until 50,000 years ago in Australia, represented by genera such as Wonambi. There are numerous debates in the systematics within the group. For instance, many sources classify Boidae and Pythonidae as one family, while some keep the Elapidae and Hydrophiidae (sea snakes) separate for practical reasons despite their extremely close relation. Recent molecular studies support the monophyly of the clades of modern snakes, scolecophidians, typhlopids + anomalepidids, alethinophidians, core alethinophidians, uropeltids (Cylindrophis, Anomochilus, uropeltines), macrostomatans, booids, boids, pythonids and caenophidians. Families Legless lizards While snakes are limbless reptiles, evolved from (and grouped with) lizards, there are many other species of lizards that have lost their limbs independently but which superficially look similar to snakes. These include the slowworm and glass snake. Other serpentine tetrapods that are unrelated to snakes include caecilians (amphibians), amphisbaenians (near-lizard squamates), and the extinct aistopods (amphibians). Biology Size The now extinct Titanoboa cerrejonensis was in length. By comparison, the largest extant snakes are the reticulated python, measuring about long, and the green anaconda, which measures about long and is considered the heaviest snake on Earth at . At the other end of the scale, the smallest extant snake is Leptotyphlops carlae, with a length of about . Most snakes are fairly small animals, approximately in length. Perception Pit vipers, pythons, and some boas have infrared-sensitive receptors in deep grooves on the snout, allowing them to "see" the radiated heat of warm-blooded prey. In pit vipers, the grooves are located between the nostril and the eye in a large "pit" on each side of the head. Other infrared-sensitive snakes have multiple, smaller labial pits lining the upper lip, just below the nostrils. A snake tracks its prey using smell, collecting airborne particles with its forked tongue, then passing them to the vomeronasal organ or Jacobson's organ in the mouth for examination. The fork in the tongue provides a sort of directional sense of smell and taste simultaneously. The snake's tongue is constantly in motion, sampling particles from the air, ground, and water, analyzing the chemicals found, and determining the presence of prey or predators in the local environment. In water-dwelling snakes, such as the anaconda, the tongue functions efficiently underwater. The underside of a snake is very sensitive to vibration, allowing the snake to detect approaching animals by sensing faint vibrations in the ground. Despite the lack of outer ears, they are also able to detect airborne