Differential Survival Across the K‐T Boundary – Why the Non‐Avian Dinosaur Eggs Didn’t Hatch, and the Reptile and Bird Eggs Did

Artist's impression of the Chicxulub impact

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Extinction in itself is, throughout geologic history, the norm rather than the exception.  The fact that many species perished in the aftermath of the many environmental calamities that occurred around 65 million years ago, (the Chicxulub asteroid impact and Deccan Volcanism) is hardly surprising.  The real question has always been the differential survival of species.  Dinosaurs were the dominant land animals for over 100 million years, the dominant herbivores, the dominant predators, ranging from chicken size to the largest land animals that ever lived, adapted to every environment, and living on every continent from pole to pole.  The question of  how this dominance ended is important, as it is fully possible that without this extinction, dinosaurs would still rule the earth, and mammals might still be rat sized animals rustling through the underbrush.
In the article, we argue that the global cooling period lasting from years to decades caused by the injection of SO2 from either asteriod impact or Deccan Volcanism differentially affected the hatching of dinosaur eggs as opposed to reptile and bird eggs.    We attempt to show that a global cooling of a few decades could have created a ‘dead generation’, where many reptile and bird eggs would still hatch, but dinosaur eggs would not, wiping out a 100 million years of dominance in a single generation.   We will examine data on the differing effects of cold on reptile and bird eggs today, along with dinosaur nesting behaviors and the stable Cretaceous climate.
It turns out the developmental temperature requirements for reptile and birds eggs are quite different.  Given the assumption that dinosaur eggs had similar developmental requirements to bird eggs, dinosaur eggs not incubated by body contact, and subjected to a global cooling of ~10C would result in dinosaur eggs that never hatched, while mammals, reptiles and amphibians would continue to reproduce.  Such a global cooling period is a predicted effect of both the  Chicxulub asteroid impact and Deccan Volcanism.  Avian dinosaurs (birds) that had evolved incubation by direct body contact would also have reproduced and survived.

A New Theory on Why the Dinosaurs Perished but Reptiles, Birds, Mammals and Amphibians Did Not

Probably no scientific mystery has captured the public imagination as broadly as the extinction of the dinosaurs. However, as many scientists have noted, the mystery isn’t so much why the dinosaurs disappeared, but rather why did other species survive? Why did every single one of the hundreds of species of dinosaurs, who were the dominant land animals for more than 100 million years, suddenly disappear about 65 million years ago, while many mammals, reptiles, birds, and amphibians survived?

Research conducted in the last few decades, while not answering the mystery of extinction, has shown a light on many issues around dinosaurs and their extinction. In one sense, all the dinosaurs did not disappear. Most researchers believe that one member of the Dinosaur family is still with us, in the shape of birds. We now know that two enormous events occurred near 65 million years ago that caused powerful environmental effects. One was an asteroid impact that occurred near present day Chicxulub, Mexico, where a ~10 km asteroid impacted the earth, and the other was Deccan volcanism, where much of modern‐day India was covered with lava. Both events are theorized to have caused short term global cooling by the injection of gigatons of  SO2 into the atmosphere. Other important research has hypothesized dinosaurs being endothermic, or ‘hot blooded’ animals as opposed to exothermic animals such as reptiles and amphibians; and many thousands of fossilized dinosaur eggs have been analyzed, giving us insight into dinosaur nesting behaviors.

However, the main question still eludes us. While the effects of the Chicxulub impact and Deccan volcanism decimated many animals, why did all the non‐avian dinosaurs, numerous and successful around the world from the equator to the polar regions, all die out while many previously less numerous and successful species survived? The answer we propose is that the global cooling period lasting from years to decades caused by injection of  SO2 into the atmosphere by either the Chicxulub impact or Deccan volcanism differentially affected the hatching of dinosaur eggs as opposed to reptile and bird eggs. We intend to show that a global cooling of a few decades could have created a ‘dead generation’, where many reptile and bird eggs would still hatch, but dinosaur eggs would not, wiping out a 100 million years of dominance in a single generation. We will examine data on the differing effects of cold on reptile and bird eggs today, along with dinosaur nesting behaviors and the stable Cretaceous climate.

Dinosaur Extinction

Numerous theories have been suggested for the demise of the dinosaurs, from the well accepted theories of an asteroid based cataclysm, massive volcanic eruptions or a more gradual climactic change; to the less accepted theories of disease, dinosaur stress, and egg‐eating mammals. In March of 2010, 41 scientists meeting at the University of Cambridge agreed, after a comprehensive review of the evidence, that the Chicxulub asteroid impact caused the K‐T boundary mass extinction event.  They state that the impact effects could have included “extended darkness, global cooling and acid rain”. However, many other scientists believe the primary culprit was the Deccan volcanism. Aside from the severe environmental effects of the volcanism, they note that earlier asteroid impacts on the scale of the Chicxulub impact do not seem to relate to known extinctions. [33]However, both events are believed to have injected gigatons of  SO2 into the atmosphere, where the SO2 would be converted to sulfate aerosols, causing short term global cooling periods that lasted for years to decades. [17] [18] [33] [34]

Cretaceous Weather Global Forecast: Warm, Same as Yesterday

The entire Mesozoic era was characterized by a complete lack of polar ice caps, an average temperature warmer than today, and a remarkably even temperature gradient from pole to pole. In contrast to the varied global climates of today, much of the planet enjoyed a similar climate. The poles, for example, were 50C (90F) warmer than today. In particular, the Cretaceous is often characterized as a ‘greenhouse climate’, where global temperatures were much more uniform than today, and only 25C separated average polar temperatures from those at the equator [6] [7] .

Dinosaurs (but interestingly not exothermic reptiles and amphibians) and deciduous forests were abundant at latitudes close to both North and South poles, latitudes today that are covered by either mile‐thick glaciers or perennial sea ice. While the question of how the dinosaurs adapted to months of darkness is still a controversial area of research today, summer months at high latitudes were characterized by warm temperatures, and of course 24 hour sunlight.

Global Weather Forecast 65 million BC: Pack that Winter Coat

The stable, warm, pole to pole climate that dinosaurs had enjoyed for the previous 150 million years was abruptly interrupted at some time around 65 millions years ago. The Chicxulub asteroid impact released an estimated 200 gigatons each of sulfur dioxide and water into the atmosphere, resulting in the production of sulfate aerosols that cooled the planet globally by perhaps 10C for years to decades. Pope [17] noted “global average surface temperatures probably dropped between 5° and 31°K, suggesting that global near‐freezing conditions may have been reached”. [17] [18]

However, the short, sharp, cooling caused by the asteroid impact may have been predated by eruptions of SO2 caused by the Deccan volcanism. Self [34] estimates that each of the 30 largest Deccan volcanic eruptions injected up to 150 gigatons of SO2, perhaps in such close succession that multiple years‐long cooling periods would overlap. [33]

Not All Eggs Are the Same

Today most bird species incubate their eggs, keeping them warm by direct body contact. This allows the eggs to reach the internal temperature required for the embryo to develop. While some species leave the nest for several hours at a time to forage for food, they return to incubate the eggs daily, and particularly to keep them warm during the nighttime hours.The only exceptions to this are birds that have found strategies that allow their eggs to incubate in warm, stable environments suitable for the narrow range of temperature required for bird egg incubation without brooding them. The brush turkey and other birds of the family Megapodiidae build huge mound nests made of earth and organic debris. The eggs are hatched by the heat of the composting mound which is tended only by the males who regulate the temperature carefully such that the eggs stay within a 33‐35°C incubation window. [19]

How does temperature affect hatching success in bird eggs? Joint research was conducted by the University of California and US Department of Agriculture [4] on the viability of avian eggs removed from their nests for periods of time prior to return and proper incubation. It was found that the minimum temperature the eggs experienced had a strong affect on their hatch rate. The hatch rate on eggs not incubated for 7 days was just 2.1%, and the temperatures never reached below 15C (59F) on any day, and much higher on most days. In any case, the developmental zero for avian eggs, being the temperature at which no development of the embryo can take place, is generally considered to be 24C‐26C (75F‐79F) [5].

Not all eggs are the same. Exothermic (cold blooded) reptile eggs are much more resilient to temperature changes than bird eggs. Eggs of many reptile species can tolerate temperatures well below the developmental zero, indeed, some can withstand temperatures close to freezing [10]. Experiments with the lizard Bassiana duperreyi that involved moving eggs to cooler regimes where overnight temperatures dropped to 0C resulted in “only a relatively short delay in hatching”. [11]. Eggs of the turtle chelydra serpentina can survive a couple of weeks at 10C, and go on to develop normally. [22]

Not only can reptile eggs survive low minimum temperatures, but their developmental zero temperature is much lower than that exhibited by bird eggs. Developmental zero temperatures for the lizards B. duperreyi and T. septentrionalis are estimated to be 14.2C (57F), and 16.0C for the turtle P. sinensis. [12]

The temperature requirements for incubation of eggs, which relate to the underlying reproductive and physiological processes of the animal (e.g.; endothermic or exothermic) seem to be little changed by evolution. Although birds have radiated over many millions of years to environments that vary from the steamiest jungles to the Antarctic, the requirements for egg incubation have apparently changed only little. Drent notes that for 25 varied bird species measured, incubation temperatures varied from 34C‐38C (93F‐100F), with most species incubating between 35C‐36C. The coldest known incubation for any bird is for the emperor penguin, which incubates its eggs at 31C (89F) in environments that are ‐60F. [20] [21]

In addition, while avian eggs must begin incubation within several days of laying, reptiles may delay egg development after laying by several months. Díaz‐Paniagua subjected chameleon eggs to initial low temperatures of 14C for up to 149 days, and still observed a hatching success of 96.4%. In an experiment that could replicate the effect of a 5C drop in average temperature following the Chicxulub impact, Qualls and Andrews subjected the eggs of the mountain lizard Sceloporus Virgatus to a 15C to 20C diurnal nesting temperatures, instead of its normal 20C to 25C diurnal nesting temperatures. The result was the hatch rate reduced from 94% to 82%, and the incubation period increased, but many viable lizards were produced. [23] [24]

Dinosaurs Nesting Behavior

Most dinosaurs could not have incubated their eggs by direct body contact. Certainly the idea of huge Cretaceous dinosaurs such as Tyrannosaurus Rex or Titanosaurus sitting lightly on a clutch of eggs is only fit for Gary Larson cartoons. It has been suggested that small dinosaurs such as Oviraptor may have incubated eggs, as several skeletons have been found associated with clutches of eggs. However, given the positions of the egg clutches, it is unlikely Oviraptor was brooding eggs similar to birds, as few of the eggs could have been in the direct contact with the body. It is more likely that Oviraptor was guarding the eggs from marauding predators. [25]

While many scientists have theorized that dinosaur eggs were laid in the open, in small depressions or ditches, this would have left them open to predation unless guarded constantly by a parent. Deeming has analyzed pore areas on dinosaur eggshells, and concluded that the large pore area must reflect an environment of very high humidity, and suggests that dinosaurs buried their eggs in mounds of earth or organic material. [26]

Why the Birds, Reptiles, Amphibians and Mammals Survived, but the Non‐Avian Dinosaurs Did Not

A global cooling of ~10C for a few decades caused by either asteroid impact or Deccan volcanism, as is well within present predictions, would have a highly differential effect on survival between mammals, reptiles, amphibians and dinosaurs if we make the following assumptions: 1) late Cretaceous birds had developed incubation of eggs by direct body contact, and 2) non‐avian dinosaurs had egg developmental temperature requirements similar to avian dinosaurs. Dinosaurs had 150 million years of unchanging, steady, and remarkably even temperatures across the globe. Non‐avian dinosaur eggs with evolved incubation temperature requirements similar to present birds would have experienced regular hatching success, even at very high latitudes, so long as eggs were laid in the warm season. Whether laid in the open, or in nesting mounds, the long summer days of 30C and higher temperatures would assure a reasonable hatch rate.

A sudden drop in temperature from 30C to 20C average temperatures, while not fatal to the dinosaurs themselves, would have been catastrophic to their eggs. For season after season, dinosaurs would have guarded cold clutches of eggs, never hatching, to finally see them decay or be eaten by scavengers. The eggs either having been killed by minimum temperatures, or never having reached developmental incubation temperatures for sufficient time. If the global cooling period was short enough, perhaps a few aging mothers finally, after years, saw their progeny emerge from their eggs, only for the youngsters to be unable to find mates in the land decimated by effects of asteroid impact or extreme volcanism. If the cooling period was long enough, the dinosaurs that were young on the day of the impact would fight, grow old, and finally fall without ever having seen a new hatchling emerge. Dinosaurs, the undisputed masters of the earth for 100 million years, were extinguished by a few decades of cold weather.

With the exception of the birds, of course; why did birds develop incubation by direct body contact? Perhaps brooding evolved because the relatively small size of their egg would subject the egg to more degradation from thermal changes than their larger cousins, and body heat incubation would help insure a good hatch rate. Perhaps avoiding predators by nesting in trees resulted in nests that forced the bird, if it were to protect the eggs, to literally sit on the eggs. Bird nests made of grass or other organic debris would have provided insulation from the surrounding cold earth, making incubation by body contact all that more effective. Dinosaur eggs partially buried in earth substrate would have perished, even if partially incubated by body contact, as the surrounding soil temperature plummeted. Dinosaur eggs buried in mounds of earth or organic material would also perish as the chill from the surrounding environment cooled the mounds.

The same catastrophe would not occur for turtles, crocodiles, lizards, snakes and other reptiles following the asteroid impact. So long as the cooling was not very severe (> 20C), the sudden cooling would have probably reduced hatch rates, and increased time to hatch, and perhaps reduced the viability of the hatched offspring, but hatched offspring would occur. In addition, the lower metabolic requirements for exothermic reptiles would have been a key survival trait in the post‐impact desolation. Some present day snakes have been known to survive a year without eating.

In a dark and cold world, the reproductive strategy of mammals would be a distinct advantage, with progeny being incubated internally, and then nursed next to the warm mother’s body. The global cooling would have no effect in itself on reproduction. The remaining egg laying mammals, the Monotremes, are a case in point. Of the two remaining species, the echidnas possess a separate pouch where they incubate their eggs and nurse their young, while the egg of the platypus spends about 28 days in the uterus, and only 10 days in warm external incubation. The global cooling could perhaps also help explain why many more placental mammal species survived the K‐T boundary extinction than marsupial mammal species. Marsupials at birth are tiny, wet, hairless things that must make the long journey up their mother’s body to the pouch, a journey that takes 3 to 5 minutes for a kangaroo joey. For a newborn Virginia Opposum, which weighs only 1/10th of a gram, so small that dozens can fit in a teaspoon, perhaps the journey in a cold environment was too much for some.

Other Theories Exploring Differential Survival across the KT Boundary

The list of theories attempting to explain the extinction of the dinosaurs is long and often silly. Many catastrophes that might befall the dinosaurs have been dreamed of, but most fail to answer the more important question, why did some survive? Alvarez [27] writes, ‘‘Many smaller land animals survived, including mammals, as well as reptiles such as crocodiles and turtles. No one really understands why these animals escaped extinction.’’ While Powell [28] notes, ‘‘No one has yet been able to explain under any theory why the crocodiles and turtles survived and the dinosaurs did not”.

One serious attempt to answer this question was made by Robertson, who argues that ballistically reentering ejecta from the Chicxulub impact caused an intense infrared radiation pulse in the sky ‘‘of the order of 10 kW / square meter over periods ranging from one to several hours after the impact. These power levels are comparable to those obtained in a domesticoven set at broil”. Robertson further argues that this IR pulse might kill any unsheltered animal, and given the potential sheltering characteristics of small mammals and potentially turtles and other small creatures, the differential character of the extinctions might be explained. [29]

Since Robertson’s paper, the strength of this IR pulse has been questioned [30], and evidence of burrowing dinosaurs has appeared [31]. In addition, the IR pulse, while clearly a potentially devastating global event, may not adequately explain the differential survival of small dinosaurs over reptiles, or even larger dinosaurs which may have sheltered from the IR pulse in deep forest environments. Robertson theorizes on bird survival that birds sheltering in dense marsh vegetation survived. Certainly if animals survived in dense marsh vegetation, they would be expected to survive sheltered in deep forest environments, if the forests did not burn, which seems likely given recent data. [32]

Conclusion – And a Warning

Extinction in itself is, throughout geologic history, the norm rather than the exception. The fact that many species perished in the aftermath of the many environmental calamities that occurred around 65 million years ago is hardly surprising. The real question has always been the differential survival of species. Dinosaurs were the dominant land animals for over 100 million years, the dominant herbivores, the dominant predators, ranging from chicken size to the largest land animals that ever lived, adapted to every environment, and living on every continent from pole to pole. The question of how this dominance ended is important, as it is fully possible that without this extinction, dinosaurs would still rule the earth, and mammals might still be rat sized animals rustling through the underbrush.

This theory, if valid, is important for another reason. Today we face the potential of rapid climate change, with unknown and potentially unexpected consequences. A sudden climate change 65 million years ago may have exposed the dinosaurs Achilles’ heel: the need for a warm summer season to reproduce. Man today is the dominant land animal, numerous, on every continent, and adapted to every environment. Every species, even the strongest, eventually suffers extinction. Do we, as a species, have an Achilles’ heel, and if so, will we recognize it in time?

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