amphibians

amphibians Amphibians are tetrapods that still retain a connection to the water because of their need to lay their eggs in moist conditions. Although modern amphibians, frogs and toads, salamanders, and caecilians (legless amphibians) are familiar organisms today, their Palaeozoic ancestors looked quite different and there are difficulties in establishing relationships between the two groups. The earliest amphibians are known from East Greenland and from Russia. They are exemplified by Ichthyostega from the Late Devonian of East Greenland, an animal about 0.5–1 m long that may have breathed air and moved about on land, but retained a fish-like tail and the sensory canal system and gills of its osteolepiform (bony fish) ancestors. These features indicate that it spent most of its time in the water. Sharp conical teeth in the jaws indicate that this animal was a predator, while a number of skeletal adaptations attest to its ability to support its body on land. These include enlarged ribs to support the viscera, and the replacement of the notochord by vertebrae that consisted of a neural arch and a ventral half-ring connected by bony nodules.

A major division of the amphibians occurred in the Early Carboniferous with the development of two groups, the labyrinthodonts and the lepospondyls (Fig. 1). Labyrinthodonts are named for the complex folding of their tooth enamel. Most of them were stocky, short-legged animals up to 2 m in length, with large skulls and jaws lined with sharp conical teeth. They had notches in the back of the skull to accommodate the tympanic membrane (ear-drum). This was joined to the inner ear by a small bone called the stapes, which had been a jaw support (the hyomandibular) in the osteolepiforms. The large body size and short legs suggest that these animals lived mostly in the water, where they adopted a crocodilian life-style. Typical labyrinthodonts of this type are called temnospondyls. This group also includes some forms that developed aquatic characteristics, such as laterally flattened tails and reduced limbs, which indicate a return to a fully aquatic way of life. Temnospondyls were important carnivores during the Carboniferous and Permian and persisted into the Mesozoic. The last examples are known from the Cretaceous of Australia, where they were contemporaries of the dinosaurs and the mammals. The other group of labyrinthodonts was the anthracosaurs, terrestrial amphibians that gave rise to the reptiles in the Early Carboniferous. In anthracosaurs such as Seymouria from the Permian of Texas, the skull and dentition are still essentially amphibian while the post-cranial skeleton shows a large number of features characteristic of early reptiles. Whether animals such as this were really amphibians or reptiles ultimately depends on whether they laid amniote (hard-shelled) eggs or not; and the answer to this question is not yet known.

The second major group of Palaeozoic amphibians, the lepospondyls, were mostly rather small animals less than 0.3 m long, in which the teeth lacked the labyrinthodont structure and the skull lacked a notch for the tympanic membrane. They are named for the spool-shaped centrum or bony disc surrounding the notochord that contrasts with the labyrinthodont vertebrae formed of several pieces. This group developed elongated bodies and reduced limbs as an adaptation to a completely aquatic existence. One group, the diplocaulids, developed skulls that were greatly extended laterally, giving them a boomerang shape. Hydrodynamic studies have shown that this shape provided lift and allowed the animal to rise rapidly from its resting-place on the bottom to attack fish swimming above it.

The modern amphibians are separated by a large gap in time and morphology from the major Palaeozoic groups. The earliest frogs and salamanders are found in the Jurassic and look very much like their modern descendents. A possible ancestral frog from the Triassic of Madagascar, Triadobatrachus, shows specializations in the ear region similar to those found in some temnospondyl labyrinthodonts, which suggests that they are likely ancestors. On the other hand, it has been argued that salamanders and caecilians were derived from the microsaurs, a lepospondyl group. If this is correct, then modern amphibians are polyphyletic (derived from more than one ancestor), despite the fact that there appear to be a number of characters linking them, including the development of respiration through the skin, cylindrical vertebral centra, and the presence of pedicellate teeth in which a zone of weakness between base and crown allows them to break easily.

David K. Elliott

Bibliography

Carroll, R. L. (1988) Vertebrate paleontology and evolution. W. H. Freeman, New York.
Colbert, E. H. and and Morales, M. (1991) Evolution of the vertebrates. Wiley-Liss, Inc., New York.