Vertebrate land invasion
The vertebrate land invasion refers to the eco-evolutionary transition of vertebrate animals, more specifically of the sarcopterygian clade Tetrapodomorpha, from aquatic/semiaquatic "fishapods" to predominantly terrestrial tetrapods during the Late Devonian period. This transition allowed these vertebrates to escape competitive pressure from other aquatic animals and explore new ecological niches on land, which eventually established the vertebrates as the supreme phylum of the terrestrial food webs. 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, more efficient lungs, air-transmitted hearing, water-retaining integuments and physiology, and robust appendicular skeleton for limb weightbearing and terrestrial locomotion.
The late Devonian vertebrate transition was preceded by the terrestrial floral invasion of plants and fungi, with invertebrates such as arthropods following afterwards. These biotic colonization on land created vast swathes of wetland coal forests during the Late Devonian and Early Carboniferous, allowing for the development of primitive terrestrial ecosystems that would be available to accommodate habitation by amphibian vertebrates such as temnospondyls and lepospondyls, latter of which is thought to give rise to the fully terrestrial amniotes. While the Late Devonian event was the first land invasion by vertebrates, newer aquatic species have continued to develop adaptations suited to terrestrial life from the Late Devonian to the Holocene. Several group of tetrapods, however, evolved secondary re-adaptation to aquatic life later on.
Overview of transition
The vertebrate species that were important to the initial water-to-land transition can be sorted into five phases: aquatic sarcopterygians, prototetrapods, stem tetrapods, semiaquatic amphibians and fully terrestrial amniotes, the last two considered true tetrapods. Many morphological changes occurred throughout this transition. Mechanical support structures changed from paired fins to limbs, the method of locomotion changed from aquatic to terrestrial, respiratory structures changed from gills to lungs, feeding mechanisms changed from suction feeding to biting/gulping, and mode of reproduction changed from larval development to metamorphosis.Evolutionary timeline
Lungfish appeared approximately 400 million years ago, undergoing 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.
An evolutionary timeline of the late Devonian vertebrate terrestrial invasion demonstrates the changes that took place. A group of fish from the Givetian stage began developing limbs, and eventually evolved into aquatic tetrapods in the Famennian stage. Pederpes, Westlothiana, Proterogyrinus, and Crassigyrinus descended from these species into the Early Carboniferous period and were the first land vertebrates, indicating the crown group originated and split in that time, around 350 Ma.
A particularly important transitional species is one known as Tiktaalik. It has a fin, but the fin has bones within it that are similar to mammalian tetrapods. It has an upper arm bone, a lower arm bone, forearm bones, a wrist, and fingerlike projections. Essentially, it is a fin that can support the animal. Similarly, it also has a neck that allows independent head movement from the body. Its ribs are also able to support the body in gravity. Its skeletal features exhibit its ability as a fish that can live in shallow water and also venture onto land.
Driving factors
It took many millions of years for vertebrates to transition out of water onto land. During this time, perhaps competitive pressures pushed species out of the water and certainly niche occupation incentives pulled species onto land. The culmination of these driving factors are what ultimately facilitated the vertebrate transition.Evolutionary pushes
Scientists believe that a long period of time where biotic and abiotic factors in the aquatic environment were unfavourable to certain aquatic organisms is what pushed their transition to shallower waters. Some of these push factors are environmental hypoxia, unsuitable aquatic temperatures, and increased salinity. Other constantly present factors such as predation, competition, waterborne diseases and parasites also contributed to the transition.A theory put forth by Joseph Barrell possibly helps explain what may have initiated these push factors to become relevant in the late Devonian. The extensive oxidized sediments that were present in Europe and North America during the late Devonian are evidence of severe droughts during this time. These droughts would cause small ponds and lakes to dry out, forcing certain aquatic organisms to move on land to find other bodies of water. Natural selection on these organisms eventually led to the evolution of the first terrestrial vertebrates.
Evolutionary pulls
The pull factors were secondary to the push factors, and only became significant once the pressures to leave the aquatic environment became significant. These were largely the niches and opportunities that were available for exploitation in the terrestrial environment, and include higher environmental oxygen partial pressures, favourable temperatures, and the lack of competitors and predators on land. The plants and invertebrates that had preceded the vertebrate invasion also provided opportunities in the form of abundant prey and lack of predators.Barriers to transition
There were many challenges that the first land vertebrates faced. These challenges allowed for rapid natural selection and niche domination, resulting in an adaptive radiation that produced many different vertebrate land species in a relatively short period of time.Anatomical
The primary anatomical barrier is the development of lungs for proper gas exchange, however other anatomical barriers also exist. The stressors of the musculoskeletal system are different in air than they are in water, and the muscles and bones must be strong enough to withstand the increased effects of gravity on land. The tongue and the three chambered heart evolved similarly for efficient digestion and blood circulation on land respectively. The vomeronasal organ is found in many living tetrapods but not any fish, suggesting it originated in tetrapods only. Similarly, all tetrapods have parathyroid glands which other animals don't.Depending on the water depth at which a species lives, the visual perception of many aquatic species is better suited to darker environments than those on land. Similarly, hearing in aquatic organisms is better optimized for sounds underwater, where the speed and amplitude of sound is greater than in air. The spiracle of their ancestors, once used for breathing, was repurposed to hold a eardrum inside, connected to the pharynx by the auditory tube and to the otic vesicle by the columella, for hearing.