There is no doubt that reptiles have developed a fearsome reputation. In particular the crocodile and snake have given rise to urban legends of cold blooded animals that are intent on killing anything that is within their vicinity. Reptiles fall within the Linnaean class known as Reptilia. One of the characteristics that are associated with them is the ability to breathe and lay shelled eggs. Their skin is covered with scutes and scales. These creatures are tetrapods in as much as they have four limbs right from the ancestral line. They are found in most continents apart from the blatantly inhospitable Antarctica.
The reptilian orders include Crocodilia. These reptiles include gavials, crocodiles, caimans and alligators. There are a total of twenty three species within this order. The order that is known as Sphenodontia includes the rare Tuataras from New Zealand. There is just one other species within this order. Squamata includes snakes, lizards and worm lizards. These are a diverse group of seven thousand and nine hundred species. Testudines includes tortoises and turtles. The number of species within this order of reptiles is three hundred. Their classification is made based on slight differences that take them to one class or the next.
Reptiles are different from amphibians in as much as they do not go through an aquatic larval stage. Generally speaking they are oviparous although there are certain species such as squamates which can give live birth. They can achieve this change through ovoviviparity where they retain the egg. Alternatively they might undergo viviparity where they produce young ones without the development of any calcified eggs. They feed their fetuses through the placenta-like organs and others may provide initial care to the young ones. Reptiles can be as small as a gecko but can include the saltwater crocodile. This crocodile can grow up to six meters in length and weighs up to one thousand kilograms.
One of the divisions of reptiles that are very interesting to scientists is synapsid. The identification of these creatures follows the Linneaean approach. Under this system they are split into four subclasses which broadly reflect their number and temporal fenestrae. This is the opening in the sides of the skulls behind the ears. This form of classification was popularized by Henry Fairfield Osborn under the Vertebrate Paleontology system. These classes include Anapsida which have no fenestrae. They contain Chelonia and cotylosaurs. The Synapsida group has one low fenestra and includes the therapsids as well as the pelycosaurs. These are erroneously referred to as mammal-like reptiles.
The Euryapsida group of reptiles has one high fenestra which is above the postorbital or squamosal. The protorosaurs are similar to lizards. There are some marine species such as ichthyosaurs and plesiosaurs. Finally the Diapsida group of reptiles has two fenestrae. It covers the largest part of these creatures including crocodiles, lizards and snakes. In the old times Reptiles included pterosaurs and dinosaurs.