The Cretaceous Period
144 to 65 Million Years Ago
The Cretaceous is usually noted for being the last portion of the "Age of Dinosaurs", but that does not mean that new kinds of dinosaurs did not appear then. It is during the Cretaceous that the first ceratopsian and pachycepalosaurid dinosaurs appeared. Also during this time, we find the first fossils of many insect groups, modern mammal and bird groups, and the first flowering plants.
The breakup of the world-continent Pangaea, which began to disperse during the Jurassic continued. This led to increased regional differences in floras and faunas between the northern and southern continents.
The end of the Cretaceous brought the end of many previously successful and diverse groups of organisms, such as non-avian dinosaurs and ammonites. This laid open the stage for those groups which had previously taken secondary roles to come to the forefront. The Cretaceous was thus the time in which life as it now exists on Earth came together.
The age of dinosaurs reached its apex during the Cretaceous period, which occurred 145 to 65 million years ago. Dinosaur and other animal diversity characterized this time, as more different types of dinosaurs emerged during the Cretaceous than in any other period. Nevertheless, ominous signs began to surface, hinting at the non-avian dinosaur downfall that would mark the end of this final phase of the Mesozoic era.
Cretaceous Geology, Climate and Plant Life
Laurasia, a portion of the former Pangea supercontinent, consisted of the present-day continents of North America, Europe and much of Asia. Another section of former Pangea was referred to as Gondwana, which included Africa, Antarctica, Arabia, Australia, India and South America. Both Laurasia and Gondwana broke into pieces during the Cretaceous, leading to the separate continents. North America, however, was substantially different than it is today. A shallow sea split it in half, turning the western portion into an island.
Although wet and dry periods existed before this time, the seasons became more distinct during the Cretaceous. Flowering plants emerged, providing another food source for dinosaurs. The first flowers were mostly like small weeds but, like weeds today, they grew and spread well, quickly moving from tropical to cooler regions. Oak, maple, walnut and other trees also emerged.
The Largest Flying Animals
Cretaceous skies were full of creatures big and small. Minuscule moths and small bees shared airspace with enormous pterosaurs, which were warm-blooded flying reptiles related to dinosaurs. The pterosaur Pteranodon, which had a wingspan of up to 33 feet, was one of the biggest of the bunch. It spent much of its time soaring over water, looking for fish, crabs, insects and mollusks to eat.
Big, small, spiked, fat and more, dinosaurs of all shapes and sizes appeared during the Cretaceous. Since the landmasses were now separated, animals on the distinct continents went their separate evolutionary ways. Beaked, plant-eating dinosaurs known as ceratopsians first appeared at this time. One of the most well-known members of this group was Triceratops, which had three facial horns and a large, bony frill. The so-called "bone-headed" dinosaurs also emerged during the Cretaceous. One of the biggest was Pachycephalosaurus, which had front teeth and might have even enjoyed meat with its plant-based diet.
Numerous carnivorous dinosaurs preyed upon the other dino bounty. There was Pelicanimimus, which had more teeth — around 220 — than any other known dinosaur. Afrovenator used its own bladelike teeth to tear at the flesh of prey. Carnotaurus had such short arms that its hands appeared to form out of its elbows, but its clawed feet and sharp teeth could still take down sauropods. Tyrannosaurus rex, however, stood out from the carnivorous dinosaur pack. This 40-foot-long beast was so strong that researchers believe it could shake victims to death, once it had sunk its teeth into their bodies.
The Cretaceous period marks the end of the age of Dinosaurs with what is known as the Great Extinction. However, this period gives us some of the most beloved dinosaurs of modern days, like the Triceratops and, of course, the Tyrannosaurus-rex, king of the dinosaurs.
Hints of Extinction
Not all dinosaurs thrived during the Cretaceous. Stegosaurs, which lacked the protective armor of other plant eaters, like ankylosaurs, went into a population decline. In northern areas, long-necked sauropods began to disappear. While minor, these changes might have foreshadowed the mass extinction that marked the end of both the Cretaceous period and the dinosaur age.
The Great Extinction
While the cause of the massive extinction brought on at the end of the Cretaceous period is debated, many theories exist. There is evidence of plant decay which would have contributed to the extinction, as all dinosaurs, whether directly or indirectly, depended on plant life. This could have been caused by large asteroid collisions or volcanic eruptions or both. These events would have suspended sunlight, causing plant life to diminish. Other theories include the regression of the sea level, or a combination of many of these possibilities. Most of the species that survived were not as dependent on plant life. Insects survived on other animals and dead organisms and mammals would often feed on insects. This allowed the Mammals to survive until the present day when mammals would replace the dinosaurs as the dominant species.
The Cretaceous ocean predators were very large. I suspect that the productivity implied by this was caused by a flow of phosphorus toward the ocean from the savannas (seasonal rainfall areas) permitted by erosion of phosphorus rich runways of plant smothering termites in the Amitermitinae starting in late Jurassic in Australia where the first ocean phosphorite deposits occurred. Anoxic conditions in the oceans were also probably caused by this. This anoxic bottom condition probably helped reduce the ammonites also, in addition to competition from phosphorus enhanced vertebrates. The savanna herbivore dinosaurs declined in armor, teeth, and quite a bit in bony structure across the Cretaceous outside of South America, especially in southeast Asia. Many even lost teeth. I suggest it was due to this same phosphorus famine created by erosion of the soil of the runways of plant smothering termites. Pterosaurs and birds probably lost teeth primarily because of the young eating iron oxide and bauxite in the flying reproductive soil borne termites’ guts, which bound the phosphates. You may see this discussed in more detail starting in http://www.angelfire.com/nc/isoptera/termites.html and its links, which links explore the possible affect that ant evolution had upon them. By the time the Cretaceous ended the world ended up with tiny savanna vertebrates, most of them mammals, which were able to give their young phosphorus in milk at that critical stage. They were a far cry from the massive, well boned Stegosaurs, etc., which roamed around the Jurassic, and had diminished tooth structure at first. They were a long time starting to increase in size (several million years).ReplyDelete
You may see the affects on soil discussed in more detail in http://www.angelfire.com/nc/isoptera/soil.html .
Sincerely, Charles Weber
PS It is conceivable that you would also find interesting a hypothesis of my son explaining the Decca (or Deccan) lava flows as disruption of the crust by the disruption of the crust at the antipode (opposite side of a sphere) by a huge meteorite impact. You may see my version in http://charles_w.tripod.com/antipode.html .
Sincerely, Charles Weber