Evolution of tetrapods
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The evolution of tetrapods began about 400 million years ago in the
Most amphibians today remain semiaquatic, living the first stage of their lives as fish-like
The change from a body plan for breathing and navigating in water to a body plan enabling the animal to move on land is one of the most profound evolutionary changes known.[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.[1]
Origin
Evolution of fish
The Devonian period is traditionally known as the "Age of Fish", marking the diversification of numerous extinct and modern major fish groups.
They did, however, have certain traits separating them from cartilaginous fishes, traits that would become pivotal in the evolution of terrestrial forms. With the exception of a pair of
It was assumed that fishes to a large degree evolved around reefs, but since their origin about 480 million years ago, they lived in near-shore environments like intertidal areas or permanently shallow lagoons and didn't start to proliferate into other biotopes before 60 million years later. A few adapted to deeper water, while solid and heavily built forms stayed where they were or migrated into freshwater.[10][11] The increase of primary productivity on land during the late Devonian changed the freshwater ecosystems. 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. Some fish also became detritivores.[12] Early tetrapods evolved a tolerance to environments which varied in salinity, such as estuaries or deltas.[13]
Lungs before land
The lung/
To function in gas exchange, lungs require a blood supply. In cartilaginous fishes and
The breath
In order for the lungs to allow gas exchange, the lungs first need to have gas in them. In modern tetrapods, three important breathing mechanisms are conserved from early ancestors, the first being a CO2/H+ detection system. In modern tetrapod breathing, the impulse to take a breath is triggered by a buildup of CO2 in the bloodstream and not a lack of O2.[24] A similar CO2/H+ detection system is found in all Osteichthyes, which implies that the last common ancestor of all Osteichthyes had a need of this sort of detection system.[24][25] The second mechanism for a breath is a surfactant system in the lungs to facilitate gas exchange. This is also found in all Osteichthyes, even those that are almost entirely aquatic.[26][27] The highly conserved nature of this system suggests that even aquatic Osteichthyes have some need for a surfactant system, which may seem strange as there is no gas underwater. The third mechanism for a breath is the actual motion of the breath. This mechanism predates the last common ancestor of Osteichthyes, as it can be observed in Lampetra camtshatica, the sister clade to Osteichthyes. In Lampreys, this mechanism takes the form of a "cough", where the lamprey shakes its body to allow water flow across its gills. When CO2 levels in the lamprey's blood climb too high, a signal is sent to a central pattern generator that causes the lamprey to "cough" and allow CO2 to leave its body.[28][29] This linkage between the CO2 detection system and the central pattern generator is extremely similar to the linkage between these two systems in tetrapods, which implies homology.
External and internal nares
The
The evolution of the tetrapods' internal nares was hotly debated in the 20th century. The internal nares could be one set of the external ones (usually presumed to be the posterior pair) that have migrated into the mouth, or the internal pair could be a newly evolved structure. To make way for a migration, however, the two tooth-bearing bones of the upper jaw, the maxilla and the premaxilla, would have to separate to let the nostril through and then rejoin; until recently, there was no evidence for a transitional stage, with the two bones disconnected. 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.[31] Kenichthys is more closely related to tetrapods than is the coelacanth,[32] which has only external nares; it thus represents an intermediate stage in the evolution of the tetrapod condition. The reason for the evolutionary movement of the posterior nostril from the nose to lip, however, is not well understood.
Into the shallows
The relatives of Kenichthys soon established themselves in the waterways and brackish estuaries and became the most numerous of the bony fishes throughout the Devonian and most of the Carboniferous. The basic anatomy of the group is well known thanks to the very detailed work on
There were a number of families:
While most of these were open-water fishes, one group, the Elpistostegalians, adapted to life in the shallows. They evolved flat bodies for movement in very shallow water, and the pectoral and pelvic fins took over as the main propulsion organs. Most median fins disappeared, leaving only a protocercal tailfin. Since the shallows were subject to occasional oxygen deficiency, the ability to breathe atmospheric air with the swim bladder became increasingly important.[6] The spiracle became large and prominent, enabling these fishes to draw air.
Skull morphology
The tetrapods have their root in the early
From fins to feet
The oldest known tetrapodomorph is
Another indication that feet and other tetrapod traits evolved while the animals were still aquatic is how they were feeding. They did not have the modifications of the skull and jaw that allowed them to swallow prey on land. Prey could be caught in the shallows, at the water's edge or on land, but had to be eaten in water where hydrodynamic forces from the expansion of their buccal cavity would force the food into their esophagus.[42]
It has been suggested that the evolution of the tetrapod limb from fins in lobe-finned fishes is related to expression of the HOXD13 gene or the loss of the proteins actinodin 1 and actinodin 2, which are involved in fish fin development.[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.[45] This type of movement, as well as changes to the pectoral girdle are similar to those seen in the fossil record, can be induced in bichirs by raising them out of water.[46]
A 2012 study using 3D reconstructions of Ichthyostega concluded that it was incapable of typical quadrupedal gaits. The limbs could not move alternately as they lacked the necessary rotary motion range. In addition, the hind limbs lacked the necessary pelvic musculature for hindlimb-driven land movement. Their most likely method of terrestrial locomotion is that of synchronous "crutching motions", similar to modern mudskippers.[47] (Viewing several videos of mudskipper "walking" shows that they move by pulling themselves forward with both pectoral fins at the same time (left & right pectoral fins move simultaneously, not alternatively). The fins are brought forward and planted; the shoulders then rotate rearward, advancing the body & dragging the tail as a third point of contact. There are no rear "limbs"/fins, and there is no significant flexure of the spine involved.)
Denizens of the swamp
The first tetrapods probably
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.[49] Currently, however, fish are stranded in significant numbers only at certain times of year, as in alewife spawning season; such strandings could not provide a significant supply of food for predators. There is no reason to suppose that Devonian fish were less prudent than those of today.[51] According to Melina Hale of University of Chicago, not all ancient trackways are necessarily made by early tetrapods, but could also be created by relatives of the tetrapods who used their fleshy appendages in a similar substrate-based locomotion.[52][53]
Palaeozoic tetrapods
This section needs additional citations for verification. (November 2012) |
Devonian tetrapods
Research by
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. Freshwater habitats were not the only places to find water filled with organic matter and dense vegetation near the water's edge. 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. Early fossil tetrapods have been found in marine sediments, and because fossils of primitive tetrapods in general are found scattered all around the world, they must have spread by following the coastal lines — they could not have lived in freshwater only.
One analysis from the University of Oregon suggests no evidence for the "shrinking waterhole" theory — transitional fossils are not associated with evidence of shrinking puddles or ponds — and indicates that such animals would probably not have survived short treks between depleted waterholes.[54] The new theory suggests instead that proto-lungs and proto-limbs were useful adaptations to negotiate the environment in humid, wooded floodplains.[55]
The Devonian tetrapods went through two major bottlenecks during what is known as the Late Devonian extinction; one at the end of the Frasnian stage, and one twice as large at the end of the following Famennian stage. These events of extinctions led to the disappearance of primitive tetrapods with fish-like features like Ichthyostega and their primary more aquatic relatives.[56] When tetrapods reappear in the fossil record after the Devonian extinctions, the adult forms are all fully adapted to a terrestrial existence, with later species secondarily adapted to an aquatic lifestyle.[57]
Lungs
It is now clear that the common ancestor of the bony fishes (Osteichthyes) had a primitive air-breathing
Fleshy lobe-fins supported on bones rather than ray-stiffened fins seem to have been an ancestral trait of all bony fishes (Osteichthyes). The lobe-finned ancestors of the tetrapods evolved them further, while the ancestors of the ray-finned fishes (Actinopterygii) evolved their fins in a different direction. The most primitive group of actinopterygians, the bichirs, still have fleshy frontal fins.
Fossils of early tetrapods
Nine genera of Devonian tetrapods have been described, several known mainly or entirely from lower jaw material. All but one were from the Laurasian supercontinent, which comprised Europe, North America and Greenland. The only exception is a single Gondwanan genus, Metaxygnathus, which has been found in Australia.
The first Devonian tetrapod identified from Asia was recognized from a fossil jawbone reported in 2002. 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.
These earliest tetrapods were not terrestrial. The earliest confirmed terrestrial forms are known from the early Carboniferous deposits, some 20 million years later. Still, they may have spent very brief periods out of water and would have used their legs to paw their way through the mud.
Why they went to land in the first place is still debated. One reason could be that the small juveniles who had completed their
At this time the abundance of invertebrates crawling around on land and near water, in moist soil and wet litter, offered a food supply. Some were even big enough to eat small tetrapods, but the land was free from dangers common in the water.
From water to land
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.
However, recent microanatomical and histological analysis of tetrapod fossil specimens found that early tetrapods like Acanthostega were fully aquatic, suggesting that adaptation to land happened later.[58]
Research by Per Ahlberg and colleagues suggest that tides could have been a driving force for the evolution of tetrapods. The hypothesis proposes that as "the tide retreated, fishes became stranded in shallow water tidal-pool environments, where they would be subjected to raised temperatures and hypoxic conditions" and then fishes that developed "efficient air-breathing organs, as well as for appendages adapted for land navigation" would be selected.[59]
Carboniferous tetrapods
Until the 1990s, there was a 30 million year gap in the fossil record between the late Devonian tetrapods and the reappearance of tetrapod fossils in recognizable mid-Carboniferous amphibian lineages. 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.
During the "gap", tetrapod backbones developed, as did limbs with digits and other adaptations for terrestrial life. Ears, skulls and vertebral columns all underwent changes too. The number of digits on hands and feet became standardized at five, as lineages with more digits died out. Thus, those very few tetrapod fossils found in this "gap" are all the more prized by palaeontologists because they document these significant changes and clarify their history.
The transition from an aquatic, lobe-finned fish to an air-breathing amphibian was a significant and fundamental one in the evolutionary history of the vertebrates. For an organism to live in a gravity-neutral aqueous environment, then colonize one that requires an organism to support its entire weight and possess a mechanism to mitigate dehydration, required significant adaptations or exaptations within the overall body plan, both in form and in function. Eryops, an example of an animal that made such adaptations, refined many of the traits found in its fish ancestors. Sturdy limbs supported and transported its body while out of water. A thicker, stronger backbone prevented its body from sagging under its own weight. Also, through the reshaping of vestigial fish jaw bones, a rudimentary middle ear began developing to connect to the piscine inner ear, allowing Eryops to amplify, and so better sense, airborne sound.
By the
The first
Carboniferous rainforest collapse
Amphibians and reptiles were strongly affected by the Carboniferous rainforest collapse (CRC), an extinction event that occurred ~307 million years ago. The Carboniferous period has long been associated with thick, steamy swamps and humid rainforests.[60] Since plants form the base of almost all of Earth's ecosystems, any changes in plant distribution have always affected animal life to some degree. The sudden collapse of the vital rainforest ecosystem profoundly affected the diversity and abundance of the major tetrapod groups that relied on it.[61] The CRC, which was a part of one of the top two most devastating plant extinctions in Earth's history, was a self-reinforcing and very rapid change of environment wherein the worldwide climate became much drier and cooler overall (although much new work is being done to better understand the fine-grained historical climate changes in the Carboniferous-Permian transition and how they arose[62]).
The ensuing worldwide plant reduction resulting from the difficulties plants encountered in adjusting to the new climate caused a progressive fragmentation and collapse of rainforest ecosystems. This reinforced and so further accelerated the collapse by sharply reducing the amount of animal life which could be supported by the shrinking ecosystems at that time. The outcome of this animal reduction was a crash in global carbon dioxide levels, which impacted the plants even more.[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.[60]
This impacted amphibians in particular in a number of ways. The enormous drop in sea level due to greater quantities of the world's water being locked into glaciers profoundly affected the distribution and size of the semiaquatic ecosystems which amphibians favored, and the significant cooling of the climate further narrowed the amount of new territory favorable to amphibians. Given that among the hallmarks of amphibians are an obligatory return to a body of water to lay eggs, a delicate skin prone to
Permian tetrapods
In the
The end of the Permian saw a major turnover in fauna during the
Mesozoic tetrapods
Life on Earth seemed to recover quickly after the Permian extinctions, though this was mostly in the form of disaster taxa such as the hardy Lystrosaurus. Specialized animals that formed complex ecosystems with high biodiversity, complex food webs, and a variety of niches, took much longer to recover.[67] Current research indicates that this long recovery was due to successive waves of extinction, which inhibited recovery, and to prolonged environmental stress to organisms that continued into the Early Triassic. Recent research indicates that recovery did not begin until the start of the mid-Triassic, 4M to 6M years after the extinction;[68] and some writers estimate that the recovery was not complete until 30M years after the P-Tr extinction, i.e. in the late Triassic.[67]
A small group of reptiles, the
Cenozoic tetrapods
The
During the Mesozoic, the prototypical mammal was a small
During the Paleocene and Eocene, most mammals remained small (under 20 kg). Cooling climate in the Oligocene and Miocene, and the expansion of grasslands favored the evolution of larger mammalian species.
Whales, seals, manatees, and sea otters have returned to the ocean and an aquatic lifestyle.
Vast herds of
have evolved to keep the herd-animal populations in check.Extant (living) tetrapods
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 Ma due to unknown reasons. The other six persisting today also include many extinct members:
- salamanders, and caecilians
- terrapins
- amphisbaenians and snakes
- caimans and gharials
- Neornithes: extant birds
- Mammalia: mammals
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Works cited
- Benton, Michael J. (5 February 2009). Vertebrate Palaeontology. John Wiley & Sons. ISBN 978-1-4051-4449-0.
- Clack, Jennifer A. (2012). Gaining Ground: The Origin and Evolution of Tetrapods. Indiana University Press. ISBN 978-0-253-35675-8.
External links
- Media related to Tetrapoda evolution at Wikimedia Commons