by Fred Bervoets
 

Dinosaurs where the dominant landanimals during the Mesozoic period also called the Age of the Dinosaurs. During that time their diversity showed fluctuations, sometimes even major ones. They survived several massextinctions during this about 165 million years long timeperiod, until they finaly disapeared from the fossilrecord at the end of the Cretaceous period, during one of the biggest mass-extinctions ever. There are several hypothesis about their extinction from diseases to the Yucatan impact event. In this article I want to bring the low-diversity at the end of the Cretaceous and the major changes in the environment under your attention as possible co-responsible circumstances that lead to their demise. Starting at the Campanian.


The dinosaur fossil record of the Campanian shows a remarkable increase of diversity compared to the Santonian, from these two timeperiods 87% of the worlds dinosaurs fossil record derives from the Campanian. If we fuse the fossil record of the Maastrichtian with that of the Campanian then we even end up at 92% of the total species, where if we look at the total fossilrecord of the Mesozoic compared to the combined Campanian and Maastrichtian the last two represent 32% of the total volume of the dinosaur fossil record, where the Maastrichtian in that view represents 15% and the Campanian 17%. Campanian fossils are most commonly found in Asian sites, with 48% of the total fossil record. North America comes in a close second with 39%. Is this coincidental, or are these numbers representative for these time periods? The chance re mains that paleontologists until now just have found more fossils from the the last episodes of the Cretaceous compared to the rest of the Mesozoic period.

However the numbers are in my view so convincing that one can easily assume that the diversity among dinosaurs during the Campanian period dramatically increased. Some argue that the Campanian diversity explosion shows that this time period. (together with the Maastrichtian period) provided a very favorable environment for dinosaurs. However it is my view that the reason for this diversity explosion may lay in the fact that at that time the environment for dinosaurs (and other animals) was changing often and perhaps sometimes radically, which forced the then living dinosaurs to adapt to a new environment in order to survive. According to William Sager of Texas A-and-M University the magnetic measures indicate that the Earth may have rolled over slightly (16 to 21degrees) during the Campanian (84 mya) over a span of about 2 million years . The wobble would have relocated the poles and put Washington D-C in the tropics, about where Cuba is now. Then, something appears to have changed the weight distribution in the Earth, causing it to get back in balance.

In the middle Cretaceous (100 mya) an initially gradual (but later faster) cooldown was taking place after a long period of a stable but specialized ecosystem. Angiosperms conquered the world. During this period the diversity among dinosaurs increased until the Maastrichtian, with an tremendous peak in the Campanian. The mass-extinction at the end of the Cretaceous even coincided with a rapid cooling of the climate.

Relative small changes in temperature on the borders of the Albian and Cenomanian (98 Million years ago), the Cenomanian Turonian border (91 million years ago) and the border of the Turonian and Coniacian (88 million years ago) brought changes in the biosphere and caused extintions, but after every event a new equilibrium originated. The changes at the end of the Cretaceous had to be much larger because the consequences were so catastrophic and definitive. Leonard Ginsburg, a paleontologist at the Museum of Natural History at Paris, thinks that the massextinction of the dinosaurs was caused by falling sea levels during the Late Cretaceous, and that this resulted in such changes in the climatic circumstances that dinosaurs could not adapt and proceeded to die out.

Excavations in Montana shows that 75 million years ago 30 dinosaur species were living, 5 million years later there were 23 left and again 2 million years later their numbers went down to 18. This decline in species count formed a continuing line that went on to the end of the Cretaceous. This proves the thought that dinosaurs where declining rapidly after the diversity explosion in the Campanian. Mass-extinctions occur at the end of various geological timeperiods such as the Permian (245 mya), Devonian (360 mya), Ordovician (438 mya) en Cambrian (510 mya).

In Ginsburg\'s opinion changes in sea levels are a collective element that caused all these mass-extinctions. Changes in sea-levels can be caused by major changes in the size of the polar caps or by continental drift. Every geological time period starts with an sealevel rise, accompanied by the origination of new faunas, and ends with a lowering of the sealevel that caused a massextinction. The most remarkable aspect of the lowering of the sealevel during the end of the Cretaceous was the extent where it happened. Water that had been covering a large part of Europe, all of North Africa, the Sahara, the Middle East and a big part of north-western South-America was pulling back. In Europe the sea withdrew thousands of miles to end in the sight of the present borders.

These dramatic changes killed about 80% of the living fauna on land as well as in sea. Know victims were the ammonites that had lived for over 300 million years before they disappeared at the end of the Cretaceous. The drop of the sealevel caused a drastic change of the land climate. The more tropical climate of the Jurassic gave way to one that varied from summer to winter. Dinosaurs where not able to adapt to these new circumstances and rapidly decreased in numbers. As the sea levels fell and sediments continued to be delivered to the coast and the seas began a slow retreat, volcanism intensified. British researchers discovered that mondial climatological changes can lead to variations in the average volcanic activity.

An ice age causes a increasing pressure on the earth\'s crust and makes magma flow more easily to the surface through volcanoes, a process which also inversely proportional would appear. The researchers also concluded that sea-mirror variations that hang close with the coming and going of ice ages can have influence on the volcanic activities even on great distance from the ice-covered areas. They investigated the possibility of an connection between variations in the level of the Middleterranian Sea and the volcanic activity in that area. Almost all volcanoes near the Middleterranian Sea are located on islands or near the coasts.

They concluded that in the past 80.000 years volcanic activity in the area has mostly kept stable, however there have been three periods that showed an increase of volcanic activity. These periods where between 8.000 and 15.000, 34.000 and 38.000 and between 55.000 and 61.000 years ago. During this periods the average sea level was undergoing a drastic decrease. There findings show that climatic changes, through the rise or incline of the sealevel, can have influence over great distances on volcanic activities. Of the now known active volcanoes, 57% are located on islands, while 38% are located less than 250 km from shore.

In 1972 geologe Peter Vogt wrote that mass-extinctions coincide with tremendous worldwide volcanic activity. The proof are to be found in the Pacific Ocean and India itself. Over a period of 500.000 years there were many volcanic eruptions that produced, an average, 2.000.000 cubic meters of lava (of which traces are still to be found at the Deccan Traps, an area south of India). Almost all around the world there is a dark trace in the strata where the Cretaceous meets the Tertiary, also called the K-T boundary. Research in the Hell Creek Formation (in Montana) showed that the last Cretaceous dinosaurs lived until 65.01 million years ago (with a margin of error of 30.000 years). This is similar to the end of the eruptions in the Deccan-area 64.96 million years ago (with a margin of error of 110.000 years). The many shallow seas of the Cretaceous had a great influence over global climate. They provided a subtropical climate in the areas where dinosaurs where dwelling. However, at the end of this period these inland seas withdrew and created a more temperate climate with hotter summers and colder winters. The volcanic eruptions threw dust into the atmosphere, which brought short- and long-term changes. Shortly after the eruptions acid rain fell down to earth and lakes and ponds became more acidic.

At first there was a temperature drop in global temperature caused by atmospheric dust; sunlight was blocked by the dust and could not shine through. This disturbde the necessary photosynthesis for plants and possibly resulted in the collaps of the food chain. As result of the accidation of the surfacewater, plankton at the bottom of the foodchain died out. Over a longer period of time a greenhouseeffect caused the temperature of the Earth to rise an average 5 degrees Celsius. In general, summers became hotter; winters colder.

All these changes may have caused the diversity of dinosaurs to drop. If the Yucatan impact had occurred earlier in time, dinosaurs might have had a better chance to survive this disaster because their
diversity was higher, and possibly some species would have been able to adapt as they did before in earlier extinctions.

However even being low in diversity and living in this extreme new environment , it is possible that they would have survived into the Tertiary. If not an extra terrestrial object would have (most likely) finished them off.

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