Dragons in the Dust: The Paleobiology of the Giant Monitor Lizard Megalania
232
Dragons in the Dust: The Paleobiology of the Giant Monitor Lizard Megalania
232Hardcover
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Product Details
| ISBN-13: | 9780253343741 |
|---|---|
| Publisher: | Indiana University Press |
| Publication date: | 03/30/2004 |
| Series: | Life of the Past |
| Pages: | 232 |
| Product dimensions: | 7.00(w) x 10.00(h) x 0.77(d) |
| Age Range: | 18 Years |
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Dragons in the Dust
The Paleobiology of the Giant Monitor Lizard Megalania
By Ralph Molnar, James O. Farlow
Indiana University Press
Copyright © 2004 Ralph E. MolnarAll rights reserved.
ISBN: 978-0-253-34374-1
CHAPTER 1
The Pleistocene World
Introduction: Back to the Ice Age
Megalania lived during the Pleistocene, the "Ice Age." Like other organisms, it evolved or faced extinction according to the dictates of the environment. Thus, understanding Megalania presupposes understanding the Pleistocene world. Furthermore, since the Pleistocene was the mother of the present, even to comprehend our own world we must know the Pleistocene. For in the Pleistocene, the present was born.
I once briefly lived in Fargo, North Dakota, a part of the North American continent so flat that people used freeway off-ramps for downhill skiing and traveled for 30 minutes to the north, toward Grand Forks, just to see a hill. And it was only about a meter high. To those of us who came from the Rockies or the Appalachians, this country was amazingly flat. One could not help but wonder how it came to be so. To the east, in Minnesota, there are odd, rounded, little gravelly hills — kames — sitting on the otherwise flat and rather monotonous plain. There also, a bench-like terrace in a hillside could be found; it marked an old Pleistocene beach — not a beach from the ocean, but from a vast lake, now drained and dry. The region around Fargo was flat because it had been the floor of Lake Agassiz, a great but (geologically) short-lived body of water that at its maximum extended from near Minneapolis, Minnesota, to near Reindeer Lake on the Manitoba-Saskatchewan border. Lakes Winnipeg, Winnipegosis, and Manitoba are probably its remnants. The lake's water came from the melting of the great ice sheet. The kames were deposits laid down on the sheet of ice that became hills when the ice melted and left them behind. Relics of the great glacier not only littered, but actually formed, the entire landscape.
It is the evidence for these glaciers of the Pleistocene that gave it the name "Ice Age." But this is just the most recent example of such an episode in the history of the earth. During this one, much of the landscape of North America and Europe was overrun by glaciers, as was Antarctica in the south, where the ice still remains. There have been several major advances and retreats of the ice — glaciations of the continents and seas — during the past two million years. Four of these occurred during the Pleistocene, separated by warmer periods, the interglacials, and punctuated by short transient warmings, the interstadials.
These different periods required names in order to be clearly kept in mind. And names required definitions, ways of distinguishing one time from another. Periods of time are divided, or defined, by events: the positions of hands or changing digits for clocks; significant and obvious (to geologists) events for geological periods. For the Pleistocene, the obvious events are the sequence of advances and retreats of the glaciers. Different names have been given to the glacial periods in different places (Table 1), because at the beginning of our understanding of the Pleistocene no one knew which periods at any one location corresponded to those elsewhere. It wasn't even known if different places had experienced the same number of advances and retreats of the ice (and hence comparable periods).
There are no names in general use for these periods in Australia, so — arbitrarily — the names for Alpine Europe are used here for the glacials, and those for northwestern (continental) Europe for the interglacials. When the Pleistocene is considered over the whole world, geologists generally use either the Alpine or North American terms. Although the correlations shown in Table 1 are generally accepted, these correlations are still not as well established as we could wish, especially regarding the pluvials (a term originally thought to denote periods of greater rainfall) of Asia and Africa. The pluvials were initially believed to have been contemporaneous with the glacials but are now correlated with interglacials: aridity characterized the glacials and apparently was especially intense during their opening and closing phases. Dust in glacial and sea-floor sediments is taken to indicate dry, windy conditions — which blew the dust onto the glacier or into the sea — and to correlate with glacial times. The ages are established by radiometric dating, that is, dating using radioactive decay — for example, the carbon 14 or potassium/argon method.
The discovery that some rocks are magnetized allowed a different and independent set of criteria to be used to correlate sequences of rocks and sediments: their magnetization. The magnetization of a rock follows and preserves the direction of the terrestrial magnetic field at the time the molten rock cooled and solidified. If small sedimentary particles are magnetized and if they contain iron, they may also be laid down so as to record the direction of the magnetic field. Thus, igneous rocks as well as some sedimentary rocks can be used to determine the directions of ancient fields. If the terrestrial field never changed, then determining the magnetization of ancient rocks would only tell us what we already knew; but in fact, the field does change from time to time. Sometimes, as now, the south magnetic pole was directed toward the north geographic pole (called "normal polarity"); at other times, toward the south pole ("reversed polarity"). The south magnetic pole is at the north geographic pole because the north pole of a compass needle was defined as the end that pointed northward. The north pole of one magnet is attracted to the south pole of another, and since the north pole of the needle is attracted toward the north geographic pole, that pole must be associated with the south magnetic pole. The direction of the field changes abruptly, geologically speaking, so these changes provide convenient "datum planes" (or "time data"), records of events occurring around the world almost instantaneously that can be used to correlate the ages of rocks in different parts of the world. "Almost instantaneously" geologically speaking, that is. The two magnetic divisions of the Pleistocene are the Matuyama (reversed polarity) and, since 730,000 years ago, the Brunhes (normal polarity).
Just as the glacial episodes had short interruptions, the interstadials, so (sometimes) did the magnetic epochs. For example, the Brunhes was interrupted by the Jaramillo (reversed) from 940,000 to 880,000 years ago. Magnetostratigraphy, the study of the temporal relationships of rocks based on their magnetization, has proved very helpful in the establishment of ages and correlation of rocks.
The temperatures of the water of the sea's surface changed with the climatic changes of the glacials and interglacials. These temperatures were reflected both in the kinds of organisms living in these waters and in the chemical processes occurring there. Since the shells of the organisms sink when the organisms die, they all collect in the sediments of the sea floor. Thus, these sediments record a history of the temperatures of the sea's surficial waters and of the air over them. Paleoclimatologists may use either the kinds of organisms or the composition of their shells (chemical processes of their construction) to infer past climates. In practice, both methods often make use of the shells of foraminifera (forams): those forams that lived in warm waters could be recognized and distinguished from those that lived in cold. David B. Ericson at Lamont Geological Observatory in New York (now the Lamont-Doherty Earth Observatory) pioneered reconstructing the history of the climate by tracing the sequence of warm- and cold-preferring forams. The proportion of two isotopes of oxygen, O and O, that were incorporated into the tests (shells) of forams depends on the water temperature, so this ratio could be used to reconstruct ancient temperatures and thus climatic histories. This work was undertaken by Cesare Emiliani. Both methods worked, but it took some time to iron out the apparent discrepancies between them (see Ericson and Wollin 1964, and Imbrie and Imbrie 1979). The two methods revealed the same history and, after some effort, were shown to correlate with climatic history as preserved in the continental deposits as well.
There had been glaciations before the Pleistocene; in fact, cool and warm times seem to have alternated throughout the history of the earth. This pattern occurs at various scales — the cool periods lasting for years, millennia, or millions of years — and it extends as far back as we can tell. There were extensive glaciations during the Precambrian — at least four of them — and a restricted glaciation near the South Pole during the Ordovician (about 445-495 million years ago). The Precambrian, that period of time from the beginning of the earth to (more or less) the appearance of multicellular life, was obviously long — around 90 percent of the history of the earth — so it is not surprising that it has a number of glaciations. What is significant is that the glaciations started early. One of these glaciations (the Varangian) seems to have been the greatest ever and may have extended into the equatorial regions, bringing glaciers about as close to the equator at that time (about 1,100 km or 680 miles) as Trinidad is today (Evans, Beukes, and Kirschvink 1997). There is extensive evidence for Permo-Carboniferous glaciations in South America, India, southern Africa, and Australia, which also convinced geologists (initially those of the Southern Hemisphere) that the continents had been arranged differently then than they are now.
During the Pleistocene glacial periods the climate was cooler than it is now — from about 5°C lower near the equator to about 15°C lower at high latitudes — and glacial ice sheets spread over the northern continents and Antarctica. Interestingly, one can think of this as a depression of the layers of the atmosphere: the climate of the ice age still exists about 2 km above our heads at sea level (Galloway 1965). It was the evidence for the ice sheets that also indicated that the climate had been cooler. Two hundred years ago most geologists did not realize that there had been any ice ages at all, much less a recent one. Swiss peasants, who were familiar with evidence of montane glaciers, recognized that the Alpine glaciers had once been much more extensive than they are now; whereas geologists still interpreted this evidence in terms of a worldwide deluge (Imbrie and Imbrie 1979). The community of geologists became convinced by a suite of features, each of which could most convincingly be explained as the results of glaciers where none now exist. When taken together, this evidence indicated unexpectedly extensive glaciation.
These features included moraines — elongate hills of rocks and rock fragments — deposited by glaciers, as well as the shorelines of certain lakes, like the Great Salt Lake, that are now well above the lake's surface. Valleys with U-shaped cross sections and amphitheater- or bowl-like valleys often found high in mountains, known as cirques, both testify to the erosive power of glaciers. So do fluted (grooved) surfaces developed in rock or sediment, and polished and scored surfaces of exposed bedrock, both caused by the abrasive action of moving ice. Drumlins — elongate, smoothly rounded hills, shaped like inverted bowls of spoons — resulted from the flow of ice over preexisting hills; and eskers — long, narrow, often sinuous ridges — proved to be deposits from glacial streams. However, perhaps the most influential features (at least in accounts of the history of how the glacial ages came to recognized) were erratics, boulders of rock unlike that found anywhere in the vicinity. Erratics posed the questions Where did they come from? and How did they get there? Initially erratics were thought to be relics of the biblical flood, but careful study made it clear that they had been moved by ice.
Sedimentary as well as geomorphological features resulted from glaciation. Drift — masses of rock and rock fragments — and loess — very fine dust often deposited in thick masses — are prominent among them. The ice sheets left a clear geological sign in the form of till. An undisputed relic of glaciers, till is composed of unsorted rocks, cobbles, gravel, sand, and mud all mixed together, not in orderly beds with the larger pieces toward the bottom and the smaller toward the top. Till is the result of detritus carried by glaciers and deposited in place by their melting.
Physical Environment of the Ice Age (Glacials)
At its greatest extent, the ice covered much of northern North America and Europe and the entire Arctic Ocean. The ice sheet did not extend out from a single source in the northern continents. It does so now in Greenland only because Greenland is a relatively small island (compared to a continent). In North America three separate sheets coalesced. The Laurentide sheet, centered on what is now Hudson Bay, was the largest and merged with the Cordilleran sheet centered on the Canadian Rockies in the west and the Greenland sheet in the east. Three major sheets, none as large as the Laurentide, also made up the Eurasian glaciers. The Fennoscandian sheet, centered on Scandinavia, was the largest and joined with the British sheet in the west, centered on the northern British Isles, and the Barents sheet, centered on the Barents Sea, in the east. The Fennoscandian sheet also merged with the Greenland sheet to the northwest (Fig. 1.1).
Recent evidence suggests that not only the adjacent land but also the Arctic Ocean was completely frozen over, with perhaps as much as a kilometer of ice covering it (Polyak et al. 2001). The thickness of the continental ice sheets is unknown, but it is generally considered to have been about 3.5 km (about 2.2 miles) for the Laurentide. Dissenting opinion suggests it was only a little over 2 km (Kerr 1994). Geologists had generally assumed that the cooler climates of the glacial times implied more precipitation, whether rain or snow. But the removal of water from the oceans and its storage as ice should have created drier climates since less water was available to evaporate because of the reduced size of the oceans. Furthermore, that water was cooler, thus requiring more energy (i.e., heat) to evaporate.
Work during the 1970s had suggested that changes in temperature during the glacials occurred preferentially in the polar regions. In other words, it was not that the whole planetary surface (which here effectively means the surface of the ocean) had cooled as such, but that the cooler (i.e., polar) regions had gotten even colder, while the equatorial regions remained more or less as warm as they are now. More recent evidence disputes this and suggests that both polar and equatorial regions cooled — by about 5°C in the Tropics (Charles 1997), although the possibility remains that some equatorial regions didn't cool significantly.
The glacial ice sheets stored not only water but also — speaking metaphorically — cold. They chilled the overlying air. Cold air is denser than warmer air and thus heavier. This alone would cause it to flow down the ice sheets, as what meteorologists term katabatic winds. Such winds occur today in Antarctica and Greenland and may blow at more than 150 km/hr (about 93 miles/hr). These strong winds also, presumably, blew off the Pleistocene ice sheets of Europe and North America and may have been important in forming the extensive loess deposits of those lands. They were clearly cold, although it has been argued that because of heating resulting from the increase in density of the air as it descends (adiabatic heating), these winds were not substantially colder than winter winds in these regions today. Judging from the winter in North Dakota, that is quite cold enough.
The ice sheets affected the general circulation of the atmosphere in the Northern Hemisphere. Simulations indicate that the jet streams over North America would have been split — the southern one bringing increased rain to the southwest and so presumably accounting for the now-dry lakes there (Fig. 1.2). The Laurentide ice sheet also affected the winds blowing out across the Atlantic, causing cooler and stronger westerlies.
(Continues...)
Excerpted from Dragons in the Dust by Ralph Molnar, James O. Farlow. Copyright © 2004 Ralph E. Molnar. Excerpted by permission of Indiana University Press.
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