5. The Evolution of Metazoa

5.5 The Great Dying

Several mass extinctions have been recorded in geologic times, of which 5 were of momentous proportions. Two of these extinctions, i.e. the late Devonian 364 million years ago and the end Triassic of 200 million years ago lost the diversity of genera more through a failure to produce new genera than through extinction. In these two cases, it is currently assumed that normal extinction was high and there was a lack of new genera to replace losses. They are not really mass extinctions but mass depletions.

The causes of the extinctions appear to be multiple. One possible cause, applying essentially to events that occurred early in the history of the planet, is bursts of gamma rays. Gamma-ray bursts occur everywhere in the universe with extremely high frequency, i.e. a burst is observed almost every day. These bursts last usually only a second or two, while some may last as much as 30 seconds. In mere seconds, each burst releases some 1051 ergs of energy, which is as much as the sun would generate if it continues shining as it does today for ten billion (1010) years. The realization that such immense explosions take place routinely throughout the universe has made paleontologists wonder whether past gamma ray bursts might, in former times when the expansion of the universe was not as pronounced as today, have caused several early mass extinctions.

Three of the ultimate five extinctions, the Cretaceous-Tertiary 65 million years ago, the Triassic-Jurassic 200 million years ago and the Permian-Triassic 251 million years ago, are linked with large volcanic eruptions that discharged millions of cubic kilometers of dark basalt lava. The whole of the Deccan plateau, that makes about a third of the surface of India and that is 2500 meters in thickness, erupted in a short interval of less than a million years about 65.5 million years ago.

The deep-sea extinction that was a turning point in mammal evolution 55 million years ago at the Paleocene-Eocene boundary coincided with massive lavas laid down when Greenland and Europe parted. These events were often associated with the release of huge amounts of methane into the atmosphere. The warming of the magma in a short period of a million years destabilized methane hydrates trapped below the sea floor as icy methane hydrates. Methane bursts occurred 55 million years ago, 90 million years ago, 120 million years ago and 183 million years ago. The methane is a powerful greenhouse gas that further heats up the earth while it oxidizes to another greenhouse gas, carbon dioxide, that brings about suffocating oceanic anoxia that leads to mass oceanic extinctions.

About 75 percent of the species living in terminal Mesozoic times vanished at the beginning of the Cenozoic era11. The extinction interval spanned about 2.5 x 105 years (250 thousand years, i.e. an extremely short time). Mesozoic vertebrate faunas prior to the Great Dying included birds, small mammals no greater than cats, and reptiles of all sizes, some of which weighed many tons. After the extinction, only the birds, the mammals and reptiles of less than 25 kilograms survived. The dinosaurs were not the only ones to disappear. In addition to the land and marine reptilian extinction, which were of global extent, many genera of foraminifers (a eukaryotic protozoan ), echinoids (sea urchins, sea cucumbers), clams and marine algae underwent catastrophic reduction in diversity and abundance at the same time.

At the terminal Mesozoic era, floras acclimatized to warmth were widespread; the width of the Cretaceous equatorial belt was about 80° in latitude, from Washington to Buenos Aires, from Madrid to Cape Town, from Peking to Melbourne. The decline of the dinosaurians was accompanied by the spectacular efflorescence of angiospermic plants, i.e. all flowering plants, grasses, vegetables and most trees, with the exception of conifers (fig. 5.23).

Figure 5.23. Angiosperms. The angiosperms, i.e. flowering plants, common trees, vegetables and grasses, rose spectacularly at the end of the Jurassic, about 135 million years ago. The lower vascular plants are club mosses, quillworts and horsetails. Gymnosperms are the evergreens.

These flowering plants superseded the spruce-fir-pine trees and other gymnosperms and ferns, and could have contributed to the decline (see section 5.4.5). The destruction of the flowering plants at the Cretaceous-Tertiary boundary precipitated a plant-insect diversity bottleneck that span the first 10 million years of the Tertiary. Most of the insects that were specialized for feeding on only one kind of plant became extinct at the boundary.

5.5.1 Seasonal contrasts

The meteoric shower that hit the earth 215 million years ago tilted its north-south axis. If the impacts had immediate dramatic consequences, the effects of the tilt per se were delayed during about 150 million years and could not be felt as long as the earth was warm. During the 100 million years that saw the dominance of dinosaurians, the near totality of all continents that was assembled into one super-continent called Pangea, broke up (fig 5.24). At -100 million years, South America, Africa and India were still close to each other but some land area was covered with water. Sea levels continued to rise later and continents to drift apart so that, by 80 to 75 million years ago, the land area covered by shallow seas was very extensive. Sedimentation led to continuous leveling of these epicontinental seafloors. They, therefore, had depths of only 100 to 200 meters. The large heat capacity of water compared to continental rocks and the atmosphere, the covering of more than 40% of the continents with water 80 million years ago, the provision of water corridors along which heat could be transferred, assured the uniformity and mildness of the Cretaceous climate. Crocodile-like dinosaurian fossils have been found in the Cretaceous deposits of the polar Arctic Circle. The fossils, Champosaurs, are cold-blooded, as are crocodiles, and indicate that, 92 to 86 million years ago, during the late Cretaceous, the Arctic had a tropical climate with a mean temperature above 14°C.

Figure 5.24. Part a illustrates the position of some continents and the extent of the epicontinental seas 100 million years ago. Part b shows that epicontinental seas, which covered large parts of North America and Europe, even further accentuated the fragmentation of continents 80 million years ago.

When the seas began to withdraw, 60 million years ago, the uncovering of land proceeded initially extremely fast because the seas were shallow. The emergence of continents had the consequence that oceanic thermal advection by water was reduced. A rapid cooling of the earth took place and seasonal contrasts appeared. This produced stresses on species adapted to stable Upper Cretaceous conditions.

The dinosaurs were large tropical animals that maintained a constant body temperature without the help of a dense superficial insulation because they presented a low surface area in comparison with their volume. They were hairless and thus obliged to keep such a large body size. This is also the case now with tropical homeotherms such as elephants, hippopotamuses, rhinoceroses, pigs and armadillos, that are all hairless. Their size allows them to resist without great difficulty short exposures to cool temperatures. When the size of the homeotherms is small, there is a greater tendency to lose heat, and insulation is required since even short cold periods would be an intolerable strain on the organism’s heat production.

5.5.2 Endothermy and cool temperatures

One explanation that comes to mind for the Great Dying is the cooling of the earth. In the face of the sudden drop in worldwide temperatures that occurred at the Cretaceous-Tertiary boundary, the big tropical quasi-homeothermic but naked dinosaurs, too big to find adequate refuge and shelter on a fragmented earth surface could have been under severe stress. They could have disappeared at that time. Such an explanation was very popular for a long time and might be satisfactory if it were not that, firstly, several other animal species were also made extinct at the same time and, secondly, that the explanation is per se invalid for Dinosaurs. Endothermy involves maintenance of body temperatures that are higher than average environmental temperatures. All types of vertebrates, be they endothermal like mammals, birds and dinosaurians, i.e. able to maintain a constant body temperature in a cool environment, or else ectothermals like present-day reptiles, amphibians and fishes, unable to maintain a constant body temperature in a cool environment, are programmed to live under cool conditions. Many mammals and birds function routinely at temperatures far lower than their body temperatures (sea lions, seals, polar bears, mammoths, Siberian tigers, caribou’s, wolves, rabbits, wolverines, penguins and all the mammals and birds living in temperate regions). Endothermy is based on heat production and conservation and is not based on cooling and heat loss. External heat in tropical and equatorial regions is difficult to combat (sweating, panting, seeking shade, nocturnal habits). This is true also for poikilotherms: desert reptiles are not known to be heat resistant; discomfort and damage can result from thermal exposures of only 3°C above the optimum. A few degrees above the optimum are fatal for alligators. If modest temperature elevations are damaging for reptiles, they tolerate temperatures far below their optimum without suffering damage. Contrary to homeotherms, they cannot maintain normal life processes at abnormally low temperatures but they survive it if the stress is not too prolonged.

A large size benefits organisms that adapt to cooling conditions, as was the case of the dinosaurians living at the end of the Mesozoic era, because body heat loss is retarded and a more stable level of body temperature can be maintained.

5.5.3 The causes of the Green House effect

The end of the Cretaceous was characterized by the regressing to the ocean basins of the shallow seas that had flooded the continents earlier in the Mesozoic era and by the termination of a 20 million-year cooling trend of the climate. Several factors have been discovered to work in synergy to aid this termination. An iridium-bearing meteorite fell 65 million years ago on Yucatan and a second hit the sea on the North shore of Australia. The North American meteorite was around 10 km in diameter and was traveling at 90,000 km/hour. Yucatan was then a shallow, tropical sea. The meteorite did not descend perpendicularly but it was in a trajectory from the southeast, which released as much energy as 100 million megatons of high explosives. The Chicxulub impact event resulted in the loss of 12% of the ejecta into space, the rest falling to the earth’s surface in daily pulses over three days. The highest concentrations occurred near Chicxulub and its antipode, roughly where India was situated at the end of the Cretaceous. The repeated intense heating of the atmosphere in these regions ignited wildfires, ensuring destruction of the fauna and flora. It resulted in an ecological devastation, with giant tsunamis sweeping the Gulf Coast clean of life. North America’s forests were flattened and the continent, for centuries after the impact, resembled a muddy field devoid of life. For thousands of years, the continent was little more than a field of ferns. Without being as complete as in North America, the ecological devastation extended to Antarctica and Australia and it took several million years for New Zealand’s flowering plants to regain the prominence they enjoyed over ferns before impact. This was concomitant with the passage of the earth during 2.5 x 105 years through an interstellar cloud that contained iridium, which dampened in addition the blue radiations of the sun. These events probably triggered the green house effect. In addition, a periodic inversion of the terrestrial magnetic field failed to occur at that time. Such an absence of inversion has also been observed for the Permian-Trias boundary, 245 million years ago. In both cases, massive extinctions occurred, which are explained by the intense volcanic activity, followed by acid rains, that such a lack of inversion induces through temperature upward shifts. One thing is certain: the dinosaurs were doing fine until after the meteorites crashed to earth 65 million years ago.

5.5.4 CO2 and temperature surges

At the close of the Mesozoic era, the coccolithophorid algae that had been vastly abundant in the Mesozoic seas underwent catastrophic reductions in abundance and also in the number of taxa. The cause of this overturn of marine coccolithophorid algae (the chrysophits) is probably due to the iridium bearing meteorites and the passage of the earth through the iridium-laden cloud. By suppressing the blue radiations, it interfered with the synthesis of vitamin D of dinosaurs as well as photosynthesis by chrysophits. The consequences of the reduction in numbers of the coccolithophorids following the hit were of an extraordinary magnitude.

In the late Mesozoic era, declining world temperatures would have allowed the oceans to store increasing quantities of CO2. Marine phytoplankton alone consumes about 25 billion tons of carbon in the form of CO2 annually. Sharp reduction of the vast coccolithophorid floras in the late Mesozoic era would have reduced total consumption. As a result, CO2 that had previously been utilized by the coccolithophorids would have accumulated in the atmosphere. Increasing quantities of CO2 in the atmosphere cause warming of the lower atmosphere and of the surface of the earth: a doubling of CO2 results in an increase of at least 2°C. Such an increase warms the oceans, whereby the CO2 solubility decreases and is driven out from the oceans into the atmosphere. Whilst the shallow seas regressed to the ocean basins, a climatic reversal from cooling to warming took place. A rapid extensive vertical mixing of the ocean layers may then have overturned the deep, cool and CO2 laden waters, liberating at once vast quantities of CO2 into the atmosphere. A consequential chain reaction of the type here described of alternating CO2 expulsion and enhanced warming may have resulted in a late Mesozoic "greenhouse" effect. The declining temperatures of about 5°C over the final 20 million years of the Mesozoic era was thereby not only halted but also reversed and overreached in a short period of between 100,000 years and a million years.

Both the CO2 increase and the temperature increase affected the survival potential of the flora and fauna.

5.5.4.1 Raising temperature and body size

Whereas a large size offers advantages to organisms that are adapting to cooling conditions, it would be disadvantageous during an abrupt climatic reversal from cooling to abnormally rising temperatures. Small body size, which allows relatively rapid loss of body heat, plus the ability to escape readily direct sunlight by physical means, would offer the greater selection advantages in this case. The animals affected would have been those heavier than 25 kilograms, regardless of whether they were endothermal or ectothermal. Marine reptiles would have been also adversely affected because the epeiric seas were retiring to the ocean basin, which reduced the number of suitable biotopes available. During the warming of the oceanic waters, they would not have been able to utilize evaporative techniques to reduce body temperatures, or to easily physically escape elevated temperatures by assuming nocturnal habits or by seeking shade.

5.5.4.2 Effect of raising temperature and CO2 concentration on egg hardness

A second effect of high temperature plus CO2 is noticed at the level of the egg hardness. Dinosaurs laid hard, calcareous eggs like the modern birds. Chickens exposed to elevated temperatures and the lowering of the pH through CO2 lay thin-shelled eggs. Dinosaur eggshells were relatively thick (i.e. 2.5 mm) in the beginning. At the close of the Mesozoic era, the eggs were thinner-shelled and fragile. Contrary to the eggs of birds, and similarly to the eggs of modern reptiles, the eggs of the dinosaurs were extremely permeable to CO2 and O2. A relative increase of CO2 in the air would have stopped air breathing and the embryo would have suffocated. Some of them have been found whole; they had not hatched.

5.5.4.3 Effect of raising temperature on germinal cells

It is known that, in the human species, the failure of the testicles to sink into the scrotum during development results in eunuchoids: sperm secretion is halted and secondary male sexual characters are absent. This failure of the gonads to function is simply due to the normal 37°C body temperature they are subject to during their location in the body cavity, because a surgical intervention that relocates them in the scrotum, where the temperature is 3°C to 4°C lower, suffices to initiate their normal functioning12. The disfunctioning of the germinal cells, the lack of secretion of sperm ultimately causes sterility at temperatures far below those that begin to cause physical discomfort to an organism.

Temperatures above normal for the testes during active spermatogenesis may be sufficient to play a significant role in evolution. A prolonged benign temperature increase throughout the breeding season, if extended over the life span of many generations, could cause extinctions. During the greenhouse effect at the end of the Mesozoic era, all animals with low thermal sensitivity thresholds to the male germinal cells, regardless of whether they were endothermal or ectothermal, regardless of whether they were big, small or minuscule, would have been eliminated. Those who managed a heat-dissipating system efficient enough to protect the male germinal cells from thermal damage were the survivors. In this respect, the sinking of the male gonads into the scrotum of mammals was an important evolutive achievement. In elephants and birds, sperm still forms at body temperature and, in many rodents, the male gonads are located in the body cavity except during the mating season, when they descend back into the scrotal sac. The development of teguments into insulating hair, feathers and fur was another sizeable advantage for protection against heat. The abandonment of a reproductive system based on the initial development of an egg enclosed within a shell, to the feeding of the embryo directly through a placenta and later milk, was another sizable advantage that could be exploited by animal species submitted to the greenhouse stress occurring at the end of the Mesozoic era.

5.5.4.4 Temperature and sex determination

Sex is, for the great majority of animal species, determined genetically. However, the majority of reptile species have no determinable sexual chromosomes. For these species, the ambient temperature controls sex determination, during the period of incubation of the eggs. This was found for the alligator: an incubation temperature inferior to 30°C produces females and a temperature superior to 34°C produces only males. The sex of the alligator offspring is thus determined by the height of the nesting burrow above the water level. If the determination of the sex of the offspring of dinosaurs were likewise dependent on temperature, then a rapid warming or cooling of the climate might have been in part the cause of their decline.

5.5.4.5 Food poisoning

The floras living in the warm environment of the Cretaceous would not have responded as drastically to a short-term warming of only a few degrees as would have temperate floras. On the contrary, short-term increases of temperature and of atmospheric CO2 enhance plant growth. These angiosperms secrete toxic substances that may have produced a mass poisoning of the huge remaining reptilian herbivores. That some of them might have survived until prehistoric times is improbable but by no means impossible. The fight of man against them would have been collectively recorded in tales of heroes and saints slaying dragons. The Cretans, relating 3,000 years BC the economic ties they had with Libya, depicted a griffin in a relief sculpture. This was very probably a triceratops whose remains are abundant in the Asian desert of Gobi, but could be also a still living species. That such an animal was not better known is also not inconceivable, since the okapi and the dragon of Komodo were not known less than 100 years ago and the whole of Australia and its fauna was not known 400 years ago.

5.5.5 Resume

The direct hit of at least two iridium-loaded meteorites, together with the passage of the earth through an interstellar cloud, with the absence of the expected reversion of the terrestrial magnetic field that should have occurred at that time, provoking an intense volcanic activity, stopped the proliferation of some calcium-needing algae. Their disappearance disrupted a whole nutrition chain and, in addition, subsequently provoked an abrupt heating up of the terrestrial atmosphere that led to a greenhouse effect. This greenhouse effect lasted long enough to wipe out those living forms that were initially not directly dependent on the algae for their survival. All in all, 75% of all animal species were wiped out in less than a million years.

References

11. For extinctions large and small, the recovery time is roughly the same: 10 million years.

12. The appearance of a eunuchoid behavior also threatens those human adult males who constantly and perpetually wear tight pants that impede the correct ventilation of their genitals.

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