Dinosaurs of the Jurassic Period

Dinosaurs of the Jurassic Period Nicole Jones Park University Abstract This paper examines the Dinosaurs of the Jurassic Period. The paper considers the holistic view of the Jurassic period in addition to the Dinosaurs that roamed the Earth during this period. Finally, historical consideration evidence leading to the demise or disappearance of the dinosaurs.

Analysis is based upon research conducted from ten academic reference sites. The paper provides an understanding of the different species that lived during the Jurassic period and the environmental and climatic conditions that supported them. Also discussed is a significant amount of information regarding palaeontologists’ discoveries of great dinosaur faunas, such as the Jurassic Morrison Formation. Why did the dinosaurs flourish during the Jurassic period and what evidence is there to support it? How did the environment and climatic conditions impact the evolution of the species? What ultimately resulted in the extinction of the species? The Jurassic Period, also known as the Age of Reptiles, and is marked from the end of the Triassic period to the beginning of the Cretaceous period; the time frames within this period are broken down into Early, Middle and Late Jurassic. There are three main extinction events in the life of the dinosaurs, none of which took place during the Jurassic period; however, the beginning of the Jurassic period is identified by the Triassic-Jurassic extinction.

The term Jurassic was linked to the Jura Mountains of Switzerland, a small range north of the Swiss Alps dividing the Rhone and Rhine rivers (Palmer, 2002). In the Jurassic period, there were many vertebrates living in the oceans, including, fish and marine reptiles; these included that of coiled ammonites, ichthyosaurs, marine crocodiles, and long-necked plesiosaurs. Herbivores roamed the earth, feeding on the lush growths of palm-like cycads and ferns, while being preyed upon by smaller but vicious carnivores; and vertebrates such as the pterosaurs took to the sky (Palmer, 2002). During this time, the dinosaurs dominated the earth and were more numerous and more extraordinary than those in the Triassic period, in fact, the Jurassic period housed the largest land animals of all time.

Palaeontologists have discovered many amazing dinosaur faunas revealing information about the dinosaurs as well as the geographical and climatical terrain of the Jurassic period, conceding the Jurassic Period to be one of the most flourishing times in the Mesozoic era. Climatic features are a very important consideration in Jurassic times, primarily temperature and precipitation; for many years it has been said that the climates of the Jurassic were similar to those of the Cretaceous, yet were considerably more equable than the climates we know today (Hallam, 1993). Palaeontologists had formed a consensus that the climate was hot and humid, with steady precipitation which presented ideal climatic conditions for the explosive spread of flora. However, this belief has recently been challenged; while the climates of the Jurassic haven’t been studied like those of the Cretaceous, enough data has been gathered to form reasonably confident hypotheses regarding the overall patterns during that time. One example in particular is shown in the large supercontinent of Pangaea, which is believed to have experienced a significantly seasonal range of temperatures (Hallam, 1993). Early in the Jurassic period, this large continent continued and accelerated to breakup, creating huge volcanoes with rivers of lava and clouds of poisonous gases; it is possible that this contributed to the significantly higher atmospheric levels of carbon dioxide which is assumed of Jurassic times (Hallam, 1993). Pangaea divided into many smaller continents; the northern half, Laurasia, divided into North America and Eurasia, and by the Middle Jurassic the southern half, Gondwana began to break up; the eastern part, to include Antarctica, India, Australia and Madagascar separated from Africa and South America, making up the western portion (Unknown, 2010). Thus, oceans engulfed the areas in between, raising the mountains on the seafloor, which raised sea levels higher, flowing onto the continents (Unknown, n. d. ). Hence, the climate changed accordingly; for example, they may have experienced “strong seasonal contrasts of temperature within large continental areas as well as some polar ice.

Monsoonal effects were dominant on the continents and rainfall in low and mid latitudes was probably strongly seasonal, with arid conditions prevailing at low latitudes” (Hallam, 1993). In the late Jurassic, scientists noted a considerable spread of aridity in southern Eurasia and attributed this to orographic effects (Hallam, 1993). Although there are no concrete conclusions regarding the climate during the Jurassic times, we do know that the forests flourished and the dinosaurs grew larger than ever before. Research has shown the flora distributions of the Jurassic period indicate a wide array of ferns, ginkgoes, conifers, bennettitaleans and cycads, many of which still exist today (Palmer, 2002). In the Jurassic life, the conifers were the most variegated of the large trees, and among those were the ginkgos. Ginkoes carpeted the mid-to high northern latitudes, and podocarps, a type of conifer, were particularly successful south of the Equator” (Unknown, n. d. ). The ostensibly palm-like cyads were very diverse and abundant; so much so that the Jurassic period could also be referred to as “the Age of the Cycads” (Kazlev, 2002). Some cyads were very tall trees with rough branches that were covered by leaf scars and fern-like fronds (Kazlev, 2002). Others, such as the beenettitaleans, were of extreme importance among the shrub-like trees, having short and stubby trunks with the fronds sprouting from the top (Kazlev, 2002). Although many different trees and plants fared well during the Triassic period, we can conclude the flora of the Jurassic was much more lush and verdant (Viegas, 2010). However, there are marked differences in the vegetation based on their geographical location. In the Lower Jurassic, scientists have mapped the northern floral zone to include such countries as Siberia and Japan, Greenland, northern and central Europe; from here, they mapped a southern zone, which extended from Mexico to the Middle East and southern China (as cited in Hallam, 1993). Therefore, it is possible the zones might reflect a wide variation of the latitudinal differentiation, however, some considered the differences were due in part to the continentality of the climate, thus exhibiting evidence to the degree of contrast between the seasons (as cited in Hallam, 1993). The climate of the northern zone was considered to be humid and moderately warm whilst that of the southern zone could be compared to the present humid tropical zone (Hallam, 1993). Conifer pollens have been used as an indicator to detect the amount of aridity during this period; the pollens were found to be the most abundant Upper Jurassic sediments, and as such indicated a time of maximum aridity during that time (as cited in Hallam, 1993). Scientists also found strong differentiation between the conifer species observed in the northern hemisphere and that of the southern emisphere; this is considered not a reflection of climate change, but more of shifting continents, as pollen, cannot cross oceans (Hallam, 1993). For example, “The Indian floras are significantly different from those in adjacent Eurasia, which presumably relates to the subcontinent’s northward migration in post-Jurassic times” (Hallam, pg. 288. 1993). In essence, the Jurassic period was a time of abundance in terms of vegetation growth and the evolutionary growth of the dinosaurs, but there was more to life than plants and dinosaurs; the seas also began to flourish. The breaking up of the supercontinent created a new environment for marine life as well; Similar to the growth on land, massive changes were taking place in the oceans in many different ways, particularly with the newly formed shallow interior seas. Fishlike ichthyosaurs were at their crest, replacing their Triassic predecessors, they shared the oceans with creatures displaying long necks and paddle fins known as plesiosaurs, in addition to giant marine crocodiles, and modern shark groups also began to appear (Kazlev, 2002). Another species prominent in the seas were the Jurassic cephalopods which included ammonites and belemnites. In the seas there was also a wide array of invertebrates, to include: sponges, corals, bryozoa, gastropods, bivalves and ammonoid and belemnite cephalopods; while they all thrived, the latter two groups becamd the dominant nektonic invertebrates for the remainder of the Mesozoic period (Kazlev, 2002). Plankton was also very abundant, more specifically, the dinoflagellates and coccolithophorids became more diverse during this time (Palmer, 2002). Indeed, the Jurassic oceans have been said to have something really special about them; “Of the dozen or so types of planktonic organisms with a fossil record, at least four either first evolved or experienced massive radiation during Jurassic” (Kazlev, 2002). There are many ideas of what that “something” might be; it could have been something as simple as an abundance of free calcium, however, red algae evolved in the Jurassic which suggests that the “something in the water” was the rhodophytes themselves, or alternatively, their chloroplasts (Kazlev, 2002). Some have gone as far to say that the plankton at that time was so copious it may have turned parts of the ocean red.

Due to the vast changes on the land and in the seas, we can assume the dinosaurs of the Jurassic evolved as well, keeping up with their surroundings. The dinosaurs of the Jurassic period were very different from each other, consisting of a greater variety, in comparison to other periods. Due to the changes in climate, the forests grew, the herbivores became immense, and the carnivores also grew larger and fiercer to go up against such huge prey. It was a time of the largest land animals ever to exist on our planet; the saurpods were enormous vegetarian feeders who fed on a variety of plants, rumbling around on four, huge, pillar like legs, had a very long necks and long tails following behind (Unknown, n. d. ). This large group of saurischian herbivores had relatively small heads, considering their overall size, and displayed peg-like teeth that were primarily used for grazing (Unknown, n. d. ). Included in this group was the Apatosaurus, also known as the Brontosaurus, weighing in at about 30 tons and was around 65 feet in length; another well-known herbivore during this time was the Diplodocus; the Diplodocus was much longer, measuring around 90 feet in total length, and an estimated weight of 11 tons (Unknown, n. d. ). Also in the Late Jurassic was the Brachiosaurus who was even more enormous, weighing an astonishing 55 to 80 tons (Unknown, n. d. ). These animals walked the great floodplains eating the leaves of tall conifers and taking water from the rivers as they traveled; they are believed to be in herd animals that traveled at a very leisurely pace. Herbivorous dinosaurs consumed a vast amount of food in order to survive; they needed to eat continuously in order to store up large surpluses of energy in order to escape their predators.

Another plant-eating dinosaur from the ornithopods, was the Stegosaurus, one of the most popular of the distinct-looking stegosaur group; famous for the row of back plates, also known as ‘dermal plates or dermal armor’, which palaeontologists believe were used to regulate body temperature (Unknown, n. d. ). The Stegosaurs was a slow moving dinosaur whose appearance may have warned off predators, and possibly swayed its tail using the spikes to defend itself. Carnosaurs, meaning “meat-eating lizards”, were theropods who reached their peak during the Jurassic period. These carnivorous dinosaurs were bipedal, meaning they walked on two legs and they were smaller in comparison to the sauropods, however they were much more vicious. One of the most formidable carnivorous dinosaurs of this time was the Allosaurus; this was a fierce and terrifying creature that had long narrow jaws filled with very sharp teeth that could rip its prey to shreds; this dinosaur was extremely well muscled, growing to about 35 feet tall, having extremely short front legs with long sharp claws and extending from its feet were three long toes with talons similar to an eagles (Smith, 2005). Another meat-eating dinosaur that was present during Jurassic times was the Dilophosaurus; this creature sported a double crest on the top of its head (which is guessed to be either ornamental or sexual), measured about 6 meters long, stood about 8 feet tall, and was quite slender (Smith, 2005). The Dilophosaurus had “strong hind legs; forelimbs with hands that were flexible, with an opposable thumb… so he can grasp his prey… his hind legs, his feet, were armed with very powerful claws and were probably used as weapons as well as for locomotion. He was bipedal of course, and probably a very rapid runner” (Smith, 2005). One of the first dinosaurs to be found and named was the Megalosaurus, meaning “great reptile”; this large carnivore also lived in the Jurassic period and was considered to be more intelligent and could run faster in comparison to the other dinosaurs (Dinosaur Facts, 2010). Standing about 10 feet tall, measuring 30 feet long, and weighing in around a ton, this fierce hunter was equipped with a big bulky body, long legs with clawed feet, sharp claws protruding from his three-fingered hands, and big powerful jaws full of long and sharp teeth, enabling it to prey on even the largest of the sauropods (Dinosaur Facts, 2010). Only one question remains, how do palaeontologists know all of this information and where exactly do dinosaur fossils come from? One of the premier places for fossils of this period is that of the Morrison Foundation, a distinct sequence of late Jurassic rock found in the Western United States. The upper Jurassic Morrison formation is considered to be one of the most opulent sites of dinosaur faunas in the world and provides large amounts of information about the geographical and climatical terrain of that time.

The Morrison dinosaur bones were initially discovered in Cimarron County, Oklahoma close to the Oklahoma- New Mexico State line. This incredible discovery revealed initially four different types of dinosaur bones; most of which came from a single genus, Brontosaurus (Stovall, 1938). Extensive quarry operations have been carried on at the site of the first discovery; over 3,500 individual bones were collected, providing an indication of the species that made up the dinosaur community during this period including: Ceratosaurus, Stegosaurus and Camptosaurus (Stovall, 1998). One of the remarkable things about the discoveries in the Morrison formation is the excellent quality of the xhibits discovered; a new quarry for Jurassic dinosaurs was found approximately 8 miles east of Cleveland, Emery County, Utah (Stokes, 1945). “Partial remains of 19 individual dinosaurs were recovered from excavations which measure in plan about 35 by 25 feet.

The specimens are referred to species within the following genera: Diplodocus, Brontosaurus, Stegosaurus, Camptosaurus, Ceratosaurus and Antrodemus” (Stokes, pg. 116, 1945). There are several theories that have been suggested by the physical characteristics of this quarry, named after Mr. Malcolm Lloyd of Philidelphia, which could possibly explain the accumulation of bones; In Morrison time, dinosaurs may have not been any more numerous than in other periods of the Mesozoic, but their bones were excellently preserved indicating the siliceous nature of the bentonitic matrix was likely to be the reason (Stokes, 1945). “Overlying the bone bed is a 3- to 4-foot layer of almost pure bentonite containing fragments of zircon, quartz, plagioclase, mica and hematite”, which suggests volcanic activity could be responsible for the death of the dinosaurs at the site of the Lloyd quarry (Stokes, 1945). An unusual discovery in this deposit was the ratio of the carnivorous dinosaurs to the herbivorous dinosaurs which was nearly two to one; compared to most other large Morrison bone beds where the ratios are reversed indicating the carnivores to be rare (Stokes, 1945). The abnormally large numbers of the carnivores in the Lloyd quarry could possibly be explained by the scavenging tendencies of the Antrodemus; in this deposit, 10 to 11 specimens of Antrodemus were found, indicating they may have congregated and died from starvation amid the herbivores bones (Stokes, 1945). Many jaws of the Antrodemus species were so well preserved that full sets of teeth were found to be in place; this provided a tremendous amount of dental information including that of tooth growth and tooth replacement, “Antrodemus, was theoretically never left with a gap of more than one tooth’s space on each maxillary or dental bone at any one time” (Stokes, 1945). For palaeontologists, the discovery of the Lloyd quarry was much like finding a buried treasure. The upper Jurassic Morrison formation produced one of the richest fauna deposits in the world, with sediments distributed over more than 1,000,000 km? across the western region of the United States; this includes Wyoming, Colorado, Montana, North Dakota, Nebraska, New Mexico, Arizona, Utah, South Dakota and Idaho. Essentially this deposit represents “a mosaic of riverine, lacustrine and floodplain environments developed on a vast alluvial plane nourished by the debris from ancestral Rocky Mountains” (Dodson, pg. 08, 1980). It appears the flora productivity was great in order to support an abundant amount of rather large herbivores indicating that some parts of the Morrison area were at times humid enough in order lush vegetation to develop; however, this fauna lacked coals but had a plethora of oxidized sediments, and small aquatic vertebrates were scarce suggesting that water was recurrently in short supply (Dodson, 1980). Dinosaurs of the Morrison were not contained within particular depositional environments but were spread across the complete distribution of accessible habitats; this suggests that these animals were a part of a land-based and cursorial community (Dodson, 1980). One model for the Morrison suggests that the paleoenvironment had both humid and semi-arid conditions, proposing “a vast alluvial plain with a climate of high average rainfall and seasonal extremes of dryness and humidity” (Dodson, p. 211, 1980). The Morrison Formation is the most copious source of the Jurassic dinosaurs that roamed North America; dating back to the Marsh and Cope wars, the Morrison was one of the first areas where the bones of dinosaurs were discovered which reveals its long history (Unknown, 2001). The Jurassic period was considered to be the Age of the Dinosaurs. It started after the Triassic period some 260 million years ago and ended some 144 million years ago, thus spanning a gap of about 60 million years in the middle of the Mesozoic era. The climate was warm, humid, moist and stable giving rise to many new plant and fauna species; the conditions were ideal for them to live and adapt.

The abundance of plant food enabled the herbivores to develop and become huge plain dwelling animals and as they became bigger, evolution similarly adapted the carnivores to become larger and more ferocious to deal with the size of its prey. As the land animals developed, equally the climatic conditions favoured the development of marine life; this creating an abundance of fish to support the development of an increasing population of predatory reptiles. It is imperative to give credit where credit is due; without the hard work, research and studies completed by scientists and palaeontologists, we wouldn’t have such a vast amount of dinosaur history available to us. Palaeontology is divided into various sub-disciplines and can be described as the study of what fossils tell us about the ecologies of he past, about evolution, and about our place, as humans, in the world; it integrates knowledge from many different disciplines such as geology, biology, anthropology, ecology, and archaeology to give us a better understanding of the processes that have led to the origination and eventual destruction of the various types of organisms since life began (Unknown, n. d. ). References Dinosaur Facts. (2010). Jurassic Period. Accessed on 20 Feb 2010 from: https://www. dinosaurfact. net/jurassic. php. Dodson, P. , Behrensmeyer, A. K. , Bakker, R. , and McIntosh, J. 980. Taphonomy and Paleoecology of the Dinosaur Beds of the Jurassic Morrison Formation. Paleobiology 6: 208-232. Hallam, A. Crame, J. A. , Mancenido, M. O. , Francis, J. ; Parrish, J. T. 1993. Jurrasic Climates as Inferred from the Sedimentary and Fossil Record [and Discussion]. Philosophical Transactions: Biological Sciences 341: 287-296. Kazlev, M (2002). The Jurassic: The Jurassic Period of the Mesozoic Era: 200 to 146 million years ago. Accessed on 22 Feb 2010 from: https://www. palaeos. com/Mesozoic/Jurassic/Jurassic. htm.

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