8. Origin and characteristics of tetrapods.pptx

Developmnt of tetrapods and their characteristics are discussed.

1. ORIGIN OF TETRAPODS Dr. Sandeep Kumar Raghuvanshi Assistant Professor Department of Zoology Bareilly College Bareilly

2. INTRODUCTION Tetrapoda is a superclass of animal kingdom, which includes all living and extinct amphibians, reptiles, birds and mammals. The  evolution of tetrapods  began about 400 mya in the Devonian Period from lobe-finned fishes. Little evidence supports the idea that any of the earliest tetrapods could move on land. The specific ancestors of tetrapods , and the process by which land colonization occurred, remain unclear.

3. Many groups of tetrapods have lost some or all of their limbs. In addition, many tetrapods have returned to partially or fully aquatic lives throughout the history of the group. The first returns to aquatic lifestyle may have occurred during Carboniferous Period, whereas other returns occurred recently during the Cenozoic (cetaceans and several modern amphibians). The change of a body plan from aquatic life to amphibious life is one of the most profound evolutionary changes known. It is also best understood, due to significant transitional fossil findings in the late 20 th century combined with improved phylogenetic analysis.

4. DENIZENS OF THE SWAMPS The first tetrapod probably  evolved  in coastal and brackish marine environments as well as in shallow and swampy freshwater habitats. Formerly, researchers thought the timing was towards the end of the Devonian. In 2010, this belief was changed to the Middle Devonian. Additionally, the tracks show that the animal was capable of thrusting its arms and legs forward, a type of motion that would have been impossible in tetrapodomorph fish like  Tiktaalik . The animal that produced the tracks is estimated to have been up to 2.5 metres (8.2 ft) long with footpads up to 26 cm (10 in) wide, although most tracks are only 15 cm (5.9 in) wide.

5. The new findings suggest that the first tetrapod may have lived as opportunists on the tidal flats, feeding on marine animals that were washed up by the tide.  Currently, however, fish are stranded in significant numbers only at certain times of year, as during spawning season. Such stranding 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. 

6. LUNGS BEFORE LAND Lungs require a blood supply for gaseous exchange. In  lungfishes ,  bowfin  and  bichirs , the swim bladder is supplied with blood by paired  pulmonary arteries  branching off from the 6 th aortic arch.  The same basic pattern is found in the lungfish  Protopterus  and in terrestrial  salamanders , and was probably the pattern found in the tetrapods' immediate ancestors as well as the first tetrapods .  In most other bony fishes the swim bladder is supplied with blood by the dorsal aorta.

7. The Breath In modern tetrapods , two important breathing mechanisms are conserved from early ancestors. The first mechanism is a CO 2 /H+ detection system. In modern tetrapods , the impulse to take a breath is triggered by a buildup of CO 2  in the bloodstream and not a lack of O 2 . A similar CO 2 /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. The second mechanism is the actual motion of the breath. In Lampreys, this mechanism takes the form of a "cough", where the lamprey shakes its body to allow water flow across its gills. When CO 2  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 CO 2  to leave its body. This linkage between CO 2  detection system and central pattern generator is extremely similar to tetrapods , which implies homology.

8. SKULL MORPHOLOGY The tetrapods have their root in the early  Devonian   tetrapodomorph fish . Primitive tetrapods developed from an osteolepid tetrapodomorph lobe-finned fish sarcopterygian-crossopterygian), with a two-lobed  brain  in a flattened  skull . The coelacanth group represents marine sarcopterygians that never acquired these shallow-water adaptations. The sarcopterygians apparently took two different lines of descent and are accordingly separated into two major groups: the  Actinistia  (including the coelacanths) and the  Rhipidistia  (which include extinct lines of lobe-finned fishes that evolved into the lungfish and the tetrapodomorphs ).

9. FROM FINS TO FEET The oldest known tetrapodomorph is  Kenichthys  from China, dated at around 395 mya . Two of the earliest tetrapodomorphs , dating from 380 mya , were  Gogonasus  and  Panderichthys . All of them used their fins to move through tidal channels and shallow waters choked with dead branches and rotting plants.  The universal tetrapod characteristics of front  limbs  that bend forward from the  elbow  and hind limbs that bend backward from the  knee  can plausibly be traced to early tetrapods living in shallow water. Pelvic bone fossils from  Tiktaalik  shows, that hind appendages and pelvic-propelled locomotion originated in water before terrestrial adaptations.

10. 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. A 2012 study using 3D reconstructions of  Ichthyostega  concluded that it was incapable of typical  quadrupedal steps . 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 . 

11. EXCRETION IN TETRAPODS When lungfishes are living in water, they produce ammonia, but when the water dries up and they forced to burrow down in the mud, they switch to urea production. Like cartilaginous fishes, the  coelacanth  can store urea in its blood, as can the only known amphibians that can live for long periods of time in salt water ( Bufo marinus  and Rana cancrivora ). These are traits they have inherited from their ancestors. If early tetrapods lived in freshwater, and if they lost the ability to produce urea and used ammonia only, they would have to evolve it from scratch again later. Terrestrial animals  that can only produce ammonia would have to drink water constantly, making a life on land impossible. This probably also was a problem at the start when the tetrapods started to spend time out of water, but eventually the urea system would dominate completely. Because of this it is not likely they emerged in freshwater, although some species never left, or returned to, the water could of course have adapted to freshwater lakes and rivers.

12. EARLIEST TETRAPODS Recent research showed that the very earliest tetrapods , similar to  Acanthostega , were wholly aquatic and quite unsuited to life on land. This is in contrast to the earlier view that fish had first invaded the land — either in search of prey or to find water when the pond they lived in dried out and later evolved legs, lungs, etc. By the late Devonian, swampy habitats like shallow wetlands, coastal lagoons and large brackish river deltas also existed, 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.

13. The Devonian tetrapods went through two major bottlenecks during what is known as the  Late Devonian extinction . These events of extinctions led to the disappearance of primitive tetrapods with fish-like features like Ichthyostega and their primary more aquatic relatives. 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 secondary adapted to an aquatic lifestyle.

14. FOSSILS OF EARLY TETRAPODS The first Devonian tetrapod identified from  Asia  was recognized from a  fossil  jaw-bone reported in 2002. The  Chinese  tetrapod  Sinostega was discovered among fossilized tropical plants and lobe-finned fish in the red  sandstone  sediments of northwest China. This finding substantially extended the geographical range of these animals. 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 .

15. Why they went to land in the first place is still debated. One reason could be that the small juveniles who had completed their  metamorphosis  had to make use of what land had to offer. Already adapted to breathe air and move around in shallow waters near land as a protection, two very different niches partially overlapped each other, with the young juveniles in the diffuse line between. One of them was overcrowded and dangerous while the other was much safer and much less crowded, offering less competition over resources.

16. The terrestrial niche was also a much more challenging place for primarily aquatic animals, but because of the way evolution and selection pressure work, those juveniles who could take advantage of this would be rewarded. Once they gained a small foothold on land, favourable variations in their descendants gradually resulted in continuing evolution and diversification. 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.

17. EXTANT (LIVING) TETRAPODS Following the great faunal turnover, only six major groups of tetrapods were left, all of which also include many  extinct  groups: Lissamphibia :  frogs , toads ,  newts, salamanders , and caecilians Testudines :  turtles  and  tortoises Lepidosauria :  tuataras ,  lizards ,  amphisbaenians  and  snakes Crocodilia :  crocodiles ,  alligators ,  caimans  and  gharials Neornithes : modern birds Mammalia : mammals

18. In  Late Devonian   vertebrate  speciation, descendants of  lobe-finned fish  — like  Eusthenopteron  — exhibited a sequence of adaptations. Panderichthys , suited to muddy shallows, Tiktaalik  with limb-like fins that could take it onto land. Early tetrapods in weed-filled swamps, such as Acanthostega , which had feet with eight digits and Ichthyostega  with limbs. Descendants also included pelagic lobe-finned fish such as  coelacanth  species.

19. Eusthenopteron  is a  genus  of  prehistoric  sarcopterygian ( lobe-finned fishes ) which has attained an iconic status from its close relationships to  tetrapods . The name derives from two Greek stems— eustheno -  "strength" and  -pteron  "wing" and thus "strongly developed fins".  P aleontologists  widely agree that it was a strictly aquatic animal. The genus  Eusthenopteron  is known from several species that lived during the Late  Devonian  period, about 385 mya . 

20. Panderichthys  is a  genus  of  extinct   sarcopterygian  (lobe-finned fish) from the late  Devonian  period, about 380  mya . It exhibits a range of features transitional between  tristichopterid  lobe-fin fishes (e.g.,  Eusthenopteron ) and early  tetrapods . Possible tetrapod tracks dating back to before the appearance of  Panderichthys  in the fossil record were reported in 2010, which suggests that  Panderichthys  is not a transitional fossil, but nonetheless shows the traits that evolved during the fish-tetrapod evolution. 

21. Tiktaalik is a  monospecific   genus  of  extinct   sarcopterygian  from the Late  Devonian  Period (375 mya ), having many features similar to tetrapods . It is technically a fish, having scales and gills - but has the flattened head like a crocodile and unusual fins. Its fins have thin ray bones for paddling like most fishes', but they also have sturdy interior bones that would have allowed Tiktaalik to prop itself up in shallow water and use its limbs for support as most four-legged animals do. It has a combination of features that show the evolutionary transition between swimming fish and their descendants, the four-legged vertebrates - a clade which includes amphibians, reptiles, birds and mammals. It and similar animals may possibly be the common ancestors of the broad swath of all  vertebrate  terrestrial  fauna : amphibians, reptiles, birds, and mammals.

22. Acanthostega  (meaning "spiny roof") is an extinct genus of  stem-tetrapod , among the first  vertebrate animals  to have recognizable  limbs . It appeared in the late  Devonian  period about 365 mya , and was anatomically intermediate between  lobe-finned fishes  and those that were fully capable of coming onto land .

23. Ichthyostega  is an early  genus  of  tetrapodomorphs  that lived at the end of the Late  Devonian  Period. It was one of the first four-limbed vertebrates in the  fossil  record. It had lungs and limbs that helped to navigate through shallow water in swamps. Until findings of other early stegocephalians and closely related fishes in the late 20th century,  Ichthyostega  stood alone as a  transitional fossil  between fish and tetrapods .

24. Tulerpeton  is an extinct  genus  of  Devonian  four-limbed vertebrate, known from a fossil that was found in the  Tula Region  of  Russia  at a site named Andreyevka . This genus and the closely related  Acanthostega  and  Ichthyostega  represent the earliest  tetrapods .

25. Crassigyrinus  is an  extinct   genus  of  carnivorous   stem tetrapod  from the  Early Carboniferous .   It  grew up to 2 meters in length, had tiny limbs and unusually large jaws. It was traditionally placed within the group  Labyrinthodontia  along with many other early tetrapods .  Large jaws were used to catch and swallow other animals. It had two rows of sharp  teeth  in its jaws, the second row having a pair of fangs.  It  had large eyes, suggesting that it was either nocturnal, or lived in very muddy water.

26. Diadectes is an extinct  genus  of large reptile -like  amphibians  that lived during early  Permian  period, between 290-272 mya . It was one of the very first  herbivorous   tetrapods , and also one of the first fully terrestrial animals to attain large size.