Zoology 10th Edition by Stephen Miller,John Harley

Exercise 4

Fossils of ancient sarcopterygian fish and early amphibians, such as Ichthyostega , provide ample evidence of the evolutionary transition between fish and amphibians. Ancient tetrapods gave rise to numerous lineages, including the Lissamphibia (modern amphibians) and the amniotes. The relationship among these lineages is controversial.
Why is the use of the "class" designation for members of the Amphibia phylogenetically incorrect?
FIGURE 19.2 Ichthyostega: An Early Amphibian. Fossils of this early amphibian were discovered in eastern Greenland in late Devonian deposits. The total length of the restored specimen is about 65 cm. Terrestrial adaptations are heavy pectoral and pelvic girdles and sturdy limbs that probably helped push the body across the ground. Strong jaws suggest that it was a predator in shallow water, perhaps venturing onto shore. Other features include a skull that is similar in structure to that of ancient sarcopterygian fishes and a finlike tail. Note that bony rays dorsal to the spines of the vertebrae support the tail fin. This pattern is similar to the structure of the dorsal fins of fishes and is unknown in any other tetrapod. The arrangement of bony elements in the distal portion of the foreleg is unknown.
FIGURE 19.3 One Hypothesis of Evolutionary Relationships among the Tetrapods. The earliest amphibians arose during the Devonian period. There are three classes of modern amphibians and numerous extinct taxa. "Amphibia" is used to designate this lineage as there are many controversies surrounding the ancestry of lissamphibians. The reptiliomorph lineage of early tetrapods gave rise to reptiles (including birds), mammals, and other extinct taxa. Daggers (†) indicate extinct taxa. Synapomorphic characters for lower taxonomic groups are not indicated. The relationships depicted here are controversial. The Tetrapoda, as represented here, includes the crown-group members of this lineage.