kräldjur

Kräldjur, or reptiles, are a class of amniote vertebrates that includes snakes, lizards, turtles, and crocodilians. They are characterized by dry, scaly skin containing keratin and by ectothermy, meaning they regulate body temperature mainly through external heat sources. Most kräldjur reproduce by laying eggs with leathery or calcified shells, although some species give birth to live young.

In terms of anatomy and physiology, their body plan generally includes lungs, a compact skeleton, and limbs that are often adapted to walking, climbing, gliding, or swimming. The heart is three-chambered in most lineages, with crocodilians possessing a four-chambered heart that allows more complete separation of oxygenated and deoxygenated blood. Skin scales serve as protection and help reduce water loss. Reproduction involves various strategies, including oviparity and viviparity, and many species exhibit parental care to some degree.

Traditionally, kräldjur comprises four extant orders: Testudines (turtles and tortoises), Squamata (lizards and snakes), Crocodilia (crocodiles, alligators, caimans), and Rhynchocephalia (tuataras). In some modern classifications, birds are placed within the broader reptile lineage or Sauropsida, reflecting their close evolutionary relationships. Reptiles originated in the late Carboniferous period with the evolution of the amniotic egg and diversified extensively during the Mesozoic era.

Ecology and distribution vary widely; reptiles inhabit deserts, forests, grasslands, and oceans, with marine forms such as sea turtles and marine iguanas. They are typically predators or herbivores, with specializations across species. Many reptiles rely on temperature cues for the sex of their offspring, a phenomenon known as temperature-dependent sex determination in several groups.

Conservation concerns include habitat loss, overexploitation, pollution, and climate change. A significant proportion of reptile species are protected by national and international regulations, and conservation efforts emphasize habitat protection, research, and captive breeding programs.