Reptile

Classification of reptiles

Reptiles are a paraphyletic group. The group can be made monophyletic by including the birds (Aves).

From the classical standpoint, reptiles included all the amniotes except birds and mammals. Thus reptiles were defined as the set of animals that includes crocodiles, alligators, tuatara, lizards, snakes, amphisbaenians and turtles, grouped together as the class Reptilia (Latin repere, "to creep"). This is still the usual definition of the term.

However, in recent years, many taxonomists have begun to insist that taxa should be monophyletic, that is, groups should include all descendants of a particular form. The reptiles as defined above would be paraphyletic, since they exclude both birds and mammals, although these also developed from the original reptile. Colin Tudge writes:

Mammals are a clade, and therefore the cladists are happy to acknowledge the traditional taxon Mammalia; and birds, too, are a clade, universally ascribed to the formal taxon Aves. Mammalia and Aves are, in fact, subclades within the grand clade of the Amniota. But the traditional class reptilia is not a clade. It is just a section of the clade Amniota: the section that is left after the Mammalia and Aves have been hived off. It cannot be defined by synamorphies, as is the proper way. It is instead defined by a combination of the features it has and the features it lacks: reptiles are the amniotes that lack fur or feathers. At best, the cladists suggest, we could say that the traditional Reptila are 'non-avian, non-mammalian amniotes'. (Tudge, p.85)

 

By the same token, the traditional class Amphibia becomes Amphibia*, because some ancient amphibian or other gave rise to all the amniotes; and the phylum Crustacea becomes Crustacea*, because it may have given rise to the insects and myriapods (centipedes and millipedes). If we believe, as some (but not all) zoologists do, that myriapods gave rise to insects, then they should be called Myriapoda*....by this convention Reptilia without an asterisk is synonymous with Amniota, and includes birds and mammals, whereas Reptilia* means non-avian, non-mammalian amniotes. (Tudge, p.85)

Recent college-level references, such as Benton (2004) [1], offer another compromise by applying traditional ranks to accepted phylogenetic relationships. In this case, reptiles belong to the class Sauropsida, and mammal-like reptiles to the class Synapsida, with birds and mammals separated into their own traditional classes.

Reptile Groups

• Class Sauropsida
  
  • Family Captorhinidae (extinct)
  • Family Protorothyrididae - Hylonomus (extinct)
  • Subclass Anapsida
    
    • Family Mesosauridae (extinct)
      Order Procolophonia - incl. Pareiasaurs (extinct)
      ?Order Testudines - Turtles
  • Subclass Diapsida
    
    • Superorder Ichthyopterygia - Ichthyosaurs (extinct)
    • Infraclass Lepidosauromorpha
      
      • Superorder Sauropterygia - Plesiosaurs (extinct)
      • Superorder Lepidosauria
        
        • Order Sphenodontia - Tuatara
        • Order Squamata - Lizards & Snakes
    • Infraclass Archosauromorpha
      
      • Order Crocodilia - Crocodilians
      • Order Pterosauria - Pterodactyls (extinct)
      • Superorder Dinosauria - Dinosaurs
        • Class Aves - Birds

Evolution of the reptiles

Young American Alligator Georgetown, South Carolina

Hylonomus is the oldest-known reptile, and was about 8 to 12 inches (20 to 30 cm) long. Westlothiana has been suggested as the oldest reptile, but is for the moment considered to be more related to amphibians than amniotes. Petrolacosaurus and Mesosaurus are other examples. The first true "reptiles" (Sauropsids) are categorized as Anapsids, having a solid skull with holes only for nose, eyes, spinal cord, etc. Turtles are believed by some to be surviving Anapsids, as they also share this skull structure; but this point has become contentious lately, with some arguing that turtles reverted to this primitive state in order to improve their armor. Both sides have strong evidence, and the conflict has yet to be resolved.

Shortly after the first reptiles, two branches split off, one leading to the Anapsids, which did not develop holes in their skulls. The other group, Diapsida, possessed a pair of holes in their skulls behind the eyes, along with a second pair located higher on the skull. The Diapsida split yet again into two lineages, the lepidosaurs (which contain modern snakes, lizards and tuataras, as well as, debatably, the extinct sea reptiles of the Mesozoic) and the archosaurs (today represented by only crocodilians and birds, but also containing pterosaurs and dinosaurs).

The earliest, solid-skulled amniotes also gave rise to a separate line, the Synapsida. Synapsids developed a pair of holes in their skulls behind the eyes (similar to the diapsids), which were used to both lighten the skull and increase the space for jaw muscles. The synapsids eventually evolved into mammals, and are often referred to as mammal-like reptiles, though they are not true members of the class Sauropsida.

Systems

Circulatory

Thermographic image of a monitor lizard.

Most reptiles have closed circulation via a three-chamber heart consisting of two atria and one, variably-partitioned ventricle. There is usually one pair of aortic arches. In spite of this, due to the fluid dynamics of blood flow through the heart, there is little mixing of oxygenated and deoxygenated blood in the three-chamber heart. Furthermore, the blood flow can be altered to shunt either deoxygenated blood to the body or oxygenated blood to the lungs, which gives the animal greater control over its blood flow, allowing more effective thermoregulation and longer diving times for aquatic species. There are some interesting exceptions among reptiles. For instance, crocodilians have an incredibly complicated four-chamber heart that is capable of becoming a functionally three-chamber heart during dives (Mazzotti, 1989 pg 47). Also, it has been discovered that some snake and lizard species (e.g., monitor lizards and pythons) have three-chamber hearts that become functional four-chamber hearts during contraction. This is made possible by a muscular ridge that subdivides the ventricle during ventricular diastole and completely divides it during ventricular systole. Because of this ridge, some of these squamates are capable of producing ventricular pressure differentials that are equivalent to those seen in mammalian and avian hearts (Wang et al, 2003).

Respiratory

All reptiles breathe using lungs. Aquatic turtles have developed more permeable skin, and even gills in their anal region, for some species (Orenstein, 2001). Even with these adaptations, breathing is never fully accomplished without lungs. Lung ventilation is accomplished differently in each main reptile group. In squamates the lungs are ventilated almost exclusively by the axial musculature. This is also the same musculature that is used during locomotion. Because of this constraint, most squamates are forced to hold their breath during intense runs. Some, however, have found a way around it. Varanids, and a few other lizard species, employ buccal pumping as a complement to their normal "axial breathing." This allows the animals to completely fill their lungs during intense locomotion, and thus remain aerobically active for a long time. Tegu lizards are known to possess a proto-diaphragm, which separates the pulmonary cavity from the visceral cavity. While not actually capable of movement, it does allow for greater lung inflation, by taking the weight of the viscera off the lungs (Klein et al, 2003). Crocodilians actually have a muscular diaphragm that is analogous to the mammalian diaphragm. The difference is that the muscles for the crocodilian diaphragm pull the pubis (part of the pelvis, which is movable in crocodilians) back, which brings the liver down, thus freeing space for the lungs to expand. This type of diaphragmatic setup has been referred to as the "hepatic piston."

How Turtles & Tortoises breathe has been the subject of much study. To date, only a few species have been studied thoroughly enough to get an idea of how turtles do it. The results indicate that turtles & tortoises have found a variety of solutions to this problem. The problem is that most turtle shells are rigid and do not allow for the type of expansion and contraction that other amniotes use to ventilate their lungs. Some turtles such as the Indian flapshell (Lissemys punctata) have a sheet of muscle that envelopes the lungs. When it contracts, the turtle can exhale. When at rest, the turtle can retract the limbs into the body cavity and force air out of the lungs. When the turtle protracts its limbs, the pressure inside the lungs is reduced, and the turtle can suck air in. Turtle lungs are attached to the inside of the top of the shell (carapace), with the bottom of the lungs attached (via connective tissue) to the rest of the viscera. By using a series of special muscles (roughly equivalent to a[diaphragm]), turtles are capable of pushing their viscera up and down, resulting in effective respiration, since many of these muscles have attachment points in conjunction with their forelimbs (indeed, many of the muscles expand into the limb pockets during contraction). Breathing during locomotion has been studied in three species, and they show different patterns. Adult female green sea turtles do not breathe as they crutch along their nesting beaches. They hold their breath during terrestrial locomotion and breathe in bouts as they rest. North American box turtles breathe continuously during locomotion, and the ventilation cycle is not coordinated with the limb movements (Landberg et al., 2003). They are probably using their abdominal muscles to breathe during locomotion. The last species to have been studied is red-eared sliders, which also breathe during locomotion, but they had smaller breaths during locomotion than during small pauses between locomotor bouts, indicating that there may be mechanical interference between the limb movements and the breathing apparatus. Box turtles have also been observed to breathe while completely sealed up inside their shells (ibid).

Most reptiles lack a secondary palate, meaning that they must hold their breath while swallowing. Crocodilians have evolved a bony secondary palate that allows them to continue breathing while remaining submerged (and protect their brains from getting kicked in by struggling prey). Skinks (family Scincidae) also have evolved a bony secondary palate, to varying degrees. Snakes took a different approach and extended their trachea instead. Their tracheal extension sticks out like a fleshy straw, and allows these animals to swallow large prey without suffering from asphyxiation.

Also, crocodiles are known to cry while eating. Many myths and folklore have grown around this astonishing fact, such as that the crocodile feels guilty for eating, but in truth, the crocodile cries to release fluid from its body, to make room for oxygen. This is also due to the fact that the crocodile's nasal cavity (nose) is exceptionally small.

Excretion

Excretion is performed mainly by two small kidneys. In diapsids uric acid is the main nitrogenous waste product; turtles, like mammals, mainly excrete urea. Unlike the kidneys of mammals and birds, reptile kidneys are unable to produce liquid urine more concentrated than their body fluid. This is because they lack a specialised structure present in the nephrons of birds and mammals, called a Loop of Henle. Because of this, many reptiles use the colon and cloaca to aid in the reabsorption of water. Some are also able to take up water stored in the bladder. Excess salts are also excreted by nasal and lingual salt-glands in some reptiles.

Nervous

Advanced nervous system compared to amphibians. They have twelve pairs of cranial nerves.

Sexual

Most reptiles reproduce sexually. All male reptiles except turtles and tortoises have a twin tube like sexual organ called the hemipenes. Turtles and tortoises have a single penis. All testudines lay eggs, none are live bearing as some lizard and snakes are. All reproductive activity occurs with the cloaca, the single exit/entrance at the base of the tail where waste and reproduction happens.

Asexual reproduction has been identified in squamates in six families of lizards and one snake. In some species of squamates, a population of females are able to produce a unisexual diploid clone of the mother. This asexual reproduction called parthenogenesis occurs in several species of gecko, and is particularly widespread in the teiids (especially Aspidocelis) and lacertids (Lacerta). Parthenogentic species are also suspected to occur among chameleons, agamids, xantusiids, and typhlopids.

Amniotic eggs are covered with leathery or calcareous shells. An amnion, chorion and allantois are present during embryonic life. There are no larval stages of development.

References

• Colin Tudge (2000). The Variety of Life. Oxford University Press. ISBN 0198604262.

• Benton, M. J. (2004), Vertebrate Paleontology, 3rd ed. Blackwell Science Ltd.

• Pianka, Eric; Vitt, Laurie (2003). Lizards Windows to the Evolution of Diversity. University of California Press, 116-118. ISBN 0-520-23401-4.

• Mazzotti, Frank; Ross, Charles (ed) (1989). "Structure And Function" Crocodiles and Alligators. Facts on File. ISBN 0-8160-2174-0.

• Wang, Tobias; Altimiras, Jordi; Klein, Wilfried; Axelsson, Michael (2003). Ventricular haemodynamics in Python molurus: separation of pulmonary and systemic pressures. The Journal of Experimental Biology 206: 4242-4245.

• Klein, Wilfried; Abe, Augusto; Andrade, Denis; Perry, Steven (2003). Structure of the posthepatic septum and its influence on visceral topology in the tegu lizard, Tupinambis merianae (Teidae: Reptilia). Journal of Morphology 258 (2): 151-157.

• Orenstein, Ronald (2001). Turtles, Tortoises & Terrapins: Survivors in Armor. Firefly Books. ISBN 1-55209-605-X.

• Landberg, Tobias; Mailhot, Jeffrey; Brainerd, Elizabeth (2003). Lung ventilation during treadmill locomotion in a terrestrial turtle, Terrapene carolina. Journal of Experimental Biology 206 (19): 3391-3404.

• Pough, Harvey; Janis, Christine; Heiser, John (2005). Vertebrate Life. Pearson Prentice Hall. ISBN 0-13-145310-6.

• Laurin, Michel and Gauthier, Jacques A.: Diapsida. Lizards, Sphenodon, crocodylians, birds, and their extinct relatives, Version 22 June 2000; part of The Tree of Life Web Project

Links

• Tree of Life Website
• The EMBL Reptile Database
• The HC Network
• Reptile Phylogeny
• The Reptipage
• Herpetological Conservation and Biology
• ReptilesWeb.com, a Non-Profit web portal for pet lovers all over the world
• Pictures of Reptils on Biocrawler.com
• Reptile Protection in India

This guide is licensed under the GNU Free Documentation License. It uses material from the Wikipedia.

Agamidae

Agamids or lizards of the family Agamidae include more than 300 species in Africa, Asia, Australia, and a few in Southern Europe. Phylogenetically they may be sister to the Iguanidae, characterized by predominantly acrodont dentition. Agamids usually have well-developed, strong legs. Their tails cannot be shed and regenerated like those of Geckoes, though a certain amount of regeneration is observed in some. Many agamid species are capable of limited change of their colours. Ecologically they range from hot deserts to tropical rainforests.

Agamids
Scientific classification
Kingdom: Animalia   Phylum: Chordata   Class: Reptilia   Order: Squamata   Suborder: Sauria   Family: Agamidae
Subfamilies
Agaminae Leiolepidinae Draconinae

There have been very few studies of the Agamidae with the first comprehensive assessment by Moody (1980) followed by a more inclusive assessment by Frost and Etheridge (1989). Subsequent studies were based mitochondrial DNA loci with Macey et al. (2000) and Honda et al. (2000) and Joger (1991)(using allozymes) sampling across the Agamidae. Few other studies focused on clades within the family, but the Agamidae have not been as well investigated as the Iguanidae.

Among the Agamidae, six clades or lineages are generally recognized including the Leiolepidinae (Leiolepis), Uromasticinae (Uromastyx), Amphibolurinae (Australian and New Guinean), Hydrosaurinae (Hydrosaurus), Draconinae (South and Southeast Asian), and Agaminae (African and Arabian). The Chamaeleons of the sister family Chamaeleonidae are sometimes discussed as sub-family Chamaeleoninae and sub-family Agaminae (referring to Agamidae, not the Agaminae mentioned above).

Link

• http://www.embl-heidelberg.de/~uetz/families/Agamidae.html

This guide is licensed under the GNU Free Documentation License. It uses material from the Wikipedia.

Archosaurs

Archosaurs  Fossil range: Early Triassic - Recent
Living archosaurs include crocodiles (pictured above) and birds.
Scientific classification
Kingdom: Animalia   Phylum: Chordata   Class: Sauropsida   Subclass: Diapsida   Infraclass: Archosauromorpha   (unranked) Archosauria Cope, 1869
Groups
Crurotarsi Aetosauria Crocodilia (crocodiles) Phytosauria Rauisuchia Ornithodira Pterosauria Marasuchus Dinosauria Aves (birds)

Archosaurs (Greek for 'ruling lizards') are a group of diapsid reptiles that first evolved from Archosauriform ancestors during the Olenekian (Lower Triassic Period). They are represented today by birds and crocodiles. Archosaurs are set apart by having socketed teeth (a feature that inspired the traditional name, 'thecodonts', for the Triassic forms) and four-chambered hearts, among other characteristics. Most early forms were carnivores, with narrow serrated meat-tearing teeth. Their "reptilian" metabolism seem to have given them a clear advantage over the mammal-like therapsids that were their contemporaries in the arid interiors and strong monsoon climates that were the natural result of the single world-continent, Pangaea. Thus, whereas the Permian was dominated by synapsids, the Triassic came to be dominated by sauropsids.

There are two primary groups of archosaurs — the Ornithodira which were insignificant during the Middle Triassic but in the Late Triassic radiated as the dinosaurs and pterosaurs; and the Crurotarsi, which were the predominant group at this time, and included a number of purely Triassic groups like the rauisuchians, the phytosaurs, and the herbivorous aetosaurs, as well as the ancestors of the crocodilians.

A number of these archosaur groups - chiefly those large Crurotarsi that are in pre-cladistic books called the Thecodonts - became extinct 195 million years ago, during the Triassic-Jurassic extinction event. The survivors - the Dinosaurs and the Pterosaurs among the Ornithodira, and first the Sphenosuchia and Protosuchia then their descendants the Crocodilia among the Crurotarsi - flourished during the Jurassic and Cretaceous Periods. The dinosaurs dominated the land, the pterosaurs and later another archosaurian group, the birds, dominated the air,and the crocodiles dominated the rivers and swamps and even invaded the seas (the Teleosaurs and Metriorhynchidae).

Most of these taxa perished 65 million years ago, during the Cretaceous-Tertiary extinction event. The only groups of archosaurs to continue through to the Tertiary and, ultimately, to the present day, are the birds (which are descended from the dinosaurs) and the crocodylia (which include all modern crocodiles, alligators, and gharials).

Birds are traditionally treated as a separate class, Aves, while the rest of the archosaurs are treated as a subclass or infraclass, Archosauria, within the class Reptilia. More recently, with the cladistic method dominating Biology, only monophyletic groups are considered valid and birds are included within the division Archosauria.

Taxonomy

• Infraclass Archosauromorpha
  • (unranked) Archosauriformes
    • (unranked) ARCHOSAURIA
      
      • (unranked) Crurotarsi ("Pseudosuchia")
        
        • Order Phytosauria
        • Order Aetosauria
        • Order Rauisuchia
          • Order Crocodilia
      • (unranked) Ornithodira ("Ornithosuchia" sensu Gauthier but without Ornithosuchidae)
        
        • Order Pterosauria
        • Superorder Dinosauria
          • Class Aves

References

• Benton, M. J. (2004), Vertebrate Paleontology, 3rd ed. Blackwell Science Ltd
• Carroll, R. L. (1988), Vertebrate Paleontology and Evolution, W. H. Freeman and Co. New York

Links

• UCMP
• Paleos
• Mikko's Phylogeny Archive Archosauria

This guide is licensed under the GNU Free Documentation License. It uses material from the Wikipedia.

Avicephalans

Avicephala is an extinct order of bizarre diapsid reptiles that lived during the Late Permian and Triassic periods. Many species had odd specialized grasping limbs and prehensile tails, adapted to arboreal (and possibly aquatic) lifestyles.

Fossil range: Permian-Triassic
Scientific classification
 Kingdom: Animalia
 Phylum: Chordata
 Class: Sauropsida
 Subclass: Diapsida
 Avicephala, Senter, 2004
Families: Longisquamidae, Coelurosauravidae, Drepanosauridae

References

• Colbert, E. H. & Olsen, P. E. (2001). "A new and unusual aquatic reptile from the Lockatong Formation of New Jersey (Late Triassic, Newark Supergroup)." American Museum Novitates: #3334, pp. 1-24.
• Harris, J. D. & Downs, A. (2002). "A drepanosaurid pectoral girdle from the Ghost Ranch (Whitaker) Coelophysis quarry (Chinle Group, Rock Point Formation, Rhaetian), New Mexico." Journal of Vertebrate Paleontology: Vol. 22, #1, pp. 70-75.
• Renesto, S. (1994). "Megalancosaurus, a possibly arboreal archosauromorph (Reptilia) from the Upper Triassic of northern Italy." Journal of Vertebrate Paleontology: Vol. 14, #1, pp. 38-52.
• Renesto, S. (2000): Bird-like head on a chameleon body: new specimens of the enigmatic diapsid reptile Megalancosaurus from the Late Triassic of northern Italy. Rivista Italiana di Paleontologia e Stratigrafia 106: 157–180. HTML abstract
• Senter, P. (2004). "Phylogeny of Drepanosauridae (Reptilia: Diapsida)." Journal of Systematic Palaeontology 2 (3): 257-268.

Links

• Monkey Lizards of the Triassic - An illustrated article on drepanosaurs from HMNH.
• Prof. Silvio Renesto—Vertebrate Paleontology at Insubria University: Research - Images and discussion of Drepanosaurus.
• Prof. Silvio Renesto—Vertebrate Paleontology at Insubria University: Research - Images and discussion of Megalancosaurus.
• Gliding Mechanism in the Late Permian Reptile Coelurosauravus (Eberhard Frey, Hans-Dieter Sues, & Wolfgang Munk) - Abstract and available full text of the article in Science.

This guide is licensed under the GNU Free Documentation License. It uses material from the Wikipedia.

Fictional reptiles

Fictional reptiles