Skeletal changes of vertebrates 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.
[1] The complete transition occurred over a period of 30 million years beginning with the tetrapodomorph diversification in the Middle Devonian (380 myr).
[2] 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.
[7] The loss of the intercranial joint was a direct functional necessity to strengthen the broad and long platybasic skull when the animal was out of the water.
In the cranium, there is a stapes derived from the hyomandibular of fishes; a single bilateral pair of nasal bones, and a fenestra ovalis in the otic capsule of the braincase.
[5] It involved the enlargement of the jugal, ceasing the contact of the maxilla with the squamosal and the single bilateral pair of nasal bones.
[12] Transitional forms prior to fully developed terrestrial tetrapods such as Acanthostega, are thought to have captured prey in the water.
These differences as well as reductions of the gill chamber and changes in the nature of the lower jaw are hypothesized to indicate a reduced reliance on suction feeding in early tetrapods in comparison to osteolepiform fish.
Acanthostega fossil records demonstrate that no strain pattern was exhibited that relate to prey capture by means of suction.
[21] This is thought to have originated in the group Sarcopterygians, including osteolipiforms like Eusthenopteron, due to the homology of the tetrapod forelimb and the osteolepiform fin endoskeleton.
[3] Like Panderichthys, the humerus of Acanthostega is flattened dorso-ventrally, the intermedium terminates level with the radius, and the endoskeleton can be divided into stylopodium, zeugopodium and autopodium segments.
The humerus and femur of Acanthostega also contain evidence of greater development of the appendicular muscles compared to more aquatic tetrapods, hinting at the presence of digits.
[3] It has a sacrum; a fundamental skeletal feature that allows the organism to transfer force produced in its hindlimbs to its axial skeleton, and move in a terrestrial environment.
[3] In contrast, Protopterus annectens (a member of lungfish, thought to be a sister group to tetrapods) has a small, anatomically simpler pelvis, a derived limb endoskeleton and a lack of digits.
[4] Yet, it shares the ability to lift itself using a solid surface as a base with its pelvic region with Acanthostega and is also observed to move with tetrapod-like locomotion in an aquatic environment.
[3] Acanthostega has a total lack of dermal fin rays and displays the presence of two or more spool-shaped bones or cartilages articulating individually in antero-posterial sets on the distal end of its limbs.
[2] Unlike previous tetrapods, who have been only partially adapted to land, Pederpes has the novel ability to bend its limbs and propel itself forwards in a terrestrial setting.
This is attributed to the symmetry of the digits and limbs in Pederpes, allowing it to rotate its hindlimbs to an anteriorly facing position and propel itself from the edge of the foot when moving forward.
