Dinosaurs appeared in the Middle to Late Triassic, quickly rose to dominance, evolved to colossal size, reigned the Earth for over 160 million years, and diversified into over 1,000 species worldwide. However, the last non-avian dinosaur species disappeared from the fossil record approximately 65 million years ago. The Cretaceous-Tertiary Boundary (KTB) mass extinction occurred 65 million years ago, causing the devastation of land vegetation and the extinction of non-avian dinosaurs, other vertebrates, marine reptiles and invertebrates, planktonic foraminifera, and ammonites (Brusatte et al., 2015). Only 12% of the land-dwelling forms survived this extinction while 90% of species in the freshwater assemblage survived (Kaiho et al., 2016). Marine plankton diversity and marine productivity decreased significantly, accompanied by a biogeochemical collapse, but 90% of benthic foraminiferal species survived the impact, because while there were significant environmental changes in the surface waters, there was little change in the deep sea (Kaiho et al., 2016).
Introduction:
The severity of extinctions, debate as to what truly caused the reign on non-avian dinosaurs to end, and whether it was gradual or sudden in geologic time, has led to years of research and disagreement within the scientific community. An asteroid impact and volcanic activity are popular speculations as the cause of non-avian dinosaur extinctions, as they roughly coincide with the timing of this extinction. Speculations that global climate, in response to these events, could explain why non-avian dinosaurs went extinct have also developed. The impact of an asteroid would have caused local and short-term consequences but also produced large amounts of dust, sulfate aerosols, and greenhouse gases affecting climate globally; likewise, large-scale volcanic eruptions would have released sulfur dioxide and carbon dioxide into the atmosphere leading to climatic changes possibly inducing the mass extinction (Brugger, Feulner, & Petri, 2017). Moreover, new hypotheses such as iridium in dinosaur eggshells and fungi virulence, to explain the extinction have also emerged to try answer the KTB extinction mystery. It is the divisiveness and unknown of this mystery that makes it imperative to continue research in this topic to achieve a better understanding of the climatic, atmospheric and environmental conditions, as well as reaching consensus within the scientific community and eliminating bias within the fossil record.
Hypotheses: Asteroid Impact:
The asteroid impact hypothesis is widely accepted because it would explain the global and simultaneously catastrophic extinction of non-avian dinosaurs and other species. Chondritic siderophile trace-element anomalies, shocked minerals and tektites have been subsequently found in the KTB layers strengthening the case for a large KT impact (Hildebrand, 1993). The Chicxulub impact crater in the Yucatan Peninsula of Mexico is believed to have been an impact site causing the extinction of non-avian dinosaurs. It is the third largest known impact crater, approximately 180-200 km in diameter, on Earth (Brusatte et al., 2015). The crater was formed by the impact of an asteroid, approximately 10km in diameter, at the KT boundary (Kaiho et al., 2016).
The effects of the impact were broad and devastating. It triggered tsunamis that may have reached >300 km inland around the Gulf of Mexico, potentially caused earthquakes, and created a global heat pulse that ignited large wildfires near the impact site (Brusatte et al., 2015). The impact occurred in a hydrocarbon and sulfur rich region, causing these to become heated and generating a vapor plume (Keller, 2011). Massive quantities of sulfate and other aerosols were then released into the atmosphere, which would have caused sulphuric acid rain and at least temporarily destroyed the ozone layer (Hildebrand, 1993). A 3- mm-thick Chicxulub ejecta layer would have globally distributed, and the aerosols would have also briefly cooled the Earth to near-freezing global conditions following the initial heat pulse (Sloan & Rigby, 1986), (Pope, 2002). Global annual mean surface air temperature decreased by at least 26°C, with 3 to 16 years subfreezing temperatures and a recovery time larger than 30 years. The surface cooling triggered vigorous ocean mixing which could have resulted in a plankton bloom due to upwelling of nutrients (Brugger et al., 2017). Dust thrown up by the impact would have formed a thick cloud that darkened the Earth and depressed and possibly lead to short-term cessation of photosynthesis (Pope, 2002). Over a slightly longer term, the injection of carbon dioxide, methane, and water vapor into the atmosphere may have caused greenhouse warming of ~10°C (Brusatte et al., 2015).
Volcanic Activity:
Tremendous volcanic activity formed the Deccan flood basalts of India. The eruptions proceeded in three main phases during the Late Cretaceous/early Paleocene. The second phase, which probably began ∼400,000 years before the KT boundary, was the largest and formed up to 80% of the volume of the Deccan Traps (Brusatte et al., 2015). This phase was similar in size to other large-scale flood basalt volcanism in the geological record, such as the Central Atlantic Magmatic Province (CAMP), which has been implicated in the end-Triassic extinction (Brusatte et al., 2015). It is this similarity to the end-Triassic extinction that has resulted in implementation of the theory of uniformitarianism to try and explain why these eruptions could have been the cause for the extinction of the dinosaurs.
Regardless of their precise timing, the Deccan eruptions would have caused major environmental perturbations in the Late Cretaceous/early Paleocene. Each eruption would have injected substantial amounts of Sulphur dioxide into the atmosphere, causing sulphuric acid rain and short-term cooling, depending on their frequency and whether the Sulphur dioxide reached the stratosphere (Brusatte et al., 2015). The radiation from these eruptions would have also generated fires destroying most of the vegetation when they reached the Earth’s surface. Evidence for a soot layer at the KTB implies such fires existed and presumably contributed to the species extinctions (Kaiho & Oshima, 2017). Furthermore, volcanic-induced environmental changes would have affected dinosaur communities in other ways during this time, such as changes in population structure or community ecology, or at regional scales that are currently undetectable in the fossil record (Keller, 2011).
non-avian dinosaur extinction implies their extinction was geologically gradual and species declination was not catastrophic within the same time frame. The globally distributed thin clay layer, rich in iridium, at the mass extinction level marking the KTB clay consists of 2 layers: a uniform ~3mm-thick global layer, dispersed by the impact asteroid, and a layer found only near the source crater composed of ballistically distributed ejecta (Hildebrand, 1993). The presence of the increased iridium levels in the clay layer implies that much of the material constituting the layer must have come from an asteroid that collided with the Earth since such heavy elements, present in asteroids, are rare in the Earth’s crust having sunk toward the core in the Earth’s early molten state (Sloan & Rigby, 1986).
Iridium anomalies have been observed in dinosaur eggshells in the KTB sections of the Nanxiong Basin, Guangdong Province, South China. Radiochemical neutron analysis of dinosaur eggshells was used in three different stratigraphic regions and the analysis of other trace elements was conducted with atomic absorption spectrometry (Zhao et al., 2002). Two types of pathological development, variation in eggshell thickness and eggshell microstructure, are observed from the basin samples. The eggshells collected at and near the fossil KTB interval show iridium increases of about 19 and 28 times, respectively, above the background level (Zhao et al., 2002). Similarly, the enrichment of other trace elements, such as Ni, Co, Zn, Pb, K, Cu, Mn, Sr and V, show similar patterns of abundance variations in the eggshells, and reach a maximum level at and near the KTB interval (Penner, 1985).
The enrichment of iridium and other trace elements in eggshells could have been caused by the assimilation of these elements into the dinosaur body through consumption of food, water, or air and then into the eggs laid by them (Penner, 1985). The increased levels would affect dinosaur’s bodily functions causing variations in thickness and alterations of the eggshells (Penner, 1985). A repeating short- and long-term geochemically induced environmental stress adversely could have also affected the reproductive process and contributed to the extinction of the dinosaurs (Zhao et al., 2002). Nonetheless, the geochemical analyses of the dinosaur eggshells indicate the extinction of the dinosaurs in the Nanxiong Basin did not occur instantaneously but were instead gradual with major extinction beginning at the KTB interval.