INTRODUCTION TO LIFE OF THE MESOZOIC ERA

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1 BACKGROUND INTRODUCTION TO LIFE OF THE MESOZOIC ERA Fossil (fos -sil): Any remains, trace, or imprint of a plant or animal that has been naturally preserved in the Earth s crust since some past geologic time; any evidence of past life. e This set contains 12 specimens of Mesozoic fossils selected to show representative fossil forms collected from rocks of Triassic, Jurassic, and Cretaceous ages. The fossils in this set provide a broad overview of the paleontological record in the rocks from each period of the Mesozoic Era. The specimens are arranged alphabetically in the Set. The accompanying activities effectively cover the basic concepts necessary for the student to identify and understand certain typical Mesozoic fossils. THE GEOLOGIC TIME SCALE Geologic column: A composite diagram that depicts in a single column the subdivisions of part or all of geologic time or the sequence of stratigraphic units of a given locality or region. The oldest rock units appear at the bottom and the youngest at the top. Each unit is arranged vertically to indicate their relative positions to each other and to the subdivisions of geologic time. In order to understand the significance of fossils and their role in geologic history, we must have some knowledge of geologic time. An understanding of the geologic time scale and the geologic column is important for the earth scientist who must be able to associate fossils, and the rocks that contain them, with specific chapters in earth history. The earth scientist, like the social scientist dealing with the development of civilization, must have some method of relating important events to one another. Consequently, earth scientists have developed a special time scale consisting of major divisions and subdivisions of geologic time. These divisions are arranged in sequence on a time scale in the order of their age. Therefore, a geologist might speak of an animal that lived during the Paleozoic Era in much the same manner that a historian might speak of a general who served during the Civil War. Each of these terms gives us an idea as to when an animal lived or a general served, but in terms of relative time rather than in a specific dated time. The geologic time scale is a geologic calendar consisting of named units of time during which various rock strata were formed. Geologists use these time units to date geologic events that took place in the past. The largest time units are eras. The eras are divided into periods, and these may be further subdivided into smaller time units called epochs. When arranged in their proper historical sequence, these time units provide a standard by which the age of rocks can be discussed. Thus, the geologic time scale might be roughly compared to the calendar in which the year is divided into months, months into weeks, and weeks into days. Unlike years, however, geologic time units are somewhat arbitrary in length and of differing duration. The geologist cannot be positive about the exact length of time spanned by each unit. The time scale does, however, provide an internationally recognized standard by which we can discuss the ages of rocks and their incorporated fossils. Geologic time has been divided into six eras. Each has been given a name of Greek derivation that is descriptive of the sequential position of that era on the geologic time scale. 1

2 1. Cenozoic (SEE-no-zo-ic) recent-life - from present to 65 million years ago. 2. Mesozoic (MES-o-zo-ic) middle-life from 65 million to 248 million years ago. 3. Paleozoic (PAY-lee-o-zo-ic) ancient-life from 248 million to 544 million years ago. 4. Proterozoic (PRO-ter-o-zo-ic) fore-life from 544 million to 2.5 billion years ago. 5. Archaean (ar-kay-en) beginning-life from 2.5 billion to 3.8 billion years ago. 6. Hadean (HAD-e-an) from 3.8 billion to 4.5 billion years ago. GEOLOGIC TIME SCALE ERA PERIOD/(EPOCH) MILLIONS OF YEARS BEFORE PRESENT Cenozoic Quaternary (Holocene Epoch) (Pleistocene Epoch) Tertiary (Pliocene Epoch) (Miocene Epoch) (Oligocene Epoch) (Eocene Epoch) (Paleocene Epoch) 1.8 to present.008 to present 1.8 to to to to to to to 55.5 If you were to begin counting at the rate of one number per second and continued nonstop for 24 hours a day, 7 days a week, it would take nearly 150 years to count to 4.5 billion! Mesozoic Cretaceous Jurassic Triassic 145 to to to 213 Paleozoic Permian Pennsylvanian Mississippian Devonian Silurian Ordovician Cambrian 286 to to to to to to to 505 Proterozoic Vendian 2,500 to to 650 Archaean 3,800 to 2,500 Hadean Time 4,500 to 3, WARD S Natural Science Establishment, Inc. All Rights Reserved

3 The Law of Superposition was first derived and used by Nicholas Steno, a 17th century Italian physician and naturalist who later became a theologian. From his geological studies in northern Italy, Steno observed that in an undisturbed sequence of sedimentary rocks, the topmost layer is always the youngest, and the bottommost layer is always the oldest. The Precambrian time (Proterozoic, Archaean, and Hadean) accounts for nearly 88% of earth history, and yet is the least understood span of geologic time. Surface outcrops of Precambrian rocks are relatively rare and consist largely of deformed metamorphic rocks devoid of fossils. Some early life existed during the Precambrian, but consisted mainly of primitive soft bodied forms not easily fossilized. The earliest era is at the bottom of the scale. Customarily, the geologic time scale is read from the bottom of the chart upward. Placing the earliest era at the bottom of the time scale, with successively younger eras above it, allows the time scale to be compared directly with the rock record (in which, according to the Law of Superposition, the oldest rock is at the bottom, with successively younger rocks above). The Mesozoic Era has been divided into three periods of geologic time. With the earliest at the bottom of the list, these periods and the sources of their names are: 1. Cretaceous (cre-tay-shus) from the Latin word creta, meaning chalky; from 65 to 145 million years ago. Chalk (calcium carbonate) deposits are common in rocks of this age. 2. Jurassic (joo-ras-ik) from the Jura Mountains of Europe; from 145 to 213 million years ago. 3. Triassic (try-ass-ik) from the Latin word triad meaning three, so called because of the prominent three-fold formations of this age in Germany; from 231 to 248 million years ago. You will note that there are three GEO-logic Topic Sets covering examples of life during the three most recent eras on the geologic time scale. Hadean, Archaean, and Proterozoic rocks are commonly grouped together and referred to as Precambrian in age. The Precambrian rocks have been greatly contorted and metamorphosed, and the record of this portion of earth history is most difficult to interpret. Precambrian time represents that portion of geologic time from the beginning of earth history until the deposition of the earliest fossiliferous Cambrian strata. If the earth is as old as is believed, Precambrian time may represent as much as 88 percent of all geologic time. Precambrian fossils are very rare and none are included in the GEO-logic System. THE GEOLOGIC COLUMN Unlike the geologic time scale, which consists of units of time, the geologic column is composed of the total succession of rocks, from the oldest to the most recent, in a given area. Thus, the geologic column of Texas, California, or New York consists of all rock divisions known to be present in those states. By referring to the geologic column previously worked out for a given area, the geologist can determine what types of rocks he might expect to find there, and at what levels. In using the geologic column, geologists usually work with units of rock strata called formations. In geology, a formation is identified and established on the basis of distinctive physical and chemical characteristics of its rock, and sometimes its fossil content. Formations are usually given geographic names, which are combined with the type of rock that makes up the bulk of the formation. For example, the Beaumont Clay was named for Beaumont, Texas. Formations of this kind of clay found in other localities and of the same age as the clay near Beaumont, are also called Beaumont Clay. 3

4 In the specimen identification list on the inside cover of the topic set box, you may note a number of geologic terms used in abbreviated form. They are: 1. Formation (Fm.) may consist of several layers called beds. 2. Group (Grp.) two or more formations. 3. Series (Ser.) two or more groups. Abbreviations for the kinds of rocks are: 1. Ls. limestone 2. Sh. shale 3. Ss. sandstone THE MESOZOIC ERA A TIME OF GREAT CHANGES As the Mesozoic Era began, the Appalachian Mountains had been uplifted, and the North American continent was going through a stage of volcanism on both the east and west coasts. The geologic processes that were set in motion at this time eventually resulted in the separation of North America from Europe and northwest Africa and the opening of the basin now occupied by the Atlantic Ocean. This event marked a turning point in the geologic history of the continents. The so-called middle era of the geologic calendar was also a turning point in the history of life, marking the transition from the relatively simple organisms of Paleozoic time to the more modern species of the Cenozoic Era. Mesozoic seas were filled with countless species of plants and animals, but terrestrial organisms were equally abundant, and the dominant group of creatures in this chapter of life were the reptiles. It is not surprising that this era is called the Age of Reptiles, for reptiles ruled the land as well as the sea and the air. TRENDS IN THE DEVELOPMENT OF MESOZOIC LIFE In the following account, the life of the era will be traced by a discussion of changes in various important groups of organisms throughout the Mesozoic rather than period-by-period. The plant kingdom was dominated by conifers in the Triassic, but ferns and palm-like plants called cycads were also common. Fossil conifers of Triassic age are well represented among the great stone tree trunks at Petrified Forest National Park in Arizona. The tree ferns, scouring rushes, and conifers (#125) all continued strongly through the Jurassic, but cycads became more abundant. The Ginkgo, probably the oldest genus of trees still living today, made its appearance in the Jurassic. In the Middle Cretaceous, angiosperms appeared. With the addition of these flowering trees and plants to the Mesozoic landscape, the whole flora took on a modern look, which persisted throughout the following era (Cenozoic) to the present day. In the ocean, there were significant changes also. The foraminifera no longer were dominated by the fusulinids, the relatively large spindleshaped forms of Late Paleozoic time. Many of the Mesozoic forams During the Mesozoic Era, ancient gymnosperms, including cycads, conifers, and ginkgoes, quickly replaced the dominant ferns and scale trees of the Late Paleozoic. Conifers were the largest plants in the Mesozoic forest. Their descendants today include the pines, firs, and junipers. Angiosperm (an -gi-o-sperm): A true flowering plant in which the seeds are enclosed in an ovary, comprising the fruit. Examples include roses, grasses, elm trees, and orchids WARD S Natural Science Establishment, Inc. All Rights Reserved

5 were of types that can be found in the oceans today. They became so numerous in many parts of the world during the late Cretaceous, that their limy shells accumulated on the ocean bottom to form great thicknesses of chalk beds. The White Cliffs of Dover are a famous example of such deposits. The Mesozoic corals were of a new kind the hexacorals (scleractinians). They replaced the extinct tetracorals and the honeycomb corals and contributed to reef building in Mesozoic seas. Specimen #129 is a good example of a hexacoral; notice the pattern of septa on this coral; unlike Paleozoic corals, its structure does not distinguish the boundaries between individual animals in the colony. Brachiopods (#128) were far less prominent in the Mesozoic seas than in the Paleozoic, except for a good representation in the Jurassic Period. Pelecypods developed many new forms during the Mesozoic; they probably replaced the brachiopods in many parts of the marine environment. A significant development among the pelecypods was the oyster group (#123), which had a particular tendency to coil one of the two valves of their shells, producing a beak much like that of the brachiopods. By Cretaceous time, this coiling was extremely pronounced and affected both valves. Other groups of oysters developed curved shells with frilled edges (#126). Marine gastropods exhibited more of a modern tendency to develop ornate spiral shells in the Mesozoic. Specimen #118 is a typical example. The major development among the mollusks was the appearance of a great variety of cephalopods, particularly the ammonites (#119, 120, 124). The sutures on the shells of the ammonites became more complex throughout Mesozoic time, and the shells of some groups became larger too. The word ammonite means Ammon s stones? The name was given to fossil shells whose wrinkled whorls resembled a rams horn, which often appeared on the Egyptian god, Ammon. The septa, or internal partitions, in ammonites grew so large that they were forced to become fluted or deeply lobed. The patterns that formed from the line of contact between the septa and the inner surface of the shell wall are called sutures, and are used to identify individual ammonites. Some ammonite shells, well over a meter in diameter, are found in Cretaceous rocks in many of the world s highest mountain ranges. Despite their great quantity and variety in the Mesozoic, the ammonites became extinct by the end of the Cretaceous Period. The belemnites (#127), another group of cephalopods, had straight, thick internal shells that are very common in some Mesozoic rocks. Some species had shells up to 1.5 meters in size. Like the ammonites, belemnites became extinct in the Late Cretaceous. Cretaceous Ammonite Placenticeras 5

6 Among the echinoderms, crinoids were less prominent during the Mesozoic than in the Paleozoic. Many species became free of the stem in the adult stage, and moved about like starfish, except that they were more agile and delicate, drifting in the waters around the reefs. Echinoids (#122) became a more prominent part of the marine fauna in the Mesozoic especially in the Cretaceous, when very large forms existed in the warm, shallow continental seas. The dawn of the Mesozoic Era also revealed changes in the vertebrate fauna in the sea. The fishes no longer dominated, although sharks and bony fishes continued to diversify and many large species existed. The reptiles were now adapted to the marine environment; notable were the ichthyosaurs, streamlined creatures that resemble modern swordfishes. The equally peculiar long-necked plesiosaurs were also present; some of these grew to 12 meters in length. Later, in the Cretaceous, mosasaurs joined this group of marine reptiles, and some species were 16 meters long. They were characterized by flattened tails, sharp teeth, and four limbs modified into paddle-like flippers. Giant turtles, up to 4 meters in size, also lived in Cretaceous seas. Aquatic Reptile Ichthyosaur On the land, the reptiles reigned supreme. In the Triassic, dinosaurs appeared, and at first were quite small (chicken sized). They left thousands of tracks in the soft brown mud in the area now occupied by the Connecticut River valley. Most of them walked or ran on long slender hind legs. In the Jurassic Period, however, some dinosaurs (#121) grew to huge sizes. Some, like Apatosaurus, were four-footed plant-eaters that grew to be over 25 meters long and weighed over 35 tons. These creatures provided food for the ferocious meat eaters like Allosaurus. This great beast of prey was about 12 meters long and its powerful jaws were well equipped with long sharp teeth. Crinoid (cri -noid) An echinoderm of the class Crinoidea, characterized by a globular body enclosed by a calyx from which arms extend radially, and by a jointed flexible stem and a root by which it is attached to the sea floor. Commonly referred to as a sea lily. Mosasaurs were wide ranging marine reptiles and inhabited many inland seas during the Cretaceous. Some of the finest fossil specimens have been found in chalk deposits in western Kansas. With bony plates on its head, lizardlike scales on its body, and massive jaws each armed with 16 to 18 sharp teeth, mosasaurs never lacked weapons to attack rivals or prey. In the Cretaceous Period, there were duck-billed dinosaurs like Anatosaurus, horned forms such as Triceratops, and tank like, armored species like Ankylosaurus. In addition to these plant-eaters, there were monstrous carnivorous (meat-eating) dinosaurs such a Tyrannosaurus rex. Standing some 6 meters tall on its huge hind legs, Tyrannosaurus was over 15 meters long and had many long dagger like teeth. The earliest known pterosaurs, or flying reptiles, appeared in the Early Mesozoic. These remarkable winged dragons had bat like membranous wings supported by arms and long thin fingers. Per WARD S Natural Science Establishment, Inc. All Rights Reserved

7 haps the best-known pterosaur of this time was Pteranodon, of the Cretaceous. Although its short, 65 centimeter body weighed only five or six kilograms, this peculiar beast had a wingspread of as much as 8 meters. Another Cretaceous pterosaur, discovered in Texas, had a wingspread of over 15.5 meters. Archaeopteryx was first discovered in 1861, and its importance as a link between reptiles and birds was soon recognized. Many paleontologists now believe that birds evolved from small carnivorous dinosaurs that ran upright on long, slim hind legs. Archaeopteryx was found in rocks of Late Jurassic age, some 150 million years old. PALEONTOLOGIST Paleontologists study fossils to learn more about the kinds of life and conditions that existed on earth in the geologic past. They use this information to better understand the nature and evolution of life on our planet, and also use fossil evidence to help locate deposits of strategically important resources, like oil, coal, and natural gas. One of the most significant biological events of the Mesozoic Era was the appearance of the first birds in the Jurassic Period. Known from two nearly complete skeletons and fragments of a third, these important fossils were collected from limestone quarries in southern Germany. Named Archaeopteryx (which literally means ancient wing ), this primitive crow-sized bird still retained certain reptile like features, such as teeth in sockets instead of a beak, vertebrae extending the full length of the tail, and claws on the forelimbs forming the front edge of the wing. Archaeopteryx Another significant development in the life of the Mesozoic was the appearance of mammals in the Triassic. Known only from fragmental fossil remains, chiefly teeth, these rare early mammals appear to have been about the size of a large rat or squirrel. The structure of their teeth indicates that some of them were herbivorous and others were carnivorous. During the Jurassic, the mammals remained rare and were small in size, but about a score of different species have been found. In the Cretaceous, insectivores and marsupials appeared. It is interesting that it was also during the Cretaceous that the earliest grasses, vegetables, and fruits appeared; the mammals very likely adapted to these foods in the Late Mesozoic. At the close of the Mesozoic Era, more great changes were taking place in the animal kingdom. Despite the long success of the reptiles during the Mesozoic, the dinosaurs along with all the flying reptiles and most marine reptiles were extinct by the end of Cretaceous time. Contrary to popular belief, this extinction of all the dinosaurs did not take place immediately; rather, it progressed throughout the Late Cretaceous. However, its cause still remains one of science s great mysteries. Toward the end of the Mesozoic Era, most of western North America was uplifted, eventually forming the Rocky Mountains. Quite likely this had a marked effect on the climate of the whole continent. These changes may have contributed to the extinction of the dinosaurs. 7

INTRODUCTION TO LIFE OF THE CENOZOIC ERA

BACKGROUND INTRODUCTION TO LIFE OF THE CENOZOIC ERA This set contains 16 specimens of Cenozoic fossils selected to show representative fossil forms collected from rocks of Tertiary and Quaternary ages.