| Pal�os: Mesozoic |  |
MesozoiC ERA |
| MESOZOIC ERA | Mesozoic - 1 |
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The supercontinent Pangea divides into Laurasia in the north and Gondwana in the south. The climate is hot and tropical worldwide. On land, the dinosaurs reign supreme. In the oceans are various kinds of marine reptiles, as well as ammonite and belemnite
molluscs and many other invertebrate groups. Plants include ferns and gymnosperms. Mammals are small and insignificant, but probably numerically common.
The Mesozoic Era lasted more than 180 million years. During this time, many modern forms of plants, invertebrates, and fishes evolved. On land, dinosaurs were the dominant animals, while the oceans were populated by large marine reptiles, and Pterosaurs ruled the air. For most of this period, the climate worldwide was warm and tropical, and shallow seas covered low-lying landmasses. At the beginning of the Mesozoic, all of the world's continents were joined into the supercontinent of Pangea, which rifted into Laurasia in the north and Gondwanaland in the south. By the end of the era most of continents had separated into their present form.
The Mesozoic Era is divided into three periods, each lasting many millions of years: the Triassic, Jurassic, and Cretaceous. The Triassic saw the emergence of many modern invertebrate groups, and on land the archosaur reptiles replaced the therapsids. In the oceans Ichthyosaurs such as Shonisaurus became as large as whales. The Jurassic was the height of the dinosaur era, with giants such as Brachiosaurus, Stegosaurus, etc, and mammals tiny and shrew-like. Distinctive plants like ferns, Cycads, Bennettitales, and Cheirolepidiaceae conifers characterized the landscape. During the Cretaceous period, the first flowering plants appeared, birds and fish diversified, and new types of dinosaurs appeared. The climate cooled and unique dinosaurs evolved on different continents.
The Mesozoic era came to an end with the great terminal extinction event known as the K-T (Cretaceous-Tertiary) event.
Image: illustration � Doug Henderson, reproduced with permission
| Range (Mya) | Duration (My) | |||
|---|---|---|---|---|
| Cretaceous K | Upper/Late K2 | Maastrichtian k6 | 70.6 - 65.5 | 5.1 |
| Campanian k5 | 83.5 - 70.6 | 12.9 | ||
| Santonian k4 | 85.8 - 83.5 | 2.3 | ||
| Coniacian k3 | 89.3 - 85.8 | 3.5 | ||
| Turonian k2 | 93.5 - 89.3 | 4.2 | ||
| Cenomanian k1 | 99.6 - 93.5 | 6.1 | ||
| Lower/Early K1 | Albian b6 | 112.0 - 99.6 | 12.4 | |
| Aptian b5 | 125.0 - 112.0 | 13.0 | ||
| Barremian b4 | 130.0 - 125.0 | 5.0 | ||
| Hauterivian b3 | 136.4 - 130.0 | 6.4 | ||
| Valanginian b2 | 140.2 - 136.4 | 3.8 | ||
| Berriasian b1 | 145.5 - 140.2 | 5.3 | ||
| Jurassic J | Upper/Late J3 | Tithonian j7 | 150.8 - 145.5 | 5.3 |
| Kimmeridgian j6 | 155.7 - 150.8 | 4.9 | ||
| Oxfordian j5 | 161.2 - 155.7 | 5.5 | ||
| Middle J2 | Callovian j4 | 164.7 - 161.2 | 3.5 | |
| Bathonian j3 | 167.7 - 164.7 | 3.0 | ||
| Bajocian j2 | 171.6 - 167.7 | 3.9 | ||
| Aalenian j1 | 175.6 - 171.6 | 4.0 | ||
| Lower/Early J1 | Toarcian l4 | 183.0 - 175.6 | 7.4 | |
| Pliensbachian l3 | 189.6 - 183.0 | 6.6 | ||
| Sinemurian l2 | 196.5 - 189.6 | 6.9 | ||
| Hettangian l1 | 199.6 - 196.5 | 3.1 | ||
| Triassic T | Upper/Late T3 | Rhaetian t7 | 203.6 - 199.6 | 4.0 |
| Norian t6 | 216.5 - 203.6 | 12.9 | ||
| Carnian t5 | 228.0 - 216.5 | 11.5 | ||
| Middle T2 | Ladinian t4 | 237.0 - 228.0 | 9.0 | |
| Anisian t3 | 245.0 - 237.0 | 8.0 | ||
| Lower/Early T1 | Olenekian t2 | 249.7 - 245.0 | 4.7 | |
| Induan t1 | 251.0 - 249.7 | 1.3 |
Some of the main outlines of Mesozoic climate are matters of general
agreement, but almost no one is very satisfied
with the explanations for what has been observed. Here's the usual story:
The Triassic, particularly the first half of the Triassic, was dry and highly seasonal, with particularly large annual temperature variations in the vast continental interior of Pangea, the world-spanning continent of the Triassic. Low sea levels probably exaggerated these temperature extremes. Water acts as a heat sink -- it takes much more heat to warm a cup of water than it does to warm a cup of rock. Water also circulates, so that heat doesn't build up in one place. The net result is that water tends to stabilize temperatures. Land areas near the ocean are warmed or cooled by winds which pass over the ocean and by rains from evaporated ocean waters. It is generally agreed (a) that the low sea levels of the Triassic contributed to temperature extremes in the interior of Pangea and (b) that the interior of Pangea probably included huge areas of desert.
During the Jurassic,
sea levels began to rise, probably due to an increase in sea-floor
spreading. This seems paradoxical, but the mechanism is explained in the
image. This caused flooding of large areas of the continents.
As a result, the deserts began to retreat, and continental temperatures
stabilized. Pangea also began to break up into smaller units, which
brought more land area in contact with the ocean. The presence of nearby
oceans also increased humidity, so that climates worldwide became wetter as well
as warmer.
During the first half of the Cretaceous, this process continued. In addition two climate trends which began in the Jurassic became quite pronounced in the Cretaceous. The mechanism for these events is not fully understood. First, the temperature gradient from North to South became almost flat -- much more so than would be predicted from ocean circulation models. In other words, average temperatures were about the same everywhere on Earth, from the poles to the equator. Second, average temperatures were much higher than today, probably by about 10C�. Higher CO2 (carbon dioxide) levels certainly played a part, but the paleoclimate data do not match theoretical predictions.
The later Cretaceous story is more complex, and more controversial. Many researchers, but not a real consensus, believe that sea temperatures near the equator may have become a bit too warm by the Aptian-Albian, perhaps actually incompatible with ocean life. In addition, some data suggest that land areas near the equator were not jungle- or forest-covered, that plant diversity was low, and that these regions were arid despite being close to the sea. Deep ocean circulation may also have broken down. That is, water continued to circulate horizontally, but not vertically. The deep oceans weren't getting oxygen, and "black shales" appeared in the Aptian-Albian and High Cretaceous. These are large volumes of organic matter in the oceans which never completely decomposed because of lack of deep ocean oxygen. Still, the north-south temperature gradient remained very flat.
Things
cooled off a little during the End-Cretaceous, but it's unclear how much or how
regularly. The climate at the very end of the Mesozoic is
particularly controversial.
Unfortunately, the data only match this story to a limited degree, and there are internal inconsistencies. Here are a few of the problem areas.
1) If temperature extremes in the Triassic were as great as general circulation models predict, one would expect rather hefty ice-build-up in at least some polar regions. Glaciers leave a rather distinct geological signature, and we simply don't have any evidence of Triassic glaciers or polar caps.
2) Conversely, there is evidence of the kind of rapid sea level changes associated with polar ice in the Mid-Cretaceous, which is rather hard to accept. Miller et al. (2003).
3) CO2 levels are usually invoked to explain Cretaceous warmth and the flat Cretaceous temperature gradient. This makes sense, since the very active mid-ocean spreading ridges might well have been associated with out-gassing of CO2 from deep within the Earth. Unfortunately, neither the geology of the period nor the stable carbon isotope records really support the idea as well as they might.
4) Even the most sophisticated quantitative models can't reconstruct the flatness of the Cretaceous temperature gradient. Either our temperature estimates are off, or some important factor is missing from the models. Since dinosaurs and semi-tropical vegetation are known from within 10� of the Cretaceous poles, the problem is likely to be with the theory. A recent study of a mid-latitude continental interior (in eastern Russia) -- far from the ocean in even Late Cretaceous times, suggest that temperatures were very even and that these regions were damp and non-seasonal even in the Mid-Cretaceous.
Links: Mesozoic Dinosaurs - Enchanted Learning Software, Lecture 24-, Global Climate Change Student Guide, Pz-Mzclimate, Global Climate and Phytogeography in the Early Mesozoic, Pangaean climate during the Early Jurassic- GCM simulations and ..., The Vilui Basin and the Late Cretaceous Continental Interior ..., MESOZOIC LAND ECOSYSTEMS, Geological Society - Abstracts.
ATW041023. Text and ocean crust image public domain. No rights reserved.
Dragonfly |
Bivalve |
Ammonite |
Belemnite |
Pterosauria |
Crinoid |
Araucariacean conifer |
