Evolution of tetrapods
[1] Tetrapods (under the apomorphy-based definition used on this page) are categorized as animals in the biological superclass Tetrapoda, which includes all living and extinct amphibians, reptiles, birds, and mammals.
While most species today are terrestrial, little evidence supports the idea that any of the earliest tetrapods could move about on land, as their limbs could not have held their midsections off the ground and the known trackways do not indicate they dragged their bellies around.
[6] It is also one of the best understood, largely thanks to a number of significant transitional fossil finds in the late 20th century combined with improved phylogenetic analysis.
When nutrients from plants were released into lakes and rivers, they were absorbed by microorganisms which in turn were eaten by invertebrates, which served as food for vertebrates.
[16] Per unit volume, there is much more oxygen in air than in water, and vertebrates (especially nektonic ones) are active animals with a higher energy requirement compared to invertebrates of similar sizes.
In lungfishes, bowfin and bichirs, the swim bladder is supplied with blood by paired pulmonary arteries branching off from the hindmost (6th) aortic arch.
Such evidence is now available: a small lobe-finned fish called Kenichthys, found in China and dated at around 395 million years old, represents evolution "caught in mid-act", with the maxilla and premaxilla separated and an aperture—the incipient choana—on the lip in between the two bones.
Since the shallows were subject to occasional oxygen deficiency, the ability to breathe atmospheric air with the swim bladder became increasingly important.
Pelvic bone fossils from Tiktaalik shows, if representative for early tetrapods in general, that hind appendages and pelvic-propelled locomotion originated in water before terrestrial adaptations.
[43][44] Robot simulations suggest that the necessary nervous circuitry for walking evolved from the nerves governing swimming, utilizing the sideways oscillation of the body with the limbs primarily functioning as anchoring points and providing limited thrust.
The fins are brought forward and planted; the shoulders then rotate rearward, advancing the body & dragging the tail as a third point of contact.
In 2010, this belief was challenged by the discovery of the oldest known tetrapod tracks named the Zachelmie trackways, preserved in marine sediments of the southern coast of Laurasia, now Świętokrzyskie (Holy Cross) Mountains of Poland.
[49] Additionally, the tracks show that the animal was capable of thrusting its arms and legs forward, a type of motion that would have been impossible in tetrapodomorph fish like Tiktaalik.
[50] The new finds suggest that the first tetrapods may have lived as opportunists on the tidal flats, feeding on marine animals that were washed up or stranded by the tide.
[52][53] Research by Jennifer A. Clack and her colleagues showed that the very earliest tetrapods, animals similar to Acanthostega, were wholly aquatic and quite unsuited to life on land.
This is in contrast to the earlier view that fish had first invaded the land — either in search of prey (like modern mudskippers) or to find water when the pond they lived in dried out — and later evolved legs, lungs, etc.
By the late Devonian, land plants had stabilized freshwater habitats, allowing the first wetland ecosystems to develop, with increasingly complex food webs that afforded new opportunities.
Swampy habitats like shallow wetlands, coastal lagoons and large brackish river deltas also existed at this time, and there is much to suggest that this is the kind of environment in which the tetrapods evolved.
The Chinese tetrapod Sinostega pani was discovered among fossilized tropical plants and lobe-finned fish in the red sandstone sediments of the Ningxia Hui Autonomous Region of northwest China.
This finding substantially extended the geographical range of these animals and has raised new questions about the worldwide distribution and great taxonomic diversity they achieved within a relatively short time.
The terrestrial niche was also a much more challenging place for primarily aquatic animals, but because of the way evolution and selection pressure work, those juveniles who could take advantage of this would be rewarded.
Once they gained a small foothold on land, thanks to their pre-adaptations, favourable variations in their descendants would gradually result in continuing evolution and diversification.
Initially making only tentative forays onto land, tetrapods adapted to terrestrial environments over time and spent longer periods away from the water.
It is also possible that the adults started to spend some time on land (as the skeletal modifications in early tetrapods such as Ichthyostega suggests) to bask in the sun close to the water's edge[citation needed], while otherwise being mostly aquatic.
It was referred to as "Romer's Gap", which now covers the period from about 360 to 345 million years ago (the Devonian-Carboniferous transition and the early Mississippian), after the palaeontologist who recognized it.
By the Triassic, this group had already radiated into the earliest mammals, turtles, and crocodiles (lizards and birds appeared in the Jurassic, and snakes in the Cretaceous).
Amphibians and reptiles were strongly affected by the Carboniferous rainforest collapse (CRC), an extinction event that occurred ~307 million years ago.
[63] The aridity and temperature drop which resulted from this runaway plant reduction and decrease in a primary greenhouse gas caused the Earth to rapidly enter a series of intense Ice Ages.
Reptiles invaded new niches at a faster rate and began diversifying their diets, becoming herbivorous and carnivorous, rather than feeding exclusively on insects and fish.
Following the great faunal turnover at the end of the Mesozoic, only seven groups of tetrapods were left, with one, the Choristodera, becoming extinct 11 million years ago due to unknown reasons.
