Middle Devonian–Present, 390–0 Ma
|Representatives of the four classes of extant tetrapods, (clockwise from upper left): a frog (an amphibian), a hoatzin (a bird), a skink (a reptile), and a mouse (a mammal)|
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The superclass Tetrapoda (Ancient Greek τετραπόδηs tetrapodēs, "four-footed"), or the tetrapods //, comprises the first four-limbed vertebrates and their descendants, including the living and extinct amphibians, reptiles, mammals , and birds. Tetrapods evolved from the lobe-finned fishes about 390-1 million years ago in the Devonian Period. The specific aquatic ancestors of the tetrapods, and the process by which land colonization occurred, remain unclear, and are areas of active research and debate among palaeontologists at present.
While most species today are terrestrial, the first tetrapods were fully aquatic. Amphibians today generally remain semiaquatic, living the first stage of their lives as fish-like tadpoles. Amniotes evolved about 340 million years ago (crown amniotes 318 mya), and their descendants drove most amphibians to extinction. One population of amniotes diverged into lizards, dinosaurs, birds and their relatives, while another diverged into mammals and their extinct relatives. Several groups of tetrapods, such as the caecilians, snakes, cetaceans, sirenians, and moas have lost some or all of their limbs. In addition, many tetrapods have returned to partially aquatic or fully aquatic lives throughout the history of the group (modern examples of fully aquatic tetrapods include cetaceans and sirenians). The first returns to an aquatic lifestyle may have occurred as early as the Carboniferous Period, whereas other returns occurred as recently as the Cenozoic, as in cetaceans, pinnipeds, and several modern amphibians.
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. It is also becoming increasingly well-understood as a result of more transitional fossil finds and improved phylogenetic analysis.
- 1 Definitions
- 2 Biodiversity
- 3 Evolution
- 4 Classification
- 5 Phylogeny
- 6 Anatomy and physiology of early tetrapods
- 7 See also
- 8 References
- 9 Further reading
Tetrapods can be defined in cladistics as the nearest common ancestor of all living amphibians (the lissamphibians) and all living amniotes (reptiles, birds, and mammals), along with all of the descendants of that ancestor. This is a node-based definition (the node being the nearest common ancestor). The group so defined is the crown group, or crown tetrapods. The term tetrapodomorph is used for the stem-based definition: any animal that is more closely related to living amphibians, reptiles, birds, and mammals than to living dipnoi (lungfishes). The group so defined is known as the tetrapod total group.
Paleontologists generally use a synapomorphy-based cladistic definition, one that is more complex and requires some explanation. Biologists are intensely interested in the question of how the ancestors of modern amphibians, reptiles, birds, and mammals evolved from fish into animals that can live on land. By the time of the nearest common ancestor (the first crown tetrapod), the transition was largely complete. Thus, the node-based definition is too recent. The stem-based definition has the opposite problem: it goes back too far, to fish with fins showing no signs of sprouting legs. Paleontologists seek an intermediate definition of tetrapod, one that begins at the beginning of the transition to life on land. Unfortunately, the beginning of this transition lies in the middle of a large fossil gap. Thus, paleontologists seek to identify traits in early tetrapods that can be compared with corresponding traits in their fish ancestors to determine which traits are key.
Among the traits that have been considered and rejected because they arose either too early or too late: lungs and the ability to breathe air (far too early), tympanic hearing and the ability to hear high-frequency sounds (far to late), a stiff spine, enabling an animal to walk on land without dragging its body on the ground (a little too late), the evolution of the neck, with the shoulder-girdle attaching to the spine instead of head (also a little too late). For other traits, the order in which the traits evolved is not known. For this reason, the provisional definition proposed by paleontologist Jennifer Clack has gained considerable acceptance: "an animal with four legs bearing digits" (e.g., fingers and toes). This definition is intended to apply only to members of the clade tetrapodomorph, as opposed to the ray-finned fishes, many of which have four legs bearing digits (the Sargassum fish for example).
Stegocephalia is an approximate synonym for the paleontologist's definition of tetrapod. It is used by scholars who prefer to reserve the word "tetrapod" for the crown group (based on the nearest common ancestor of living forms). Such scholars use the term Stem tetrapods to refer to those tetrapods that are not members of the crown group, as well as the tetrapodomorph fishes  Stegocephalians may also refer to an evolutionary grade: those members of the clade Stegocephalia (in the above sense) that are neither amniotes nor lissamphibians.
The two subclades of crown tetrapods are Batrachomorpha and Reptiliomorpha. Batrachomorphs are all animals sharing a more recent common ancestry with living amphibians than with living amniotes (reptiles, birds, and mammals). Reptiliomorphs are all animals sharing a more recent common ancestry with living amniotes than with living amphibians.
Tetrapoda includes four classes: amphibians, reptiles, mammals, and birds. Overall, the biodiversity of lissamphibians, as well as of tetrapods generally, has grown exponentially over time; the more than 30,000 species living today are descended from a single amphibian group in the Early to Middle Devonian. However, that diversification process was interrupted at least a few times by major biological crises, such as the Permian–Triassic extinction event, which at least affected amniotes. The overall composition of biodiversity was driven primarily by amphibians in the Palaeozoic, dominated by reptiles in the Mesozoic and expanded by the explosive growth of birds and mammals in the Cenozoic. As biodiversity has grown, so has the number of niches that tetrapods have occupied. The first tetrapods were aquatic and fed primarily on fish. Today, the Earth supports a great diversity of tetrapods that live in many habitats and subsist on a variety of diets.
The first tetrapods probably evolved in the Emsian stage of the Early Devonian from Tetrapodomorph fish living in shallow water environments. The very earliest tetrapods would have been animals similar to Acanthostega, with legs and lungs as well as gills, but still primarily aquatic and unsuited to life on land.
The earliest tetrapods inhabited saltwater, brackish-water, and freshwater environments, as well as environments of highly variable salinity. These traits were shared with many early lobed-finned fishes. As early tetrapods are found on two Devonian continents, Laurussia (Euramerica) and Gondwana, as well as the island of North China, it is widely supposed that early tetrapods were capable of swimming across the shallow (and relatively narrow) continental-shelf seas that separated these landmasses.
Since the early 20th century, several families of tetrapodomorph fishes have been proposed as the nearest relatives of tetrapods, among them the rhizodonts (notably Sauripterus), the osteolepidids, the tristichopterids(notably Eusthenopteron), and more recently the elpistostegalians (also known as Panderichthyida) notably the genus Tiktaalik.
Among the notable features of Tiktaalik are the absence of bones covering the gills. These bones would otherwise connect the shoulder girdle with skull, making the shoulder girdle part of the skull. With the loss of the gill-covering bones, the shoulder girdle is separated from the skull, connected to the torso by muscle and other soft-tissue connections. The result is the appearance of the neck. With the exception of Tiktaalik, this feature is found only in tetrapods, not in tetrapodomorph fishes. Tiktaalik also had a pattern of bones in the skull roof (upper half of the skul) that bears similarity to the end-Devonian tetrapod Ichthyostega. The two also shared a semi-rigid ribcage of overlapping ribs which could have served as a substitute for a rigid spine. In conjunction with robust forelimbs and shoulder girdle, both Tiktaalik and Ichthyostega may have had the ability to locomote on land in the manner of a seal, with the forward portion of the torso elevated, the hind part dragging behind. And finally, the fin bones of Tiktaalik are somewhat similar to the limb bones of tetrapods.
There are, however, issues, among them Tiktaalik's long spine with far more vertebra than in any known tetrapod or any other known tetrapodomorph fish. Of even greater concern is the date: the oldest tetrapod trace fossils (tracks and trackways) predate Tiktaalik by a considerable margin. Several hypotheses have been proposed to explain the date discrepancy: 1) The nearest common ancestor of tetrapods and Tiktaalik dates to the Early Devonian. By this hypothesis, the Tiktaalik lineage is the closest to tetrapods, but Tiktaalik itself was late-surviving relict. 2) Tiktaalik represents a case of parallel evolution. 3) Tetrapods evolved more than once.
|Euteleostomi (bony vertebrates)||
The oldest evidence for the existence of tetrapods comes from trace fossils, tracks (footprints) and trackways found in Zachełmie, Poland, dated to the Eifelian stage of the Middle Devonian, 390-1 mya. The adult tetrapods had an estimated length of 2.5 m (8 feet). They lived in a lagoon with an average depth of 1–2 m, although it is not known at what depth the underwater tracks were made. The lagoon was inhabited by a variety of marine organisms and was apparently salt water. The average water temperature was 30 degrees C (86 F). The second oldest evidence for tetrapods, also tracks and trackways, date from 386 mya (Valentia Island, Ireland)
The oldest partial fossils of tetrapods date from the Frasnian beginning ~380 mya. These include Elginerpeton and Obruchevichthys. Some paleontologists dispute their status as true (digit-bearing) tetrapods.
All known forms of Frasnian tetrapods became extinct in the Late Devonian extinction, also known as the end-Frasnian extinction. This marked the beginning of a gap in the tetrapod fossil record known as the Famennian gap, occupying roughly the first half of the Famennian stage.
The oldest near-complete tetrapod fossils, Acanthostega and Ichthyostega, date from the second half of the Fammennian. Although both were essentially four-footed fish, Ichthyostega is the earliest known tetrapod that may have had the ability to pull itself onto land and drag itself forward with its forelimbs. There is no evidence that it did so, only that it may have been anatomically capable of doing so.
The end-Fammenian marked another extinction, known as the end-Fammenian extinction or the Hangenberg event, which is followed by another gap in the tetrapod fossil record, Romer's gap, also known as the Tournaisian gap. This gap, which was initially 30 million years, but has been gradually reduced over time, currently occupies much of the 13.9-million year Tournaisian, the first stage of the Carboniferous period.
Tetrapods first appeared in the early Devonian period. These early tetrapods would have been animals similar to Ichthyostega, with legs and lungs as well as gills, but still primarily aquatic and unsuited to life on land. The Devonian tetrapods went through two major bottlenecks during what the Late Devonian extinctions, also known as the end-Frasnian and end-Fammenian extinctions. These extinction events led to the disappearance of primitive tetrapods with fish-like features. When tetrapods reappear in the fossil record in early Carboniferous deposits, some 10 million years later, the adult forms of some tetrapods are somewhat adapted to a terrestrial existence. Why they went to land in the first place is still debated.
During the early Carboniferous, the number of digits on hands and feet became standardized at no more than five, as lineages with more digits died out. By mid-Carboniferous times, the early tetrapods had radiated into at least three main branches. Modern amphibians are derived from the either the temnospondyls or the lepospondyls (or possibly both), whereas the anthracosaurs were the relatives and ancestors of the amniotes.
The first amniotes are known from the early part of the Late Carboniferous. Amphibians must return to water to lay eggs; in contrast, amniote eggs have a membrane ensuring gas exchange out of water and can therefore be laid on land. A reptile is defined as any amniote that is neither a mammal nor a bird, so these early amniotes were primitive reptiles.
Amphibians and reptiles were affected by the Carboniferous Rainforest Collapse (CRC), an extinction event that occurred ~300 million years ago. The sudden collapse of a vital ecosystem shifted the diversity and abundance of major groups. Reptiles were more suited to the new conditions. They invaded new ecological niches and began diversifying their diets to include plants and other tetrapods, previously having been limited to insects and fish.
In the Permian period, in addition to temnospondyl and anthracosaur clades, there were two important clades of amniotes, the sauropsids and the synapsids. The latter were the most important and successful Permian animals.
The end of the Permian saw a major turnover in fauna during the Permian–Triassic extinction event. There was a protracted loss of species, due to multiple extinction pulses. Many of the once large and diverse groups died out or were greatly reduced.
The diapsids (a subgroup of the sauropsids) began to diversify during the Triassic, leading to the turtles, crocodiles and dinosaurs. In the Jurassic, birds first appeared as a derived clade of theropod dinosaurs, and lizards developed from other diapsids. In the Cretaceous, snakes developed from lizards. By the late Mesozoic, the large labyrinthodont groups that first appeared during the Paleozoic such as temnospondyls and reptile-like amphibians had gone extinct. Many groups of synapsids, such as anomodonts and therocephalians, that once comprised the dominant terrestrial fauna of the Permian also became extinct during the Mesozoic; however, during the Triassic one group (Cynodontia) gave rise to the mammals, which survived through the Mesozoic to later diversify during the Cenozoic.
Extant (living) tetrapods
Following the great faunal turnover at the end of the Mesozoic, only six major groups of tetrapods were left, all of which also include many extinct groups:
- Lissamphibia: frogs and toads, newts and salamanders, and caecilians
- Testudines: turtles and tortoises
- Lepidosauria: tuataras, lizards, amphisbaenians and snakes
- Crocodilia: crocodiles, alligators, caimans and gharials
- Neornithes: modern birds
- Mammalia: mammals
The classification of tetrapods has a long history. Traditionally, tetrapods are divided into four classes based on gross anatomical and physiological traits. Snakes and other legless reptiles are considered tetrapods because they are sufficiently like other reptiles that have a full complement of limbs. Similar considerations apply to caecilians and aquatic mammals. Newer taxonomy is frequently based on cladistics instead, giving a variable number of major "branches" (clades) of the tetrapod family tree.
As is the case throughout evolutionary biology today, there is debate over how to properly classify the groups within Tetrapoda. Traditional biological classification sometimes failrs to recognize evolutionary transitions between older groups and descendant groups with markedly different characteristics. For example, the birds, which evolved from the dinosaurs, are defined as a separate group from them, because they represent a distinct new type of physical form and functionality. In phylogenetic nomenclature, in contrast, the newer group is always included in the old. For this school of taxonomy, dinosaurs and birds are not groups in contrast to each other, but rather birds are a sub-type of dinosaurs.
History of classification
The tetrapods, including all large- and medium-sized land animals, have been among the best understood animals since earliest times. By Aristotle's time, the basic division between mammals, birds and egg-laying tetrapods (the "herptiles") was well known, and the inclusion of the legless snakes into this group was likewise recognized. With the birth of modern biological classification in the 18th century, Linnaeus used the same division, with the tetrapods occupying the first three of his six classes of animals. While reptiles and amphibians can be quite similar externally, the French zoologist Pierre André Latreille recognized the large physiological differences at the beginning of the 19th century and split the herptiles into two classes, giving the four familiar classes of tetrapods: amphibians, reptiles, birds and mammals.
With the basic classification of tetrapods settled, a half a century followed where the classification of living and fossil groups was predominately done by experts working within classes. In the early 1930s, American vertebrate palaeontologist Alfred Romer (1894–1973) produced an overview, drawing together taxonomic work from the various subfields to create an orderly taxonomy in his Vertebrate Paleontology. This classical scheme with minor variations is still used in works where systematic overview is essential, e.g. Benton (1998) and Knobill and Neill (2006). While mostly seen in general works, it is also still used in some specialist works like Fortuny & al. (2011). The taxonomy down to subclass level shown here is from Hildebrand and Goslow (2001):
- Superclass Tetrapoda - four-limbed vertebrates
- Class Amphibia - amphibians
- Class Reptilia - reptiles, (ancestors of both birds and mammals)
- Class Aves - birds
- Class Mammalia - mammals
This classification is the one most commonly encountered in school textbooks and popular works. While orderly and easy to use, it has come under critique from cladistics. The earliest tetrapods are grouped under Class Amphibia, although several of the groups are more closely related to amniotes than to modern day amphibians. Traditionally, birds are not considered a type of reptile, but crocodiles are more closely related to birds than they are to other reptiles, such as lizards. Birds themselves are thought to be descendents of theropod dinosaurs. Basal non-mammalian synapsids ("mammal-like reptiles") traditionally also sort under Class Reptilia as a separate subclass, but they are more closely related to mammals than to living reptiles. Considerations like these have led some authors to argue for a new classification based purely on phylogeny, disregarding the anatomy and physiology.
Stem tetrapods are all animals more closely related to living terrestrial vertebrates than to living Dipnoi (lungfish), excluding the tetrapod crown group. Each item in the section "Tetrapodomorph fishes" represents a nominal family, with some representative genera shown in parentheses. These "families" are generally not monophyletic. The group marked "Frasnian forms" straddles the dividing line between tetrapods and tetrapodomorph fishes, as they are known only from fragmentary fossils. The dividing line between stem tetrapods and crown tetrapods is uncertain, as indicated by the question marks in the cladogram.
Crown tetrapods are defined as the nearest common ancestor of all living tetrapods (amphibians, reptiles, birds, and mammals) along with all of the descendants of that ancestor.
There are currently three notable hypotheses on the origins of lissamphibians. In the temnospondyl hypothesis (TH) lissamphibians are most closely related to dissorophoid temnospondyls. In the lepospondyl hypothesis (LH) lissamphibians are the sister taxon of lysorophian lepospondyls. In the polyphyletic hypothesis (PH) frogs and salamanders come out of dissorophoid temnospondyls while caecilians come out of microsaur lepospondyls.
Temnospondyl hypothesis (TH)
This hypothesis comes in a number of variants, most of which have lissamphibians coming out of the dissorophoid temnospondyls, usually with the focus on amphibamids and branchiosaurids.
The Temnospondyl Hypothesis is the currently favored or majority view, supported by Ruta et al (2003a,b), Ruta and Coates (2007), Coates et al (2008), Sigurdsen and Green (2011), and Schoch (2013,2014).
total group Lissamphibia
total group Amniota
Lepospondyl hypothesis (LH)
total group Amniota
total group Lissamphibia
Polyphyly hypothesis (PH)
This hypothesis has batrachians (frogs and salamander) coming out of dissorophoid temnospondyls, with caecilians out of microsaur lepospondyls. There are two variants, one developed by Carroll, the other by Anderson.
Anatomy and physiology of early tetrapods
|This section does not cite any references or sources. (November 2012)|
The tetrapod's ancestral fish, tetrapodomorph, possessed similar traits to those inherited by the early tetrapods, including internal nostrils and a large fleshy fin built on bones that could give rise to the tetrapod limb. Their palatal and jaw structures were similar to those of early tetrapods, and their dentition was similar too, with labyrinthine teeth fitting in a pit-and-tooth arrangement on the palate.
The major difference between early sarcopterygians and early tetrapods was in the relative development of the front and back skull portions; the snout is much less developed than in most early tetrapods and the post-orbital skull is exceptionally longer than an amphibian's.
To propagate in the terrestrial environment, animals had to overcome certain challenges. Their bodies needed additional support, because buoyancy was no longer a factor. Water retention was now important, since it was no longer the living matrix, and could be lost easily to the environment. Finally, animals needed new sensory input systems to have any ability to function reasonably on land.
A notable characteristic that make a tetrapod's skull different from a fish's are the relative frontal and rear portion lengths. The fish had a long rear portion while the front was short; the orbital vacuities were thus located towards the anterior end. In the tetrapod, the front of the skull lengthened, positioning the orbits farther back on the skull. The lacrimal bone was no longer in contact with the frontal, having been separated from it by the prefrontal bone. This left the skull was now free to rotate from side to side, independent of the spine, on the newly forming neck.
A diagnostic character of temnospondyls is that the tabular bones (which formed the posterior corners of the skull-table) were separated from the respective left and right parietals by a sutural junction between the postparietals and supratemporals. Also, at the rear of the skull, all bones dorsal to the cleithrum were lost.
The lower jaw of, for example, Eryops resembled its early sarcopterygians ancestors in that on the outer surface lay a long dentary that bore teeth. There were also bones below the dentary on the jaw: two splenials, the angular and the surangular. On the inside were usually three coronoids that bore teeth and lay close to the dentary. On the upper jaw was a row of marginal labyrinthine teeth, located on the maxilla and premaxilla. In Eryops, as in all early amphibians, the teeth were replaced in waves that traveled from the front of the jaw to the back in such a way that every other tooth was mature, and the ones in between were young.
The "labyrinthodonts" had a peculiar tooth structure, from which their name derives—and though not exclusive to the group, the labyrinthine dentition is a useful indicator as to proper classification. The important feature of the tooth is that the enamel and dentine fold into a complicated corrugated pattern when viewed in cross section. This infolding strengthened the tooth and increased wear resistance. Such teeth survived for 100 Ma, first among crossopterygian fish, then stem reptiles. Modern amphibians no longer have this type of dentition, but rather pleurodont teeth, in fewer numbers of the whole group.
With the move from water to land, the spine had to resist the bending caused by body weight and had to provide mobility where needed. Previously, it could bend along its entire length. Likewise, the paired appendages had not been formerly connected to the spine, but the slowly strengthening limbs now transmitted their support to the axis of the body.
The pectoral girdle of early tetrapods, such as Eryops, was highly developed, with a larger size for both increased muscle attachment to it and to the limbs. Most notably, the shoulder girdle was disconnected from the skull, resulting in improved terrestrial locomotion. The early sarcopterygians cleithrum was retained as the clavicle, and the interclavicle was well-developed, lying on the underside of the chest. In primitive forms, the two clavicles and the interclavical could have grown ventrally in such a way as to form a broad chest plate, although such was not the case in Eryops. The upper portion of the girdle had a flat, scapular blade, with the glenoid cavity situated below performing as the articulation surface for the humerus, while ventrally there was a large, flat coracoid plate turning in toward the midline.
The pelvic girdle also was much larger than the simple plate found in fishes, accommodating more muscles. It extended far dorsally and was joined to the backbone by one or more specialized sacral ribs. The hind legs were somewhat specialized in that they not only supported weight, but also provided propulsion. The dorsal extension of the pelvis was the ilium, while the broad ventral plate was composed of the pubis in front and the ischium in behind. The three bones met at a single point in the center of the pelvic triangle called the acetabulum, providing a surface of articulation for the femur.
The main strength of the ilio-sacral attachment of Eryops was by ligaments, a condition structurally, but not phylogenetically, intermediate between that of the most primitive embolomerous amphibians and early reptiles. The condition that is more usually found in higher vertebrates is that cartilage and the fusion of the sacral ribs to the blade of the ilium are utilized in addition to ligamentous attachments.
Fleshy lobe-fins supported on bones seem to have been an ancestral trait of all bony fishes (Osteichthyes). The ancestors of the ray-finned fishes (Actinopterygii) evolved their fins in a different direction. The Tetrapodomorph ancestors of the Tetrapods further developed their lobe fins. The paired fins had bones distinctly homologous to the humerus, ulna, and radius in the fore-fins and to the femur, tibia, and fibula in the pelvic fins.
The paired fins of the early sarcopterygians were smaller than tetrapod limbs, but the skeletal structure was very similar in that the early sarcopterygians had a single proximal bone (analogous to the humerus or femur), two bones in the next segment (forearm or lower leg), and an irregular subdivision of the fin, roughly comparable to the structure of the carpus / tarsus and phalanges of a hand.
In typical early tetrapod posture, the upper arm and upper leg extended nearly straight horizontal from its body, and the forearm and the lower leg extended downward from the upper segment at a near right angle. The body weight was not centered over the limbs, but was rather transferred 90 degrees outward and down through the lower limbs, which touched the ground. Most of the animal's strength was used to just lift its body off the ground for walking, which was probably slow and difficult. With this sort of posture, it could only make short broad strides. This has been confirmed by fossilized footprints found in Carboniferous rocks.
Early tetrapods had a wide gaping jaw with weak muscles to open and close it. In the jaw were fang-like palatal teeth that, when coupled with the gape, suggests an inertial feeding habit. This is when the amphibian would grasp the prey and, lacking any chewing mechanism, toss the head up and backwards, throwing the prey farther back into the mouth. Such feeding is seen today in the crocodile and alligator. A study of these jaws shows that they were used for feeding underwater, not on land.
The tongue of modern adult amphibians is quite fleshy and attached to the front of the lower jaw, so it is reasonable to speculate that it was fastened in a similar fashion in primitive forms, although it was probably not specialized like it is in a frog.
Modern amphibians breathe by inhaling air into lungs, where oxygen is absorbed. They also breathe through the moist lining of the mouth and skin, known as cutaneous respiration. Eryops also inhaled, but its ribs were too closely spaced to suggest that it did this by expanding the rib cage. More likely, it breathed by buccal pumping in which it opened its mouth and nostrils, depressed the hyoid apparatus to expand the oral cavity, closed its mouth and nostrils finally and elevated the floor of the mouth to force air back into the lungs — in other words, it gulped, then swallowed. It probably exhaled by contraction of the elastic tissue in the lung walls. Other special respiratory methods probably existed.
Lungs originated as an extra pair of pouches in the throat, behind the gill pouches. They were probably present in the last common ancestor of bony fishes. In some fishes they evolved into swim bladders for maintaining buoyancy. Lungs and swim bladders are homologous (descended from a common ancestral form) as is the case for the pulmonary artery (which delivers deoxygenated blood from the heart to the lungs) and the arteries that supply swim bladders. Air was introduced into the lungs by a process known as buccal pumping.
Early tetrapods probably had a three-chambered heart, as do modern amphibians and reptiles, in which oxygenated blood from the lungs and de-oxygenated blood from the respiring tissues enters by separate atria, and is directed via a spiral valve to the appropriate vessel — aorta for oxygenated blood and pulmonary vein for deoxygenated blood. The spiral valve is essential to keeping the mixing of the two types of blood to a minimum, enabling the animal to have higher metabolic rates, and be more active than otherwise.
Ligamentous attachments within the limbs were present in Eryops, being important because they were the precursor to bony and cartilaginous variations seen in modern terrestrial animals that use their limbs for locomotion.
The difference in density between air and water causes smells (certain chemical compounds detectable by chemoreceptors) to behave differently. An animal first venturing out onto land would have difficulty in locating such chemical signals if its sensory apparatus was designed for aquatic detection.
Fish have a lateral line system that detects pressure fluctuations in the water. Such pressure is non-detectable in air, but grooves for the lateral line sense organs were found on the skull of early tetrapods, suggesting either an aquatic or largely aquatic habitat. Modern amphibians, which are semi-aquatic, exhibit this feature whereas it has been retired by the higher vertebrates. The olfactory epithelium would also have to change to detect airborne odors.
Changes in the eye came about because the refractive index of light differs between air and water, so the focal length of the lens altered to function in air. The eye was now exposed to a relatively dry environment rather than being bathed by water, so eyelids developed and tear ducts evolved to produce a liquid to moisten the eyeball.
Four cone opsins were present in the first vertebrate, inherited from invertebrate ancestors:
- LWS/MWS (long- to medium-wave sensitive) - green, yellow, or red
- SWS1 (short-wave sensitive) - ultraviolet or violet - lost in monotremes (platypus, echidna)
- SWS2 (short-wave sensitive) - violet or blue - lost in therians (placental mammals and marsupials)
- RH2 (rhodopsin-like cone opsin) - green - lost separately in amphibians and mammals, retained in reptiles and birds
A single rod opsin, rhodopsin, was present in the first jawed vertebrate, inherited from a jawless vertebrate ancestor:
- RH1 (rhodposin) - blue-green - used night vision and color correction in low-light environments
Animals retained the balancing function of the middle ear from fish ancestry. However, delicate air vibrations could not set up pulsations through the skull as in a proper auditory organ. The spiracle was retained as the otic notch, eventually closed in by the tympanum, a thin, tight membrane.
The hyomandibula of fish migrated upwards from its jaw supporting position, and was reduced in size to form the stapes. Situated between the tympanum and braincase in an air-filled cavity, the stapes was now capable of transmitting vibrations from the exterior of the head to the interior. Thus the stapes became an important element in an impedance matching system, coupling airborne sound waves to the receptor system of the inner ear. This system had evolved independently within several different amphibian lineages.
The impedance matching ear had to meet certain conditions to work. The stapes had to be perpendicular to the tympanum, small and light enough to reduce its inertia, and suspended in an air-filled cavity. In modern species that are sensitive to over 1 kHz frequencies, the footplate of the stapes is 1/20th the area of the tympanum. However, in early amphibians the stapes was too large, making the footplate area oversized, preventing the hearing of high frequencies. So it appears they could only hear high intensity, low frequency sounds—and the stapes more probably just supported the brain case against the cheek.
- Grzegorz Niedźwiedzki, Piotr Szrek, Katarzyna Narkiewicz, Marek Narkiewicz, Per E. Ahlberg (2010). "Tetrapod trackways from the early Middle Devonian period of Poland". Nature 463 (7277): 43–8. Bibcode:2010Natur.463...43N. doi:10.1038/nature08623. PMID 20054388.
- Clack 2012, pp. 125–7
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