an introduction to dinosaurs
6. the cretaceous period
THE CRETACEOUS
The Cretaceous period began 145 million years ago and ended 66-65 million years ago, making it the longest and last period of the Mesozoic era. The earth's climate was relatively warm, however a cooling trend that began at the end of the Jurassic continued into the early Cretaceous. There is evidence of snowfall at higher latitudes and the tropics became wetter than during the Jurassic. However this changed as the period progressed, with temperatures increasing again in the Late Cretaceous.
The Cretaceous period saw the development and radiation of flowering plants, with some of the oldest dating to around 122 million years. Birds also diversified during this period, as did mammals who, whilst generally still small in size, became a very relivant component of the fauna.
Dinosaurs were at their most diverse during the Cretaceous. While some of the large Sauropods such as Diplodocus went extinct at the end of the Jurassic, other giant sauropods like the Titanosaurs emerged and flourished in the Cretaceous. With their origins in the Jurassic, Tyrannosaurs went on to diversify and eventually become apex predators in the Late Cretaceous. Large herds of herbiverous ornithischians such as hadrosaurs, ceratopsians and ankylosaurs also thrived.
earth during the early cretaceous
Earth 120 million years ago during the Early Cretaceous. The world is warm and has no polar ice caps. Large reptiles dominate and mammals remained small. Flowering plants evolve and spread throughout the world. Marine transgression gradually floods much of southern England throughout the early Cretaceous, creating shallow seas.
The Wessex Formation
The Wessex Formation of the Isle of Wight (located off the southern coast of England) yields the richest and most diverse Early Cretaceous fauna in Europe. The formation dates to between 145-125 million years and was deposited in a floodplain environment. It contains many types of dinosaur; most notably, near the top of the formation is what is known as the "Hypsilophodon bed", a 1 meter thick bed containing an abundance of Hypsilophodon dinosaur fossils. Over one hundred complete skeletons have been recovered from this bed and they are believed to have accumulated due to a mass mortality event caused when a river broke its banks and deposited a large amount of sediment on the floodplain. Other dinosaurs in the formation include Iguanodon, Baryonyx, Neoventor and several types of sauropod.
Baryonyx
Baryonyx, fossils of which are found in the Wessex Formation, was a bipedal Spinosaurid dinosaur measuring at least 25-33ft (but probably larger). It had a large number of finely serrated, conical teeth and a 12 inch claw on its first finger.
earth during the mid cretaceous
Earth 90 million years ago during the mid-Cretaceous. Ceratopsian and pachycephalosaurid dinosaurs evolve. Modern mammal, bird, and insect groups emerge. The continent of North America splits into two landmasses, seperated by The Western Interior Seaway.
The Kem Kem Beds
The Kem Kem Beds are located in southeastern Morocco and date to around 95 million years. They yield an exceptional number of dinosaur fossils, such as Spinosaurus, Carcharodontosaurus, abelisaurids and sauropods. These beds are poorly understood and limited thorough fieldwork has been conducted to date despite the abundance of fossils. Aside from dinosaurs, the beds also contain numerous crocodile, turtle, pterosaur and fish fossils. Many questions remain about the Kem Kem Beds: why is there an abundance of predatory dinosaurs and an apparent lack of herbivores? Were these dinosaurs all sharing the same habitat or were the beds laid down over a longer period of time?
Spinosaurus
Spinosaurus fossils are found regularly in the Kem Kem Beds. It was among the largest of all known carniverous dinosaurs, with adults reaching over 50ft in length. It's unserrated, peg-like teeth suggest it mostly preyed upon fish in the rivers, deltas and estuaries of modern day North Africa. It also had a large "sail" on its back and a massive fin-like tail which suggests it was semi-aquatic.
earth during the Late cretaceous
Earth 66 million years ago during the Late Cretaceous. The Western Interior Seaway which had split North America is displaced by the uplift of the continent and eventually disappears.
Dinosaurs of Mongolia
Almost 100 years ago American scientist Roy Chapman Andrews discovered the first dinosaur eggs in Mongolia. Over the last century scientists from all over the world have returned to Mongolia's Gobi Desert to collect and study its treasure trove of Late Cretaceous dinosaur fossils. The fossils found here are roughly 75-71 million years old, although the exact ages of the relevant rock formations are uncertain. The Gobi Desert hasn't changed considerably in the last 75 million years and was a mix of sand dunes, valleys and freshwater lakes in the Late Cretaceous.
Velociraptor
Velociraptor was a mid-sized dromaeosaur, measuring just over 6.5ft in length with three strongly curved manus (hand) claws for slashing at or gripping prey. It's thought Velociraptor and other similar Dromaeosaurs may have attacked and killed their prey in a similar way to modern day birds of prey; leaping onto their victim, pinning it down with their body and gripping it tightly with its large sickle-shaped claws. They would then feed upon their prey while it was still alive, the animal eventually dying of blood loss.
The Fighting Dinosaurs
Mongolia has yielded some incredible dinosaur fossils, with one of the most famous being the 'Fighting Dinosaurs'. The fossils were discovered in 1971 by a Polish-Mongolian team and preserve a Velociraptor and Protoceratops in the midst of battle. It is believed they died mid-fight when they were either engulfed by a sandstorm or buried by a collapsing sand dune.
Tarbosaurus
An apex predator, Tarbosaurus bataar was one of the largest Tyrannosaurs of the Cretaceous, reaching up to 39ft in length. There is some debate as to whether Tabosaurus is actually an Asian relative of the North American Tyrannosaurus rex and it has previously been assigned the species name Tyrannosaurus bataar. The first fossils of Tarbosaurus were discovered in 1946 by a Soviet-Mongolian expedition to the Gobi Desert.
Protoceratops
Protoceratops was a small quadrupedal dinosaur measuring up to 6ft in length. It had a disproportionately large head for its small body size and also had a distinctive neck frill. The discovery of the first Protoceratops nest in 2011 has provided evidence that this dinosaur cared for its young at the nest during at least the early stages of life.
Oviraptor
Oviraptor was a bird-like, non-Avian theropod dinosaur. It was a small dinosaur, measuring around 3.3ft in length and, whilst discovered almost 100 years ago in 1923, it was poorly understood for most of that time. The first Oviraptor fossil was found atop a nest of what were believed to be Protoceratops eggs, which is why it was assigned the name Oviraptor (latin for "egg taker"). However, with the discovery of similar oviraptorids atop nests in the 1990s, it soon became clear that the eggs in question actually belonged to Oviraptor itself.
The Hell Creek Formation
The Hell Creek Formation is Late Cretaceous in age and stretches over several North American states including Montana, North Dakota, South Dakota and Wyoming. The formation was deposited over a period of around 2 million years, from around 67.5 to 65.5 million years. An iridium-enriched boundary marks the top of the Hell Creek Formation and was deposited by the Chicxulub meteorite impact. The Hell Creek Formation and the overlying (younger) Fort Union Formation offer a continuous record of depositation, from the Late Cretaceous through to the Paleocene.
Hell Creek is famous for its abunance of dinosaur fossils and has been intensively studied. It was laid down by rivers, deltas and occasionally peaty swamps along the edge of the Western Interior Seaway. The climate was subtropical and the rivers and deltas were inhabited by crocodiles, champsosaurs, turtles, frogs and lizards.
Among some of the best known dinosaurs from this formation are Triceratops, Tyrannosaurus rex, Edmontosaurus, Anylosaurus and Pachycephalosaurus.
DINOSAURS OF THE HELL CREEK FORMATION
Triceratops
Triceratops is perhaps one of the best known dinosaurs. It was a large herbivore, growing to around 30ft in length and had a large skull featuring a frill and horns above its eyes and nose. Its teeth were arranged into groups called batteries, with up to 40 tooth columns in each side of the jaw, 3 to 5 teeth per column. Triceratops replaced its teeth throughout its life and its teeth are some of the most common dinosaur fossils found in the Hell Creek Formation.
Tyrannosaurus rex
Tyrannosaurus rex was one of the largest land carnivores to ever exist. One of the largest skeletons found to date (nicknamed "Sue") measured 42ft in length. Tyrannosaurus had a large skull measuring up to 5ft in length, with a thin snout for good binocular vision, large and robust teeth and a powerful bite. The center of many of the bones in its skeleton exhibit a honeycomb like texture to help reduce weight.
Nanotyrannus
Nanotyrannus is a Tyrannosaurid dinosaur from the Hell Creek Formation. It was first discovered in 1946, but was originally assigned to the genus Gorgosaurus. That was changed in 1988 when Robert T. Bakker reassigned the skull to a new genus, Nanotyrannus. There is currently a lot of debate as to whether Nanotyrannus is a juvenile form of Tyrannosaurus rex.
Pachycephalosaurus
Pachycephalosaurus was a medium sized herbivor, growing up to 15ft in length. It is famous for having a large bony dome atop its skull which in some cases was up to 10 inches thick. Its skull was short with eye sockets that faced forward, suggesting good binocular vision. Based on other Pachycephalosaurs, it was probably bipedal, but to date only skull remains have been found in Hell Creek.
Edmontosaurus
Edmontosaurus was a large, herbivorous, hadrosaurid dinosaur. It was one of the largest known hadrosaurs, growing to over 40ft in length. It had bulky hind legs, a laterally flattened tail and a duck-like beak. Like Triceratops, it had batteries of teeth which were continually replaced. Some Edmontosaurus skeletons have been found with exceptional preservation, including several "mummified" specimens.
The K-T Extinction event
The reign of the dinosaurs comes to an end.