Snake
Reference: Snake
A tetrapod that gave up all four limbs — and still carries the hardware. Pythons and boas have visible pelvic spurs on the outside of the body.
Scored on every axis
Limbs lost, evidence retained
Snakes descend from four-limbed lizards, and the loss is visible in the living animals: pythons and boas retain vestigial pelvic girdles and small external spurs beside the vent, used in courtship. Fossil snakes with hind legs are known.
Internally the same reduction runs through: most snakes have one functional lung, the left being reduced or absent, because a long thin body has no room for a pair.
Smelling in stereo, seeing in heat
The forked tongue collects molecules and delivers them to a paired sensory organ in the roof of the mouth — two samples, two points, so the animal reads a chemical gradient directionally, the way we localise a sound.
Pit vipers, pythons and boas add facial pit organs that image infrared. A pit viper hunting at night has a thermal channel running alongside vision, which is a sensory dimension humans have no equivalent of.