Biology : Semester II


IntroductionSection 1 | Section 2 | Section 3 | Section 4 | Section 5

  Section Three:

Part 1 | Part 2 | Part 3 | Part 4 | Part 5 | Part 6 | Part 7 | Part 8 | Part 9 | Part 10 | Part 11 | Part 12 | Part 13 | Part 14

Biology : The Time of Ancient Life : Part Thirteen

The Permian:
Prelude to Crisis

The Permian period spanned the time interval from 286 to 245 million years ago. During the Permian the assembly of Pangaea was completed and a whole host of new groups of organisms evolved. The Permian ended in the greatest of the mass extinctions, where over 90% of all species were extinguished. With the assembly of Pangaea and resulting mountain building, many of the shallow seas retreated from the continents.

Life in the Water

Fusulinid foraminiferans, which had appeared during the Carboniferous, continued their diversification. Trilobites were rarely encountered, although brachiopods and crinoids had some recovery of species diversity after the Carboniferous extinction. Marine environments were much restricted due to the Pangaea-related mountain-building and resulting uplift as the supercontinent continued its assembly.

Reconstruction of the seafloor near a Permian coast.

Life on Land

At first glance it might appear that the only evolutionary changes of note were occurring on the land during the Permian. Part of this illusion results from the relative scarcity of Permian-aged marine sediments as well as the tendency of land-dwellers to view themselves as the pinnacle of evolution.

A Permian [terrestrial] scene. Two Dimetrodon stroll past a pool with an emerging Eryops. The trees on the left are Calamites.

Dimetrodon and lunch!

Plant life of the Permian took on an increasingly modern "look" with the rise of a number of gymnosperm (naked seeded) plants during the late Carboniferous and their diversification during the Permian. Indeed, the late Carboniferous "extinction" had almost no impact on land plants. The arborescent lycopods of the Carboniferous coal swamps disappeared before the end of the Carboniferous. The Permian saw the spread of conifers and cycads, two groups that would dominate the floras of the world until the rise of the flowering plants 144 million years ago. The first conifers had small leaves similar to those seen in the modern plant Araucaria, the Norfolk Island pine.

Paleobotany of the Permian is also marked by an interesting group of seed ferns completely restricted to Gondwana, the southern part of Pangaea. This group, the glossopterids, produced a distinctive leaf type classified under the leaf-genus Glossopteris. Glossopteris occurred over all of the former parts of Gondwana, the present-day continents of India, Australia, Africa, South America and Antarctica. When Alfred Wegener proposed his continental drift hypothesis he used fossil evidence including Glossopteris and its distribution to argue for the former existence of a super continent he termed Pangaea. When the bodies of the ill-fated British expedition to the South Pole led by Captain Robert Falcon Scott (1868-1912) were found in Antarctica, leaf fossils belonging to Glossopteris were in the tents.

Glossopteris leaf fossils from Australia are more than 250 million years old.

Reconstruction of Gondwana is supported by the distribution of various fossils, such as the plant fossil Glossopteris.

The amniote lineage (the first truly terrestrial vertebrates that did not need to return to water to lay eggs), especially the fossils, are often classified based on the number of openings in their skulls. These openings allow for muscle attachment and have traditionally been used to separate the "reptiles" into several groups, including the anapsids, synapsids, and diapsids. The anapsids, which include the turtles, lack any openings. Synapsids include the mammals and their predecessor and related groups the pelycosaur (the non-mammalian synapsids). Diapsids, with two openings on each side of their skulls, include the birds, dinosaurs, and most of the traditional reptile groups. The synapsids used to be called the mammal-like reptiles. Recent studies suggest that many animals in this group were not as strongly reptilian as the obsolete term "mammal-like reptiles" implied.

The amniotes that had appeared during the late Carboniferous diversified from the protothyrid stock along two lineages: one leading to the quadrupedal pelycosaurs (stem synapsids) of the Carboniferous-Permian and then to the therapsids (advanced synapsids) of the Permian-Triassic; and the other leading to the bipedal thecodonts of the Permian-Triassic. Dimetrodon, shown above, was a member of the pelycosaurs, or non-mammalian therapsids. By the end of the Permian the therapsids had developed, a group possibly characterized by some degree of endothermy (warm-bloodedness). The early mammals of the Triassic period are possibly an offshoot or descendant group of the therapsids.

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