Well, the time has come for the next installment of my focus articles on a cool period in geologic time. This one will look at the events that transpired during the Cretaceous period. The Cretaceous period ran from
145.5 - 65.5 million years ago and is the last period of the Mesozoic Era. It was a time of great evolutionary and tectonic change for the Earth and has certainly earned its place in history as the period that marked the end of dinosaurs.
The position of the continents in the Cretaceous in no way resembles today's orientation. In fact, the Cretaceous was a time of major tectonic changes as the super-continent of Pangea was undergoing breakup into pieces similarly shaped to today's continents. The breakup of Pangea began during the late Triassic and early Jurassic and continued on into the Cretaceous as the mid-Atlantic Ridge began to separate proto-North America from proto-Europe and Africa.
The continents in the Late Jurassic and early Cretaceous. (www.scotese.com)
The position of the continents in the Late Cretaceous. (www.scotese.com)
The tectonic changes that occurred over the course of the Cretaceous are very noticeable. Some of the major things to notice are that the Atlantic Ocean had started to form. There was a large shallow sea in western North America which was known as the mid-Cretaceous seaway and is responsible for hosting massive coal and oil deposits in western Canada as well as gigantic potash deposits in Saskatchewan that would not exist otherwise. Also, notice that India had broken off from Africa and Antarctica and was making its way north where it eventually encountered the rest of Asia. Finally, a few island arcs, which are basically small, volcanic continents, crashed into the western side of North America and started forming the Rockies and Sierra Nevadas.
Evolutionary Advances and Declines (One step forward and two steps back)
When I say Cretaceous period everyone thinks one thing: huge dinosaurs! However, there were other important evolutionary advances besides the existence and eventual demise of the dinosaurs (which I will come to). The first major development of the Cretaceous was the appearance of angiosperms, or flowering plants. Up until the Cretaceous the only plant life on land was gymnosperms, such as ferns. The major difference between angiosperms and gymnosperms is that angiosperms have flowers which attract pollinating insects and birds to them which in turn carry the seeds of the angiosperms to other locations. This was a major advance in plant reproduction!
Throughout the Cretaceous angiosperms, dinosaurs and insects co-evolved to create a complex and new ecosystem. By the end of the Cretaceous dinosaurs had reached their peak in terms of diversity. In fact, they were filling almost every possible ecological niche from top predator to basic herbivore and everything in between. There were two major groups of dinosaurs: the saurischians, which included all of the giant herbivorous dinosaurs and the carnivorous dinosaurs. The other group was the ornischians, or "bird hipped" dinosaurs that included a variety of herbivorous dinosaurs and all of the armoured dinosaurs.
A comparison of dinosaur hip bones.
Most people have heard about some of the big names in the Saurischian world such as Tyrannosaurus Rex and the gigantic Brontosaurus, which was 23 metres long and weighed a whopping 27,000 kilograms and was one of the largest animals to ever live. There were also much smaller dinosaurs such as Compsognathus, a relative of T. Rex, that was only about the size of a chicken. Another relative of the T.Rex, all of which belong in the theropod family, was Deinonychus a 3m long predator that could run very quickly and had a strong forelimbs with an oversized claw on each. The discovery of Deinonychus paved the way for the revelation that modern birds are descended from theropods.
The big names of the Ornithischians were armoured dinosaurs such as Stegosaurus and the “tanks” of the dinosaur world: the Ankylosaurs, which were completely covered by a thick coat of armour. Other important members of this group were the duckbilled dinosaurs or Hadrosaurs that were very common fixtures in late Cretaceous. They are easy to identify due to their large head crests used for species recognition and in some cases producing a loud, booming call. Hadrosaurs are commonly found in Dinosaur Provincial Park in Alberta, Canada. The ornithischian family also included the bone headed and horned dinosaurs such as Triceratops and Protoceratops.
A Cretaceous ecosystem
Finally, if you look in the picture above near the large flower you will notice a small mammal. The Cretaceous was a time when most mammals were small and lived in the underbrush, preying on small lizards and generally escaping notice from most other creatures. The only major evolutionary advance made by mammals in the Cretaceous is the divergence of placentals and marsupials as mammal reproductive behaviour evolved from earlier egg-laying predecessors. The most remarkable thing about mammals in the Cretaceous does not have to do with their evolutionary advances however, but is related to their survival through the impending mass extinction that would take place at the end of the Cretaceous.
While there was some pretty cool stuff happening on land, there was also some pretty amazing stuff happening in the oceans as well. The Cretaceous can be thought of as a marine arms race. The predators were getting bigger, stronger and faster, however, the prey were matching these advances by getting better defence mechanisms. This resulted in an ecosystem populated by predators with strong shell-crushing teeth and prey with thick, spiny shells or excellent escape strategies. It was survival of the fittest at its best. The reefs that we know today were also non-existent, at least in the sense that they were formed by corals. In the Cretaceous “reef-like” mounds created by large clams called rudists, which would attach themselves to hard substrates by the thousands and create huge mounds and colonies. The Cretaceous was also the age of the ammonite. Ammonites were predatory, squid-like creatures with large spiral shells. Our nautiloids of today are descended from the ammonites, which were very common in the Cretaceous and occupied a middle space in the food chain, that is to say they could be both predator and prey.
A giant ammonite
There were also lots of marine reptiles and fish, many related to the fish of today. Some of the most dominant marine animals were the marine reptiles such as the Plesiosaurs that could grow to huge sizes. Some had short bodies and long necks, up to 8m, ideal for catching fish, while other more closely resembled whales of today with short necks and large bodies. Another dominant aquatic reptile was the Icthyosaurs, which had bodies similar to sharks with long beaks full of teeth. Other marine reptiles of the Cretaceous included crocodiles and sea turtles.
The K-T Boundary – The end of the party
At around 65 million years ago a giant cataclysm rocked the Earth and spelled out doom for the dinosaurs and most other organisms that had thrived in the Cretaceous and spent the Mesozoic evolving. The cause has been the subject of much debate in the geologic community and for over a century there was no consensus. In fact, many crazy suggestions were proposed such as UFO invasions or rampant constipation and diarrhea. But, by 1980, evidence of a meteorite impact was discovered and led to more logical discussion on the K/T extinction (Cretaceous-Tertiary mass extinction event). Whatever the cause though it must have been an event that would decimate both life on land and in the ocean as more than just the dinosaurs were affected. In fact, the ammonites were almost entirely wiped out as well as marine reptiles, the rudists, many types of plankton-like organisms and many land plants. As things stand now there are two main views on the cause of the K-T extinction and no one hypothesis has been proven conclusively correct.
The Impact Hypothesis
Oddly enough the discovery of the K-T meteorite impact was made accidentally. Geologist Walter Alvarez and his father, Nobel laureate, Luis Alvarez were investigating the rate of sedimentation for a clay layer at the K-T boundary by looking at the cosmic dust contained within it. They were analyzing for the element iridium, a metal that is very rare on Earth, but more common in meteorites. They found that within this clay was a very high concentration of iridium that could not be explained by any Earthly cause and must have come from space. They hypothesized that a meteorite impact had spread a massive cloud of dust, enriched in iridium, that would have led to global cooling and a catastrophic extinction. A likely site for the impact was the Chicxulub crater in the Yucutan Peninsula, Mexico, which dating of melt rock at the site shows to be exactly at the K-T boundary.
|Geophysical survey of the Chicxulub Crater|
The Volcano Hypothesis
Another possible explanation for the K-T extinction is a massive volcanic eruption that occurred only half a million years before the K-T boundary. The eruptions occurred in India and was one of the biggest ever. It is called the Deccan Traps and it erupted ~10,000 cubic kilometers of lava. These eruptions would have released huge amounts of volcanic ash into the atmosphere, which also would have led to global cooling and a subsequent change in ocean chemistry. The ash also would have contained iridium from the Earth's mantle, where it is more plentiful.
|The area covered by the Deccan Traps, which are up to 250m thick in places.|
Which ever the actual cause of the K-T mass extinction the combination of a large meteorite impact and huge volcanism could not have been good for life on Earth so it is likely that the extinction is a result of these two factors working in concert.
I hope you have enjoyed this post. Feel free to comment on which extinction hypothesis you prefer.
Reference: Prothero and Dott, Evolution of the Earth , 7th Edition, 2004.