From Water To Land: Salamanders as a Model for Understanding the Evolution of Tetrapod Locomotion

Wake Forest University, Winston Salem NC

Investigators

Abstract

From our perspective as humans, one of the most important events in vertebrate history was the evolution of the tetrapod limb. Only after the lobed fins of some Sarcopterygian fishes had been modified into limbs could the tetrapods (four-footed animals, including amphibians, reptiles, birds and mammals) begin to occupy terrestrial habitats. Undergraduate textbooks typically present a scenario where limbs evolved specifically to facilitate terrestrial locomotion. However, fossil evidence indicates that this notion is incorrect, and that the earliest tetrapods, though possessing appendages that were recognizable as limbs rather than fins, were fully aquatic organisms. The logical conclusion is that the tetrapod limb evolved for underwater locomotion. Little is known about aquatic locomotion involving the limbs compared to what is known about the involvement of the axial musculoskeletal system. Among the living tetrapod groups, salamanders most closely approach the body form and posture of primitive tetrapods. Therefore, they are the most appropriate choice for a model group with which to investigate this question. Terrestrial and underwater locomotion using the limbs, swimming, and the transition between swimming and walking will be examined in representative species of several salamander families. The techniques to be used include high-speed video recording to quantify limb movements, electromyography to examine the patterns of muscle activation, and histochemical assays to quantify the composition of locomotor muscles. The results of this research will yield insight into how the same anatomical structures function in different environments and, more generally, into the evolution of vertebrate gait patterns and the neural control of this major mode of locomotion.