Amphibians II

In my last article, I talked about the extant (still living) groups of amphibians. There were also two groups of amphibians that did very well for millions of years, but are now extinct.

The early amphibians differ somewhat from our modern amphibians in that their skeletons were well-ossified (their bones were composed mainly of hydroxyapatite, a calcium-containing salt), whereas modern amphibians have bones that are mostly cartilage, and do not fossilize well. The extinct amphibians also had well-developed rib cages, while extant amphibians have ribs that are greatly reduced, and cannot be used to help expand their lungs. The extinct amphibians also had a complete set of teeth, in comparison to the teeth of modern amphibians that are reduced both in size and in number. They also had strong, bony skulls that resembled the lobe-finned fish, from whom they are descended. In contrast, modern amphibians have skulls with bones that are reduced in size and number from the lobe-finned fish.

There were two main groups of extinct amphibians. The Labyrinthodonts were large, up to 4 meters (13 feet) in length. They lived from the Devonian Period (395 to 345 million years ago) to the end of the Triassic Period (230 to 195 million years ago). It is possible that they may have continued to live into the Jurassic Period (195 to 140 million years ago). Their name comes from the labyrinth-like shape of their teeth, which resembles the shape of the teeth of lobe-finned fish.

The Labyrinthodonts had massive skulls with many ornamental bumps and ridges. They had a sprawling gait, in which their legs were not directly under their shoulders and hips, but stuck out to the sides. (This is in contrast to the upright stance of dinosaurs, see my article "What do You Mean It's Not a Dinosaur?" for more information.)

Modern amphibians have a moist permeable skin, but we do not know if the Labyrinthodonts did as well. Thus far, paleontologists have not found any skin impressions that can be identified as Labyrinthodont. However, scales have been found associated with Labyrinthodont skeletons, indicating that they may have been at least partially covered with scales.

The second group of extinct amphibians are the Lepospondyls. The Lepospondyls get their name from the spool-like shape of their vertebrae. They were small animals, with simple teeth, somewhat reduced skulls, and some degeneration of the strongly ossified skeletons of their predecessors. Some paleontologists have speculated that they are the ancestors of our modern amphibians, or a middle point in the development from the Labyrinthodonts to modern amphibians. The Lepospondyls arose during the early Carboniferous Period (345 to 280 million years ago), and began to disappear toward the end of the Permian Period (280 to 230 million years ago), sometimes called the "Age of Amphibians."

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