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| Introduction | Last Updated • May 7, 2019 | ||||||||||||||
After the Triassic, life in the remainder of the Mesozoic was dominated on land by dinosaurs. dinosaurs thrived through the Jurassic and expanded and diversified in the Cretaceous. As you remember, most of today's continents were grouped in what we call Pangea, a giant landmass surrounded by the global ocean, Panthalassa. Pangea started to break up in the Jurassic and continued to do so in the Cretaceous. Since dinosaurs are land reptiles, they need solid grounds to move, and if an ocean comes in between continents, dinosaur populations remain isolated. That is exactly what happened during the last part of the Mesozoic, when North America started to separate from Europe, and South America from Africa. Then the northern continents moved away from the southern ones, allowing the expansion the Tethys Ocean westward.
Despite all the evolutive steps that occurred because of this continental breakup, dinosaurs - together with several other species - became extinct at the end of the Cretaceous. This extinction wiped away about 85% all of known species, somehow less than the Permo-Triassic extinction (which was at 95%) but still a powerful one.
Here is a map of the world during the Late Jurassic, the time of the Morrison Formation.
Here is a map of the world during the Late Cretaceous, about 30 million years before the K/T boundary extinction. Here is a map of the world at the Cretaceous/Paleogene boundary, right after or towards the end of the deposition of the Hell Creek Formation.
 The Jurassic and the Cretaceous: the Morrison Formation and the Hell Creek Formation in relation to the Geological Time Scale Since both of these Lagerstätten deal with the expansion (and successive extinction) of dinosaurs, they are dealt with together in this quiz. | The Morrison Formation
| Last Updated • May 7, 2019 | As we said above, land life during the Mesozoic was dominated by dinosaurs. Dinosaurs evolved and diversified making it possible to observe different fossil assemblages, and because of this, we need to learn some nomenclature. Here is a table that might help you retain this information:
 Fossil evidence for terrestrial life during much of the Cretaceous is quite poor, but towards the later part of the period there are some exceptionally rich deposits. The Morrison Formation of North America is a vast and highly productive sequence known for its spectacular dinosaur skeletons. The Morrison Formation outcrops along the Front Range of the Rocky Mountains in Montana, North Dakota, South Dakota, eastern Idaho, Wyoming, Nebraska, Utah, Colorado, western Kansas, northeastern Arizona, north New Mexico and small parts of Oklahoma and Texas. Read this section for a nice tale of discovery and for completeness of information, but I will not ask any question out of this paragraph The Morrison Formation has been divided into the upper Brushy Basin Member and the lower Salt Wash Member in 1998, and it dates approximately to 150 million years ago. Its thickness reaches 188 meters within the boundaries of Dinosaur National Monument, in Utah. The Morrison Formations is followed in time by the Lower Cretaceous Cloverly Formation, which is highly resistant and protects the Morrison Formation from erosion. The Morrison Formation itself is found on top the Middle Jurassic Sundance formation, which represents the marine deposits of the Sundance Sea (the arm of the ocean that split North America in two giant islands, see above). This sequence clearly indicates a regression, marking the northward withdrawal of the Sundance Sea. There is enormous variations in the environment of the Morrison Formation but most bone accumulation occurs in immature sandstones probably deposited as a consequence of cataclysmic flash floods. Upon the retreat of the Sundance Sea, coastal areas were slowly replaced by floodplains traversed by slow, meandering rivers, crossed by herds of dinosaurs looking for food. Episodes of severe droughts caused the dinosaurs to gather around the remnant water pools, where they eventual died of dehydration, as it happened in the deposits of the Chinle Group. When rains started again, the raging rivers entered the pool areas where the dinosaurs had met their destiny, picking up their now disarticulated bones and burying them in immature sands, representing the fill of rivers channels. When we see concentration bones such as it is observed in the Morrison Formation, we should always wonder if they form as a consequence of catastrophic or non-catastrophic episodes. In the first case, an external agent kills all animals without discrimination, while in the second the killing agent might be selective. Based on what is observed in the field in the Morrison Formation, geologists and paleontologists are oriented towards the second hypothesis, The preservation of disarticulated bones is favored by the arid climate of the time. Possibly, dinosaur carcasses decomposed on dry open land or in channel beds before the successive flooding episode. The bones do not show evidence of scavenging or predation, indicating an environment that was hostile to other living creatures. At the same time though the bones do not show extreme exfoliation or cracking, indicating that the arid condition possibly lasted only a few years, possibly not beyond 10, before burial by flash flood. Many of the dinosaurs found in the Morrison Formation have populated the imagination generations of kids and young adults, and you will be able to recognize some of these names. Read this section for completeness of information, particularly if you are curious about these creatures, as I would expect you to be, but the only things that you might find in the next quiz would be about what kinds of creatures you would find in this Lagerstätten: dinosaurs, but also other reptiles (frogs, lizards, crocodiles, turtles), amphibians, mammals, fish, invertebrates (mollusks such as gastropods and bivalves, but also ostracod and others), and plants (mosses, horsetails, ferns, cycads, ginkgoes, and conifers) The Morrison formation constitutes a land deposit formed in arid to semi-arid environment occasionally interrupted by heavy precipitation and subsequent flooding. Arid conditions were probably enhanced by mountains to the west of the area, and this is also supported by the existence of evaporites, desert sand dunes, and saline lake facies. Still, we observe freshwater fish and invertebrates, and a variety of subtropical plants that require humid conditions and some perennial water in lakes and rivers crossing this semi-desertic land, similarly to what we see in the actual Colorado Plateau of the American Southwest or in the Nile River valley of Egypt today. Likely, herbivorous dinosaurs followed the streams and their vegetation-covered marginal areas, and carnivorous dinosaurs hunted them in packs. Other reptiles, and mammals too, thrived closed to these water bodies. Mammals were small and probably nocturnal and arboreal, in order to avoid becoming the carnivorous dinosaurs' favorite preys. Dinosaurs fossils form the Jurassic have also been retrieved from the Tendaguru Formation of Tanzania, in Africa. read this paragraph for completeness of information but I will ask no questions out of it. | The Hell Creek Formation
| Last Updated • May 7, 2019 | We have already observed (see top of this page) how the progressive break-up of Pangea led to the evolution of different species of organisms on different continental landmasses that were previously united. This is true for all land organisms, and also for dinosaurs. During the Cretaceous for instance, we went from a moment (Early Cretaceous) when Europe and North America were part of the same land mass and dinosaurs could walk from one side to the other, to another moment (Middle Cretaceous) when an ocean had started to develop between Europe and North America, and dinosaurs could not cross it, thus evolving in time into different species. Something similar happened between the northern (Europe and North America) and the southern (South America and Africa) continents. So, it seems like life was going normally for these creatures, and for many others too, when, at the end of the Cretaceous, a widespread extinction event. This event wiped out about 85% of all living creatures, at least as reconstructed from the fossil record, including of course dinosaurs. This extinction event was chosen as the changing moment that marked the end of the Mesozoic and the beginning of the Cenozoic. Together with dinosaurs, all land animals bigger than 1 m in size were wiped out, all the flying reptiles, and even 33% of higher-level plans taxa. In the ocean, most marine reptiles, ammonites and many brachiopods and bivalves also disappeared. A Jurassic ammonite from the Italian Alps
According to the most likely theory, the extinction of all these creatures was triggered by an extra-terrestrial impact, whose recognition is based on the identification of a so-called "boundary clay", first identified at Gubbio, Italy by Walter Alvarez. This clay contains incredibly high amounts of the metal iridium (Ir) that are only compatible with an extraterrestrial origin. The Cretaceous / Tertiary (K/T) boundary at Gubbio, Perugia, Italy The boundary clay is clearly visible along the surface with the coin.
The International Commission on Stratigraphy (ICS) is a scientific body whose primary objective is to precisely define global units (systems, series, and stages) of the International Chronostratigraphic Chart that, in turn, are the basis for the units (periods, epochs, and age) of the International Geologic Time Scale.
Not long ago, the ICS has decided to get rid of the old term "Tertiary", so technically the K/T boundary is now the Cretaceous / Paleogene (K/Pg) boundary. The K/T boundary in Italy and in Denmark is in deep marine sequences. As such it is relatively undisturbed, and the boundary clay is easily recognizable. In other parts of the world, including the Hell Creek succession of Montana, North Dakota and South Dakota, the K/T boundary occurs in terrestrial sediments. The beds below the K/T boundary have yielded some of the most amazing and controversial dinosaurs ever found. Dinosaurs, and of course many other organisms were found here, but only BELOW the K/T boundary. This Lagerstätte has become the most valuable section in the world in which to evaluate the enormous changes in fauna and flora that occurred (on land) across the K/T boundary. As usual, read this part for completeness of information, but there will be no questions out of it on the quiz.
You have read in the previous section about the problem related to the exact positioning of the K/T boundary between the Hell Creek Formation and the Fort Union Formation. The K/T boundary was normally identified by the boundary between the Hell Creek and the Fort Union Formations. In reality, after Alvarez's theory made it clear, iridium was found in the lower part of the Fort Union Formation, so that the contact between these two lithostratigraphic units is actually fully in the Cretaceous. There are about 3 meters (10 feet) between the contact and the actual K/T boundary. Taphonomically, the Hell Creek Formation can be regarded as a Concentration Lagerstätte but therein some evidence that it may also be regarded as a Conservation Lagerstätte: cartilaginous structures such as beaks, vertebral spines fragments, ribs, have been reported along with evidence of vascular structures. Skeletons with skin impressions are also known, and it ha been suggested that the relatively organic, anoxic, and slow-moving waters of coastal wetlands may provide unusual opportunities for such preservation. Read this part, except for the sections on mammals (pages 180 and 181) and plants (page 182), which you should know. Study this part in its entirety Read this section for completeness of information, but I will ask you no question out of it.
© Alessandro Grippo, since 1994 | |||||||||