Facies selectivity of benthic invertebrates in a Permian/Triassic boundary microbialite succession: Implications for the "microbialite refuge" hypothesis

Ostracodal evolution during the Permian–Triassic transition at the Youping section of the Nanpanjiang Basin

The Permian–Triassic mass extinction has been considered the largest bio-crisis of the Phanerozoic, with more than 90% of marine species extinct. Previous studies showed that ostracods suffered various extinction patterns in different localities and were relatively enriched in the lowermost Triassic shallow marine microbialites. Multiple hypotheses have been put forward to explore the reasons for this phenomenon. Abundant ostracod fossils were collected from the microbialite-bearing Youping section in the Nanpanjiang Basin. 45 species in 22 genera from Wujiaping Formation increased dramatically to 104 species in 33 genera from the microbialites of basal Luolou Formation. However, Ostracods from the Youping section suffered severe extinction during the second phase of the Permian–Triassic crisis, i.e., the earliest Triassic mass extinction (ETME), rather than the first phase, i.e., the latest Permian mass extinction (LPEM). In addition, the Sørenson coefficient has been used to examine the similarity of faunal associations among different sections of the Permian–Triassic transitional beds. There was no significant differences for ostracods between microbialites and non-microbialites sections based on similarity analysis. Thus, we proposed that “Shallow marine refuge” hypothesis could explain the high diversity and high abundance of ostracods of the Permian–Triassic transitional beds. Besides, ostracods showed remarkable geographical differentiation at both regional and global scales during the Permian–Triassic transitional beds and were presumably controlled by geographical isolation.

Towards quantifying the mass extinction debt of the Anthropocene

To make sense of our present biodiversity crises, the modern rate of species extinctions is commonly compared to a benchmark, or 'background,' rate derived from the fossil record. These estimates are critical for bounding the scale of modern diversity loss, but are yet to fully account for the fundamental structure of extinction rates through time. Namely, a substantial fraction of extinctions within the fossil record occurs within relatively short-lived extinction pulses, and not during intervals characterized by background rates of extinction. Accordingly, it is more appropriate to compare the modern event to these pulses than to the long-term average rate. Unfortunately, neither the duration of extinction pulses in the geological record nor the ultimate magnitude of the extinction pulse today is resolved, making assessments of their relative sizes difficult. In addition, the common metric used to compare current and past extinction rates does not correct for large differences in observation duration. Here, we propose a new predictive metric that may be used to ascertain the ultimate extent of the ongoing extinction threat, building on the observation that extinction magnitude in the marine fossil record is correlated to the magnitude of sedimentary turnover. Thus, we propose that the ultimate number of species destined for extinction today can be predicted by way of a quantitative appraisal of humanity's modification of ecosystems as recorded in sediments-that is, by comparing our future rock record with that of the past. The ubiquity of habitat disruption worldwide suggests that a profound mass extinction debt exists today, but one that might yet be averted by preserving and restoring ecosystems and their geological traces.

Suppressed competitive exclusion enabled the proliferation of Permian/Triassic boundary microbialites

During the earliest Triassic microbial mats flourished in the photic zones of marginal seas, generating widespread microbialites. It has been suggested that anoxic conditions in shallow marine environments, linked to the end-Permian mass extinction, limited mat-inhibiting metazoans allowing for this microbialite expansion. The presence of a diverse suite of proxies indicating oxygenated shallow sea-water conditions (metazoan fossils, biomarkers and redox proxies) from microbialite successions have, however, challenged the inference of anoxic conditions. Here, the distribution and faunal composition of Griesbachian microbialites from China, Iran, Turkey, Armenia, Slovenia and Hungary are investigated to determine the factors that allowed microbialite-forming microbial mats to flourish following the end-Permian crisis. The results presented here show that Neotethyan microbial buildups record a unique faunal association due to the presence of keratose sponges, while the Palaeotethyan buildups have a higher proportion of molluscs and the foraminifera Earlandia. The distribution of the faunal components within the microbial fabrics suggests that, except for the keratose sponges and some microconchids, most of the metazoans were transported into the microbial framework via wave currents. The presence of both microbialites and metazoan associations were limited to oxygenated settings, suggesting that a factor other than anoxia resulted in a relaxation of ecological constraints following the mass extinction event. It is inferred that the end-Permian mass extinction event decreased the diversity and abundance of metazoans to the point of significantly reducing competition, allowing photosynthesis-based microbial mats to flourish in shallow water settings and resulting in the formation of widespread microbialites.

Echinoids from the Tesero Member (Werfen Formation) of the Dolomites (Italy): implications for extinction and survival of echinoids in the aftermath of the end-Permian mass extinction

The end-Permian mass extinction (∼252 Ma) was responsible for high rates of extinction and evolutionary bottlenecks in a number of animal groups. Echinoids, or sea urchins, were no exception, and the Permian to Triassic represents one of the most significant intervals of time in their macroevolutionary history. The extinction event was responsible for significant turnover, with the Permian–Triassic representing the transition from stem group echinoid-dominated faunas in the Palaeozoic to Mesozoic faunas dominated by crown group echinoids. This turnover is well-known, however, the environmental and taxonomic distribution of echinoids during the latest Permian and Early Triassic is not. Here we report on an echinoid fauna from the Tesero Member, Werfen Formation (latest Permian to Early Triassic) of the Dolomites (northern Italy). The fauna is largely known from disarticulated ossicles, but consists of both stem group taxa, and a new species of crown group echinoid, Eotiaris teseroensis n. sp. That these stem group echinoids were present in the Tesero Member indicates that stem group echinoids did not go extinct in the Dolomites coincident with the onset of extinction, further supporting other recent work indicating that stem group echinoids survived the end-Permian extinction. Furthermore, the presence of Eotiaris across a number of differing palaeoenvironments in the Early Triassic may have had implications for the survival of cidaroid echinoids during the extinction event.