Skeletal changes of vertebrates transitioning from water to land

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Innovations conventionally associated with terrestrially first appeared in aquatic elpistostegalians such as Panderichthys rhombolepis, Elpistostege watsoni, and Tiktaalik roseae. Phylogenetic analyses distribute the features that developed along the tetrapod stem and display a stepwise process of character acquisition, rather than abrupt.^[1] The complete transition occurred over a period of 30 million years beginning with the tetrapodomorph diversification in the Middle Devonian (380 myr).^[2]

By the Upper Devonian period, the fin-limb transition as well as other skeletal changes such as gill arch reduction, opercular series loss, mid-line fin loss, and scale reduction were already completed in many aquatic organisms.^[3] As aquatic tetrapods began their transition to land, several skeletal changes are thought to have occurred to allow for movement and respiration on land. Some adaptations required to adjust to non-aquatic life include the movement and use of alternating limbs, the use of pelvic appendages as sturdy propulsors, and the use of a solid surface at the organism's base to generate propulsive force required for walking.^[4]