Eighth Edition
by Harold L. Levin

Chapter 14 - page 2

Life of the Mesozoic

Mesozoic Climate and its Effect on Life

Factors influencing climate include:

• Distribution and size of continents and oceans
• Development and location of mountain ranges
• Development and location of land bridges between continents
• Changes in snow cover, cloud cover, or vegetation cover
• Carbon dioxide content of the atmosphere
• Location of the poles and equator (with respect to continents and oceans)
• Amount of particulate matter and aerosols released into the atmosphere by volcanic eruptions
• Astronomical factors concerning the Earth's orbit and tilt of the rotational axis

Climate and Plate Tectonics

Overall, climates in the Mesozoic Era were warm. This was in contrast to the cool dry climates which characterized many continental areas near the end of the Paleozoic Era. Evidence for warming includes the disappearance of glaciers which were common in many areas during the Permian.

Distribution of glacial tillites (blue triangles), coal (red circles) and evaporites (green areas) during the Permian Period in the Late Paleozoic Era, about 250 million years ago.

Climatic warming was related to continental drift and the breakup of Pangea during the Mesozoic Era. As the continents moved away from the South Pole, conditions were no longer favorable for glaciers to exist.

Fossil plants from the Jurassic and Cretaceous indicate that tropical climates existed in areas that today have temperate climates. Subtropical plants were living in areas that were 70^o from the equator during the Cretaceous: a latitude similar to that of northern Alaska, Canada, and Siberia today. This suggests that temperatures were much warmer during the Cretaceous than they are today. (Note that during the Jurassic and Cretaceous, the continents were at roughly the same latitudes that they occupy today.)

Triassic

Jurassic

Early Cretaceous

When the continents were clustered together to form Pangea, much of the land area was far from sea, and was arid. Evaporite deposits accumulated as dry areas became intermittently flooded by the sea as the continents began to rift apart in the Late Triassic.

As the continents separated, water was able to circulate in the equatorial and mid-latitudes. Water retains heat better than land, and the circulation of seawater distributed the warmth between the separating continents and around the globe.

Climate and Sea Level Affect Diversity

With the disappearance of the glaciers, sea level rose (with some fluctuations) from the Early Triassic to the Late Jurassic, and continued rising to a maximum during the Cretaceous.

Vail sea-level curve showing major cycles of sea level changes. Note the rise in sea level during the Jurassic and Cretaceous.

Sea level rise was also related to the rifting and fragmentation of Pangea. The mid-Atlantic ridge system developed as the Atlantic Ocean widened. The basaltic rocks that were extruded along the mid-ocean ridge system were hot and thermally inflated. As a result, they displaced a considerable volume of sea water onto the continents.

In the Late Jurassic and Cretaceous, epicontinental (or epeiric) seas flooded large areas of North America and Europe. These epicontinental seas also contributed to the warmer climate because the water carried heat.

These epicontinental seas provided an extensive habitat for shallow marine organisms and probably led to the increase in diversity.

Note the similarity between the sea level curve (above) and the graph of marine diversity (below) during the Jurassic and Cretaceous.

Diversity of marine animals through geologic time, as indicated by number of known fossil genera.

Both curves show an increase through the Jurassic and Cretaceous, a dip at the end of the Jurassic and a peak in the Late Cretaceous.

The paleogeographic maps below show the spread of epicontinental seas over North America through the Mesozoic.

Triassic paleogeography of North America. Note absence of epicontinental seas in the central part of the continent.

Jurassic paleogeography of North America. Note the open seas in the Great Plains area, characterized by sandy and shaley deposits, limestones and evaporites.

Cretaceous paleogeography of North America. Note the epicontinental seas covering the western interior of North America as well as the Coastal Plain areas in the south and east.

Terminal Cretaceous Cooling

The climate began to cool toward the end of the Cretaceous Period.

Evidence for cooling includes:

1. Regression of the epicontinental seas
2. Paleomagnetic data show that continental drift moved Antarctica onto the Late Cretaceous South Pole and parts of North America were near the North Pole
3. Changes occurred in terrestrial plant populations:
   • Tropical cycads were sharply reduced
   • Ferns declined in North America and Eurasia
   • Hardier plants such as conifers and angiosperms became more widespread
4. Oxygen isotope ratios of planktonic organisms, which can be used as paleothermometers, indicate a decline in ocean temperatures beginning about 80 million years ago.

The climatic cooling near the end of the Cretaceous may have contributed to the extinctions of plants and animals at the end of the Mesozoic Era.