Jessica H. Whiteside

Compound-specific carbon isotopes from Earth’s largest flood basalt eruptions directly linked to the end-Triassic mass extinction

A leading hypothesis explaining Phanerozoic mass extinctions and associated carbon isotopic anomalies is the emission of greenhouse, other gases, and aerosols caused by eruptions of continental flood basalt provinces. However, the necessary serial relationship between these eruptions, isotopic excursions, and extinctions has never been tested in geological sections preserving all three records. The end-Triassic extinction (ETE) at 201.4 Ma is among the largest of these extinctions and is tied to a large negative carbon isotope excursion, reflecting perturbations of the carbon cycle including a transient increase in CO2. The cause of the ETE has been inferred to be the eruption of the giant Central Atlantic magmatic province (CAMP). Here, we show that carbon isotopes of leaf wax derived lipids (n-alkanes), wood, and total organic carbon from two orbitally paced lacustrine sections interbedded with the CAMP in eastern North America show similar excursions to those seen in the mostly marine St. Audrie’s Bay section in England. Based on these results, the ETE began synchronously in marine and terrestrial environments slightly before the oldest basalts in eastern North America but simultaneous with the eruption of the oldest flows in Morocco, a CO2 super greenhouse, and marine biocalcification crisis. Because the temporal relationship between CAMP eruptions, mass extinction, and the carbon isotopic excursions are shown in the same place, this is the strongest case for a volcanic cause of a mass extinction to date.

Implications of the Newark Supergroup-based astrochronology and geomagnetic polarity time scale (Newark-APTS) for the tempo and mode of the early diversification of the Dinosauria

The Newark-APTS established a high-resolution framework for the Late Triassic and Early Jurassic. Palaeomagnetic polarity correlations to marine sections show that stage-level correlations of continental sequences were off by as much as 10 million years. New U–Pb ages show the new correlations and the Newark basin astrochronology to be accurate. Correlation of Newark-APTS to the Chinle Formation/Dockum Group, Glen Canyon Group, Fleming Fjord Formation and Ischigualasto Formation led to the following conclusions: (1) there are no unequivocal Carnian-age dinosaurs; (2) the Norian Age was characterised by a slowly increasing saurischian diversity but no unequivocal ornithischians; (3) there was profound Norian and Rhaetian continental provinciality; (4) the classic Chinle-, Germanic- and Los Colorados-type assemblages may have persisted to the close of the Rhaetian; (5) the distinct genus-level biotic transition traditionally correlated with the marine Carnian–Norian is in fact mid-Norian in age and within published error of the Manicouagan impact; (6) the end-Triassic marine and continental extinctions as seen in eastern North America were contemporaneous; and (7) compared to Triassic communities, Hettangian and Sinemurian age terrestrial communities were nearly globally homogenous and of low diversity. Consequently, the complex emerging picture of dinosaur diversification demands biostratigraphically-independent geochronologies in each of the faunally-important regions.

Delayed recovery of non-marine tetrapods after the end-Permian mass extinction tracks global carbon cycle

During the end-Permian mass extinction, marine ecosystems suffered a major drop in diversity, which was maintained throughout the Early Triassic until delayed recovery during the Middle Triassic. This depressed diversity in the Early Triassic correlates with multiple major perturbations to the global carbon cycle, interpreted as either intrinsic ecosystem or external palaeoenvironmental effects. In contrast, the terrestrial record of extinction and recovery is less clear; the effects and magnitude of the end-Permian extinction on non-marine vertebrates are particularly controversial. We use specimen-level data from southern Africa and Russia to investigate the palaeodiversity dynamics of non-marine tetrapods across the Permo-Triassic boundary by analysing sample-standardized generic richness, evenness and relative abundance. In addition, we investigate the potential effects of sampling, geological and taxonomic biases on these data. Our analyses demonstrate that non-marine tetrapods were severely affected by the end-Permian mass extinction, and that these assemblages did not begin to recover until the Middle Triassic. These data are congruent with those from land plants and marine invertebrates. Furthermore, they are consistent with the idea that unstable low-diversity post-extinction ecosystems were subject to boom–bust cycles, reflected in multiple Early Triassic perturbations of the carbon cycle.

Climatically driven biogeographic provinces of Late Triassic tropical Pangea

Although continents were coalesced into the single landmass Pangea, Late Triassic terrestrial tetrapod assemblages are surprisingly provincial. In eastern North America, we show that assemblages dominated by traversodont cynodonts are restricted to a humid 6° equatorial swath that persisted for over 20 million years characterized by “semiprecessional” (approximately 10,000-y) climatic fluctuations reflected in stable carbon isotopes and sedimentary facies in lacustrine strata. More arid regions from 5–20°N preserve procolophonid-dominated faunal assemblages associated with a much stronger expression of approximately 20,000-y climatic cycles. In the absence of geographic barriers, we hypothesize that these variations in the climatic expression of astronomical forcing produced latitudinal climatic zones that sorted terrestrial vertebrate taxa, perhaps by excretory physiology, into distinct biogeographic provinces tracking latitude, not geographic position, as the proto-North American plate translated northward. Although the early Mesozoic is usually assumed to be characterized by globally distributed land animal communities due to of a lack of geographic barriers, strong provinciality was actually the norm, and nearly global communities were present only after times of massive ecological disruptions.

Episodic photic zone euxinia in the northeastern Panthalassic Ocean during the end-Triassic extinction

Severe changes in ocean redox, nutrient cycling, and marine productivity accompanied most Phanerozoic mass extinctions. However, evidence for marine photic zone euxinia (PZE) as a globally important extinction mechanism for the end-Triassic extinction (ETE) is currently lacking. Fossil molecular (biomarker) and nitrogen isotopic records from a sedimentary sequence in western Canada provide the first conclusive evidence of PZE and disrupted biogeochemistry in neritic waters of the Panthalassic Ocean during the end Triassic. Increasing water-column stratification and deoxygenation across the ETE led to PZE in the Early Jurassic, paralleled by a perturbed nitrogen cycle and ecological turnovers among noncalcifying groups, including eukaryotic algae and prokaryotic plankton. If such conditions developed widely in the Panthalassic Ocean, PZE might have been a potent mechanism for the ETE.

Extreme ecosystem instability suppressed tropical dinosaur dominance for 30 million years

Significance This is, to our knowledge, the first multiproxy study of climate and associated faunal change for an early Mesozoic terrestrial ecosystem containing an extensive vertebrate fossil record, including early dinosaurs. Our detailed and coupled high-resolution records allow us to sensitively examine the interplay between climate change and ecosystem evolution at low paleolatitudes during this critical interval of Earths history when modern terrestrial ecosystems first evolved against a backdrop of high CO2 in a hothouse world. We demonstrate that these terrestrial ecosystems evolved within a generally arid but strongly fluctuating paleoclimate that was subject to pervasive wildfires, and that these environmental conditions in the early Mesozoic prevented large active warm-blooded herbivorous dinosaurs from becoming established in subtropical low latitudes until later in the Mesozoic. A major unresolved aspect of the rise of dinosaurs is why early dinosaurs and their relatives were rare and species-poor at low paleolatitudes throughout the Late Triassic Period, a pattern persisting 30 million years after their origin and 10–15 million years after they became abundant and speciose at higher latitudes. New palynological, wildfire, organic carbon isotope, and atmospheric pCO2 data from early dinosaur-bearing strata of low paleolatitudes in western North America show that large, high-frequency, tightly correlated variations in δ13Corg and palynomorph ecotypes occurred within a context of elevated and increasing pCO2 and pervasive wildfires. Whereas pseudosuchian archosaur-dominated communities were able to persist in these same regions under rapidly fluctuating extreme climatic conditions until the end-Triassic, large-bodied, fast-growing tachymetabolic dinosaurian herbivores requiring greater resources were unable to adapt to unstable high CO2 environmental conditions of the Late Triassic.

Mercury evidence for pulsed volcanism during the end-Triassic mass extinction

Significance The end of the Triassic Period (∼201.5 million years ago) witnessed one of the largest mass extinctions of animal life known from Earth history. This extinction is suggested to have coincided with and been caused by one of the largest known episodes of volcanic activity in Earth’s history. This study examines mercury concentrations of sediments from around the world that record this extinction. Mercury is emitted in gaseous form during volcanism, and subsequently deposited in sediments. We find numerous pulsed elevations of mercury concentrations in end-Triassic sediments. These peaks show that the mass extinction coincided with large-scale, episodic, volcanism. Such episodic volcanism likely perturbed the global environment over a long period of time and strongly delayed ecological recovery. The Central Atlantic Magmatic Province (CAMP) has long been proposed as having a causal relationship with the end-Triassic extinction event (∼201.5 Ma). In North America and northern Africa, CAMP is preserved as multiple basaltic units interbedded with uppermost Triassic to lowermost Jurassic sediments. However, it has been unclear whether this apparent pulsing was a local feature, or if pulses in the intensity of CAMP volcanism characterized the emplacement of the province as a whole. Here, six geographically widespread Triassic–Jurassic records, representing varied paleoenvironments, are analyzed for mercury (Hg) concentrations and Hg/total organic carbon (Hg/TOC) ratios. Volcanism is a major source of mercury to the modern environment. Clear increases in Hg and Hg/TOC are observed at the end-Triassic extinction horizon, confirming that a volcanically induced global Hg cycle perturbation occurred at that time. The established correlation between the extinction horizon and lowest CAMP basalts allows this sedimentary Hg excursion to be stratigraphically tied to a specific flood basalt unit, strengthening the case for volcanic Hg as the driver of sedimentary Hg/TOC spikes. Additional Hg/TOC peaks are also documented between the extinction horizon and the Triassic–Jurassic boundary (separated by ∼200 ky), supporting pulsatory intensity of CAMP volcanism across the entire province and providing direct evidence for episodic volatile release during the initial stages of CAMP emplacement. Pulsatory volcanism, and associated perturbations in the ocean–atmosphere system, likely had profound implications for the rate and magnitude of the end-Triassic mass extinction and subsequent biotic recovery.

“Imperfections and oddities” in the origin of the nucleus

Abstract  “Dual terminologies should be reserved for the exclusive use of those who prefer confusion to clarity.” L. R. Cleveland, 1963 We outline a plausible evolutionary sequence that led from prokaryotes to the origin of the first nucleated cell. The nucleus is postulated to evolve after the archaebacterium and eubacterium merged to form the symbiotic ancestor of amitochondriate protists. Descendants of these amitochondriate cells (archaeprotists) today thrive in organic-rich anoxic habitats where they are amenable to study. Eukaryosis, the origin of nucleated cells, occurred by the middle Proterozoic Eon prior to the deposition in sediments of well-preserved microfossils such as Vandalosphaeridium and the spiny spheres in the Doushantou cherts of China.

The first phytosaur (Diapsida, Archosauriformes) from the Late Triassic of the Iberian Peninsula

The Triassic was first defined based on the characteristic three-fold sequence of rocks that crops out across much of Europe, and many of the first records of Triassic dinosaurs, crocodile-line arc...

Newly integrated approaches to studying Late Triassic terrestrial ecosystems

The Late Triassic (~235–201.3 Ma) is one of the critical intervals of Phanerozoic earth history. Against a backdrop of marked climatic differences across Pangea (Sellwood and Valdes, 2006) and changing global atmospheric conditions (e.g., Berner, 2006), the Late Triassic saw the final recovery from the Permo–Triassic mass extinction, just before the end-Triassic mass extinction, and was punctuated by at least one major extraterrestrial bolide impact (Hodych and Dunning, 1992; Ramezani et al., 2005). Late Triassic terrestrial ecosystems are of particular interest, because they saw the origin of many modern vertebrate groups (e.g., Hugall et al., 2007), including the origin and early diversification of dinosaurs, while continents drifted through distinctly different climate zones (Kent and Tauxe, 2005). A variety of recent research examining the terrestrial realm with new analytical tools has dramatically changed our understanding of this time interval on land.