The cretaceous is one geological period, the third and the last of the Mesozoic era. It started 145 million years ago and lasted till 65 million years ago. It is one of the longest periods of history. It was identified by a Belgium geologist Jean d’Omalius d’Halloy in 1822. It is believed that ended with the fall of a meteorite that killed many of the species including the dinosaurs.
But how was life in this era?
The reptiles were the most important animals, noted the dinosaurs but they were the only ones, the first birds emerge. At first they were reptiles with wings but over time they came to have feathers, something similar happened with mammals since some reptiles evolve some such that would become furry mammals. Amphibians almost disappear while mollusks doubled, highlighting some of these invertebrates called ammonite. The angiosperms (flowering plants) appeared, they are the dicotyledons and then the monocotyledons.
The terrestrial fauna
In terrestrial animals we can distinguish several groups of reptiles, mammals and birds. The reptiles were the animals of mayor number highlighting the dinosaurs. The main reptiles were: Anapsos (actual turtles), archosaurs (flying reptiles, dinosaurs and thecodont – today birds and crocodiles), the lepidosaurs (current reptiles), synapsides (were mammals come) and pterosaurs (large flying dinosaurs). The first mammals were the marsupials that appeared in Australia and South America.
· Archaeopteryx: they are the best example of the transition from reptile to bird. They measured 35cm, very small compared with the rest. It had similarities with the theropod reptiles: long bony tail, claws on fingers and teeth in the jaws. They had well-developed feathers. Their wings are similar to that of modern birds. It is believed that they were insectivores.
- The therapsides: are the ancestors of mammals; they had quadruped posture and were carnivorous or herbivorous depending on the class. They developed a secondary palate. Teeth are specialized in various functions and their size was reduced. The phalanges of the hands and feet had the same shape than now 2-3-3-3-3.
· Eomaia: is the name given to the first mammal evolving in a placenta that is known. It measured 14 cm and weighed 25 grams. It has five upper incisors and four lower ones. It lived in the lower branches of trees or shrubs. It is similar to a squirrel or a mouse of today.
· The Tyrannosaurus rex: rex mean king in Latin, was a two-foot carnivorous dinosaurs with a big cranium as well as most of his body and tail, except the upper members who were small but with great force, ending with two finger claws. Coming to measure 13 m long and 4m high, weighing 6 tons was the largest and predator of the Tyrannosaurus.
· The Velociraptor: was a two legs carnivorous dinosaur with a long and rigid tail and with three claws. He had very long back legs that gave him great speed. Measuring 2 m long and half tall and weighed 15 kg. Was small compared with others of their family. It had a long and rigid tail. It was covered in feathers but could not fly.
The marine fauna
Plankton had changed and was practically the same as today. Some algae were expanded in large numbers. On top of the surface the ammonites, belemites and fish of various types swam with a new generation of aquatic reptiles. Sharks (Xiphactinus), large sea turtles (Archelon) and sea stripes already existed at this time. Reptiles such elasmosauridae, with their long neck, and mosasaurs, that were the most ferocious marine predators, existed in the seas. Creatures like lobsters and crabs had appeared. The bivalves were buried deeper and deeper in the sediment to scape from the predators, or developed very solid shells or spines to deter them. A group of bivalves, the inoceramid, had shells of almost 1.8 meters of length.
· Ammonites: they belong to a subclass of mollusks; the shell could be very large up to 2 m in diameter or too small, simple centimeters. It is believed that had tentacles on the head showing through the opening of the shell while the body was inside it. They were predators and became extinct at the end of the era.
· Elasmosauridae:They were marine reptiles that had a very long neck. They can measure from 3m to 14m and weighed 2 tons. It has six teeth per premaxilla and 71 neck vertebrae. They were incapable of raising anything more than its head above the water. Elasmosaurus was a slow swimmer and may have stalked schools of fish. The long neck would allow Elasmosaurus to conceal itself below the school of fish. It then would have moved its head slowly and approached its prey from below. They can´t lay eggs in land.
· Mosasaurs: were reptiles of 17 m. Being the most ferocious marine predators, they were big lizards related with the actual snakes. They had large jaws with sharp teeth, with long, slender body and limbs with the form of popsicles. They were the second largest marine animals. They feed with fish, turtles and ammonites as it is believed that they lived near the surface but could come to dive to great depths.
The flora was bigger than the fauna. Flowering plants (angiosperms) spread during this period, although they did not become predominant until near the end of the era. The first to appear were the dicotyledonous and then the monocotyledonous. Most of the actual trees that are today like the oak, the beech and the maple appeared in this era and did not suffer the crisis of the end of the cretaceous era. Some fern such as Gleicheniales appeared in this era.
· Williamsonia Sewardiana: was a cycadeoidphyte. It had a woody stem and simple leaves. It lived from the Jurassic period through the late Cretaceous period. Williamsonia was a bennettitalean, had a long, thin, branching, woody trunk covered with spirals of broken-off leaf scars.
· Horsetails were an important source of nutrition for plant-eating dinosaurs. These primitive vascular plants were fast-growing and resilient (they could propagate using underground runners which a grazing dinosaur wouldn't eat). This meant that a hungry dinosaur could eat the plant without killing it, since the plant would regrow from the rhizome (the underground stem).