GEOL 102 Historical Geology

Spring Semester 2007
The Late Paleozoic Era IV: Permian Life and the Permo-Triassic Extinction

Arthropods on Land:
Devonian hexapods still wingless. Scorpions common, many still partly aquatic. Spider- like arachnids (but not true spiders) also present.

During the Carboniferous:
Arthropod diversity increases during Mississippian, but especially during Pennsylvanian (in concert with spread of forests). Some new aspects of Carboniferous arthropods:

Pennsylvanian arthropods include many gigantic forms:

Why so giant?

Permian terrestrial arthropods:
Few new groups, although the giant forms disappear.

The Permo-Triassic extinction is the ONLY mass extinction to effect insect diversity: sail through K-T extinction, etc., with no discernible effect, yet many clades killed off by Permo-Triassic event.

Other Terrestrial Invertebrates:
During later Paleozoic, freshwater snails and clams appear.

Late Paleozoic terrestrial vertebrates:
During Devonian, one branch of the sarcopterygians (lobe-fins) develop first wrists, then digits (fingers and toes). These represent the first stegocephalians ("terrestrial" vertebrates).

So by the later part of the Devonian Period there were vertebrates which had a bony skeleton to support their bodies; bony limbs with wrists, ankles, and digits to push along on land or on the lakebed; lungs to breath air (but still had gills to breath in the water).

Many of these Late Devonian stegocephalians still lived their lives essentially only in the water (such as Acanthostega), and thus were essentially fish-with-fingers. Others (such as Ichthyostega may have gotten most of their food from land. It was from these latter sort that the more fully terrestrial vertebrates--the Tetrapoda ("four footed ones"), would evolve.

High diversity of Carboniferous stegocephalians:

Ecological breakthrough during Middle Pennsylvanian:

Amniotes divided into two main branches:

  • Small-headed, heavily-built herbivores
  • Semi-aquatic fish-eaters
  • Sail-backed plant and/or mollusk-eaters
  • Sail-backed carnivores with different-sized teeth in different parts of the jaw

    These early synapsids would have had the sprawling stance found in primitive tetrapods in general. They almost certainly would have been "cold-blooded" (the ancestral state for vertebrates). So traditionally these animals have been considered "reptiles". However, they lack the shared derived features of reptiles (see below), and are instead simply primitive synapsids. (In traditional taxonomy, these were put in the group "Pelycosauria", but this is a paraphyletic group: all synapsids except for therapsids). New discoveries show that even early synapsids had some parental care.

    The early synapsids evolved themselves into extinction: that is, they were replaced in the Middle and Late Permian Epoch by the Therapsida: the advanced synapsids. Once called the "mammal-like reptiles", they are not true reptiles. Instead, they are the advanced branch of the synapsid phylogeny. They differed from earlier synapsids by:

    The Middle and Late Permian therapsids included:

    Ancestrally, all vertebrates are cold-blooded (warm their bodies primarily using sunlight). However, some evidence suggests that the advanced therapsids of the Late Permian may have had elevated metabolisms (that is, were at least partially warm-blooded):

    True reptiles tended to be relatively rare in the Carboniferous and Permian Periods. Reptilia is characterized by a number of particular skeletal features (which we aren't going to deal with here, as they are fairly technical). Modern reptiles (and by inference, their concestor and all of its descendants) share a number of soft-tissue features:

    In general, compared to typical Mesozoic and Cenozoic ecosystems, the late Paleozoic land vertebrates were smaller (few ox- or hippo-sized, none larger), slower (no real speed specialists), and close to the ground (only a few gliders and no powered fliers; few tree-climbing specialists).

    The Permo-Triassic Extinction:
    Largest mass extinction of Phanerozoic. Total of all Permo-Triassic events may be 95% of species.

    Victims include:

    Was once thought to be gradual (extended over 9 Myr), but now seems be in two main pulses. The first, between the Capitanian Age of the Middle Permian (or Guadalupian) Epoch and the Wuchiapingian Age of the Late Permian (or Lopingian) Epoch. It was smaller, but still powerful: 34% genus level extinction in the seas (comparable to the Cretaceous-Paleogene extinction!) and fairly powerful on land, too. The event showed a marked reduction of members of the Paleozoic fauna, but not the loss of entire major clades.

    The BIG ONE is at the end of the Changhsingian Age of the Lopingian Epoch, was the worst mass extinction in the history of multicellular life:

    Regardless of precise scenario, extinction reorganizes the world. After the event, the Paleozoic marine evolutionary fauna becomes subordinate to the Modern marine evolutionary fauna. Also, the size and diversity of the animals present is greatly reduced.

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    Last modified: 10 January 2008