Introduction
North America has the most dinosaur fossils. At the time of ancient lizards, this continent was connected with Asia where the Bering Strait is located today. It appears that the dinosaurs of North America and Asia lived on the same piece of land. Therefore, in both parts of the world, scientists find the remains of related pangolins. The landmasses were not divided by the ocean, and the dinosaurs moved in search of food. However, the remains of some ancient reptiles have only been found in North America. For example, the bones of tyrannosaurs did not come across to paleontologists on other continents. Apparently, these dinosaurs preferred to live only in North America. It is necessary to identify the reason for the extinction of dinosaurs on the territory of the continent, namely, the state of Texas.
Texas
In a detailed analysis of the state of Texas, it is worth highlighting that this territory is characterized by the volumetric presence of flightless species. This will be key in identifying the causes of the extinction of ancient animals. One of the most famous dinosaurs living in Texas is the tyrannosaurus rex (Wignall, 2019). In addition, it is worth highlighting tehacephalos, gravtilos, pachycephalosaurs and colepiocephals. The list of species is much larger, but this is enough to conclude that the predominant number of animal species in the state were flightless.
Background of Geology Research
Since the appearance of animals with a mineral skeleton on Earth about 550 million years ago, when dead animals began to be preserved in sedimentary rocks in the form of diagnosable remains that can be confidently classified as taxa, the diversity of such taxa has been increasing. However, against the background of a general increase in the diversity of the animal world, extinction of certain organisms is constantly taking place (Wignall, 2019). This is explained by the fact that they cannot adapt to constantly changing conditions – the changing configurations of continents and oceans. In 1982, David Raup and John Sepkosky published a famous article in which they analyzed the most comprehensive database at that time, which included about 3300 marine animal families, including 2400 extinct (Bromhm & Cardillo, 2019, p. 77). In this database, they recorded five events of dramatic decline in biodiversity over time, dubbed the “Great Mass Extinctions” (Bromhm & Cardillo, 2019). The youngest of them marks the stratigraphic boundary between the Cretaceous and the Paleogene, which the dinosaurs just did not survive. When discussing the reasons for the extinction of dinosaurs, one cannot leave the general context of the extinction of other animals at the boundary of the Cretaceous and Paleogene, as well as the general context of mass extinctions in other periods. In the very south of North America, scientists have found a trace of a giant meteorite – Chicxulub. Paleontologists believe that it was his fall that caused strong climate change on Earth (Bromhm & Cardillo, 2019). After this global catastrophe, all dinosaurs and many other ancient animals became extinct. However, it is impossible to designate this reason as unambiguous.
Establishing an Eventfulness
The evolution of living organisms is fixed by their remains in sedimentary rocks. The substance from which such sedimentary rocks were formed must have certain properties that make it possible to preserve organic remains, their imprints and traces of animal life. In general terms, the preservation in clay is better than in sand, and over time they will turn into solid rocks – mudstone and sandstone, respectively. However, it is not enough to find well-preserved remains; it is also necessary that the record be as continuous as possible and have a noticeable duration on the geological time scale (Wignall, 2019). Obviously, the probability of finding a preserved, continuous, and long-term record is noticeably higher for deposits of large water basins than for land. This is one of the methodological problems in establishing the causes of mass extinctions.
One way to restore relative sequence is to find a marker of a catastrophic event in the sedimentary section. Anomalies of elevated concentrations of elements of the platinum group, which are rare in the earth’s crust, but enriched in meteorites, or anomalous concentrations, such as mercury or nickel, as a marker of strong volcanic eruptions. The situation is even more complicated with the definition of the “absolute” age, with fixing when the mass extinction event occurred, leading to it a catastrophic volcanic or impact event on a time scale in years. The vast majority of geochronological methods do not allow dating events with an accuracy better than 1% of the age of the dated event (Fletcher, 2019, p. 66). Thus, the task of the international initiative EarthTime is to calibrate the main stratigraphic boundaries of the Phanerozoic with an accuracy of 0.1% of their age. To date, only two and a half methods have come closest to this accuracy:
- The method of astronomical chronology, or cyclostratigraphy, which is based on the detection of cyclic changes in one or another parameter in the sedimentary section, for example, changes in oxygen isotope ratios, and the subsequent binding of the detected cycles to the orbital parameters of the Earth’s orbit (Fletcher, 2019). To construct an absolute age scale, it is necessary to have a continuous sedimentary section from the present into the depths of time.
- U-Pb dating of single grains of zircon or baddeleyite with isotopic dilution using thermal ionization mass spectrometry. The abbreviation is important because other common U-Pb dating methods with local analysis of the substance within the dated crystal, the data of which are often found in modern literature, do not even come close to the required accuracy. Zircon and baddeleyite crystallize, respectively, in silicon-rich and relatively silicon-poor magmas, which can erupt on the Earth’s surface in the form of lava or form ejecta of pyroclastic material, including that carried over long distances in the form of ash, sometimes present in sedimentary rocks in the form of layers (Bromhm & Cardillo, 2019). When a large meteorite falls, melting of terrestrial rocks can occur with the crystallization of new zircon and baddeleyite, or with the recrystallization of these minerals already present, usually with a partial or, ideally, a complete restart of the isotope clock.
- Another method that is suitable for dating volcanic potassium-bearing minerals should be mentioned, the method using radioactive transformation. In the literature, including those related to the problem of the Cretaceous-Paleogene mass extinction, one can find dates obtained by this method with an accuracy better than 0.1% of the age (Bromhm & Cardillo, 2019, p. 13). However, the decomposition constants of the necessary minerals are known with an accuracy of no better than 1%. Additionally, in the method, the age is calculated relative to the standard – a mineral with a known age, determined by some other method.
Mass Extinction – Meteorite Impact or Volcanism
The boundary of the Cretaceous and Paleogene is marked by the disappearance from marine sections of predominantly large planktonic forms of foraminifera, unicellular animals with a calcareous skeleton. In terms of detailed stratigraphy, dinosaurs are not as informative as these single-celled animals. On Earth, there is a global clayey layer occurring between Cretaceous deposits with large foraminifers and Paleogene deposits with small ones (Fletcher, 2019). This layer contains anomalously high concentrations of iridium for terrestrial rocks. This observation allowed Luis Alvarez, a Nobel laureate in physics, his geologist son Walter and two colleagues, analytical chemists Frank Asaro and Helen Michel, to put forward a hypothesis about a large meteorite impact that led to a mass extinction at the Cretaceous-Paleogene boundary. Later, the site of the impact of this meteorite was discovered in the form of a giant, 180 km in diameter, Chicxulub crater on the Yucatan Peninsula in Mexico (Benton, 1990). It was the meteorite that was considered the main and only reason for the disappearance of dinosaurs from the continent, as well as from the state of Texas. This is also explained by the fact that the territorial location of the habitat of dinosaurs and the crash site is extremely close.
However, this hypothesis did not become absolutely true, and experts continued to study the issue. Hertha Keller, a Princeton University paleontologist and expert on foraminifera, criticized this hypothesis on the website of the Geological Society of London (Wignall, 2019). She did not dispute that the global distribution clay layer was due to the meteorite impact, but she questioned that this event led to the mass extinction, including the dinosaurs. Instead of a meteorite disaster, Hertha Keller has been arguing for years – and notably succeeding in this – that the anomalously intense volcanism of the Deccan traps was the cause of the mass extinction. This volcanism began before the fall of the Chicxulub meteorite and continued after it. Between the lava flows there are sedimentary interlayers with an iridium anomaly, although the number of deposits with an iridium anomaly in Late Cretaceous and early Paleogene sediments worldwide is more than one.
Traces of Deccan volcanism are clearly visible in sedimentary sections of the Late Cretaceous and Early Paleogene in terms of mercury anomalies. An important point in favor of the volcanic hypothesis is that before the fall of the Chicxulub meteorite on Earth, there was a sharp warming of the climate by ~2.5–10°C (Alderton & Elias, 2020, p. 83). Comparison of cyclostratigraphic dates of sedimentary rocks, which were used to estimate temperatures, showed that this warming was preceded by the main bulk phase of volcanism. It is important that this warming was several hundred thousand years before the fall of the meteorite (Fastovsky & Welshampel, 2021). Therefore, this is one of the most effective and constructive counterarguments to the meteorite impact theory or the impact hypothesis.
When narrowing the issue and concentrating on the state of Texas, you can see a number of evidence of volcanism as one of the reasons. American scientists conducted a study during which they found out that some dinosaurs could move on their forelimbs. Scientists from Purdue University and the Houston Museum of Natural Sciences explained that a few years ago in Texas, traces of large sauropods, about 110 million years old, were found that moved exclusively on their front legs (Fletcher, 2019, p. 94). They argue that the front legs of dinosaurs were much more powerful than the hind legs, so they were able to push off them when moving. At the same time, the scientists explained the absence of traces of the hind limbs by the fact that they accounted for an insignificant body weight (Fletcher, 2019). Similar prints were found by scientists in 1940, and this is the discovery of another flightless dinosaur in the state (Fastovsky & Welshampel, 2021). The experts drilled through the rock to a depth of 500 to 1,300 meters below the seabed and retrieved new samples for chemical analysis (Alderton & Elias, 2020, p. 77). When studying samples, scientists drew attention to the complete absence of sulfur. From this, scientists concluded that as a result of the catastrophe, sulfur-containing minerals evaporated and a large amount of sulfate aerosols were released into the atmosphere, which led to the death of dinosaurs.
Here it is important to analyze the detection of a large amount of sulfates in the earth. The fact is that this is a highly probable consequence of lucanic explosions, since it is precisely such a catastrophe that is characterized by a large emission of sulfur. As mentioned above, most of the dinosaurs of Texas could not fly, but moved on their limbs (Fastovsky & Welshampel, 2021). Accordingly, they simply did not have the opportunity to avoid both toxic precipitation and surface heating. This does not mean that the meteorite did not play a role, but it certainly was not the only reason.
Impact Hypothesis
In the scientific geological community, a consensus began to form that the volcanic hypothesis of the Cretaceous-Paleogene extinction, as well as other mass extinctions, which will be discussed later, is the most promising. For mass extinctions, the coincidence of a volcanic and impact event is important. However, based on 40Ar/39Ar dating, that the fall of the Chicxulub meteorite occurred just prior to the onset of bulk volcanism in the Deccan traps (Alderton & Elias, 2020, p. 107). In other words, a meteorite that fell on the opposite side of the Earth provoked the destruction of the walls of deep magma chambers under the territory of the then not yet Indian Peninsula. In turn, their association into a large focus followed, which ultimately led to volumetric eruptions on the Earth’s surface and a global environmental catastrophe. This view has been de facto criticized by a group of U-Pb geochronologists who have shown that the most voluminous volcanism of the Deccan traps preceded the meteorite impact rather than following it (Fastovsky & Welshampel, 2021). Accordingly, vulcanism continued to be the main cause of the extinction of the dinosaurs.
Nevertheless, continuing the discussion, scientists are engaged in active research on this issue. Thus, recently the impact hypothesis has received a new development, the Cretaceous-Paleogene extinction occurred after all as a result of a meteorite impact. It shows, based on simulations, that variations in boron isotopes measured in foraminifera are in better agreement with an increase in ocean acidity due to a meteorite impact than due to volcanic eruptions (Benton, 1990). A few years earlier, it was shown that at the end of the Cretaceous there were two warming events – the first, stronger, in time associated with volumetric eruptions of the Deccan traps, and the second, less noticeable, with the fall of the Chicxulub meteorite. That is why these discoveries did not make significant changes to the existing hypotheses and findings.
Similarity of Mass Deaths
One of the ways to establish the truth can be considered the analysis of all mass extinctions. David Raup and John Sepkosky identified five “Great Mass Extinctions”. The three youngest of them – namely the Cretaceous-Paleogene, Late Triassic and Permian-Triassic – correspond in age to their anomalously voluminous, so-called trap volcanism (Fastovsky & Welshampel, 2021). Accordingly, the Deccan traps in India, the Central Atlantic igneous province, spaced by plate tectonics into the territory of South America, North America, Africa and Europe, and the Siberian traps (Alderton & Elias, 2020). In addition to these three “Great Mass Extinctions”, about a dozen more are recorded, and most of them also correspond in age to some kind of trap province.
However, not every trap province corresponds to its own mass extinction, just as not every mass extinction event is accompanied by the same marking isotopic and chemical anomalies recorded in sedimentary rocks. For example, the Cretaceous-Paleogene boundary has a layer with anomalously high concentrations of iridium, while there is no such anomaly at the Permo-Triassic boundary. This suggests that either there was no impact event or the impactor was an icy comet lacking iridium (Fastovsky & Welshampel, 2021). The Permian-Triassic mass extinction event was accompanied by a sharp increase in the light carbon isotope in the atmosphere. It is usually believed that light carbon entered the atmosphere during the combustion of coal, oil, and coal shale due to the thermal effect of hot magma on them (Fastovsky & Welshampel, 2021). At the same time, combustion makes a greater contribution to the load on the atmosphere than volcanism itself. An extremely original and worthy of careful consideration explanation of the sharp injection of organic carbon into the atmosphere in the form of methane at the Permian-Triassic boundary was proposed by MIT professor Dan Rothman and colleagues. According to their hypothesis, the methanogenic archaea of the genus Methanosarcina appeared as a result of the mutation. Having no natural enemies, it began to divide uncontrollably, increasing the concentration of the greenhouse gas, methane, uncontrollably. The reason for the appearance of methanosarcina was a sharp injection into the environment of nickel from giant deposits associated with the Siberian traps and now being developed in Norilsk. That is, in this case, the root cause was volcanism.
It should be noted that the analysis of soil data gives the following results in Texas. Together with the modeling, it appears that during the global catastrophes considered as possible causes of extinction, there was a massive amount of precipitation. More than 85% of such precipitation is characteristic of volcanic eruptions and the contact of their chemical elements with soil heated air (Alderton & Elias, 2020, p. 317). Based on this, most of the flightless dinosaurs died out due to global pollution of the areal by emissions, and the rest were destroyed by a meteorite. Thus, the most likely answer to the question of the extinction of the dinosaurs in Texas will be a combination of catastrophes, namely, warming, volcanic eruptions, environmental damage and a meteorite fall.
Conclusion
Most of the mass extinction events coincided in time with anomalously large volcanic eruptions; the Cretaceous-Paleogene, during which the dinosaurs disappeared, is no exception. At the same time, only one clear coincidence in age with the fall of a meteorite has been documented, just Cretaceous-Paleogene with dinosaurs. In any case, in general, for the cause of mass extinctions, the volcanic hypothesis turns out to be preferable to the meteorite one, it explains more facts. It cannot be ruled out that the extinction of the dinosaurs was caused simultaneously by two reasons – volcanism and a meteorite impact.
References
Alderton, D. and Elias, S. (Eds.). (2020). Encyclopedia of geology. Elsevier Science.
Benton, M. J. (1990). Origin and relationships of the dinosaurs. In Weishampel, D.B., Dodson, P., and Osmolska, H. (eds.),The Dinosauria, University of California Press, Berkeley, p. 11-30.
Bromhm, L. and Cardillo, M. (2019). Origins of biodiversity. An introduction to macroevolution and macroecology. Oxford University Press.
Fastovsky, D. E. and Welshampel, D. B. (2021). Dinosaurs. A concise natural history. Cambridge University Press.
Fletcher, A. L. (2019). De-extinction and the genomics revolution. Life on demand. Springer International Publishing.
Wignall, P. B. (2019). Extinction. A very short introduction. Oxford University Press.