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What you’ll learn to do: Identify characteristics of reptiles
Reptiles are tetrapod animals in the class Reptilia. The study of these traditional reptile orders, historically combined with that of modern amphibians, is called herpetology.
In more recent years, scientists have discovered that some reptiles are more closely related to birds than they are to other reptiles (e.g., crocodiles are more closely related to birds than they are to lizards). For this reason, many modern scientists prefer to consider the birds part of Reptilia as well. In this course, we will study them separately.
Modern reptiles do not have the capacity for the rapid sustained activity found in birds and mammals. It is generally accepted that this lower capacity is related to differences in the circulatory and respiratory systems. Before the origin of lungs, the vertebrate circulatory system had a single circuit: in the fishes, blood flows from heart to gills to body and back to the heart. The heart consists of four chambers arranged in a linear sequence.
With the evolution of lungs in early tetrapods, a new and apparently more efficient circulatory system evolved. Two chambers of the heart, the atrium (or auricle) and ventricle, became increasingly important, and the beginnings of double circulation appeared. An early stage in this evolution can be seen in amphibians today, where one of the main arteries from the heart (the pulmonary artery) goes directly to the lungs, whereas the others (the systemic arteries) carry blood to the general body. In amphibians the blood is aerated in the lungs and carried back to the atrium of the heart. From the left side of the atrium, which is at least partially divided, the aerated blood is pumped into the ventricle to mix with nonaerated blood nonaerated blood from the body is returned to the heart via the right half of the atrium. Then the cycle begins again. One aspect of the amphibian system is that the blood leaving the heart for the body is only partially aerated part of it is made up of deoxygenated blood returned from the body.
All groups of modern reptiles have a completely divided atrium it is safe to assume, therefore, that this was true of most, if not all, extinct reptiles. In the four major living groups of reptiles, the ventricle is at least partially divided. When the two atria of a lizard’s heart contract, the two streams of blood (aerated blood from the lungs in the left atrium and nonaerated blood from the body in the right atrium) flow into the left chamber of the ventricle. As pressure builds up in that chamber, the nonaerated blood is forced through the gap in the partition into the right chamber of the ventricle. Then, when the ventricle contracts, nonaerated blood is pumped into the pulmonary artery and thence to the lungs, whereas aerated blood is pumped into the systemic arteries (the aortas) and so to the body.
In snakes all three arterial trunks come out of the chamber of the ventricle that receives the nonaerated blood of the right atrium. During ventricular contraction a muscular ridge forms a partition that guides the nonaerated blood into the pulmonary artery, while the aerated blood received by the other chamber of the ventricle is forced through the opening in the ventricular septum and out through the aortas.
In crocodiles the ventricular septum is complete, but the two aortas come out of different ventricular chambers. A semilunar valve at the entrance to the left aorta prevents nonaerated blood in the right ventricle from flowing into the aorta. Instead, part of the aerated blood from the left ventricular chamber pumped into the right aorta flows into the left by way of an opening.
The ventricle of the turtle is not perfectly divided, and some slight mixing of aerated and nonaerated blood can occur.
Despite the peculiar and complex circulation, lizards, snakes, and crocodilians have achieved a double system. Tests of the blood in the various chambers and arteries have shown that the oxygen content in both systemic aortas is as high as that of the blood just received by the left atrium from the lungs and is much higher than that of the blood in the pulmonary artery. Except for the turtles, limitation of activity in reptiles cannot be explained on the basis of heart circulation. An explanation may lie in the chemistry of the blood. The blood of reptiles has less hemoglobin and thus carries less oxygen than that of mammals.
Full Title Name: Biological Information: Reptile Biology and Physiology
This overview describes the fundamental characteristics of reptile biology and physiology.
Reptiles Are Animals
Reptiles are animals, as are amphibians. Physiologically, they are similar and are sometimes collectively called “herpetofauna.” All of the excepted scientific classification systems regard reptiles as such. Under the two most common classification systems, reptiles are either grouped as the Class Reptilia or the Class Diapsida under the Kingdom Animalia, meaning “animals”. Moreover, under most legal definitions, reptiles are considered to be animals as well. For example, Connecticut defines an animal as including “birds, quadrupeds, reptiles and amphibians.” Con. Gen. Stat. §26-1(1) (2004). Wisconsin is equally specific: "’Animal’ includes every living: (a) Warm-blooded creature, except a human being (b) Reptile or (c) Amphibian.” Wis. Stat. §951.01 (2004). Florida, albeit in an unflattering way, also defines “animal” to include reptiles as “the word ‘animal’ shall be held to include every living dumb creature.” Fla. Stat. §828.02 (2005). An unsettling discrepancy lies in the Federal Government’s Animal Welfare Act. 7 U.S.C. §§2131 et seq. There, the “term ‘animal’ means any live or dead dog, cat, monkey (nonhuman primate mammal), guinea pig, hamster, rabbit, or such other warm-blooded animal, as the Secretary may determine is being used, or is intended for use, for research, testing, experimentation, or exhibition purposes, or as a pet” but excludes birds, mice, rats, horses and farm animals. 7 U.S.C. §2132(g). Conspicuously absent from the list are fish, amphibians and reptiles. Id.
Unfortunately, many people, unaware of the backings from science and the law, do not consider reptiles to be animals. In his article entitled “Herpetofauna Keeping By Secondary School Students: Causes For Concern”, David Bride complied the results of some alarming graduate studies that currently remain unpublished. See http://www.psyeta.org/sa/sa6.1/bride.html . His compilation refers to a study by Martin and Nicholls (Martin, D. & Nicholls, M. (1993). The importance of children's provenance in the understanding of "animal" - a comparison of town and village primary school children in Kent. Christ Church College, Canterbury: Ecology Research Group) of 400 five- to eleven-year-olds found between 10-40% of those surveyed did not recognize either snakes or frogs as animals. Similarly, Tinkler (Tinkler, D. (1993) Zoo visitors' perceptions of animals - and the short-term effect of a zoo visit upon them. Unpublished M.S. dissertation. University of Kent at Canterbury, DICE) recorded 60% of 150 adult zoo visitors failed to classify a lizard as an animal. From his investigation into unpublished studies, Bride hypothesized that this may be due to a confusion of term "mammal" with "animal." He recently found that of 228 respondents to a questionnaire about wildlife, at least 25% appeared to confuse the two. Bride found this view, that "animal" equals "mammal," to be interesting as it gives an entirely new perspective to what many people's interpretations of such concepts as "animal protection," "animal welfare," and "animal rights" may entail. See http://www.psyeta.org/sa/sa6.1/bride.html. Knowledge of what a reptile actually is may go far in soliciting the sympathy of the public.
A very common myth is that reptiles are “cold-blooded.” In addition to describing a general temperature of reptilian blood, the term also entails the negative connotations of evil and lifelessness. To be sure, in the Biblical story of Adam and Eve, it was a serpent that deceived mankind. However, reptiles are not “cold-blooded.” On occasion, they are quite the contrary and can even be “hot-blooded.” This gradation results in reptiles being “poikilothermic.” Poikilotherms have a body temperature that is variable with environmental conditions. If the ambient temperature is warm or even hot, that leads to a reptile having warm or hot blood. Another physiological term that accurately depicts reptiles is “ectothemy.” Ectotherms control the uptake of heat from the environment as a way to control internal body temperature. Reptiles are both poikilothermic and ectothemic, but are not cold-blooded. Moreover, some larger reptiles, such as large crocodilians, sea turtles and large monitor lizards approach a level of homeothermy. That is, their temperature does not fluctuate as much based upon the environment. This results from a physiology process known as gigantothermy, where a very large animal will maintain a constant body temperature with little input from the environment.
Another popular assumption is that since reptiles are “cold-blooded,” they therefore feel little or no pain. In fact, they have little physiological control over their internal body temperature and are instead almost completely reliant on external heat sources to provide them with enough warmth for their natural activities and for metabolic processes to operate. This makes these animals extremely sensitive even to subtle changes in temperature and humidity in their captive environment.
Added to the problems arising from their “cold-blooded” reputation, reptiles lack the repertoire of facial expressions and vocalizations that would alert keepers to their pain and distress. A sick, hurt, or chronically stressed reptile will typically suffer in silence. The suffering will often be far more prolonged than that experienced by mammals, due to reptiles' slow metabolic rate. Blood loss and the healing of injuries are both relatively slow, as are the consequent risk of infection and further complications.
Most reptiles have a preferred optimum temperature zone, a zone of temperature that they try to maintain while performing daily activities. Their entire physiology, including their immune defense mechanism, is temperature dependent and operates optimally at this optimal zone. Reptiles in captivity often are maintained at suboptimal temperatures, which results in a compromised immune system. Such animals are subject to infection by a great variety of secondary invaders, including the gram-negative microorganisms commonly isolated from their oral cavity. A reptile that is kept at its preferred optimum temperature (with all other environmental conditions being ideal) and receives proper nutrition is often a healthy reptile. For a more complete discussion of reptile physiology, see 18 Carl Gans & David Crews, Biology of the Reptilia, Physiology E, (1991). The cumulative effect of these common misconceptions may play a large factor in the cruel treatment and neglect of reptiles in captivity.
Reptiles are ectothermic, meaning that the heat that they require to maintain physiological processes is derived externally, directly or indirectly from the sun. Reptiles are the ancestors of birds and the mammals, both of which are endothermic, meaning that body heat is primarily generated within their bodies. Their dependence on the external heat sources limits reptiles to tropical and temperate regions of the earth and they are especially numerous in the tropics. Reptiles are found in all continents of the world, except Antartica, living in a wide variety of habitats, including the world's driest deserts. Many are aquatic, living in both fresh water and the sea.
The skin of reptiles is covered in impermeable scales and they are able to retain water very effectively. Their eggs are covered by a hard shell, providing a sealed environment for the developing embryo, which does not depend on external sources of water. These two features have enabled reptiles successfully to colonize very dry habitats, including deserts. While tortoises are typically slow and ponderous in their movements, many reptiles are very agile and capable of moving very fast, often over large distances. Marine turtles, in particular, routinely undertake enormous journeys across the world's oceans.
Reptiles are divided into six major groups: The worm-lizards (c. 181 species), the lizards (c. 5461 species), the snakes (c. 3315 species), the turtles and tortoises (c. 317 species), the alligators and crocodiles (c. 23 species), and the tuataras (two species).
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Most reptiles have a continuous external covering of epidermal scales. Reptile scales contain a unique type of keratin called beta keratin the scales and interscalar skin also contain alpha keratin, which is a trait shared with other vertebrates. Keratin is the main component of reptilian scales. Scales may be very small, as in the microscopic tubercular scales of dwarf geckos (Sphaerodactylus), or relatively large, as in the body scales of many groups of lizards and snakes. The largest scales are the scutes covering the shell of a turtle or the plates of a crocodile.
Other features also define the class Reptilia. The occipital condyle (a protuberance where the skull attaches to the first vertebra) is single. The cervical vertebrae in reptiles have midventral keels, and the intercentrum of the second cervical vertebra fuses to the axis in adults. Taxa with well-developed limbs have two or more sacral vertebrae. The lower jaw of reptiles is made up of several bones but lacks an anterior coronoid bone. In the ear a single auditory bone, the stapes, transmits sound vibrations from the eardrum (tympanum) to the inner ear. Sexual reproduction is internal, and sperm may be deposited by copulation or through the apposition of cloacae. Asexual reproduction by parthenogenesis also occurs in some groups. Development may be internal, with embryos retained in the female’s oviducts, and embryos of some species may be attached to the mother by a placenta. However, development in most species is external, with embryos enclosed in shelled eggs. In all cases each embryo is encased in an amnion, a membranous fluid-filled sac.
22.214.171.124: Introduction to Reptiles - Biology
2 - pharynx with pouches or slits in wall (at least in the embryo)
3 - dorsal, hollow nervous system
Notochord = rod of living cells ventral to central nervous system & dorsal to alimentary canal
- Head region - incorporated into floor of skull
- Trunk & tail - surrounded by cartilaginous or bony vertebrate (except in Agnathans)
- Fishes & amphibians - notochord persists the length of the trunk & tail but is constricted within the centrum of each vertebra
- Reptiles, birds, & mammals - notochord almost disappears during development (e.g., remains as a pulpy nucleus in the vertebrae of mammals)
- Protochordates - notochord remains as the chief axial skeleton
- Agnathans - lateral neural cartilages are located on notochord lateral to the spinal cord
- permanent slits - adults that live in water & breathe via gills
- temporary slits - adults live on land
- fishes that occurred in the late Cambrian period (see The Cambrian Explosion) through the Devonian (about 400 - 525 million years before present)
- had bony plates and scales (&, therefore, were easily fossilized)
- jawless vertebrates called 'armored fishes'
- Myllokunmingia fengjiaoa (pictured below) & Haikouichthys ercaicunensis- primitive fish that have many similarities to living hagfishes and are the oldest vertebrates (530 mybf) ever found.
- Cathaymyrus diadexus(literally the 'Chinese eel of good fortune') is not the fossil of an eel. At just 5 cm long, but 535 m.y. old, it is the earliest known chordate (fossil shown below for a 'reconstruction' check http://www.gs-rc.org/GOODS/GOOD_3e.HTM). Researchers think that Cathaymyrus is a fossil relative of modern lancelets (amphioxus).
- sessile (like adult tunicates)
- tail evolved as adaptation in larvae to increase mobility
- 'higher forms' - came about by retention of tail (neoteny)
- notochord is confined to the tail
- notochord is lost during metamorphosis into sessile adult
- possess pharyngeal slits
- dorsal, hollow nervous system
- pharyngeal gill slits
- 'circulatory' system - vertebrate pattern with 'pumping vessels' (but no heart)
- semicircular canals
- agnathans have 1 or 2
- gnathostomes have 3
- agnathans have none
- gnathostomes do
- agnathans have none
- gnathostomes do
6 - Petromyzontia (lampreys)
- 1 - extinct Paleozoic (Cambrian to Devonian) jawless fish with an external skeleton of bone ('bony armor')
2 - oldest known vertebrates
3 - many had flattened appearance (some may have been bottom-dwellers)
- gill slits have a fleshy operculum & the spiracle is closed
- few scales
- common ancestor with sharks but an independent line
- most primitive ray-fins
- chiefly Paleozoic (300-400 mybp)
- include present-day Sturgeons & Paddlefish (below)
- dominant Mesozoic fishes
- possess ganoid scales
- two extant genera:
- Lepidosteus - predatory includes present-day gars
- Amia - includes present-day bowfins (or dogfish)
- recent bony fishes
- 95% of all living fish
- about 40 living orders
- well-ossified skeleton
- cycloid & ctenoid scales (flexible & overlapping)
- pelvic fins often located far forward
- no spiracle
- African & South American species have inefficient gills & will drown if held under water
- Australian species (Neoceratodus spp.) relies on gills unless oxygen content of water is too low
- ancestry uncertain due to lack of fossil evidence
- probably on a 'side branch' of vertebrate evolution
3 - skull similar to that of early amphibians
- O. Dipnoi - lungfish (3 living genera Africa, Australia, & South America)
- Oldest known = subclass Labyrinthodontia
- Fish-like features:
1- small bony scales in the skin
2- fin-rays in the tail (for swimming)
3- a skull similar to that of some Crossopterygians
Labyrinthodonts are distinguished by deeply folded structure of enamel and dentine layers in the teeth, that look like an intricate labyrinth in the cross section, hence the name of this group. Labyrinthodonts were probably similar to fishes in their mode of living. Labyrinthodonts, like fishes and most modern amphibians, laid eggs in the water, where their larvae developed into mature animals. All labyrinthodonts had special sense organs in the skin, that formed a system for perception of water fluctuations. Moreover, some of them possessed well developed gills. In contrast, many labyrinthodonts seemingly had primitive lungs. They could breath atmospheric air, that was a great advantage for residents of warm shoals with low oxygen levels in the water. The air was inflated into the lungs by contractions of a special throat sac. Primitive members of all labyrinthodont groups were probably true water predators, and only advanced forms that arose independently in different groups and times, gained an amphibious, semi-aquatic mode of living. Mature individuals of advanced labyrinthodonts could live on land, feeding mainly on insects and other small invertebrates. Well ossified robust skeletons in some Late Carboniferous and Early Permian labyrinthodonts prove their adaptation to the terrestrial mode of life. It suggests that amphibians had successfully 'organized' actual terrestrial assemblages prior to the wide expansion of reptiles.
The most diverse group of the labyrinthodonts was the batrachomorphs ('similar to a frog'). Though these animals looked more like crocodiles, they most probably gave rise to the order Anura, the amphibians without tails, which include, in particular, the modern frogs. Batrachomorphs appeared in the Late Devonian, but they had worldwide distribution in the continental shallow basins of the Permian (Platyoposaurus, Melosaurus) and Triassic Periods (Thoosuchus, Benthosuchus, Eryosuchus). Some batrachomorphs existed until the end of the Cretaceous.
- Subclass Lepospondyli
- Subclass Lissamphibia - modern amphibians
- O. Anura - frogs & toads
O. Urodela - tailed amphibians
O. Gymnophiona (apodans) - wormlike, burrowing amphibians
- unchanged for about 175 million years
- identified by bony dermal plates to which ribs & trunk vertebrae are fused
- 1 - earliest known gnathostomes (Silurian about 440 mybp)
2 - probably related to modern bony fishes
3 - small (less than 20 cm long) with large eyes
The relationships of acanthodians to other vertebrates has been the subject of considerable debate. Early researchers considered them to be most closely related to the ray-finned fishes, but most scientists during the mid-20th Century considered acanthodians to have a closer affinity to the sharks. Opinion has now generally swung back in favor of a closer relationship with ray-fins, but this is far from universally accepted.
- 1 - Silurian (about 420 million years before present)
2 - probably off the main line of vertebrate evolution
3 - many had bony dermal shields
- 1 - ancestors had bony skeletons so cartilaginous skeleton is specialized
2 - pelvic fins of males are modified as claspers
Subclass Elasmobranchii - most common cartilaginous fishes
- O. Cladoselachii - primitive sharks (300-400 mybp)
O. Selachii - 'modern' sharks
- 1 - 1st pharyngeal slit modified as a spiracle
2 - naked gill slits (no operculum)
- Subclass Holocephali
- O. Chimaeriformes (photo & drawing)
- 1 - skeleton is partly or chiefly bone
2 - gill slits are covered by a bony operculum
3 - skin has scales with, typically, little bone
4 - most have a swim bladder
Subclass Actinopterygii - ray-fins
- Superorder Chondrostei
- Superorder Neopterygii
- Order Semionotiformes
- 1 - aquatic larval stage with external gills
2 - middle ear cavity with ear ossicle (columella)
- Reptile Subclasses:
- 1 - Anapsida
- O. Cotylosauria - stem reptiles
O. Chelonia - turtles & tortoises
- O. Rhynchocephalia (Sphenodonta) - only living representative is the Tuatara
O. Squamata - lizards, geckos, & snakes
- Synapsid type = mammal-like reptiles
- Anapsid type = stem reptiles & turtles
- Diapsid type = rhynchocephalians, lizards, & snakes
- Euryapsid type = extinct plesiosaurs.
- UC Berkeley's Introduction to the Turtles and Their Kin
- Overview of Anapsid Fossil Diversity
- Comparison of Anapsid Skulls
- The Oldest Known Anapsid Reptile: Acleistorhinus (ah-kles-toe-RYE-nuss) from the Oklahoma Lower Permian (270 Mya)
- An Example of an Early Anapsid Group Related to Turtles: Pareiasauria
- Other Anapsid Group Related to Turtles: Procolophonoids (PRO-col-oh-phon-oids)
- The Oldest Known Sea Turtle (110 Mya) from Brazil (Plus a Slide Show of Extinct Marine Reptiles!)
- University of Michigan's Species Accounts for Testudines (Turtles)
- Note that Cowen's "Phylogram" of Reptiles (p. 173) Places Turtles within Diapsida (= Sauria), Whereas Formerly Authorities Place Turtles as Sister Taxon of Diapsida (= Sauria)
- Cowen's Cladogram Reflects Accumulating Evidence Based on Mitochondrial DNA Sequence Comparisons Supporting the Monophyly of a Turtle + Archosaur Clade Exclusive of Lepidosaurs (More Links: 1 - 2 - 3 - 4 - 5 - 6 - 7)
- Seaworld's Overview of Sea Turtles
- The EuroTurtle Introduction to Sea Turtles
- Turtle Web Links
- Charles Darwin's Description of the Galapagos Tortoises
- California Turtle and Tortoise Club Website
- Crocodile Web Links
- Chris Bochu's Website on Crocodile Systematics
- Hear Crocodiles Talk
- Recently Discovered Gigantic Super Crocodile, Sarcosuchus (110 Mya)
- Dan Varner's Painting of a Middle to Late Jurassic 'Fish-like' Crocodile Geosaurus Unrelated but Convergent to Ichthyosaurs
- Lecture Notes on Crocodiles and Other Aquatic Reptiles
- Excellent Overview of Archosaurs including Crocodiles
- Introduction to the Ichthyosaurs
- Image of Ichthyosaur with Skin
- Some of the Best Preserved Ichthyosaurs are from the Saurian Expedition of 1905 to West Humboldt Range in Nevada
- Berlin-Ichthyosaur Fossil State Park Near Reno, Nevada (Also See July '01 Natural History Article)
- The Jurassic Ichthyosaur Stenopterygius
- Nature's Special Multimedia Introduction to Discovery of Largest Ever Ichthyosaur Fossil or Summary
- Oliver Rieppel's Home Page
- Kid's Guide to "Nothosaurs" and Plesiosaurs
- Dan Varner's Images of Late Jurassic Crytoclidus Plesiosaurs Cruising
and a Nothosaur (Triassic) prowling the shallow sea for food
- A Small Fossil Skeleton of a Nothosaur
- A Semi-Aquatic Placodont, Placodus, (Triassic) Grubbing for Clams
- Year 1 of Digging Up a Plesiosaur in South Dakota and Year 2
- Good Overview of Plesiosaurs
- On Mosasaurs and Plesiosaurs
- "Plesiosaur Carcass" Netted in 1977 was More Likely a Basking Shark
Non-vertebrate chordates still alive today include tunicates (or sea squirts urochordates) & amphioxus (or branchiostoma). (cephalochordates)
2 - pharyngeal pouches or slits
3 - dorsal, hollow nervous system
- Chordate 'ancestor' of vertebrates:
A 530 million-year-old (although perhaps as old as 560 million years) creature, Cheungkongella ancestralis, probably a tunicate, found in the Chengjiang fauna in China's southwest Yunnan Province, might be the earliest known fossil evidence of primitive chordates (Shu, D.-G., L. Chen, J. Han, X.-L. Zhang. 2001. An early Cambrian tunicate from China. Nature 411:472 - 473.)
- Subphylum Cephalochordata= Amphioxus (or Branchiostoma)
- 1 - a dorsal, hollow nervous system
- O. Thecodontia - stem archosaurs
O. Pterosauria (check this short video & this one)
O. Saurischia - 2 major groups: sauropods & theropods (check this short video)
O. Ornithischia (like Iguanodon)
- 5 - Synapsida
- O. Pelycosauria - first stage in evolution to mammals
Saurischia (sawr-RIS-kee-ah) & Ornithischia are the two orders of dinosaurs, with the division based on the shape of the pelvic bone. The saurischian pubis (left) juts forward, and its ischium points backward. The ornithischian pubis and ischium (right) both point backward. The ornithischians were all herbivorous, and included some of the most interesting-looking dinosaurs. Ornithischian dinosaurs include three suborders: Ornithopoda, Marginocephalia and Thyreophora. The famous carnivorous dinosaurs were from the saurischian order, as were the largest herbivorous dinosaurs. The saurischian dinosaurs include two suborders: Theropoda and Sauropodomorpha.
The first vertebrates to evolve true flight were the pterosaurs, flying archosaurian reptiles. After the discovery of pterosaur fossils in the 18th century, it was thought that pterosaurs were a failed experiment in flight a humorous mishap or that they were simply gliders, too weak to fly. More recent studies have revealed that pterosaurs were definitely proficient flyers, and were no evolutionary failure as a group they lasted about 140 million years (about as long as birds have)! Pterosaurs are thought to be derived from a bipedal, cursorial (running) archosaur in the late Triassic period (about 225 million years ago). No other phylogenetic hypothesis has withstood examination however, the early history of pterosaurs is not yet fully understood because of their poor fossil record in the Triassic period. We can infer that the origin of flight in pterosaurs fits the "ground up" evolutionary scenario, supported by the fact that pterosaurs had no evident arboreal adaptations.
The pterosaur wing was supported by an elongated fourth digit (imagine having a "pinky finger" several feet long, and using that to fly!). Pterosaurs had other morphological adaptations for flight as a keeled sternum for the attachment of flight muscles, a short and stout humerus (the first arm bone), and hollow but strong limb and skull bones. Pterosaurs also had modified scales that were wing-supporting fibers, and that possibly formed hairlike structures to provide insulation -- bird feathers are analogous to the wing fibers of pterosaurs, and both are thought to possibly have been evolved originally for the primary purpose of thermoregulation (which implies, but does not prove, that both pterosaurs and the earliest birds were endothermic).
Early pterosaurs (such as Dimorphodon) had long tails that assisted balance, but later pterosaurs had no tails, and may have been more adept flyers. The most derived pterosaurs, such as Pteranodon and Quetzalcoatlus, were so large that soaring was the only feasible option these were the largest flyers ever to cast a shadow on the Earth's surface.
Temporal fenestration has long been used to classify amniotes. Taxa such as Anapsida, Diapsida, Euryapsida, and Synapsida were named after their type of temporal fenestration. Temporal fenestra are large holes in the side of the skull. The function of these holes has long been debated. Many believe that they allow muscles to expand and to lengthen. The resulting greater bulk of muscles results in a stronger jaw musculature, and the longer muscle fibers allow an increase in the gape.
2 - lost several dinosaur characteristics (e.g., long tail & teeth) but retained others (e.g., claws, scales, diapsid skull, single occipital condyle &, perhaps, feathers) (see AMNH website & ABC News website)
- Genera: Archaeopteryx & Archaeornis
- 1 - solid bones
An invited contribution to the special feature ‘Biology of extinction: inferring events, patterns and processes’ edited by Barry Brook and John Alroy.
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126.96.36.199: Introduction to Reptiles - Biology
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Sauropterygians (Including Plesiosaurs)
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Parthenogenesis: Meaning, Types and Significance
Usually an un-fertilised ovum develops into a new individual only after the union with the sperm or fertilisation but in certain cases the development of the egg takes place without the fertilisation. This peculiar mode of sexual reproduction in which egg development occurs without the fertilisation is known as the parthenogenesis (Gr., parthenos = virgin genesis = origin).
The phenomenon of parthenogenesis occurs in different groups of the animals as in certain insects (Hymenoptera, Homoptera, Coleoptera), crustaceans and rotifers.
Types of Parthenogenesis:
The parthenogenesis may be of two types:
1. Natural parthenogenesis
2. Artificial parthenogenesis
1. Natural Parthenogenesis:
In certain animals the parthenogenesis occurs regularly, constantly and naturally in their life cycles and is known as the natural parthenogenesis.
The natural parthenogenesis may be of two types, viz., complete or incomplete:
(i) Complete Parthenogenesis:
Certain insects have no sexual phase and no males. They depend exclusively on the parthenogenesis for the self-reproduction. This type of parthenogenesis is known as the complete parthenogenesis or obligatory parthenogenesis.
(ii) Incomplete Parthenogenesis:
The life cycle of certain insects includes two generations, the sexual generation and parthenogenetic generation, both of which alternate to each other. In such cases, the diploid eggs produce females and the un-fertilised eggs produce males. This type of parthenogenesis is known as the partial or incomplete or cyclic parthenogenesis.
The complete or incomplete type of natural parthenogenesis may be of following two types:
1. Haploid or arrhenotokous parthenogenesis
2. Diploid or thelytokous parthenogenesis.
1. Haploid or arrhenotokous parthenogenesis:
In the arrhenotokous parthenogenesis, the haploid eggs are not fertilised by the sperms and develop into the haploid individuals.
In these cases, the haploid individuals are always males and the diploid individuals are the females, e.g.
(i) Hymenoptera (bees and wasps),
(iii) Coleoptera (Micromalthus debilis),
(iv) Thysanoptera (Anthothrips verbasi).
Arachnids, e.g., ticks, mites and certain spiders (Pediculoides ventricusm),
Rotifers, e.g., Asplanchne amphora.
2. Diploid or thelytokous parthenogenesis:
In the diploid parthenogenesis, the young individuals develop from the un-fertilised diploid eggs.
Following types of the thelytoky have been recognised:
(i) Ameiotic Parthenogenesis:
Sometimes during the oogenesis, first meiotic or reduction division does not occur but second meiotic division occurs as usual. Such eggs contain diploid number of chromosomes and develop into new individuals without the fertilisation.
This type of parthenogenesis is known as apomictic or ameiotic parthenogenesis and occurs in Trichoniscus (Isopoda), Daphnia pulex (Crustacea), Campelona rufum (Mollusca), weevils and long-horned grasshoppers.
(ii) Meiotic Parthenogenesis:
Certain eggs develop by the usual process of oogenesis but at certain stages diplosis or doubling of chromosome number and production of diploid eggs occur. Such eggs develop into the diploid individuals and this phenomenon is known as the meiotic parthenogenesis.
The diplosis of the diploid thelytoky may occur by the following methods:
(i) By Autofertiiisation:
In certain cases, the oocyte divides meiotically up to the formation of ootid or ovum and secondary polocyte. But the ootid and the secondary polocyte unite together to form a diploid egg which develops into a new individual, e.g., Artemia salina (Crustacea) and various other organisms.
(ii) By Restitution:
Sometimes in primary oocyte, karyokinesis forms a nucleus of the secondary oocyte and nucleus of the first polocyte. But the karyokinesis is not followed by the cytokinesis. The chromosomes of both daughter nuclei are arranged on the equator and undergo second meiotic division to form a diploid ootid and a diploid polocyte.
The diploid ootid or ovum develops into a parthenogenetic diploid individual. This type of diplosis is known as the restitution, e.g., insects of order Hymenoptera (Nemertis conesceus) and Lepidoptera.
2. Artificial Parthenogenesis:
The eggs which always develop into the young individuals by the fertilisation sometimes may develop parthenogenetically under certain artificial conditions. This type of parthenogenesis is known as artificial parthenogenesis. The artificial parthenogenesis may be induced by various chemical and physical means.
The following physical means cause the parthenogenesis:
(i) Temperature the range of temperature may induce parthenogenesis in the eggs. For instance, when the egg is transferred from the 30°C to 0-10°C, the parthenogenesis is induced.
(ii) Electrical shocks can cause parthenogenesis.
(iii) Ultraviolet light can cause parthenogenesis.
(iv) When the eggs are pricked by the fine glass needles the development of young ones takes place parthenogenetically.
The following chemicals have been found to cause parthenogenesis in the normal eggs:
3. Hypertonic and Hypotonic sea waters
4. Chlorides of K + , Ca ++ , Na + , Mg ++ , etc.
5. Acids such as butyric acid, lactic acid, oleic acid and other fatty acids
6. Fat solvents, e.g., toluene, alcohol, benzene and acetone
The artificial parthenogenesis has been induced by above mentioned physical and chemical means by various workers in the eggs of most echinoderms, molluscs, annelids, amphibians, birds and mammals.
Significance of Parthenogenesis:
1. The parthenogenesis serves as the means for the determination of sex in the honey bees, wasps, etc.
2. The parthenogenesis supports the chromosome theory of inheritance.
3. The parthenogenesis is the most simple, stable and easy process of reproduction.
4. The parthenogenesis eliminates the variation from the populations.
5. The parthenogenesis is the best way of high rate of multiplication in certain insects, e.g., aphids.
6. The parthenogenesis causes the polyploidy in the organisms.
7. The parthenogenesis encourages development of the advantageous mutant characters.
8. The parthenogenesis checks the non-adaptive combination of genes which may be caused due to the mutation.
9. Due to the parthenogenesis, there is no need for the organisms to waste their energy in the process of mating but it allows them to utilise that amount of energy in the feeding and reproduction.