Vivi Vajda

The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary

The Fall of the Dinosaurs According to the fossil record, the rule of dinosaurs came to an abrupt end ∼65 million years ago, when all nonavian dinosaurs and flying reptiles disappeared. Several possible mechanisms have been suggested for this mass extinction, including a large asteroid impact and major flood volcanism. Schulte et al. (p. 1214) review how the occurrence and global distribution of a global iridium-rich deposit and impact ejecta support the hypothesis that a single asteroid impact at Chicxulub, Mexico, triggered the extinction event. Such an impact would have instantly caused devastating shock waves, a large heat pulse, and tsunamis around the globe. Moreover, the release of high quantities of dust, debris, and gases would have resulted in a prolonged cooling of Earths surface, low light levels, and ocean acidification that would have decimated primary producers including phytoplankton and algae, as well as those species reliant upon them. The Cretaceous-Paleogene boundary ~65.5 million years ago marks one of the three largest mass extinctions in the past 500 million years. The extinction event coincided with a large asteroid impact at Chicxulub, Mexico, and occurred within the time of Deccan flood basalt volcanism in India. Here, we synthesize records of the global stratigraphy across this boundary to assess the proposed causes of the mass extinction. Notably, a single ejecta-rich deposit compositionally linked to the Chicxulub impact is globally distributed at the Cretaceous-Paleogene boundary. The temporal match between the ejecta layer and the onset of the extinctions and the agreement of ecological patterns in the fossil record with modeled environmental perturbations (for example, darkness and cooling) lead us to conclude that the Chicxulub impact triggered the mass extinction.

Fossilized nuclei and chromosomes reveal 180 million years of genomic stasis in royal ferns.

Cytologically Informative Fossils Fossilization processes tend to destroy fine-cell structure but, exceptionally, Bomfleur et al. (p. 1376) have found examples of fossil ferns from the Jurassic in which subcellular structures, including organelles such as nuclei and chromosomes, are well-preserved. Comparative and quantative analyses show that these cells closely resemble the fossil nuclei of extant cinnamon ferns, Osmundastrum cinnamomea, which indicates that this group of ferns has remained virtually unchanged for 180 million years. Fern fossils provide evidence that nuclear shape, size, and chromosomal content have changed little since the Jurassic. Rapidly permineralized fossils can provide exceptional insights into the evolution of life over geological time. Here, we present an exquisitely preserved, calcified stem of a royal fern (Osmundaceae) from Early Jurassic lahar deposits of Sweden in which authigenic mineral precipitation from hydrothermal brines occurred so rapidly that it preserved cytoplasm, cytosol granules, nuclei, and even chromosomes in various stages of cell division. Morphometric parameters of interphase nuclei match those of extant Osmundaceae, indicating that the genome size of these reputed “living fossils” has remained unchanged over at least 180 million years—a paramount example of evolutionary stasis.

Pollen and spores in marine Cretaceous/Tertiary boundary sediments at mid-Waipara River, North Canterbury, New Zealand

Abstract Terrestrial pollen and spores in late Maastrichtian to early Paleocene marine strata at mid‐Waipara, New Zealand, permit reconstruction of contemporary vegetation and paleoclimates. During the latest Cretaceous, spore‐pollen assemblages reflect a temperate rainforest with a prominent podocarp and tree ferns component, angiosperm pollen being mainly represented by Nothofagus and Proteaceae. Disruption of the vegetation at the Cretaceous/Tertiary (K/T) boundary is recorded by an increase in fern spores, reduction in gymnosperm pollen, and temporary loss of angiosperm pollen both in mid Waipara and in the terrestrial sections of Moody Creek Mine and Compressor Creek. Following an interval of fern dominance, gymnosperms and later angiosperms return to the palynological record. The floral turnover at the K/T boundary is comparable to palynological records from North America and Japan, indicating that disruption of vegetation was global. Fern dominance is estimated to have lasted several thousands of years, based on foraminiferal biostratigraphy of immediate post‐K/T boundary strata. This is orders of magnitude greater than seen in normal seral successions following deforestation. We suggest that the observed vegetation succession is due to a prolonged period of low ambient light levels, sufficient for photosynthesis but favouring plants already adapted to these levels (such as forest ground stratum), accompanied by a moderate temperature and moisture regime.

Early Jurassic annelid cocoons from eastern Australia

A small assemblage of clitellate annelid cocoons (egg cases) is described from the Early Jurassic Marburg Subgroup, Clarence-Moreton Basin, eastern Australia. Two complete specimens are preserved on bedding planes, and numerous fragments were recovered from organic residues prepared for mesofossil analysis. The complete specimens are assigned to Burejospermum Krassilov emend. Manum, Bose & Sawyer, 1991, having a well-developed mesh-like hapsine and a solid layer of underlying alytine. The fragments include a form assigned to Dictyothylakos Horst, 1954 emend. Manum, Bose & Sawyer, 1991 having a mesh-like hapsine but lacking an alytine layer. The cocoons are preserved in floodbasin facies represented by leaf-rich, dark shales. Palynological data indicate a Pliensbachian to earliest Toarcian age for the assemblage. Similarities between the new specimens, congeneric material from the Triassic to Neogene of other continents, and modern egg cases highlight the long-term morphological conservatism of clitellate cocoons. The new material extends the distribution of such fossils, which generally have a poor macrofossil record.

Onshore Jurassic of Scandinavia and related areas

Jurassic strata are extensively distributed in offshore areas of Scandinavia, but onshore exposures are mostly restricted to southern Sweden (Skåne), the Danish island of Bornholm, East Greenland, northern Norway (Andøya) and Svalbard. The latest Triassic and Jurassic saw active tectonism in Scandinavia associated with the break-up of Pangaea and rifting in the North Atlantic region and the North Sea. Rifting and the gradual rise in sea level controlled the structural and sedimentological architecture of Scandinavian basins throughout the Jurassic. The Upper Triassic is represented by continental red beds (claystones and arkosic conglomerates) indicative of arid conditions (until the Norian) and by coal measures characteristic of humid conditions (in the Rhaetian). Early Jurassic sedimentation in the region was dominated by fluvial–estuarine systems. Basin subsidence combined with the supply of huge volumes of sediments led to the accumulation of thick sand units on vast coastal plains in the Early and Middle Jurassic. During the Late Jurassic, transgressions led to deposition of extensive marine mud, although sandstones are locally preserved. Paralic depositional environments prevailed during the Late Jurassic and into the Early Cretaceous in southern Scandinavia. Scandinavia hosts a rich Jurassic palaeontological record including fossil plants, sharks, dinosaur footprints, ammonites, belemnites, ichthyosaurs and pliosaurs. Miospores provide the primary tool for biostratigraphic subdivision and correlation of the continental Jurassic sediments, whereas ammonites, dinoflagellates and foraminifera are the main groups employed for marine biostratigraphy. However, much work remains to be completed to achieve a highly resolved zonation scheme that integrates both marine and terrestrial indices.

A palynological investigation of plesiosaur-bearing rocks from the Upper Cretaceous Tahora Formation, Mangahouanga, New Zealand

High-palaeolatitude plesiosaur, mosasaur and, more rarely, dinosaur fossils are well known from the Maungataniwha Sandstone Member of the Tahora Formation in Mangahouanga Stream, Hawkes Bay, New Zealand. A palynological investigation of strata exposed along Mangahouanga Stream and of transported boulders hosting vertebrate fossils reveals well-preserved assemblages dominated by terrestrial pollen and spores but also including marine dinoflagellate cysts in some samples. The palynofacies are strongly dominated by wood fragments including charcoal; one outcrop sample and the sample taken from a boulder hosting plesiosaur vertebrae contain entirely terrestrially derived palynoassemblages, suggesting a freshwater habitat for at least some of the plesiosaurs. The host unit spans the Santonian to lowermost Maastrichtian, while the key pollen taxa Nothofagidites senectus and Tricolpites lilliei, together with the dinocyst Isabelidinium pellucidum and the megaspore Grapnelispora evansii, indicate a late Campanian to early Maastrichtian age for the fossiliferous boulders. The palynoflora indicates a mixed local vegetation dominated by podocarp conifers and angiosperms with a significant tree-fern subcanopy. The presence of taxa with modern temperate distributions, such as Nothofagus (southern beech), Proteaceae and Cyatheaceae (tree-ferns), indicates a mild-temperate climate and lack of severe winter freezing during the latest Cretaceous, providing an ecosystem that most probably made it possible for polar dinosaurs to overwinter in this part of the world.

Triassic-Jurassic climate in continental high-latitude Asia was dominated by obliquity-paced variations (Junggar Basin, Ürümqi, China).

Significance Geological records of paleoclimate provide the only constraints on Solar System orbital solutions extending beyond the ∼50-Ma limit imposed by chaotic diffusion. Examples of such constraints are coupled high and low latitude, Triassic–Jurassic (∼198–202 Ma) sedimentary cyclicity in coal-bearing outcrops from the ∼60° N-paleolatitude Junggar Basin (Western China), and contemporaneous tropical basins. Analysis reveals climate variability dominated by obliquity-scale cyclicity in the Junggar Basin and precession-scale cyclicity in the tropics. Together, these geological records empirically constrain orbital solutions by providing joint g4 − g3 and s4 − s3 secular frequency estimates of the Earth–Mars orbital resonance. These results demonstrate the opportunity for developing a new class of solutions grounded by geological data extending hundreds of millions of years into the geologic past. Empirical constraints on orbital gravitational solutions for the Solar System can be derived from the Earth’s geological record of past climates. Lithologically based paleoclimate data from the thick, coal-bearing, fluvial-lacustrine sequences of the Junggar Basin of Northwestern China (paleolatitude ∼60°) show that climate variability of the warm and glacier-free high latitudes of the latest Triassic–Early Jurassic (∼198–202 Ma) Pangea was strongly paced by obliquity-dominated (∼40 ky) orbital cyclicity, based on an age model using the 405-ky cycle of eccentricity. In contrast, coeval low-latitude continental climate was much more strongly paced by climatic precession, with virtually no hint of obliquity. Although this previously unknown obliquity dominance at high latitude is not necessarily unexpected in a high CO2 world, these data deviate substantially from published orbital solutions in period and amplitude for eccentricity cycles greater than 405 ky, consistent with chaotic diffusion of the Solar System. In contrast, there are indications that the Earth–Mars orbital resonance was in today’s 2-to-1 ratio of eccentricity to inclination. These empirical data underscore the need for temporally comprehensive, highly reliable data, as well as new gravitational solutions fitting those data.

Palynostratigraphy of dinosaur footprint-bearing deposits from the Triassic–Jurassic boundary interval of Sweden

The Triassic–Jurassic boundary (c. 200 Ma) marks one of the five largest Phanerozoic mass extinction events and is characterized by a major turnover in biotas. A palynological study of sedimentary rock slabs bearing dinosaur footprints from Rhaeto–Hettangian strata of Skåne, Sweden was carried out. The theropod dinosaur footprints (Kayentapus soltykovensis) derive from the southern part of the abandoned Vallåkra quarry (Höganäs Formation) and were originally dated as earliest Jurassic (Hettangian) based on lithostratigraphy. Our results reveal that two of the footprints are correlative with the latest Triassic (latest Rhaetian) disaster zone typified by a high abundance of the enigmatic gymnosperm pollen Ricciisporites tuberculatus and Perinopollenites elatoides together with the key taxon Limbosporites lundbladii and fern spores. Two footprints are dated to correlate with the Transitional Spore-spike Interval. One footprint is interpreted as Hettangian in age based on the relatively high abundance of Pinuspollenites spp. together with the presence of the key taxa Retitriletes semimuris and Zebrasporites intercriptus. Our new palynological study suggests that the Kayentapus ichnogenus already appeared in the end of Triassic, and our study highlights the use of palynology as a powerful tool to date historical collections of fossils in museums, universities and elsewhere. The Hettangian footprint reflects a marine influence while all other studied ichnofossil specimens occur in non-marine (floodplain and delta interdistributary) sediments. The sediments associated with the Hettangian footprint include a significant proportion of charcoal transported from land after wildfires. The Rhaeto–Hettangian vegetation was otherwise characterized by multi-storey gymnosperm–pteridophyte communities.

Early Miocene pollen and spores from western Jylland, Denmark - environmental and climatic implications

Abstract A palynological analysis of a Lower Miocene cored section from Sønder Vium in western Jylland, Denmark, provides new data regarding the vegetation and climate during the earliest Neogene. Most samples yielded well-preserved palynomorphs. Terrestrial pollen and spores dominate, with lesser proportions of dinoflagellates. A fluvial input into the marine setting is corroborated by the presence of freshwater algae, indicating an inner-neritic setting. A level containing comparatively abundant dinoflagellate cysts probably represents a transgressional event. The late Aquitanian age of the sequence as suggested by previous studies is supported by the composition of the palynoflora, e.g., by the presence of Ephedripites, Platycarya, and the relatively frequent occurrence of Engelhardtia. The pollen record is dominated by Taxodiaceae-Cupressaceae suggesting that swamp forests dominated the onshore region, which is consistent with previous results from central and northern Europe. Besides Taxodium, the swamp forest also contained angiosperm taxa such as Myricaceae, Nyssa, Betula, and Alnus. Elevated or better drained hinterland areas hosted a diverse mesophytic forest, with a ground cover of reeds, sedges and pteridophytes. Abundant pollen taxa derived from mesophytic forests indicates the presence of evergreen conifers, such as Pinus, Sequoia and Sciadopitys, and deciduous angiosperms, including Fagus and Quercus. A decrease in relative abundances of thermophilous elements such as Arecaceae (palms), Ilex, Mastixiaceae and Engelhardtia, in the middle part of the studied succession indicates a possible correlation to the late Aquitanian climatic deterioration. The composition of the palynological assemblages including widely distributed Taxodium swamps, suggests a warm, frost-free temperate climate during the Aquitanian in Denmark.

A new Maastrichtian-Paleocene Azolla species from of Bolivia, with a comparison of the global record of coeval Azolla microfossils

A new heterosporous fern species, Azolla boliviensis sp. nov., is described from latest Maastrichtian (latest Cretaceous) to Paleocene (earliest Palaeogene) terrestrial sediments of the Eslabón and Flora Formations, Subandean belt, Bolivia. The species is represented by dissociated but abundantly co-preserved megasporocarps, megaspores, microsporangia, massulae and microspores. The genus consistently characterizes warm-climate lacustrine settings. Fossil Azolla is first identified around the Early to mid-Cretaceous but the genus apparently underwent dramatic radiation during the Late Cretaceous. Abundant Azolla remains in Bolivia add to this portrait of rapid geographie dispersai and diversification near the close of the Cretaceous. The ranges of many Azolla species span the Cretaceous- Palaeogene boundary and the potential of Azolla to withstand altered environmental conditions, such as periodic frost damage, drought, and salinity change, and its ability to undergo rapid vegetative regeneration in association with nitrogen-fixing cyanobacterial symbionts, suggest that the survival of this group was favoured during the adverse conditions of the end-Cretaceous event.