structure and affinities of Gymnophiona and Stegocephalia.pptx

gymnophiona

1. Structure and affinities of Stegocephalia AND Gymnophiona DR. SANDEEP KUMAR RAGHUVANSHI ASSISTANT PROFESSOR DEPARTMENT OF ZOOLOGY BAREILLY COLLEGE, BAREILLY U.P.

2. STEGOCEPHALIA (LABYRINTHODONTIA) Stegocephalia  is a term used for stem- tetrapods and their  amphibian - grade  descendants, and in  phylogenetic nomenclature  for all  tetrapods . Distinguished by a heavily armoured   skull roof  (hence the name Stegocephalia ), and complex vertebrae. The term was coined by American  paleontologist   Edward Drinker Cope (1868).  Labyrinthodontia  is an  extinct   amphibian  subclass, constituted some dominant animals of late  Paleozoic  to early  Mesozoic  eras (390 to 150 mya ). They evolved from  lobe-finned fishes , ancestral to all  land-living vertebrates . Labyrinthodonts do not form a monophyletic group, so many modern researchers have abandoned the term. However, some have continued to use the group in their classifications, pending more study of their relationships.

3. GENERL ORGANIZATION Labyrinthodonts were generally amphibian-like in build. Short-legged and mostly large headed, with moderately short to long tails. Many groups, and all the early forms, were large animals. Primitive labyrinthodonts were probably true water predators, and various degrees of amphibious, semi-aquatic and semi terrestrial modes of living arose independently in different groups. Some lineages remained water-bound or became secondarily fully aquatic with reduced limbs and elongated,  eel -like bodies.

4. SKULL The skulls were massive. The broad head and short neck may have been a result of respiratory constraints. Jaws were lined with small, sharp, conical teeth and the roof of the mouth bore larger tusk-like teeth. 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. All teeth were labyrinthodont. The sole exception were the chisel-like teeth of some of the advanced  herbivorous   diadectomorphs . The skull had prominent optic notches behind each eye and a  parietal eye .

5. HUNTING AND FEEDING Like sarcopterygian  ancestors, the labyrinthodonts were carnivorous. However, due to the broad, flat skulls and short jaw muscle, they could not open their mouth to any great extent. Majority of labyrinthodonts employed a sit-and-wait strategy, similar to many modern amphibians. When suitable prey swam or walked within reach, the jaw would slam shut, the palatine tusks stabbing the hapless victim. Swallowing was done by tipping the head back, as seen in many modern amphibians and in  crocodiles . Evolution of a deeper skull, better jaw control and a reduction of the palatine tusks is only seen in the more advanced reptile-like forms, possibly in connection with the evolution of more effective breathing, allowing for a more refined hunting style.

6. RESPIRATION The early labyrinthodonts possessed well developed internal gills as well as primitive  lungs , derived from the  swim bladders  of their ancestors. They could breathe air, which had a great advantage for residents of warm waters with low oxygen levels. There was no  diaphragm  and the ribs in many forms were too short or spaced too closely to aid in expanding the lungs. Exhalation with the aid of the ribs probably evolved only in the line leading to  amniotes .  Due to the high atmospheric oxygen and CO 2 pressure in Carboniferous, the primitive throat sac breathing was sufficient for obtaining oxygen. Removal of CO 2 was a greater problem on land. Larger labyrinthodonts probably combined a high tolerance for blood  carbonic acid  with returning to the water to dissipate the CO 2 through the skin.  The loss of the armour of  rhomboid   scales  of their piscine ancestors allowed for this as well as additional respiration through the skin as in modern amphibians.

7. Sensory apparatus The eyes of most Labyrinthodonts were situated at the top of the skull, offering good vision upwards, but very little lateral vision. The  parietal eye  was prominent, although there is uncertainty as to whether it was a true image producing organ or one that could only register light and dark. Most labyrinthodonts had special sense organs in the skin, forming a  lateral line  organ for perception of water flow and pressure. This would enable them to pick up the vibration of their prey and other waterborne sounds while hunting in weed filled waters. Early labyrinthodont groups had massive  stapes , primarily anchoring the brain case to the  skull roof . It is a question whether early terrestrial labyrinthodonts had the stapes connected to tympanum  covering their otic notch, and if they had an aerial sense of hearing.  The tympanum in  anurans  and  amniotes  appear to have evolved separately, indicating most, if not all, Labyrinthodonts were unable to pick up airborne sound.

8. Reproduction The labyrinthodonts had an amphibious reproduction — they laid eggs in water, where they would hatch to  tadpoles . They would remain in water throughout the larval stage until  metamorphosis . Only the metamorphosed individuals would eventually venture onto land on occasion. Fossil tadpoles from several species are known as neotenic  adults with feathery  external gills  similar to those found in modern  lissamphibian  tadpoles and in the fry of  lungfish . The existence of a larval stage as the primitive condition in all groups of labyrinthodonts can be fairly safely assumed, in that tadpoles of  Discosauriscus , a close relative of the  amniotes , are known.

9. GROUPS OF LABYRINTHODONTS The systematic placement of groups within Labyrinthodontia is notoriously changeable. Several groups are identified, but there is no consensus of their  phylogenetic  relationship.  Many key groups were small with moderately ossified skeletons, and there is a gap in the fossil record in the early  Carboniferous  ( Romer's gap ) when most of the groups appear to have evolved.  Further complicating the picture is the amphibian larval-adult life cycle, with physical changes throughout life complicating phylogenetic analysis. The Labyrinthodontia appear to be composed of several nested  clades . The two best understood groups, the  Ichthyostegalia  and the  reptile-like amphibians  have from the outset been known to be  paraphyletic . 

10. Ichthyostegalia The early labyrinthodonts are known from the Devonian and possibly extending into the  Romer's Gap  of the early  Carboniferous . These labyrinthodonts are often grouped together as the  order   Ichthyostegalia . Thay were predominantly aquatic and most show evidence of functional internal  gills  throughout life, and only occasionally ventured onto land. Their  polydactylous  feet were paddle-like, having more than the usual five digits. The tail had true  fin rays , like fish. The  vertebrae  were complex and rather weak. At the end of Devonian, forms with progressively stronger legs and vertebrae evolved, and the later groups lacked functional gills as adults. All were however predominantly aquatic and some spent all or nearly all their lives in water. Acanthostega , a fish-like early  ichthyostegalian

11. Reptile-like amphibians An early branch was the terrestrial reptile-like amphibians, variously called  Reptiliomorpha .  Tulerpeton  has been suggested as the earliest member of this line, indicating the split occurred before the  Devonian -Carboniferous transition. Their skulls were relatively deep and narrow compared to other labyrinthodonts. Front and hind feet bore five digits on most forms. Several of the early groups are known from brackish or even marine environments, having returned to a more or less fully aquatic mode of living. The terrestrial forms were moderately sized creatures that appeared in the early  Carboniferous . The most well known genus is  Seymouria . Some members of the most advanced group, the Diadectomorpha , were  herbivorous  and grew to several meters in length, with great, barrel-shaped bodies. Small relatives of the diadectomorphs gave rise to the first  reptiles  in the  Late Carboniferous . Seymouria , a terrestrial  reptiliomorph  from the  Permian .

12. Temnospondyli The most diverse group of labyrinthodonts was Temnospondyli , appeared in Late  Devonian  and varying from small salamander-like  Stereospondyli , to giant, well armoured   Archegosauroidea  that looked more like crocodiles.  Prionosuchus  was an exceptionally large member of the Archegosauridae , estimated to be the largest amphibian ever known (up to 9 m long). Temnospondyls typically had large heads and heavy shoulder girdles with moderately long tails. A temnospondyl's fore-foot had only four toes, and the hind-foot five, similar to the pattern seen in modern amphibians. All were more or less flat-headed with either strong or secondarily weak vertebrae and limbs. There were also fully aquatic forms, like the  Dinosauria , and even marine forms such as the  Trematosauridae . The Temnospondyli may have given rise to the modern  frogs  and  salamanders  in the late  Permian  or early  Triassic . Platyoposaurus , an advanced crocodile-like  temnospondyl  from the  Permian .

13. LEPOSPONDYLI A small group of uncertain origin, the  Lepospondyli  evolved mostly small species that can be found in European and North American Carboniferous and early Permian strata. They are characterized by simple spool-shaped vertebrae formed from a single element, rather than the complex system found in other labyrinthodont groups. Most were aquatic and external  gills  are sometimes found preserved. The Leposondyli were generally salamander-like, but one group, the  Aïstopoda , was snakelike with flexible, reduced skulls, though whether the families belong with the other lepospondyls is uncertain. Some  microsaur   lepospondyls were squat and short-tailed and appear to have been well adapted to terrestrial life. The best known genus is  Diplocaulus , a  nectridean  with a  boomerang -shaped head. The position of Lepospondyli in relation to other labyrinthodont groups is uncertain, and it is sometimes classified as a separate  subclass . The teeth were not labyrinthodont, and the group has classically been seen as separate from the Labyrinthodontia . There is some doubt as to whether the lepospondyls form a  phylogenetic  unit at all, or is a  wastebin taxon  containing the  paedamorphic  forms and  tadpoles  of other labyrinthodonts, notably the reptile-like amphibians, or even very small primitve amniotes with reduced skulls. Hyloplesion , a salamander-like  lepospondyl .

14. ORDER-GYMNOPHIONA Gymnophiona  is the group of  amphibians  that includes the legless  caecilians  and their closely related amphibians, than to  frogs  or  salamanders  (stem-caecilians). The name derives from the Greek words ( gymnos , naked) and ( ophis , snake), as the caecilians were originally thought to be related to snakes. Gymnophiona or Apoda , is one of the three major living orders of class  Amphibia . Its members are known as caecilians, means “sightless” or “blind.” They live underground in humid tropical regions throughout the world. Because of their relatively hidden existence, caecilians are not usually considered in discussions about amphibians. They are nevertheless a fascinating group of highly specialized amphibians about which there is still much to be learned. The body is cylindrical dark brown or bluish black in colour . The skin is slimy and bears grooves or ringlike markings, with minute dermal scales.

15. EVOLUTION Little is known about the evolution of the caecilian lineage. The first fossil was a vertebra from Paleocene , discovered in 1972. The first pre-Cenozoic genus,  Eocaecilia , had small limbs and well-developed eyes. Anderson and others (2008) suggested that caecilians arose from the  Lepospondyl  group of ancestral  tetrapods , may be more closely related to  amniotes  than to frogs and salamanders, which arose from  Temnospondyl  ancestors. The morphological studies on Permian and Carboniferous lepospondyls placed the early caecilian ( Eocaecilia ) among these groups. Divergent origins of caecilians and other living amphibians may help to explain the discrepancy between fossil dates for the origins of modern amphibia, which suggest  Permian  origins, and the earlier dates, in the  Carboniferous , predicted by some molecular clock studies of DNA sequences. Most recent molecular study based on multi-locus data suggest a Late  Carboniferous –Early  Permian  origin of extant amphibians. The Late Triassic stem-caecilian  Chinlestegophis  from the  Chinle Formation  of Colorado bolsters the proposed pre-Triassic origin of  Lissamphibia  suggested by molecular clocks by filling a gap in the fossil record of early caecilians and suggesting that groups of stereospondyls , including  Metoposauridae , are closely related to caecilians.

16. Size and Range Several species of caecilians in the South American genus  Caecilia  exceed 1 metre (about 3.3 feet) in total length; the largest known caecilian is  C. thompsoni , at 152 cm (about 60 inches). The smallest caecilians are  Idiocranium russeli  in  West Africa  and  Grandisonia brevis  in the Seychelles; these species attain lengths of only 98–104 mm (3.9–4.1 inches) and 112 mm (4.4 inches), respectively.

17. Distribution and Abundance Caecilians are found in tropical areas throughout the world. Of the 10 known families, 5 occur in the Americas, whereas Africa and mainland Asia harbour 3 families each. Caecilians are also found in Indonesia,  Sri Lanka , the  Philippines , and the  Seychelles . On the Seychelles there are three genera native to the islands, although caecilians are not found on any other islands in the  Indian Ocean . Approximately 180 caecilian species are known to exist, and up to 5 species have been found to inhabit the same area in the  Amazon rainforest . No caecilians have been found on Madagascar or  New Guinea .

18. Breeding behaviour Limited knowledge about reproductive  patterns in caecilians. The breeding period of some Asiatic  ichthyophiids  seems to be aseasonal , while  Ichthyophis glutinossus  in Sri Lanka, mates only during the rainy season. Females of  viviparous  species have a biennial reproductive cycle; the viviparous  Dermophis mexicanus  in Guatemala mates in the early rainy season, and  gestation  takes one year. All caecilians are believed to have internal  fertilization . This is achieved by means of the phallodeum, a copulatory organ in males that is modified from the  cloacal wall .  Eggs  of the families Ichthyophiidae and Rhinatrematidae are deposited in burrows in mud, close to water. The females watch over these clutches, which may hold up to 54 eggs. Upon hatching, the  larvae  leave the burrows to make their homes in ponds and streams. Some caecilians deposit eggs on land, and in different species these hatch as larvae or small adults. Three families have viviparous species to which usually no more than four young are born at one time. Aquatic typhlonectids are viviparous and produce larvae. The caecilian  fetus  emerges from the egg membrane as soon as its meagre  yolk  supply is exhausted; it uses its deciduous teeth, adapted for scraping, to obtain secretions and  epithelial tissues  from the oviduct lining.

19. Feeding habits The diet of terrestrial caecilians is mainly  earthworms  and other soft-bodied prey. Feeding either aboveground or in subterranean burrows, terrestrial caecilians are believed to locate their quarry by means of a chemosensory tentacle on each side of the head. They capture their  prey  with their powerful recurved teeth, masticate, and swallow. Aquatic caecilians, the typhlonectids, prey on  fishes ,  eels , and aquatic  invertebrates .

20. Form And Function Caecilians have long, limbless, cylindrical bodies that abruptly end behind the  cloaca  or short tail.  Annuli in the skin encircle the body and form segments; in some taxonomic groups, secondary and tertiary grooves partially circumscribe the body. Within the tissue of the annuli, bony  scales  of dermal origin usually occur. The heads of caecilians are blunt with bony and compact skull. The number of independent cranial bones in caecilians has reduced in comparison with  anurans  and  salamanders . Teeth are found on all jaw bones, and a palatal series of teeth appears in addition, medial to the maxillary series. A U-shaped facet, which  articulates  with the quadrate and also has a long retroarticular process that serves as an attachment site for three major  jaw  muscles, is located on the lower jaw. Vertebral column  is made up of an atlas (first vertebra) and 95-285 trunk vertebrae; no  differentiated  sacral vertebrae are present. Double-headed ribs are found on all vertebrae except the atlas and the terminal three to six vertebrae. Among all living amphibians, only caecilians have an  axial  musculature in which all the hypaxial components, excluding the subvertebral musculature, form an outer muscular sheath. This sheath, which is anchored to the skin by fibrous  connective tissue , is all but disconnected from the vertebral musculature and thereby allows the skin and superficial muscles to move as a single unit. The degenerate eyes are covered with bone or skin. These  adaptations  make it possible for the caecilian to feed, reproduce, and avoid enemies within their subterranean realm. The features of aquatic caecilians of the family Typhlonectidae are representative of secondary adaptations.

21. Annotated classification ORDER GYMNOPHIONA (APODA) about 180 species. Family  Caeciliidae : Paleocene (65.5–55.8 mya ) to present; tail absent; usually no aquatic larval stage; 2 genera, 42 species; adult size 10–152 cm (4–60 inches); South and  Central America . Family  Chikilidae : Jurassic (200–145.5 mya ) to present; lower jaws possess two rows of teeth; 1 genus, 7 species; northeastern  India . Family  Dermophiidae : Cretaceous (145.5–65.5 mya ) to present; secondary annuli and annular scales present; viviparous; 4 genera, 13 species; Africa and Central and  South America . Family  Herpelidae : Cretaceous (145.5–65.5 mya ) to present; 2 genera, 9 species; Africa. Family  Ichthyophiidae : Cretaceous (145.5–65.5 mya ) to present; tail present; mouth subterminal; aquatic larvae; 3 genera, 50 species; adult size 40–50 cm (16–20 inches);  Southeast Asia , peninsular India,  Sri Lanka , Sumatra,  Borneo , and  Philippines . Family  Indotyphlidae : Cretaceous (145.5–65.5 mya ) to present; viviparous forms lack scales and secondary annuli; some forms are oviparous; 7 genera, 21 species; Africa,  Seychelles , and India. Family  Rhinatrematidae : Cretaceous (145.5–65.5 mya ) to present; tail present; mouth terminal; aquatic larvae; 2 genera, 11 species; adult size 25–32 cm (10–13 inches); South America. Family  Scolecomorphidae : Jurassic (200–145.5 mya ) to present; tail absent; no aquatic larval stage; 2 genera, 6 species; adult size 40–45 cm (16–18 inches); Africa. Family  Siphonopidae : Cretaceous (145.5–65.5 mya ) to present; no inner mandibular teeth; oviparous; 7 genera, 19 species; South America. Family  Typhlonectidae : Cretaceous (145.5–65.5 mya ) to present; tail absent; mouth recessed; young possess gills; no larval stage; adults aquatic; 5 genera, 13 species; adult size 50–72 cm (20–28 inches); South America.

22. ORIGIN OF MODERN AMPHIBIANS A general consensus is that all modern amphibians, the  Lissamphibia , have their origin from Labyrinthodont stock. The fragile bones of the Lissamphibians are extremely rare as fossils, and the modern amphibians are highly derived, making comparison with fossil Labyrinthodonts difficult. Traditionally, the  Lepospondyli  has been favored as lissamphibian ancestors. Like the modern amphibians, they were mostly small with simple vertebrae, resembling lissamphibians in many aspects of external anatomy and presumably  ecological niches . At a  subclass  level, it was thought that labyrinthodonts gave rise to lepospondyls , and lepospondyls to lissamphibians . Several studies also favour the lepospondyl link, though placing Lepospondyli as close relatives or even derived from  reptile-like amphibians . 

23. The temnospondyl family  Amphibamidae  is considered a possible candidate for ancestors of lissamphibians . The amphibamid   Gerobatrachus , described in 2008, was proposed to be a  transitional form  between temnospondyls and  anurans  (frogs and toads) and  caudatans  (salamanders). It possessed a mixture of anuran and caudatan features, including a broad skull, short tail, and small  pedicellate teeth . Complicating the picture is the question of whether Lissamphibia itself may be  polyphyletic . During 20th century, many studies argued that Amphibia as a whole is  biphyletic , based on details of the nasal capsule and cranial nerves. Robert L. Carroll  suggested the tailed amphibians (salamanders and caecilians) are derived from lepospondyl   microsaurs  and frogs from temnospondyls. The analysis of  Gerobatrachus  suggested salamanders and frogs evolved from temnospondyl stock and caecilians being the sister group of the reptile-like amphibians, rendering Lissamphibia itself an evolutionary grade relative to the remaining tetrapod classes. Amphibamid   temnospondyl   Gerobatrachus  from the  Permian , proposed ancestor of lissamphibians

24. T H A N K Y O U