My favourite extinction theory is the one based on crocodiles. Apparently, when crocodile eggs are maintained at a certain temperature, something like 36º I seem to remember, the sex of the babies will tend to be evenly split between male and female. A couple of degrees higher and males will predominate. Two degrees lower and females will be favoured.

It's also known that, 1., the climate was cooling during the Upper Cretaceous, and 2., dinosaurs were related to crocodiles, as indicated by both being classed as archosaurs.

Maybe the dinosaur birth ratio was also steered by temperature. As the weather became cooler, the proportion of females increased until, eventually, only lesbisaurs could be sexually fulfilled. There were no more potential fathers, no more babies and then no more dinosaurs.

I find this theory wonderful. It clearly explains why the crocodiles died out, thus invalidating itself with great elegance.

The extinction is sometimes refered to as the K-T event, K representing kreta, (chalk and therefore Cretaceous in Greek), with T standing for Tertiary, the subsequent geological age. Implicit is one event, a monocause, which should account not only for the disappearance of the dinosaurs, but also for the demise of plesiosaurs, ichthyosaurs, mosasaurs, pterosaurs, belemites, ammonites and so on, whilst not eradicating the animal classes of today, including crocodiles. Amongst the many are the meteorite / comet, the volcanic and the death by natural causes groups of theories.

The Meteorite / Comet
A giant meteorite hit the earth, turning day to night, poisoning the atmosphere and annihilating the plants and animals. For this to be more convincing than the crocodile comparison there must surely be some evidence available. And there is, lots of it.

There's a great big crater, 65 million years old, just off the coast of the Yucatan peninsular, Mexico. Deposits of shocked quartz, created by the meltdown of stone under the enormously high temperatures and pressure provoked by impact, are well documented. Rock samples of that age have been tested for iridium content all round the world, and over a hundred sites have revealed unusually high levels. Iridium is extremely rare on the earth, but well known from meteorites. The fossil rich Hell Creek formation in Montana and northwest Dakota suggests a landscape suddenly denuded of vegetation, combined with traces of acid rain. A centimetre or so higher is a layer of lignite, low grade coal, featuring an abnormal concentration of fern spores. Ferns are often amongst the first recolonizers of devestated areas.

It could be argued, of course there's a 65 million year old crater. The earth's littered with craters. Over 150 have been recorded, with many more unrecognized in the less developed regions of the world. That's true. But this crater, Chicxulub, is indisputably unusual.
But first, as an illustration of the potential of uninvited visitors from space:

A meteorite of a few tens of metres exploded about ten kilometres above Tunguska, Siberia, in 1908. Being a thinnly populated area, nobody is known to have died as a result. However, 2 000 square kilometres of forest were flattened.

There's a crater at Nördlingen, Ries in Germany. It was formed 15 million years ago by a meteorite and has a diameter of about 24 km. The explosion can only have been catastrophic. All complex life forms within 100 km would have been wiped out and a far wider area strongly blitzed. This impact threw up between 50 to 100 cubic kilometres of pulverized debris, which covered an area of 2 500 km² to a depth of up to 100 metres. It was powerful enough to have blown a hole in the atmosphere and send thick clouds of dust around the globe. A 'Nuclear Winter' is conceivable. The ecological recovery time can be estimated in terms of centuries. Devastating, yes, but too short and too small to leave a scratch on the fossil record. Such impacts have occured frequently, on average perhaps every two million years.

Chicxulub was of an altogether different magnitude. This crater measures 170 km, seven times wider. It shows an impact that was near the equator, where the concentration of species, and the potential for disruption of ecosystems, is at its greatest. An explosive force of around 100 million megatons has been estimated. But perhaps this was merely the last, and most powerful of a number of attacks from space.

Craters of a similar size to Nördlingen are found during the Upper Cretaceous, which is no surprise. About 89 Ma Dellen in Sweden was hit. Others can be found at Boltysh in the Ukraine, (c. 88 Ma), and at Lappajärvi, Finland, (c. 77 Ma). But around 73 million years ago all hell appears to have been let loose. There's a 35 km wide crater at Manson, Iowa. In asiatic Russia the testimony is even more dramatic. Two impacts, identically dated and presumably simultaneous, at Ust-Kara (25 km) and Kara (65 km).

The evidence of death from meteorite impact is impressive but.... The evidence against OJ Simpson was supposed to have been watertight, and he was found innocent.

Iridium is not as rare in the earth as it is on it. The source might well have been sub- rather than extra-terrestrial. It could have been spread volcanically.

The Volcanic
Volcanic activity isn't all about spectacular eruptions like Krakatoa. Much, as along the depths of the mid Atlantic, is more a quiet seapage of lava, unobtrusively laying down new oceanic crust. But spectacular eruptions are what tend to come to mind. Some, like Vesuvius, command a prominent place in history books two thousand years later.

As an illustration of the potential power of volcanoes:

Mount Saint Helens in Washington, USA, erupted on May 18th, 1980. A 500 degree Centigrade rush of ash and gasses stormed from the breach, devastating an area of 600 square kilometres. The ash plume reached a height of 25 km. An ash cloud brought darkness at noon 250 km to the east. In all, a cubic kilometre of material was deposited into the environment by a blast equivalent to a 25 megaton bomb, 1 300 times more powerful than the one dropped on Hiroshima.

But what would happen if the power of Krakatoa, Vesuvius, Mt. St. Helens and hundreds of other volcanoes were all combined in one event, the K-T event perhaps? Day would probably be turned to night, the atmosphere filled with poison and the plants and animals annihilated. Unusual elements like iridium might well be spread worldwide.

The evidence is provided by the Deccan Traps in western India, one of the largest volcanic landscapes on the planet. Lava rock today still covers an area of nearly 500.000 km². Originally, the area of coverage was possibly three times as great. At least half a million cubic km of basalt lava were produced. The height of the activity has been dated to 65 million years ago.

The dinosaurs arose during the Triassic, around 235 Ma. The geological period immediately before, from about 300 until 250 Ma, is termed the Permian. The Permian ended with signs of a mass extinction, a P-T event, which seems to have extinguished 95% of marine and 70% of all land animal species on the earth, even more devastating than the K-T extinctions. And there's another similarity.

The Permian period takes its name from Perm in Siberia. In this region, 250 Ma, another massive bout of volcanic activity laid down the Siberian Traps. This episode was even more substantial than Deccan. Perhaps 1½ million cubic kilometres of basalt lava were deposited.

The extraordinary quantities of ash, lava, steam, sulphur and carbon dioxide released by both episodes have been identified, by some, as the common cause of two mass extinctions, the P-T and the K-T.

There are differing views on the duration of the creation of the Deccan Traps. What is certain is that this was far from being a normal volcanic event. Whether it was a short or a longer term episode, it must have had an enormous impact on the environment and climate. This was volcanic activity of a cataclysmic degree.
Especially as the earth was hit by a massive meteorite at about the same time.

With a radical alteration of climatic and ecological conditions, it is reasonable to expect a high tally of extinctions. Every animal heavier than 25 kilos appears not to have survived the K-T border, be it dinosaur, mammal or even crocodile. If the new conditions made bigness deadly, then it's clear that Tyrannosaurus and Triceratops (both late Upper Cretaceous) were for the chop.

However, not all dinosaurs were big. Dinosaurness has nothing intrinsicly to do with size. It's to do with anatomy; how the bones fit together. Diplodocus was no more a dinosaur than Compsognathus, (both Upper Jurassic and already extinct 80 million years before the K-T line), merely because it was thirty times as large. There were a number of dinosaurs which obviously weighed less than 25 kilogrammes.

Death by Natural Causes
The world of the Upper Cretaceous was very different to how it had been at the end of the Jurassic. There were a number of reasons for this.

The continents were drifting further apart, on course towards their present configuration. This brought about an increase in continental shelf area, disrupted long established tidal flows and enhanced isolation. The development of a variety of new habitats encouraged the emergence of new forms of life. The angiosperm (flowering) plants are first known from the Lower Cretaceous, but they spread rampantly during the Upper. These are the most extensive floral groups of today and provided an unprecedentedly rich supply of nourishment for plant eating animals.

And the increasing isolation of landmasses encouraged a localization of fauna. It was no longer possible for land animals to radiate worldwide, as the iguanadons had in the Lower Cretaceous. Migration routes were blocked by ocean. The animals of the northern and southern hemispheres became increasingly differentiated. Upper Cretaceous sauropods are found almost exclusively in the south, whilst the reverse is true for ankylosaurs, hadrosaurs and Ceratopsia, the horned dinosaurs such as Triceratops.

It has been suggested, the dinosaurs were driven to extinction by these very factors. Mammals were better able to adapt to the new possibilities and challenges. The dinosaurs died out because of obsolesence, old age. However, individuals die from old age, not species and certainly not whole orders. Crocodiles are about as old as the dinosaurs in origin.

Furthermore, Dinosauria was far from obsolete. Mammals certainly adapted to the new conditions, but not as spectacularly as dinosaurs did. The rate of development of new dinosaur groups during the Upper Cretaceous is impressive. About three quarters of all named genera are dated from this time, although this may in part be due to a chance bias amongst the known significant fossil sites. New discoveries from espcially China and Patagonia are redressing this imbalance to some degree.

Adaptation enabled dinosaurs not only to cope with, but to thrive on the newly available plants, as shown by the sheer variety of herbivores which evolved and the different types of habitat in which they were able to exist. The Upper Cretaceous dinosaurs were vibrant animals, not fossilis.

And though mammals diversified too, the largest only reached about the size of a badger. There is little evidence which shows these clever newcomers outcompeting their ancient, stupider rivals. Mammals have also been around for a long time. They are descended fom cynodonts, therapsids of the Middle Triassic, from about 220 Ma.

Furthermore, the notion of dinosaur stupidity is a myth. Stegosaurus famously had a brain the size of a walnut in a body the size of an elephant, but that is not typical. Troodon, of the Upper Cretaceous, had a brain-body ratio that is 'mammalian'. And some carnivores hunted in packs, further evidence for more than reptilian intelligence.

Analysis of the fossils at Hell Creek, Montana suggests that, over the last ten million years of the Cretaceous, mammals did become more numerous and dinosaurs scarcer. This finding is interesting, possibly indicative of something but not conclusive. Global trends cannot be defined by the evidence at only one site. Hopefully, new discoveries and research will come along to broaden and deepen the picture. If, as suggested, mammals were becoming dominant before the K-T border, that could be because the K-T event was a sequence of events, played out over millions of years, rather than a sudden occurance.

What Hell Creek does not show is a dramatic diversification of mammals. That first becomes apparent after the K-T line. In all, eight families of late Cretaceous mammals have been identified. During the Tertiary period, the number grew to seventy.

Another explanation offered for the demise of the dinosaurs is the cooling of the climate. Given the presence of dinosaurs at the Arctic and Antarctic, however, this is hard to credit, especially as some were probably endothermic, ('warm blooded'), and may even have had body insulation, (feathers?), eg. Sinosauropteryx, Lower Cretaceous.

The Extinction Event
"It wasn't only the dinosaurs which became extinct at the end of the Upper Cretaceous. A great many other animal groups died out at the same time."

I've read sentences like that quite often. They're misleading.

'The' dinosaurs did not become extinct at that time. Most were already dead. Chronologically, we are nearer to Triceratops and Tyrannosaurus than Herrerasaurus (c. 225 Ma) or Plateosaurus (c. 200 Ma) or even Diplodocus (c. 145 Ma) were. The age of the dinosaurs lasted a very long time.

It's also not certain that dinosaurs did not survive the K-T event(s). Isolated remains do turn up in Tertiary rocks. This could be due to fossils coming to the surface through erosion, only to be reburried, or perhaps not. However, as small mammals survived the crisis there is a reasonable chance that some small dinosaurs may also have done so, a matter which would only be resolved by the discovery of substantial remains.

And if the presently popular interpretation is correct, that birds are dinosaurs, than there are nine thousand species of maniraptor still singing sweetly or squawking horribly.

Some of the oft cited "other animal groups" did not become extinct at the same time. Oeceanic extinctions may well have begun millions of years before the K-T border, prompted by various factors. The changing ocean currents and temperatures, for example, would have had deadly consquences for many tropical species.

The ichthyosaurs were already long gone. They first appeared in the Lower Triassic and were subsequently widespread, but only one line, Platypterygius, is known from the Cretaceous. The most recent icthyosaur fossils are 93 million years old. There is no persuasive evidence of a catastrophic cause for their extinction.

Likewise,the plesiosaurs would also appear to have been the victims of a gradual decline. The final age of the Cretaceous period is called the Maastrichtian. It lasted for six or seven million years. All known plesiosaur remains from the Maastichtian seem to belong to one remaining family, the Elasmosauridae. These would appear to be the last survivors of Plesiosauria, with the possible exception of Nessie and friends.

The less well known Mosasauridae, however, are reasonably well represented amongst Maastrichtian fossils. Mosasaurs were predatory aquatic reptiles, related to monitor lizards, and radiated exclusively during the Upper Cretaceous. They do not seem to have been in decline. Their extinction might be linked to oceanic regression, or equally might not. During the whole period many continental areas, (eg. in Australia, Europe and North America), had been covered by shallow seas, epieric seas. These provided the mosasaurs with their primary habitats. These stretches of water had largely gone by the end of the Cretaceous.

However, eras of oceanic trans- and regression are hardly geologically unusual. The North Sea was dry land only ten thousand years ago. There was also an increase in continental shelf habitat available.

Size may be a significant factor. Present day monitor lizards are not exactly small. Komodo dragons can reach four metres in length. Small mosasaurs were about half as large, but some kinds were considerably bigger. Tylosaurus was a fifteen metre monster.

Crocodiles: why didn't they die out?

Pterosauria, the flying reptiles, are infrequently found amongst the fossils of the Late Cretaceous. Only two families have been recorded; the Ornithocheiridae, (eg. Pteranodon), and the Azhdarchidae, (eg. Quetzalcoatlus). These animals were remarkably large. The two above named genera had wingspans of up to seven and twelve metres respectively.

As birds became more numerous, so pterosaurs seem to have become rarer and more specialized, suggesting a long term trend. Notable bird groups of the Upper Cretaceous include the enantiornithes, the hesperonithiformes and the ichthyonithiformes, all now extinct. By the end of the period, these may have been joined by ancestral geese, ducks, loons, albatrosses, petrels, gulls and doubtless other modern groups. (However, reported finds of such groups are subject to dispute.) Birds had occupied the main biotopes of previous known pterosaur generations.

Whilst the whole Mesozoic is sometimes known as the age of the dinosaurs, the Cretaceous takes its name from the remains of a different biological group. Cretaceous is derived from the Latin for chalk, and was chosen because of the vast sediments of this rock found at, for example, Dover and Heligoland. These are massive monuments to countless billions of microscopic plankton; the coccolithophorids, whose remains built up these cliffs.

Many of these plankton species also went extinct at or near the K-T border. Climate change might well have contributed, because their modern counterparts are especially sensitive to temperature, (as are reef building coral). They require tropical conditions as at the Caribbean, where chalk is presently being laid down. The carbon dioxide released into the environment at Deccan has been cited as another possible killer.

The ocean depths are relatively CO² rich and the deep seabeds calcium carbonate poor, in relation to shallower waters. This is because CO² dissolves calcium carbonate, both in limestone on land and in microshells as they sink. In deep sea areas the shells of the surface dwelling coccolithophorids and foraminifera tend to dissolve before being able to settle. An increase of carbon dioxide in the biosphere would inevitably promote this process at higher levels. Furthermore, CO² is known to have disruptive consequences on some metabolic functions.

Many other shelled groups were also strongly affected, but not neccesarily simultaneously. A series of extinctions has been suggested, extending from the uppermost Cretaceous, (inoceramid bivalves, reef facies and other benthic molluscs), through to the K-T border, (ammonites and the coccolithophorids), and into the lowest Tertiary deposits, (benthic foraminifera). If this interpretation is accurate, it suggests tides of extinction breaking against the sands of hundreds of thousands of years.

'The extinction', the 'KT-event' are well worn phrases, but it's far from clear that they're valid. It's necessary to try and unravel the seperate strands, and not habitually assume that the causes are the same in each case. 'The extinctions', the 'KT-events'.

Conclusions
I don't much like theories that depend on cataclysm. They might make for great, or dreadful, science fiction but they rarely contribute much to understanding. Rarely.

Long term tectonic activity was transforming the planet, promoting isolation and creating new biotopes, as well as changing ocean depths and currents and also climate patterns. These are profound alterations which, on balance, can be expected to have promoted biodiversity.

73,8 Ma (+/-0,3), an unusually large meteorite laid waste to Iowa, causing regional disruption presumably for several thousand years, too short to make a dent on the fossil record.

73 Ma (+/3), a monster meteorite, despite fracturing in the atmosphere, was still powerful enough to carve out one big and one enormous crater in Siberia. That means, within seconds, one explosion in the air followed by two at ground level. This is not a frequent geological event. The only other known instance is at Clearwater in Canada, 290 Ma (+/20). But even this had no discernable impact on the fossil record.

65 Ma, an even bigger monster meteorite slammed into Yucatan, creating the largest known crater of the entire Phanerozoic; Paleo-, Meso- and Neozoic combined. This is not a frequent geological event. The only larger identified impact structures are at Sunbury, Canada, (250km, 1,8 Billion BP) and Vredefort, South Africa, (300km, 2 Billion BP).

A short and extraordinarily, or a sedater and extraordinarily profound volcanic event was underway in India, with the height of its activity dated to 65 Ma. This is not a frequent geological event.

Despite tectonic movements favouring, and a demonstrable level of biodiversity during the Upper Cretaceous, there is scant evidence of the further radiation of species at the end of the period, or in the following epochs. The radiation of mammals and birds is dramatic but, according to the fossil evidence, not until later.

Why did the dinosaurs go extinct? This is perhaps the wrong question. How on earth did anything else survive?

ktdykes@arcor.de