Evolution of reptiles
Reptiles, in the traditional sense of the term, are defined as animals that have scales or scutes, lay land-based hard-shelled eggs, and possess ectothermic metabolisms.
A definition in accordance with phylogenetic nomenclature, which rejects paraphyletic groups, includes birds while excluding mammals and their synapsid ancestors.
Reptiles have an extremely diverse evolutionary history that has led to biological successes, such as dinosaurs, pterosaurs, plesiosaurs, mosasaurs, and ichthyosaurs.
[5] A series of footprints from the fossil strata of Nova Scotia, dated to 315 million years, show typical reptilian toes and imprints of scales.
The function of the holes in both synapsids and diapsids was to lighten the skull and give room for the jaw muscles to move, allowing for a more powerful bite.
[15] The rationale for this classification was disputed, with some arguing that turtles are diapsids that reverted to this primitive state in order to improve their armor (see Parareptilia).
Important developments in the transition from reptile to mammal were the evolution of warm-bloodedness, of molar occlusion, of the three-ossicle middle ear, of hair, and of mammary glands.
In the mid-Permian period, the climate turned drier, resulting in a change of fauna: The primitive pelycosaurs were replaced by the more advanced therapsids.
[21] Late in the period, the diapsid reptiles split into two main lineages, the archosaurs (ancestors of crocodiles and dinosaurs) and the lepidosaurs (predecessors of modern tuataras, lizards, and snakes).
The Mesozoic is often called the "Age of Reptiles", a phrase coined by the early 19th-century paleontologist Gideon Mantell who recognized the dinosaurs and the ancestors of the crocodilians as the dominant land vertebrates.
Tyrannosauroids Compsognathids Ornithomimosaurs Ornitholestes Therizinosauroids Alvarezsaurids Oviraptorosaurs Avialae (birds and their closest relatives) Troodontids Dromaeosaurs Simplified cladogram from Senter (2007).
Along with massive amount of volcanic activity at the time, the meteor impact that created the Cretaceous–Paleogene boundary is accepted as the main cause for this mass extinction event.
Of the large marine reptiles, only sea turtles are left, and, of the dinosaurs, only the small feathered theropods survived in the form of birds.
Sphenodontians arose in the mid Triassic and now consists of a single genus, tuatara, which comprises two endangered species that live on New Zealand and some of its minor surrounding islands.
The most recent order of reptiles, squamates,[32] are recognized by having a movable quadrate bone (giving them upper-jaw movement), possessing horny scales and hemipenes.
The latter superorder, together with some extinct animals like the plesiosaurs, constitute the Lepidosauromorpha, the sister infraclass to the group, the Archosauromorpha, that contains crocodiles, turtles, and birds.
The first organisms that showed similar characteristics of Crocodilians[35] were the Crurotarsi, who appeared during the early Triassic 250 million years ago.
The complete loss of bipedalism was traded for a generally low quadrupedal stance for an easy and less noticeable entrance to bodies of water.
Mimicry is evident, as the backs of all crocodilia resemble some type of floating log and their general color scheme of brown and green mimics moss or wood.
