Growth of a prehistoric time scale

Where were the continents during the time of the dinosaurs?

There can be little doubt that geology's greatest contribution to human knowledge is the development of the Geological Time Scale. This is the extremely complicated framework that reveals deep time to us, as well as the means by which we develop our understanding of the grand pageant that is the history of life on earth. The job is hardly finished: the eminent stratigrapher W. B. N. Berry, writing a book on geological time, entitled it Growth of a Prehistoric Time Scale, thus emphasizing the idea that concepts about time and the placement of events in earth history are constantly in a state of refinement. As things stand today, outcrops of rocks in virtually every part of the world are integrated into the time scale by means of a complex interplay of litho-stratigraphic, biostratigraphic, and chronostratigraphic methods.

Because we are interested in dinosaurs, we will take the luxury of bypassing a mere 3.77 billion years of continental evolution and shoot right to the last time that all the continents coalesced into a single landmass, now known as Pangaea (Figure 2.5). Like any large land mass, Pangaea had many great mountain ranges; however, it was at least theoretically possible to walk on land from any continent to any other. This, as you can imagine, had important ramifications for the possibility of migration by terrestrial organisms, as well as for the similarity of faunas all around the world. As we shall see below, it also had an important influence on global weather patterns.

Figure 2.5. The positions of the present-day continents during the LateTriassic. Earth was dominated by the unified landmass Pangaea. Dots indicate locations of major fossil finds,Afh,Africa;Ant.,Antartica;As.,Asia;Aus.,Australia; Eu„ Europe; N.Am., North America; I., India; S. Am., South America. (Figures 2.5-2.10; reconstruction by Paleogeographic Information System, M. I. Ross and C. R. Scotese.)

Figure 2.5. The positions of the present-day continents during the LateTriassic. Earth was dominated by the unified landmass Pangaea. Dots indicate locations of major fossil finds,Afh,Africa;Ant.,Antartica;As.,Asia;Aus.,Australia; Eu„ Europe; N.Am., North America; I., India; S. Am., South America. (Figures 2.5-2.10; reconstruction by Paleogeographic Information System, M. I. Ross and C. R. Scotese.)

-180° -135° -90° -45° 0° 45" 90° 135° 180°

Figure 2.6. The positions of the present-day continents during the Middle Jurassic. Dots indicate locations of major fossil finds. For abbreviations, see caption to Figure 2.5.

-180° -135° -90° -45° 0° 45" 90° 135° 180°

Figure 2.6. The positions of the present-day continents during the Middle Jurassic. Dots indicate locations of major fossil finds. For abbreviations, see caption to Figure 2.5.

135'

180'

Figure 2.7. The positions of the present-day continents during the Early Jurassic.The dismemberment of Pangaea began, probably, as a rift between the northern continental mass, and the southern supercontinent. Dots indicate locations of major fossil finds. For abbreviations, see caption to Figure 2.5.

135'

180'

Figure 2.7. The positions of the present-day continents during the Early Jurassic.The dismemberment of Pangaea began, probably, as a rift between the northern continental mass, and the southern supercontinent. Dots indicate locations of major fossil finds. For abbreviations, see caption to Figure 2.5.

Plates, oceans, and seas during the time of dinosaurs

The initial rifting (break-up) of Pangaea took place in the Early Jurassic (Figure 2.6). The effect has been likened to an "unzipping" of the great supercontinent, from south to north. Sediments in the eastern seaboard and Gulf Coast regions of North America and Venezuela, as well as regions in West Africa, record the opening and widening of a seaway.

Also at this time, some of the earliest epicontinental (or "epeiric") seas of the Mesozoic Era first made their appearances. Epicontinental seas are shallow marine waters that cover parts of continents. In the past, epicontinental seas have been considerably more widespread than they are today, because in the past eustatic (or global) sea levels have been higher than they are now.

Eustatic sea level is controlled by many factors, but two of the most common are ice at earth's poles and tectonism. Obviously, if there is a great deal of polar ice (or glaciation, such as took place during the past million years or so), much seawater may be bound up in ice, lowering sea levels worldwide. Likewise, during tectonically active intervals, mid-oceanic spreading centers are topographically elevated, decreasing the volume of the ocean basins and thus displacing ocean water up onto the continents. During the Middle Jurassic, fluctuating epicontinental seas covered large parts of what is now western North America, eastern Greenland, eastern Africa, and Europe, where there developed a complex system of islands and seaways (Figures 2.7 and 2.8).

Although the Middle Jurassic was an important interval of time in dinosaur evolution, terrestrial sediments that record it are rare. Why this is the

Where were the continents? | 33

135'

Figure 2.8. The positions of the present-day continents during the Late Jurassic. Dots indicate locations of major fossil finds. For abbreviations, see caption to Figure 2.5.

135'

180'

Figure 2.8. The positions of the present-day continents during the Late Jurassic. Dots indicate locations of major fossil finds. For abbreviations, see caption to Figure 2.5.

case is uncertain, but probably it is just preservational "luck of the draw."

In the Late Jurassic (Figure 2.8) and Early Cretaceous (Figure 2.9), continental separation was well underway. A broad seaway, the Tethyan Seaway (after the Greek goddess Tethys, Goddess of the Sea), ran between two supercontinents, one in the north known as "Laurasia" and one in the south called "Gondwana". Eustatic sea level was relatively low, and hence epicontinental seas were not very predominant. Yet, the then-western (present-day northern) coast of Australia seems to have been bathed in a broad, shallow, epeiric sea, as were, periodically, western North America and parts of Europe and Asia.

The mid-Cretaceous was in tectonic terms a swinging time of active mountain-building, sea-floor spreading, high eustatic sea levels, and broad epeiric seas (Figure 2.9). The Tethyan Seaway remained a dominant geographical feature, as rifting between the Europe and North America was initiated from the south to the north. The effects of active tectonism were even more marked in the southern continents. Here, a stable continental merger dating back to the Early Paleozoic Era - the supercontinent of Gondwana - finally underwent rifting involving two of its largest constituents - Africa and South America - as well as two smaller constituents, India and Madagascar. India spent the next 50 million years motoring at breakneck speed (for a continent) across what became the Indian Ocean, to smash head-on into southern Asia and produce the Himalayan Mountains. But this is getting ahead of our story. While India and Madagascar were in the first bloom of unconfinement, Australia and Antarctica remained firmly united (a continental marriage that would not end until 50 Ma), and a land connection remained, as it almost does today, between South America and Antarctica.

135'

Figure 2.9. The positions of the present day continents during the Early Cretaceous. Note the large number of seas covering continents where today we find dry land. Dots indicate locations of major fossil finds. For abbreviations, see caption to Figure 2.5.

135'

180'

Figure 2.9. The positions of the present day continents during the Early Cretaceous. Note the large number of seas covering continents where today we find dry land. Dots indicate locations of major fossil finds. For abbreviations, see caption to Figure 2.5.

Mid-Cretaceous Europe was an archipelago (group of islands), dissected by small epeiric seaways. Extensive seas developed over both northern Africa and western South America. Finally, a series of North American Western Interior seas bisected the North American craton, deriving their waters from the Arctic Ocean to the north and from the newly formed Gulf of Mexico to the south. These seas came and went, sometimes completely dividing the craton in half, and at other times only partially dividing it.

The global positions of continents during the Late Cretaceous are almost familiar to us (Figure 2.10). North America became nearly isolated, connected only by a newly emergent land bridge across the modern Bering Straits to the eastern Asiatic continent. This land bridge has come and gone several times since the Cretaceous, but it was clearly a significant feature at the time. Africa and South America were fully separated, the former retaining its satellite, Madagascar, and the latter retaining a land bridge to the Antarctica/Australia continent. India was by now well along its way toward southern Asia.

But, if Late Cretaceous global continental positions are somewhat familiar to modern humans, the geography of the Late Cretaceous would still seem strange. The Western Interior Seas that bisected North America remained important geographical features up until the very latest Cretaceous, when they finally receded (only to nostalgically re-emerge briefly, one last time in the early Cenozoic). Europe remained a complex of islands and seaways. Africa is believed to have had extensive internal seaways, as are South America and Asia.

-180° -135° -90° -45° 0° 45° 90° 135° 180°

Figure 2.10. The positions of the present day continents during the Late Cretaceous. The positions of the continents did not differ significantly from their present-day distribution. Note the land bridge between Asia and North America, as well as the European archipelago. Dots indicate locations of major fossil finds. For abbreviations, see caption to Figure 2.5.

-180° -135° -90° -45° 0° 45° 90° 135° 180°

Figure 2.10. The positions of the present day continents during the Late Cretaceous. The positions of the continents did not differ significantly from their present-day distribution. Note the land bridge between Asia and North America, as well as the European archipelago. Dots indicate locations of major fossil finds. For abbreviations, see caption to Figure 2.5.

What were climates like during the time of the dinosaurs?

0 0

Post a comment