From Academic Kids
A tetrapod (Greek tetrapoda, "four-legged") is a vertebrate animal having four feet, legs or leglike appendages. Since amphibians, reptiles, dinosaurs and mammals are all tetrapods, and even birds and snakes are tetrapods by descent, the term is only really useful in describing the earliest tetrapods, which radiated from the Sarcopterygii, or "lobe-finned" fishes, into air-breathing amphibians in the Devonian period.
The Latin version of "tetrapod" is "quadruped," meaning any four-legged creature.
The first tetrapods evolved in shallow and swampy freshwater habitats, towards the end of the Devonian, a little more than 360 million years ago. 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.
Primitive tetrapods developed from a lobe-finned fish (an "osteolepid Sarcopterygian"), with a two-lobed brain in a flattened skull, a wide mouth and a short snout, whose upward-facing eyes show that it was a bottom-dweller, and which had already developed adaptations of fins with fleshy bases and bones. The "living fossil" coelacanth is a related lobe-finned fish without these shallow-water adaptations. These fishes used their fins as paddles in shallow-water habitats choked with plants and detritus. The universal tetrapod characteristics of front limbs that bend backward at the elbow and hind limbs that bend forward at the knee can plausibly be traced to early tetrapods living in shallow water.
The evolution of the air-breathing lung from the primitive swim bladder of lobe-finned fishes has not yet been worked out in detail. However, functioning internal gills were present in at least one late Devonian tetrapod, Acanthostega.
Nine genera of Devonian tetrapods have been described, several known mainly or entirely from lower jaw material. All of them were from the European-North American supercontinent that 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.
- Cladistic analysis of osteolepiform Sarcopterygians (http://www.palaeos.com/Vertebrates/Units/140Sarcopterygii/140.800.html#Osteolepiformes)
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", 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. The very few tetrapod fossils found in the "gap" are all the more precious.
The transition from an aquatic lobe-finned fish to an air-breathing amphibian was a momentous occasion in the evolutionary history of the vertebrates. For an animal to live in a gravity-neutral, aqueous environment and then invade one that is entirely different required major changes to the overall body plan, both in form and in function. Eryops is an example of an animal that made such adaptations. It retained and refined most 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, by utilizing vestigial fish jaw bones, a rudimentary ear was developed, allowing Eryops to hear airborne sound.
By the Visean age of mid-Carboniferous times, recognizable basal-group Amphibia (frogs, salamanders and caecilians) are represented by the labyrinthodonts, which are comprised of the temnospondyls (e.g. Eryops) and similarly primitive Amniota that now include mammals, turtles, crocodiles, birds, lizards and snakes, represented by the anthracosaurs. As living members of the tetrapod clan (that is, "crown-group"), tetrapods represent the phylogenetic end-points of these two divergent lineages. A third Palaeozoic group, the baphetids, left no modern survivors.
In the Permian period, as the separate tetrapod lineages each developed in their own way, the term "tetrapoda" becomes less useful. Each lineage, however, remains grouped with the tetrapoda, just as Homo sapiens could be considered a very highly-specialized kind of lobe-finned fish.
Most tetrapods today are terrestrial, at least in their adult forms, but some species, such as the axolotl, remain aquatic. Tetrapods that returned to the sea include ichthyosaurs and modern whales and dolphins.
Classification of Tetrapods
There are four main categories of living ("crown group") tetrapods:
- frogs and toads, newts and salamanders
- only extant examples are turtles
- many extinct species and all mammals
- dinosaurs, most modern reptiles, and birds
Note that snakes are considered tetrapods because they are descended from ancestors who had a full complement of limbs. Similar considerations apply to aquatic mammals.
Anatomical features of early tetrapods
The amphibian's ancestral fish must have possessed similar traits to those inherited by the early amphibians, including internal nostrils (to separate the breathing and feeding passages) and a large fleshy fin built on bones that could give rise to the tetrapod limb. The rhipidistian crossopterygians fulfill every requirement for this ancestry. Their palatal and jaw structures were identical to those of amphibians, and their dentition was identical too, with labyrinthine teeth fitting in a pit-and-tooth arrangement on the palate. The crossopterygian paired fins were smaller than tetrapod limbs, but the skeletal structure was very similar in that the crossopterygian 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.
The major difference between crossopterygians and amphibians was in relative development of front and back skull portions; the snout is much less developed than in most amphibians and the post-orbital skull is exceptionally longer than an amphibian's.
A great many kinds of early amphibians lived during the Carboniferous period. Therefore, their ancestor would have lived earlier, during the Devonian period. Devonian Ichthyostegids were the earliest of amphibians, with a skeleton that is directly comparable to that of rhipidistian ancestors. Early Labyrinthodonts (Late Devonian to Early Mississippian) still had some ichthyostegid features such as similar skull bone patterns, labyrinthine tooth structure, the fish skull-hinge, pieces of gill structure between the cheek and shoulder, and the vertebral column. They had, however, lost several other fish features such as the fin rays in the tail.
In order to propagate in the terrestrial environment, certain challenges had to be overcome. The animal's body needed additional support, because buoyancy was no longer a factor. A new method of respiration was required in order to extract atmospheric oxygen, instead of oxygen dissolved in water. A means of locomotion would need to be developed to traverse distances between waterholes. Water retention was now important since it was no longer the living matrix, and it could be lost easily to the environment. Finally, new sensory input systems were required if the animal was to have any ability to function reasonably while on land.
Labyrinthodontia Diagnostic features unique to the Labyrinthodontia are hard to find at first glance; the complex dentine infolding tooth structure was shared with crossopterygian fish. The labyrinthodonts are divided into the Temnospondyli and the Anthracosauria, the main difference between the two groups being their respective vertebral structures. The Anthracosauria had small pleurocentra, which grew and fused, becoming the true centrum in later vertebrates. In contrast, the Temnospondyli had a conservative vertebral column in which the pleurocentra remained small in primitive forms, vanishing entirely in the more advanced ones. The intercentra are large and form a complete ring.
Temnospondyls A diagnostic feature of the Temnospondyli was that the tabular bone in the skull roof is relatively small and had no contact with the parietal, whereas contact between the two bones was present in all anthracosaurs.
Although the temnospondyls flourished in many forms in the Late Palaeozoic and Triassic, they were an entirely self-contained group and did not give rise to any later tetrapod groups. It was the sister group Anthracosauria that gave rise to the reptiles.
Within the Temnospondyli are the two suborders Rachitomi and Stereospondyli, also distinguished by their vertebrae. There were three distinct successive stages within the Rachitomi, the first occurring in the Carboniferous. The second happened mostly in the Pennsylvanian, continuing into Permian, of which Erydops is characteristic. The third and final stage was in the Late Carboniferous and Early Permian, from which Eryops of the Texas Permian red beds is best known. Just as there were numerous side branches throughout the evolution of the temnospondyls, so too were there many of the rachitomes.
Of special interest in regards to the Rachitomi, is Branchiosaurus. This relatively tiny amphibian lived from the Late Pennsylvanian to the Early Permian and was very similar to the Rachitomi, differing only in its small size. However, it had a much less ossified skeleton, a short skull and other distinguishing features. Clear traces of gills are present in many fossilized samples, hence the name. Thought to have vertebra differing from rachitomous vertebrae, it was placed in a separate order named Phyllospondyli. Only later was it realized, by studying growth stages and seeing increasing ossification in larger specimens, that it was in fact the larval stage of a much larger rachitome like Eryops.
The most notable characteristic that makes an amphibian skull different from a fishes' 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 amphibian, the front of the skull lengthened, positioning the orbits farther back on the skull. The lacrimal bone was not in contact with the frontal anymore, having been separated from it by the prefrontal bone. Also of importance is that 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 crossopterygian ancestors in that on the outer surface lay a long dentary which bore teeth. There were also bones below the dentary on the jaw: two splenials, the angulary and the surangular. On the inside were usually three coronoids which 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 which 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 Labyrinthodontia had a peculiar tooth structure from which their name was derived and, although not exculsive 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 were folded in such a way as to form a complicated corrugated pattern when viewed in cross section. This infolding resulted in strengthening of 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.
There is a density difference between air and water that 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 which 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 labyrinthodonts, suggesting a partially 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 be modified in order to detect airborne odours.
In addition to the lateral line organ system, the eye had to change as well. This change came about because the refractive index of light differs between air and water, so the focal length of the lens was altered in order to properly function. 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, moistening the eyeball.
The balancing function of the middle ear was retained from the fish ancestry, but delicate air vibrations could not set up pulsations through the skull in order for it to function a proper auditory organ. Typical of most labyrinthodonts, the spiracular gill pouch was retained as the otic notch, 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.
In order for the impedance matching ear to work, certain conditions had to be met. The stapes must have been perpendicular to the tympanum, small and light enough to reduce its inertia and suspended in an air-filled cavity. In modern species which 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 that only high intensity, low frequency sounds could be detected, with the stapes more probably being used to support the braincase against the cheek.
The pectoral girdle of early tetrapods such as Eryops was highly developed, with a larger size forboth 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 crossopterygian 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 comprised 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 fusion of the sacral ribs to the blade of the ilium are utilized in addition to ligamentous attachments.
The humerus was the largest bone of the arm, its head articulating with the glenoid cavity of the pectoral girdle, distally with the radius and ulna. The radius resided on the inner side of the forearm and rested directly under the humerus, supporting much of the weight, while the ulna was located to the outside of the humerus. The ulna had a head, which muscles pulled on to extend the limb, called the olecranon that extended above the edge of the humerus.
The radius and the ulna articulated with the carpus which was a proximal row of three elements: the radiale underlying the radius, the ulnare underneath the ulna and an intermedium between the two. A large central element was beneath the last and may have articulated with the radius. There were also three smaller centralia lying to the radial side. Opposite the head of each toe lay a series of five distal carpals. Each digit had a first segment, the metacarpal, lying in the palm region.
The pelvic limb bones were essentially the same as in the pectoral limb, but with different names. The analogue to the humerus was the femur which was longer and slimmer. The two lower arm bones corresponded to the tibia and fibula of the hind leg, the former being the innermost and the latter the outermost bones. The tarsus is the hind version of the carpus and its bones correspond as well.
Early amphibians had a wide, gaping jaw with weak muscles with which to open and close it. Within the jaw were fang-like palatal teeth which, when coupled with the gape, suggests an intertial 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.
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.
It is taken that early amphibians were not very active, thus a predatory lifestyle was probably not the norm. It is more likely that it fed on fish either in the water or on those which became stranded at the margins of lakes and swamps. Also abundant at the time was a large supply of terrestrial invertebrates which may have provided a fairly adequate food supply.
Modern amphibians breathe by inhaling air into lungs, where oxygen is absorbed. They also breathe through the moist lining of the mouth and skin. So too did Eryops, but its ribs were too closely spaced to suggest that it simply expanded the rib cage. More likely, it depressed the hyoid apparatus to expand the oral cavity and elevated the floor of the mouth while it and the nostrils were closed. This forced air back into the lungs. Air could then be forced back out by contraction of the elastic tissue in the lung walls. Other special respiratory methods were probably also made use of.
Typical early amphibian posture is exhibited by the upper arm and upper leg extending nearly straight out from its body, while 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 contacted the ground. Most of the animal's strength was used to just elevate its body off the ground for walking, which was probably slow and difficult. With this sort of posture, only short, broad strides could be achieved. This has been confirmed by fossilized footprints found in Carboniferous rocks.
Ligamentous attachments within the limbs were present in Eryops, being important because they were the precursor to bony and cartilagenous variations seen in modern terrestrial animals that use their limbs for locomotion.
Of all body parts, the spine was the most affected by the move from water to land. It now had to resist the bending caused by body weight and had to provide mobility where needed. Previously, it was able to bend along its entire length. Likewise, the paired appendages had not been formerly related to the spine, but the slowly strengthening limbs now transmitted their support to the axis of the body.
- What is a tetrapod? (http://www.palaeos.com/Vertebrates/Units/150Tetrapoda/150.000.html#What's a "Tetrapod"?)
- UCMP Taxonomy page. (http://www.ucmp.berkeley.edu/vertebrates/tetrapods/tetraintro.html)
- Tetrapod cladograms. (http://www.palaeos.com/Vertebrates/Units/150Tetrapoda/150.000.html) These are like genealogical family trees.
- Sinostegia discovery reported in National Geographic. (http://news.nationalgeographic.com/news/2002/12/1218_021218_tetrapod.html)
- Devonian tetrapods. (http://www.mdgekko.com/devonian/opportunity/tetrapodsAnswer.html)
- Devonian tetrapod trackways. (http://www.mdgekko.com/devonian/Order/trackways.html)
- Detailed account of early tetrapod evolution. (http://www.origins.tv/darwin/tetrapods.htm) This website, geared to the layman, also describes Jennifer Clack's role in untangling the early evolution of tetrapods.
- Abstract of Jennifer A. Clack, An early tetrapod from "Romer's Gap", Nature July 2002. (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=12097908&dopt=Abstract)
- Abstract of Jennifer Clack's identification of a primitive baphetid tetrapod. (http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v394/n6688/abs/394066a0_r.html)
- "The evolution of tetrapods and the closing of Romer's Gap." (http://www.origins.tv/darwin/tetrapods.htm)