Temnospondyls - Description

Description

Dorsal (left), ventral (center), and posterior (right) views of the skull of Metoposaurus

Many temnospondyls are much larger than living amphibians, and superficially resemble crocodiles. Others are smaller and resembled salamanders. Most have broad, flat heads that are either blunt (brevirostrine) or elongated (longirostrine). The skulls are rounded or triangular in shape when viewed from above, and are usually covered in pits and ridges. The rugged surfaces of bones may have supported blood vessels, which could transfer carbon dioxide to the bones to neutralize acidic buildup in the blood (early semiaquatic tetrapods would have had difficulty expelling carbon dioxide from their bodies while on land, and these dermal bones may have been an early solution to the problem). Many temnospondyls also have canal-like grooves in their skulls called sensory sulci. The sulci, which usually run around the nostrils and eye sockets, are part of a lateral line system used to detect vibrations in water. As semiaquatic animals, most temnospondyls have small limbs with four toes on each front foot and five on each hind foot. Terrestrial temnospondyls have larger, thicker limbs, and some even have claws. One unusual terrestrial temnospondyl, Fayella, has relatively long limbs for its body, and probably lived as an active runner able to chase prey.

Homologues of most of the bones of temnospondyls are also seen in other early tetrapods, aside from a few bones in the skull, such as interfrontals, internasals, and interparietals, that have developed in some temnospondyl taxa. Most temnospondyls have tabular horns in the backs of their skulls, rounded projections of bone separated from the rest of the skull by indentations called otic notches; in some temnospondyls, such as Zatrachys, they are pointed and very prominent. Among the most distinguishing features of temnospondyls are the interpterygoid vacuities, two large holes in the back of the palate. Another pair of holes, choanae, are present in front of these vacuities, and connect the nasal passage with the mouth. Temnospondyls often have teeth on their palates, as well as in their jaws. Some of these teeth are so large, they are referred to as tusks. In some temnospondyls, such as Nigerpeton, tusks in the lower jaw pierce the palate and emerge through openings in the top of the skull.

Very little is known of the soft tissue of temnospondyls. A block of sandstone, described in 2007 from the Early Carboniferous Mauch Chunk Formation of Pennsylvania, included impressions of the bodies of three temnospondyls. These impressions show, when alive, they had smooth skin, robust limbs with webbed feet, and a ridge of skin on their undersides. Trackways referable to small temnospondyls have also been found in Carboniferous and Permian rocks. The trackways, called batrachichni, are usually found in strata deposited around freshwater environments, suggesting the animals had some ties to the water.

Unlike modern amphibians, many temnospondyls are covered in small, closely packed scales. The undersides of most temnospondyls are covered in rows of large ventral plates. During early stages of development, they first have only small, rounded scales. Fossils show, as the animals grew, the scales on the undersides of their bodies developed into large, wide ventral plates. The plates overlap each other in a way that allows a wide range of flexibility. Later semiaquatic temnospondyls, such as trematosaurs and capitosaurs, have no evidence of scales. They may have lost scales to make movement easier under water or to allow cutaneous respiration, the absorption of oxygen through the skin.

Several groups of temnospondyls have large bony plates on their backs. One temnospondyl, Peltobatrachus, has armor-like plating that covers both its back and underside. The temnospondyl Laidleria also has extensive plating on its back. Most members of the family Dissorophidae also have armor, although it only covers the midline of the back with two narrow rows of plates. Other temnospondyls, such as Eryops, have been found with small, disc-like bony scutes that were in life probably embedded in the skin. All of these temnospondyls were adapted to a terrestrial lifestyle. Armor may have offered protection from predators in the case of Peltobatrachus. The scutes may have provided stability for the spine, as they would have limited flexibility and may have been connected by strong ligaments. Temnospondyls such as Sclerothorax and Eryops that may have been at least partly terrestrial also have long neural spines on top of their vertebrae that would have stabilized the spine. Bony scutes are also seen in plagiosaurs, but unlike Peltobatrachus, Laidleria, Eryops, and dissorophids, these animals are thought to have been fully aquatic. Plagiosaurs may have inherited their armor from a terrestrial ancestor, as both Peltobatrachus and Laidleria have been considered close relatives of the group.

Temnospondyls' vertebrae are divided into several segments. In living tetrapods, the main body of the vertebra is a single piece of bone called the centrum, but in temnospondyls, this region was divided into a pleurocentrum and intercentrum. Two types of vertebrae are recognized in temnospondyls: stereospondylous and rhachitomous vertebrae. In rhachitomous vertebrae, the intercentra are large and wedge-shaped, and the pleurocentra are relatively small blocks that fit between them. Both elements support a spine-like neural arch, and well-developed interlocking projections called zygapophyses strengthen the connections between vertebrae. The strong backbone and strong limbs of many ratchitomous temnospondyls allowed them to be partially, and in some cases fully, terrestrial. In stereospondylous vertebrae, the pleurocentra have been lost entirely, with the intercentra enlarged as the main body of the vertebrae. This weaker type of backbone indicates the stereospondylous temnospondyls spent more time in water.