated primary productivity. We suggest that this process, together with the burial of organic carbon, allowed the rapid recovery and stabilization from the greenhouse state.

Neritic ecosystem response to Oceanic Anoxic Event 2 in the Cretaceous Western Interior Seaway, USA

Aim Resolving the spatio‐temporal diversification patterns and systematic relationships of endemic Australian land snails against the backdrop of Neogene aridification through analyses of a combined nuclear and mitochondrial DNA sequence dataset. Location Western Australia. Taxon Mollusca, Stylommatophora, Bothriembryontidae, Bothriembryon. Methods We employed Bayesian Inference and Maximum Likelihood to reconstruct the phylogenetic relationships in a group of Australian land snails using mitochondrial (COI, 16S) and nuclear (ANT) DNA sequences. Divergence times have been estimated by employing optimised molecular clocks using BEAST2, and RelTime in MEGA12. Speciation and net‐diversification rates have been modelled using revBayes to visualise diversification dynamics in lineage‐through‐time (LTT) plots. We employed automated species delimitation methods ASAP and bPTP to estimate taxonomic diversity. Results Our final sequence dataset contained 1052 new DNA sequences from 374 individuals, representing 97% of all accepted species plus 26 putatively new species based on morphology and distribution. Recognizing almost three times as many candidate species, both DNA‐based species delimitation methods have excessively inflated diversity estimates, casting doubt on the usefulness of these methods in groups with marked phylogeographic structure. Nine well‐supported principal clades were recovered. Fossil‐calibrated chronograms revealed an early bifurcation of Bothriembryon followed by an accumulation of lineages over time. LTT plots revealed a relative flattening of the speciation curve from 15 to 10 Ma on. However, we also detected a steep increase in intraspecific lineage diversification during the last approx. 1 Ma. The modeled speciation and net diversification rates have continuously declined over the last 25 Ma, while extinction rates have remained relatively steady until about 5 Ma, when they also started to climb. Main Conclusions Declining diversification rates during much of the Neogene, followed by increasing extinction rates, coincided with increasing aridity throughout Western Australia. A more recent increase in lineage diversification rates, driven by intraspecific differentiation, coincides with the rise of mesic conditions since the end of the Pleistocene. Both trends imply that through influencing extinction and diversification rates, historical climate change has likely contributed to shaping the current distribution patterns in Bothriembryon land snails that are characterised by fragmentation. Moreover, by uncovering many undescribed taxa, including multiple short‐range endemics, this study highlights the importance of continued conservation efforts in this globally important biodiversity hotspot. Key habitats, such as lithorefugia, in an otherwise harsh and exposed landscape are important strongholds for Bothriembryon to weather the impact of aridification.