Suggestions already made cover a huge range, from simplistic to erudite, from commonsense to comedy. The dinosaurs died out because the Earth got too hot, or too cold, or too much radiation (frizzling them up) or too little (not enough mutations occurring to let them adapt). The climate became too wet, or too dry. Their food was eaten by the newly-evolved caterpillars of butterflies and moths, or their eggs were eaten by the cunning small mammals. The Earth was bombarded by meteorites or passed through the tail of a comet, with many dire effects. The list goes on and on -- parasites, diseases, slipped discs, shrinking brains, overspecialization, racial old age, sunspots -- even boredom!
One of the more recent theories is based on the discovery that a fine layer of material rich in the rare metal iridium is found close to the Cretaceous-Paleocene boundary in many locations scattered about the world. A layer of finely-divided carbon is also found at the same boundary. It is accepted that the creatures referred to as dinosaurs also disappeared at this time, which marks the change from the second (Mesozoic) to the third and current (Cenozoic) great period of life, some 70 my ago.
John Cramer [21] supports the suggestion that a huge chondritic meteor collided with the Earth at this time, that this meteor was rich in iridium which was scattered throughout the atmosphere in a very dense dust-storm, bringing on a sort of 'nuclear winter' which was connected with the extinction of the dinosaurs. Cramer also adds the suggestion that the carbon layer is soot from an immense world-wide fire which was promoted by the high oxygen levels he hypothesizes to exist in the Cretaceous.
No theory put forward to date has received anything approaching general acceptance. I will put forward three more. The first one is really only an extension of an existing theory, while the second and third may be new.
The No-Disappearance Theory
The first theory is that the dinosaurs didn't die out. This is not a novel suggestion, but is one which has gained increasing support in recent years. The old idea of dinosaurs as just big lizards, typical reptiles, is falling into disrepute in the light of new discoveries and research.The first nail in the coffin of the older theories was when it was realised that many of the dinosaurs were 'warm-blooded' (that is, maintained a relatively constant body temperature), like modern mammals [60]. They had to be warm-blooded to maintain levels of activity which were clearly much above those thermodynamically possible for a 'cold blooded' reptile. A clear example is of the flying dinosaurs, such as the pterodactyls. No modern reptile can fly, presumably because this mode of travel is not possible with sluggish reptile metabolism -- but see the comment which appears towards the end of NU011.
In addition, it is believed that at least some of the pterodactyls were covered in fur, again a feature not found in any known modern reptile. The icthyosaurs, huge marine dinosaurs, apparently bore live young-- they were not egg-layers [77]. In fact, on close examination it gets harder and harder to find features which clearly distinguish the dinosaurs, or at least some of the later ones, from modern warm-blooded animals -- birds and mammals.
One suggestion, in fact, has been that modern birds are the current dinosaurs, so that dinosaurs are not extinct at all, only their older forms are gone. The situation gets even more interesting when one looks at the most primitive mammals, the monotremes of Australia. There are only two animals in this ancient group, the Echidnas and the Duckbilled Platypus.
This platypus has a combination of features so bizarre as to make it understandable that the first specimens brought to Europe were widely assumed to be hoaxes, stitched together from different animals. The duck bill and webbed feet, coupled with fur, were very striking at the first encounter. Then it was found that the platypus lays eggs, and has a single passage for both excretion and copulation, just like birds. And, although a mammal, it does not have teats for the milk, this just oozes out through a network of pores.
Later came the discovery that males have poison glands, unlike all other mammals and birds. Recently it has been demonstrated that the platypus has an electrical detection system, like that of some fishes. But a more subtle and very recent discovery concerns body temperatures of the platypus and the other primitive monotreme, the echidna. These creatures do not maintain typical mammalian constancy of body temperature, instead they vary dramatically by some 10°C. A variation of this size could be enough to kill one of the higher mammals.
What it comes down to, is that there are no obvious fundamental differences between some dinosaurs, some ancient mammals like the platypus, and some modern birds like penguins. The inference is that there is no difference; the dinosaurs were just early forms of modern mammals and birds. The only point that remains, and is undisputed, is that all the big ones disappeared towards the end of the Mesozoic.
Proposition 12A
Dinosaurs as a class are not extinct, they were only early
forms of modern birds and mammals. Mass extinction was
limited to larger forms of these classes
The Great Extinction -- Which One?
If Proposition 12A is true, this still leaves the matter of explaining why all the bigger animals became extinct towards the end of the Mesozoic. It appears that all animals with a mass of over around 40 kg were affected. Plants were not involved in any mass changes, although of course they continued to evolve. The large marine animals, such as the ichthyosaurs, did disappear -- modern marine giants like the whales are believed to have developed from land-based ancestors during the Cenozoic.To put some background in place for another proposition, we should look at another great extinction, one which is taking place today. This is one directly due to the activities of man.
Dramatic extinctions of species by man, such as that of the dodo, the large flightless bird of Mauritius, are a well-known cause of public concern. But behind these dramatic cases stands a huge, all-pervading influence on all species of life on Earth which stretches back well beyond modern man, well beyond civilization, to the beginnings of man.
This realization is comparatively recent, but evidence and suggestions supporting this trend are coming in thick and fast. Huge changes wrought directly and indirectly by man have affected this planet to an extent far exceeding those due to effects such as major climate shifts. In fact it appears that the influence of man on the isocons, those envelopes which define ecological niches, is now greater than that of any 'natural' factors.
Historical changes such as the conversion of the middle eastern 'Fertile Crescent' of the Bible into desert, and the degradation of the great grain fields of Carthage in North Africa into useless arid lands are well documented. But there is far more.
Fig. 12.1. Records of past distribution of the
Elephant in North Africa
Figure 12.1 (from [3]) shows recorded evidence of the past distribution of the elephant in North Africa. Clearly the elephant was once native over the whole of this huge area, right up to the shores of the Mediterranean. There is no way that the elephant could survive in large numbers under present Saharan conditions, and the inescapable conclusion is that these conditions have changes dramatically since the time when the elephant ranged freely over this huge area.
Was it way back in the distant past when these conditions were found? No, it was only yesterday, in the scale we are used to. Almost all the elephant records referred to are less than 10,000 years old, and some are as young as 4000 years, within the time of known civilizations and cities.
Around the Earth, extinctions of large animals of every sort have taken place under circumstances which show an increasing correlation with the development of mankind. We can reckon that man, as an evolved and intelligent thinker, has been active on Earth for around 100,000 years, with the emergence of what we can regard as the beginnings of civilizations going back more than 12,000 years. It is within these spans that the use of fire has been harnessed, and far-reaching changes have overtaken the Earth.
In North America, this period has seen the disappearance of a host of large animals. These included mastodons (types of mammoth), giant ground sloths, camels, giant armadillos, sabretooths, llamas, and glyptodons, all of genera now extinct [3]. Some genera became extinct in North America but survived elsewhere, such as the horse and the yak (still found in Asia) and the capybara and speckled bear (still alive in South America).
Europe saw the disappearance of the mammoth and the elephant, the hippopotamus, the rhinoceros, and many large species of horse, bear, ox, and deer. South America lost many large animals, including the giant ground sloth, which stood up to 6 m tall. Africa apparently suffered least from these extinctions, and so has the most survivors -- including the gorilla and the chimpanzee.
Australia was perhaps the heaviest sufferer of all [78]. It once had several species of giant kangaroos, a wombat relative as big as a rhinoceros, and a massive creature, the procoptodon, which stood up to 2.6m tall. It has been estimated that within the last 100,000 years, Africa lost 5% of its large mammals, Europe 30%, North America 73%, South America 80%, and Australia 94%. The classification of 'large' means creatures weighing over about 40kg.
Not all these extinctions took place at the same time. However, in North and South America, many were quite tightly clustered around 11,000 years ago. In Europe, the time was the same, but the extinctions were more spread out. And in Australia, the main peak was much earlier, around 30,000 years ago [77]. In all these cases, man is becoming more and more implicated in the extinctions.
Proposition 12B
Extinctions of creatures weighing over about 40kg in the last
100,000 years were mostly due to the activities of man
The earlier age of the Australian extinctions has caused some concerns in tying in with the accepted time of settlement of the country by aboriginals. Until recently, the oldest human relics known, dated to about 40,000 years ago, were a set of 900 stone artefacts found on the banks of the Upper Swan near Perth. However, much earlier remains have now been found near Lake Eyre [51] which push this date back to as much as 80,000 years ago. Australia has never been subjected to intense archeological scrutiny, and there may be ample evidence awaiting discovery which would indicate a much more active role for man in the country's prehistory than has previously been supposed.
The Relentless Invasion
No doubt many of the extinctions were directly caused by man, as a result of hunting. This is particularly true for larger animals, which are attractive objects for hunters in that a big haul is obtained from a single kill. Twentieth-century man is not usually regarded as a hunter any more, but he still is. In the last few hundred years alone, advances in technology have let him invade a whole new realm, and bring many species of whale to the verge of extinction.The direct actions of hunting large animals have produced major changes. But many much more significant effects have occurred through indirect actions.
The clearing of forested land for agriculture is an activity for which public acceptance has completely somersaulted, and this in as short a period as the last three decades. It is now accepted more and more widely that the loss of tree cover to create cropping and grazing lands is the root cause of a range of social and economic ills, directly including soil erosion by water and wind, salt build-up, and declining soil fertility. These problems lead indirectly to phenomena such as floods and famines which, while they can be combatted with modern science, are so much more expensive than the natural ecosystems which have been replaced as to be economically unthinkable in less developed economies.
The question of man's responsibility for the environment is regarded, quite justly, as one of the leading philosophical and ethical issues of the day. But, as I have suggested earlier [57], it is much more than that, it is an economic topic too.
The ratio of food-raising efficiencies between a well-integrated tree-crop based ecology and one based on extended cattle ranging is astounding; the former is some ten thousand times more efficient. That is why countries with extensive tree-based industries, such as the Philippines and Indonesia, are virtually proof against famines, while in places such as Ethiopia, with the tree cover reduced from the original 80% down to less than 10%, there is a constant danger of this scourge.
Proposition 12C
Countries with economies having extensive integrated tree-based
industries enjoy much more stable economic and environmental
conditions than those without
Involuntarily through the use of fire, and purposely for use in agriculture and to extract natural products, man has been clearing the forests and scrub throughout his history. This, more than anything else, has been responsible for vast changes in the environment, for immense changes to the isocons. These changes extend wherever man has laid his hand. The vast central plains of North America which are the habitat of bison may have been promoted, at least in part, by the activities of the early American Indians.
In Australia it has been suggested that the whole nature of the landscape has changed since the arrival of the Aboriginals. It may be that our sweeping plains and arid deserts were actually created by the actions of the aboriginals from scrub and forest, largely through the use of fire. Many studies show that regular burning-off changes the whole ecology of forested sites, tending to move them over to grasslands.
Naturally enough, these man-made alterations in the isocons have caused great changes to all life-forms. We tend to think of the rest of nature as relatively static, until we are confronted with evidence such as that of the great extinctions. There is only one species of gorilla today, with three sub-species. How many were there a million years ago, when man was in early evolution? We assume the same, but there may have been twenty species then -- such is the rate of change.
We know that there have been major and involuntary changes in man himself, over periods of only a few hundred years -- few modern Europeans are small enough to fit into an average medieval suit of armour. When the changes are intended, as with breeding races of dogs or wheat, the pace is even more rapid. And when the environment is drastically altered, whether by act of man or by some 'natural' cause, it is inevitable that animal and plant species in that environment will also alter drastically, or else die out. All those plant species marvellously adapted to the deserts may only be as old as man's influence in creating those deserts.
Proposition 12D
Man's actions over the last 100,000 years have caused major
changes in the composition of animal and plant species
The Civilized Dinosaurs
Back now to the dinosaurs, and the relevance of what has just been put forward to them. We are accustomed to regard the dinosaurs as generally being large and stupid, or at least as having brains very small in relation to their size. In many cases this generalization may hold. We are interested here in the exceptions (Fig. 12.2).
Fig. 12.2. "Are you sure their brains are only the size of peas?"
(Humour, from [19])
Sometime towards the end of the Cretaceous a new family of dinosaurs developed. These are commonly called the Ostrich Dinosaurs or Ornithomimids, and they are represented by eight known species of Ornithomimus. This is roughly the same as the current number of anthropoid apes -- man, gorilla, chimpanzee, orangutan, and four or five species of gibbon.
The Ornithomimids were very definitely exceptions to the general stupid-dinosaur rule, just as the anthropoid apes can be regarded as exceptions to a rule of general stupidity in mammals, based on observations of species of rats and rabbits. They were also exceptional in their physical form; they were bipeds, with large brains, large eyes, and long, strong fingers.
The suggestion is irresistible that one of these species of Ornithomimus made the not very great leap into an intelligence level sufficient to support civilization. The time available was quite sufficient for this -- recent evidence based on DNA analyses, for example, suggests that of all the anthropoid apes, man is closest to the chimpanzee, and the two species evolved from a common ancestor over just 5 my [63]. The Ornithomimids were around a good deal longer than this.
We have seen all the evidence that 'civilization' has led to the extinction of larger animals, over about 40 kg, in the case of man. What more natural than that the same extinction, of animals over about 40 kg, should occur in the case of civilized Ornithomimids? And since our own civilization has teetered on the brink of world mutually-assured destruction through the agency of nuclear bombs, could there be a more dire warning than in the Ostrich Dinosaurs' development and use of the Iridium Bomb to wipe themselves out after they had finished off the others?
Proposition 12E
At the end of the Cretaceous, a species of Ornithomimid
developed intelligence and civilization, caused the mass
extinction of large animals associated with this, then wiped
itself out in a nuclear war
Death by Thermodynamics
As with any dinosaur theory, objections can be raised to the Iridium Bomb suggestion just made. Some can be countered quite easily, for example, if a previous civilization existed, where are its remains? We need only point to the very scanty evidence of our own evolution in the last million years to excuse the lack of Ornithomimid relics from 70 my ago. As for their buildings, remember the fine layer of sooty carbon left behind with the iridium?There are, however, some more subtle objections which turn out to be more serious. It is true that the dinosaurs, as commonly defined, did not survive a reasonably sharp boundary in the fossil record. But they did not all go out overnight. According to Stanley [77], there was a progressive decline of dinosaur species during the final 10 my of Cretaceous time.
This is a short time geologically, but a long time anthropologically -- man has made his extinctions in less than 1% of this. Is there a more slow-acting mechanism which can better account for the dinosaur extinctions?
I believe that there is. It is a subtle matter, and I cannot positively identify its exact nature, but I suggest that it is a question of the thermodynamics of some biophysical or biochemical process which is related to body size. There is ample evidence of external conditions altering sufficiently to cause some such thermodynamic threshold to be reached -- changes in carbon-dioxide levels (Proposition 11J), changes in pressure (11K, 11L), and changes in water-vapour content (11M).
It could be argued that these are only changes in degree, and not in kind. But remember the hard-boiled egg on Mount Everest? At some particular altitude, the chemical action needed to allow the egg to set is no longer possible, the thermodynamic threshold for the reaction can no longer be reached.
Proposition 12F
Changes in external conditions close to the Mesozoic-Cenozoic
boundary adversely affected the thermodynamics of biochemical /
biophysical processes dependent on body size and caused the
extinction of creatures heavier than about 40 kg
The important factor may have been the clearing of cloud cover suggested in Proposition 11N. Such a change would have immense repercussions on such things as winds, humidity, input of radiation from space, and bodily insulation. One of these may have primed the thermodynamic time fuze which gradually and inexorably brought the dinosaurs down.
"This is the way the world ends: Not with a bang but a whimper."
(Full list of references at NURefs)
[3]. Daniel I Axelrod. Quaternary extinction of large mammals. University of California Press, 1967.
[19]. Alan Charig. A new look at the dinosaurs. Heinemann, London, 1979.
[21]. John G Cramer. Dinosaur breath. Analog Science Fiction/ Science Fact/ July p140-143, 1988.
[51]. Geoff Maslen. Rediscovering the history of the first Australian settlers. The West Australian/ Feb 4, 1989
[57]. David Noel. Blame it on Henry Ford: the story behind home-acre farming. West Australian Nut & Tree Crop Assn Yearbook/ p91-96, 1985.
[60]. John H Ostrom. A new look at dinosaurs. Nat Geographic/ 154(2) p152-185, 1978.
[63]. Graham Phillips. A new look at missing link. The West Australian/ Feb 27, 1989.
[77]. Steven M Stanley. Extinction. Scientific American Books, 1987.
[78]. Anthony Stuart. Who (or what) killed the giant roo? Weekend Australian/ Jul 26, 1986.
NU013: The Origins Of Fossil Fuels
NU011: The Earth's Atmosphere
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Version 2.0, 2004, PDFs etc on World Wide Web (http://www.aoi.com.au/matrix/Nuteeriat.htm)
Version 3.0, 2014 Sep 27, Reworked from Chapter 12 of "Nuteeriat" as one article in a suite on the World Wide Web.