In mammals, membranes that are homologous to the extra-embryonic membranes in eggs are present in the placenta. Additional derived characteristics of amniotes include waterproof skin, due to the presence of lipids, and costal rib ventilation of the lungs.
Amniotic eggs : The key features of an amniotic egg are the chorion, amnion, and allantois. Modern amniotes, which includes mammals, reptiles, and birds, evolved from an amphibian ancestor approximately million years ago. The first amniotes evolved from their amphibian ancestors approximately million years ago during the Carboniferous period.
The early amniotes diverged into two main lines soon after the first amniotes arose. The initial split was into synapsids and sauropsids. Synapsids include all mammals, including extinct mammalian species. Synapsids also include therapsids, which were mammal-like reptiles from which mammals evolved. Sauropsids include reptiles and birds and can be further divided into anapsids and diapsids.
The key differences between the synapsids, anapsids, and diapsids are the structures of the skull and the number of temporal fenestrae behind each eye.
Temporal fenestrae are post-orbital openings in the skull that allow muscles to expand and lengthen. Anapsids have no temporal fenestrae, synapsids have one, and diapsids have two. Anapsids include extinct organisms and may, based on anatomy, include turtles Testudines , which have an anapsid-like skull with one opening.
However, this is still controversial, and turtles are sometimes classified as diapsids based on molecular evidence. The diapsids include birds and all other living and extinct reptiles.
Tempral fenestrae : The image illustrates the differences in the skulls and temporal fenestrae of anapsids, synapsids, and diapsids. Anapsids have no openings, synapsids have one opening, and diapsids have two openings. The lepidosaurs include modern lizards, snakes, and tuataras. Clade Dinosauria includes birds, which evolved from a branch of dinosaurs. Evolution of amniotes : This chart shows the evolution of amniotes.
The placement of Testudines turtles is currently still debated. In the past, the most common division of amniotes has been into the classes Mammalia, Reptilia, and Aves. Birds are descended, however, from dinosaurs, so this classical scheme results in groups that are not true clades. Birds are considered as a group distinct from reptiles with the understanding that this does not completely reflect phylogenetic history and relationships. Reptiles are ectothermic tetrapods that lay shelled eggs on land and possess scaly skin and lungs.
Reptiles are tetrapods. Limbless reptiles snakes and other squamates have vestigial limbs and, as with caecilians, are classified as tetrapods because they are descended from four-limbed ancestors. Reptiles lay on land eggs enclosed in shells. Even aquatic reptiles return to the land to lay eggs.
They usually reproduce sexually with internal fertilization. Other species are viviparous, with the offspring born alive. One of the key adaptations that permitted reptiles to live on land was the development of their scaly skin which contains the protein keratin and waxy lipids, reducing water loss from the skin.
Due to this occlusive skin, reptiles cannot use their skin for respiration, as do amphibians; all breathe with lungs. Reptiles are ectotherms: animals whose main source of body heat comes from the environment.
This is in contrast to endotherms, which use heat produced by metabolism to regulate body temperature. In addition to being ectothermic, reptiles are categorized as poikilotherms: animals whose body temperatures vary rather than remain stable. Reptiles have behavioral adaptations to help regulate body temperature, such as basking in sunny places to warm up and finding shady spots or going underground to cool down. The advantage of ectothermy is that metabolic energy from food is not required to heat the body; therefore, reptiles can survive on about 10 percent of the calories required by a similarly-sized endotherm.
In cold weather, some reptiles, such as the garter snake, brumate. Brumation is similar to hibernation in that the animal becomes less active and can go for long periods without eating, but differs from hibernation in that brumating reptiles are not asleep or living off fat reserves. Rather, their metabolism is slowed in response to cold temperatures; the animal becomes very sluggish.
Ectotherms : Reptiles, such as these sunbathing Florida redbelly turtles, are ectotherms: they rely on their environment for body heat. Reptiles originated approximately million years ago during the Carboniferous period. One of the oldest-known amniotes is Casineria , which had both amphibian and reptilian characteristics. One of the earliest undisputed reptiles was Hylonomus. Soon after the first amniotes appeared, they diverged into three groups synapsids, anapsids, and diapsids during the Permian period.
The Permian period also saw a second major divergence of diapsid reptiles into archosaurs predecessors of crocodilians and dinosaurs and lepidosaurs predecessors of snakes and lizards. These groups remained inconspicuous until the Triassic period when the archosaurs became the dominant terrestrial group due to the extinction of large-bodied anapsids and synapsids during the Permian-Triassic extinction. About million years ago, archosaurs radiated into the dinosaurs and the pterosaurs.
Although they are sometimes mistakenly called dinosaurs, the pterosaurs were distinct from true dinosaurs. Pterosaurs had a number of adaptations that allowed for flight, including hollow bones birds also exhibit hollow bones, a case of convergent evolution. Their wings were formed by membranes of skin that attached to the long, fourth finger of each arm and extended along the body to the legs.
Pterosaurs : Pterosaurs, which existed from the late Triassic to the Cretaceous period to Instead, they may have been able to soar after launching from cliffs.
The dinosaurs were a diverse group of terrestrial reptiles with more than 1, species identified to date. Paleontologists continue to discover new species of dinosaurs. Some dinosaurs were quadrupeds; others were bipeds. Once on land, further diversification took place as the animals adapted to a plethora of terrestrial food sources; amniotes could eat terrestrial plants; perhaps more importantly, the insects that had already colonized the land.
Most frogs and salamanders engage in external fertilization; as a female deposits eggs in the water, the male releases sperm to fertilize them. The development of amniotic eggs freed animals from having to deposit their eggs in the water, but it came at a cost: External fertilization of shelled eggs is not possible. The shell of a reptile's egg is impervious to sperm, so the eggs must be fertilized before they have formed this layer and are deposited. This caused "intromittent" organs to evolve, allowing males to internally fertilize females.
By using the site, you agree to the uses of cookies and other technology as outlined in our Policy, and to our Terms of Use. The Amphibian Egg The amphibian egg -- termed anamniotic -- is largely similar to the fish egg. The Amniotic Egg In contrast to the simple anamniotic or amphibian egg, the amniotic egg contains a number of distinct structures. Amniotic Freedom Whereas most frogs and salamanders are bound to waterscapes for reproductive purposes, the development of the amniotic egg allowed creatures to venture far from the water.
Varying Fertilization Most frogs and salamanders engage in external fertilization; as a female deposits eggs in the water, the male releases sperm to fertilize them. The proportion of yolk differs between altricial and precocial birds. The former, which hatch so undeveloped that they require significant parental care and thus need less stored energy, generally have eggs that contain about 25 percent yolk.
Precocial birds, which can walk and feed themselves shortly after hatching, have eggs with about 40 percent yolk 67 percent in megapodes, inhabitants of Australia and Pacific islands which upon hatching are virtually ready to fly. Interestingly, in spite of this difference, and although bird eggs range in weight from about one hundredth of an ounce small hummingbird to three and a half pounds ostrich , all bird eggs lose water amounting to about 15 percent of their original weight during incubation.
This careful control is probably a result of the necessity to keep the water content of the developing chick's tissues constant even though metabolic water is continually being produced. Small birds tend to lay proportionately large eggs; the egg of a wren weighs about 13 percent of the wren's weight, while an ostrich egg weighs less than 2 percent of an adult's weight. As might be expected, the eggs of precocial birds tend to be heavier in proportion to body weight than those of altricial birds -- the parents must "invest" more in the egg to give the chick the energy and materials required for more advanced development within the confines of the shell.
Left-hand column: Ruby-throated Hummingbird, Great Auk extinct. Although most are "egg-shaped," some eggs, such as those of owls, are nearly spherical.
Fast-flying, highly streamlined birds such as swifts and hummingbirds tend to lay long, elliptical eggs, while those of auks, guillemots, and shorebirds are more pointed at the narrow end. Such "top-shaped" eggs can be closely packed with the pointy ends inward, helping adults to efficiently cover them during incubation.
0コメント