Cross-section of a labyrinthodont tooth
https://en.wikipedia.org/wiki/Tetrapod#/media/File:Labyrinthodon_Mivart.png
Dentition
Tetrapods had a tooth structure known as "plicidentine" characterized by infolding of the enamel as seen in cross-section. The more extreme version found in early tetrapods is known as "labyrinthodont" or "labyrinthodont plicidentine". This type of tooth structure has evolved independently in several types of bony fishes, both ray-finned and lobe finned, some modern lizards, and in a number of tetrapodomorph fishes. The infolding appears to evolve when a fang or large tooth grows in a small jaw, erupting when it still weak and immature. The infolding provides added strength to the young tooth, but offers little advantage when the tooth is mature. Such teeth are associated with feeding on soft prey in juveniles.[84][85]
Axial skeleton
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.
Girdles
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. The upper portion of the girdle had a flat, scapular blade (shoulder bone), 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.
Limbs
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.[86]
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.
Locomotion
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.
Feeding
Early tetrapods had a wide gaping jaw with weak muscles to open and close it. In the jaw were moderate-sized palatal and vomerine (upper) and coronoid (lower) fangs, as well rows of smaller teeth. This was in contrast to the larger fangs and small marginal teeth of earlier tetrapodomorph fishes such as Eusthenopteron. Although this indicates a change in feeding habits, the exact nature of the change in unknown. Some scholars have suggested a change to bottom-feeding or feeding in shallower waters (Ahlberg and Milner 1994). Others have suggesting a mode of feeding comparable to that of the Japanese giant salamander, which uses both suction feeding and direct biting to eat small crustaceans and fish. A study of these jaws shows that they were used for feeding underwater, not on land.[87]
In later terrestrial tetrapods, two methods of jaw closure emerge: static and kinetic inertial (also known as snapping). In the static system, the jaw muscles are arranged in such a way that the jaws have maximum force when shut or nearly shut. In the kinetic inertial system, maximum force is applied when the jaws are wide open, resulting in the jaws snapping shut with great velocity and momentum. Although the kinetic inertial system is occasionally found in fish, it requires special adaptations (such as very narrow jaws) to deal with the high viscosity and density of water, which would otherwise impede rapid jaw closure.
The tetrapod tongue is built from muscles that once controlled gill openings. The tongue is anchored to the hyoid bone, which was once the lower half of a pair of gill bars (the second pair after the ones that evolved into jaws).[88][89][90] The tongue did not evolve until the gills began to disappear. Acanthostega still had gills, so this would have been a later development. In an aquatically feeding animals, the food is supported by water and can literally float (or get sucked in) to the mouth. On land, the tongue becomes important.
Respiration
The evolution of early tetrapod respiration was influenced by an event known as the "charcoal gap", a period of more than 20 million years, in the middle and late Devonian, when atmospheric oxygen levels were too low to sustain wildfires.[91] During this time, fish inhabiting anoxic waters (very low in oxygen) would have been under evolutionary pressure to develop their air-breathing ability.[92][93][94]
Early tetrapods probably relied on four methods of respiration: with lungs, with gills, cutaneous respiration (skin breathing), and breathing through the lining of the digestive tract, especially the mouth.
Gills
The early tetrapod Acanthostega had at least three and probably four pairs of gill bars, each containing deep grooves in the place where one would expect to find the afferent branchial artery. This strongly suggests that functional gills were present.[95] Some aquatic temnospondyls retained internal gills at least into the early Jurassic.[96] Evidence of clear fish-like internal gills is present in Archegosaurus.[97]
Lungs
Lungs originated as an extra pair of pouches in the throat, behind the gill pouches.[98] They were probably present in the last common ancestor of bony fishes. In some fishes they evolved into swim bladders for maintaining buoyancy.[99][100] Lungs and swim bladders are homologous (descended from a common ancestral form) as is the case for the pulmonary artery (which delivers de-oxygenated blood from the heart to the lungs) and the arteries that supply swim bladders.[101] Air was introduced into the lungs by a process known as buccal pumping.[102][103]
In the earliest tetrapods, exhalation was probably accomplished with the aid of the muscles of the torso (the thoracoabdominal region). Inhaling with the ribs was either primitive for amniotes, or evolved independently in at least two different lineages of amniotes. It is not found in amphibians.[104][105] The muscularized diaphragm is unique to mammals.[106]
Recoil aspiration
Although tetrapods are widely thought to have inhaled through buccal pumping (mouth pumping), according to an alternative hypothesis, aspiration (inhalation) occurred through passive recoil of the exoskeleton in a manner similar to the contemporary primitive ray-finned fish Polypterus. This fish inhales through its spiracle (blowhole), an anatomical feature present in early tetrapods. Exhalation is powered by muscles in the torso. During exhalation, the bony scales in the upper chest region become indented. When the muscles are relaxed, the bony scales spring back into position, generating considerable negative pressure within the torso, resulting in a very rapid intake of air through the spiracle. [107] [108] [109]
Cutaneous respiration
Skin breathing, known as cutaneous respiration, is common in fish and amphibians, and occur both in and out of water. In some animals waterproof barriers impede the exchange of gases through the skin. For example, keratin in human skin, the scales of reptiles, and modern proteinaceous fish scales impede the exchange of gases. However, early tetrapods had scales made of highly vascularized bone covered with skin. For this reason, it is thought that early tetrapods could engage some significant amount of skin breathing.[110]
Carbon dioxide metabolism
Although air-breathing fish can absorb oxygen through their lungs, the lungs tend to be ineffective for discharging carbon dioxide. In tetrapods, the ability of lungs to discharge CO2 came about gradually, and was not fully attained until the evolution of amniotes. The same limitation applies to gut air breathing (GUT), i.e., breathing with the lining of the digestive tract.[111] Tetrapod skin would have been effective for both absorbing oxygen and discharging CO2, but only up to a point. For this reason, early tetrapods may have experienced chronic hypercapnia (high levels of blood CO2). This is not uncommon in fish that inhabit waters high in CO2.[112] According to one hypothesis, the "sculpted" or "ornamented" dermal skull roof bones found in early tetrapods may have been related to a mechanism for relieving respiratory acidosis (acidic blood caused by excess CO2) through compensatory metabolic alkalosis.[113]
Circulation
Early tetrapods probably had a three-chambered heart, as do modern amphibians and lepidosaurian and chelonian 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.[114]
Senses
Olfaction
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 had evolved in the context of aquatic detection. The vomeronasal organ also evolved in the nasal cavity for the first time, for detecting pheromones from biological substrates on land, though it was subsequently lost or reduced to vestigial in some lineages, like archosaurs and catarrhines, but expanded in others like lepidosaurs.[115]
Lateral line system
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.
Vision
Changes in the eye came about because the behavior of light at the surface of the eye differs between an air and water environment due to the difference in refractive index, 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.
Early tetrapods inherited a set of five rod and cone opsins known as the vertebrate opsins.[116][117][118]
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 (rhodopsin) - blue-green - used night vision and color correction in low-light environments
Balance
Tetrapods retained the balancing function of the inner ear from fish ancestry.
Hearing
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 of connective tissue also called the eardrum (however this and the otic notch were lost in the ancestral amniotes, and later eardrums were obtained independently).
The hyomandibula of fish migrated upwards from its jaw supporting position, and was reduced in size to form the columella. Situated between the tympanum and braincase in an air-filled cavity, the columella was now capable of transmitting vibrations from the exterior of the head to the interior. Thus the columella 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 columella 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 columella is 1/20th the area of the tympanum. However, in early amphibians the columella 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 columella more probably just supported the brain case against the cheek.
Only in the early Triassic, about a hundred million years after they conquered land, did the tympanic middle ear evolve (independently) in all the tetrapod lineages.[119] About fifty million years later (late Triassic), in mammals, the columella was reduced even further to become the stapes.
See also
- Body form
- Geologic timescale
- Hexapoda
- Marine tetrapods
- Octopod
- Prehistoric life
- Quadrupedalism § Quadrupeds vs. tetrapods
https://en.wikipedia.org/wiki/Tetrapod
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