Permian

From Palaeos.org

Permian reef diorama. This image is from the Earth History Resources website

Introduction

The Permian period was named in 1841 by the geologist Murchison after a tour of Imperial Russia to include the "vast series of beds of marls, schists, limestones, sandstones, and conglomerates" that overlay the Carboniferous formations in the eastern part of the country. He named it after the ancient kingdom of Permia and the present city of Perm near the Ural mountains.

Geography

During the Permian all the world's land masses joined together into a single supercontinent, Pangea. The collision between Laurasia and Siberia-Kazakhstania and China finalized assembly of Pangaea by end of Permian. This was the first time since the late Proterozoic supercontinent of Rodinia that such a landmass had formed. Pangea was shaped sort of like a giant "Pacman", with the mouth on the east. There was a correspondingly large single ocean, called Panthalassa. The body of water enclosed by the pacman mouth constituted a smaller sea, the Tethys, which covered much of what is now southern and central Europe.

Throughout the Permian, Europe was covered by a very salty inland sea, the Zechstein sea, which advanced and receded at least twice. This was home to an impoverished fauna, mainly brachiopods and bivalves, which were able to cope with the hypersaline conditions.

Due to the formation of the supercontinent Pangea, the sea level drops and the warm shallow seas decline in extent. This is one of the factors that may have led to the extinction of many life-forms at the end of the period

Stratigraphy

Unlike most other geological periods which have a three-part division into early, middle, and late, the Permian Period was conventionally divided into early and late only. The more recent arrangement has a three-fold division. Although the earlier part is well documented, there have recently been some controversies regarding the relative dating of the late Permian, and of the Permian-Triassic boundary in general. The following table presents the various periods, epochs, and ages. The dating (in millions of years ago) is of course approximate, as are all such ancient dates. We have used the most recent ICS dates, which quite plausibly allow rather more time to the later Permian ages then some of the earlier timescales.

Climate

As the Permian opened, the Earth was still in the grip of an ice age, so the polar regions were covered with deep layers of ice. Glaciers continued to cover much of Gondwanaland, as they had during the late Carboniferous . At the same time the tropics were covered in swampy forests.

Towards the middle of the period the climate became warmer and milder, the glaciers receded, and the continental interiors became drier. Much of the interior of Pangea was probably arid, with great seasonal fluctuations (wet and dry seasons), because of the lack of the moderating effect of nearby bodies of water. This drying tendency continued through to the late Permian, along with alternating warming and cooling periods.

Permian Sites

The Permian has few sites with exceptional preservation. Some exceptional arthropod specimens are known from Kansas and Oklahoma (south-central US). The Permian is best known for its vertebrate fossils .

Three important areas for Permian vertebrate fossils are the Early to middle Permian equatorial Red Beds of Texas and Oklahoma, the middle to late Permian Kazanian and Tatarian zones on the Russian platform, and the Late Permian Karoo series (Lower Beaufort) of southern Africa.

The Red Beds are full of the fossil remains of pelycosaurs like the finbacks like Dimetrodon, which was clearly the dominant predator of these environments for some 20 million years. The Russian and South African sites contain the remains of many therapsids. These creatures succeeded the basal synapsids as the rulers of the land, until they in turn were supplanted by the archosaurs during the Early Triassic.

Permian Life

The warm shallow oceans swarmed with many kinds of life, basically very similar to Carboniferous forms). Sedentary organisms like stromatolites, algae, foraminifers, sponges, corals, bryozoa, and brachiopods built great reefs which in turn provided homes and shelter for active animals like ammonoids, nautiloids, gastropods and fish. Ammonoids differed from their Carboniferous predecessors in that they had far more complex suture lines, frequently with many-pointed lobes and rounded saddles.

The giant Carboniferous insects continued for a while, before also disappearing during the Guadalupian. Meanwhile, important new groups of insects like beetles and flies, with more complex life cycles, emerged. There was a major extinction event at end of period, due in part to continental shelf environments being reduced; trilobites and many other groups of organisms became extinct

Plants

This was a period of transition. The early Permian saw the continuation of the Carboniferous biomes, with polar tundra regions and warm wet tropical swamp forests. But the drying climatic tendency during the mid Permian spelled death for the mighty swamp forests. Water loving plants like lycopods and sphenopsids were greatly reduced in size, becoming mere shrubs. The old tropical coal swamps (with their giant lycopods, calamites, and cordaitales) declined and disappeared with the drier and cooler climate, surviving only in China and in high latitudes of Pangaea. Plant life consisted mainly of ferns and seed-ferns, with new plants like conifers and ginkgos coming into prominence. The Glossopteris flora predominating in Gondwanaland (the southern portion of Pangaea). It is gradually replaced by the seed-fern Dicroidium as the climate dries in the Late Permian.

Vertebrates

The great diversity of near-shore and fresh water chondrichthyans which characterized the Carboniferous began to decline during the Permian. In the oceans, xenacanth sharks dominated until the Guadalupian, when they were replaced by hybodonts. A few acanthodians also lingered into the Cisuralian. Lungfishes and coelacanths were more diverse than they are today, but all of the other sarcopterygian fishes had already become extinct. The Permian oceans were dominated by a diverse group of spiny-finned (actinopterygian) fishes, most of which had thick, heavy scales and rather basic jaw structures. Neopterygians with more derived jaw structures probably began to appear by the end of the Lopingian.

The increasing aridity of the Permian not only affected plants. The tetrapods suffered as their swamps and pools shrunk and dried out. Those surviving forms included big-headed temnospondyls two to three meters in length, as well as long-snouted forms (the archegosaurs) superficially resembling small crocodiles. Many of the non-amniote reptilomorphs, such as anthracosaurs and embolomeres, continued in to the Permian.

But it was the amniotes that took over as the dominant land animals, adapted to life on land (thanks to water-retentive dry skin and the amniotic egg). Although there were a number of different types of amniotes, the largest and most diverse belonged to the Synapsida, which were ancestral to the mammals.

There were several distinct evolutionary dynasties of synapsids as the Permian progressed:

The first, the pelycosaur dynasty, included the large finbacks of the early Permian such as Dimetrodon, Edaphosaurus, Ctenospondylus, and Secodontosaurus, all of which attained a lengths some 3 meters, as well a similar types that lacked a "sail". The large dorsal "sails" were most certainly thermoregulatory devices that would heat up the animal in the cold morning, making it more active and giving it an advantage over it's more sluggish sail-less relatives. These animals were limited to the equatorial tropics.

Following this was the dinocephalian dynasty of the middle Permian (Guadalupian epoch). The dinocephalians were among the most primitive of the therapsids or "mammal-like reptiles". Some grew to huge size (5 to 6 meters) with meter long heads full of wicked teeth (the name dinocephalian means "fearsome head"). These creatures succeeded the pelycosaurs, being both larger in size and more metabolically active. There were several different types, the primitive anteosaurs being carnivores, and the ox-sized Tapinocephalia being herbivores.

The dinocephalians all died out suddenly, perhaps as a result of unusual climatic factors, at the end of the Guadalupian. The therapsids that followed them were smaller, and more mammal-like. Some may even evolved fur and the ability to control their temperatures metabolically. These included the large gorgonopsians (the Permian equivalent of the "saber-toothed tiger"), the small to medium-sized Therocephalia, and the herbivorous dicynodonts. These creatures had previously lived alongside the giant dinocephalians, but came into their own when the latter had died out.

In addition there were many types of non-syanapsid amniotes. The pareiasaurs, a group of big, armored herbivores probably related to turtles, reached enormous sizes (length up to 3 meters). Smaller, lizard-like reptiles were probably common, but they are very poorly known. These included the bolosaurs and procolophonids - both also turtle relatives. During the Permian, the crocodile-bird lineage (Archosauromorpha) had not yet diverged from the lizard-snake (Lepidosauromorpha) clade. The primitive reptiles were represented by lizard-like captorhinids and basal diapsids. The latter included a number of marine or amphibious forms: younginiforms, Claudiosaurus, and perhaps the earliest members of the ichthyosaur group.

© MAK 2002. Revised ATW050604.

Extinction

Main page: Permian extinction

The Permian is perhaps best known for the great mass extinction at its close, sometimes known as "The Great Dying". Species extinction rates of 90% or more have frequently been quoted, but it now seems a significant proportion of the extinctions took place earlier, at the Guadalupian-Lopingian boundary, leaving the terminal Permian extinction proper with a marine species extinction rate of "only" 80% or so.

Casualties included most therapsids (only cynodonts and dicynodonts continuing), the last trilobites, various primitive insect lineages, and the blastoids. Almost every group suffered a reduction in diversity.

  Palaeos.com: Permian

credits: MAK061101