LIFE × EVOLUTION

How Snakes Lost Their Legs

~128 to 95 million years ago

Phanerozoic · Mesozoic · Cretaceous

Snakes did not shed their limbs in one move. A whole early branch kept a pelvis and working hind legs while the body was already long and the skull was already coming apart, bone by bone, into the loose jaw that lets a snake swallow prey wider than its own head. Where they started is the open question: limbed snakes from the Cretaceous seas of the Middle East argued for a swimming ancestor, and a Patagonian snake with a sacrum and functional hind legs argues for a burrowing one on land.

Why it matters

Vertebrates have lost their limbs dozens of times over, and it almost always leads nowhere: a handful of species, in one habitat, and no further. Snakes are the exception. The same loss, combined with a skull that comes apart, produced four thousand species and predators that swallow animals larger than themselves.

Dating & uncertaintyDisputed

Where snakes came from is the argument. Limbed snakes from the marine Cretaceous of the Middle East supported a sea-going ancestor; a Patagonian snake with a sacrum and working hind legs, and later three-dimensional skulls of the same animal, support a burrowing land ancestor instead. The land reading currently has the better fossils, and an early radiation of snakes that kept their hind limbs for a long time.

Sources

1. Apesteguía & Zaher (2006), Nature · A Cretaceous terrestrial snake with robust hindlimbs and a sacrum
2. Garberoglio et al. (2019), Science Advances · New skulls and skeletons of the Cretaceous legged snake Najash, and the evolution of the modern snake body plan
3. Caldwell & Lee (1997), Nature · A snake with legs from the marine Cretaceous of the Middle East

Earth history as one day

23:19:2440 min 36 s before midnight

if Earth’s 4.54 billion years were compressed into a single day