Skeletal changes of organisms transitioning from water to land

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).

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.

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.

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.

...
Wikipedia