Vertebrate land invasion

The aquatic to terrestrial transition of vertebrate organisms occurred in the late Devonian era and was an important step in the evolutionary history of modern land vertebrates. The transition allowed animals to escape competitive pressure from the water and explore niche opportunities on land. Fossils from this period have allowed scientists to identify some of the species that existed during this transition, such as Tiktaalik and Acanthostega. Many of these species were also the first to develop adaptations suited to terrestrial over aquatic life, such as neck mobility and hindlimb locomotion.

The late Devonian vertebrate transition was not the only terrestrial invasion in evolutionary history. The vertebrate transition was preceded by the plant and invertebrate terrestrial invasion. These invasions allowed for the appropriate niche development that would ultimately facilitate the vertebrate invasion. Furthermore, the late Devonian event is only significant in that it was the first land invasion by vertebrate organisms, and that it resulted in an explosion of vertebrate biodiversity due to the many different niches that these species were able to exploit. However, aquatic species have continued to develop adaptations suited to terrestrial life (and vice versa) from the late Devonian to the Holocene.

The vertebrate species that were important to the initial water to land transition can be qualified as being one of five groups: Sarcopterygian fishes, prototetrapods, aquatic tetrapods, true tetrapods, and terrestrial tetrapods. Many morphological changes occurred throughout this transition. Mechanical support structures changed from fins to limbs, the method of locomotion changed from swimming to walking, respiratory structures changed from gills to lungs, feeding mechanisms changed from suction feeding to biting, and mode of reproduction changed from larval development to metamorphosis.

Lungfish appeared approximately 400 million years ago. It is a species that endured rapid evolution during the Devonian era, which became known as the dipnoan renaissance. The Acanthostega species, known as the fish with legs, is considered a tetrapod by structural findings but is postulated to have perhaps never left the aquatic environment. Its legs are not well-suited to support its weight. The bones of its forearm, the radius and ulna, are very thin at the wrist and also unable to support it on land. It also lacks a sacrum and strong ligaments at the hip, which would be integral to supporting the animal against gravity. In this sense, the species is considered a tetrapod but not one that has adapted well enough to walk on land. Furthermore, its gill bars have a supportive brace characterized for use as an underwater ear because it can pick up noise vibrations through the water. Tetrapods that adapted to terrestrial living adapted these gill bones to pick up sounds through air, and they later became the middle ear bones seen in mammalian tetrapods. Ichthyostega, on the other hand, is considered to be a fully terrestrial tetrapod that perhaps depended on water for its aquatic young. Comparisons between the skeletal features of Acanthostega and Ichthyostega reveal that they had different habits. Acanthostega is likely exclusive to an aquatic environment, while Ichthyostega is progressed in the aquatic to terrestrial transition by living dominantly on the shores.

...
Wikipedia