<P> 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 . The complete transition occurred over a period of 25 million years beginning with the tetrapodomorph diversification in the Middle Devonian (380 myr). </P> <P> 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 . 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 . </P> <P> The Osteolepiformes and Elpistostegalia are two crown groups of rhipidistians with respect to the tetrapods . The development of skull roof and cheekbone patterns in these organisms match those found in the first tetrapods . Palatal and nasal skeletal features like choanae are present in these groups and are also observed in modern amphibians . This indicates that incipient air breathing was developed, as well as modification of the hyoid arch towards stapes development . These characteristics account for why osteichthyans are accepted as the sister group of tetrapods . </P> <P> The elpistostegalid fish are considered the most apomorphic of fish in comparison to tetrapods . From well - preserved fossils, it is observed that they share a paltybasic skull with eye ridges, and external nares situated on the margin of the mouth . Development of eye ridges and flatting of the skull are also observed in primitive fossil amphibians and reptiles . The most likely reason for the traits to be adaptive was for their use in aerial vision above the waterline . The traits enabled animals to check area on land for safe spots if being chased by a predator in water, as well as being useful for searching for prey items above the water . The water - based lateral line system was used substantially by these aquatic tetrapods to detect danger from predators . Within the Osteichthyan diversification, there were no changes related to respiration in the transition as can be seen by the nasal region and palatal morphology in elpistostegalid fishes . The primary change from basic ostelepiform ancestors to the first elpistostegalid in the middle Devonian was to the pre-existing roof skulls . </P>

Explain how the development of the skeleton has allowed organisms to live successfully on land