LIFE × EVOLUTION

How Snakes Lost Their Legs

~128 to 95 million years ago

Phanerozoic · Mesozoic · Cretaceous

Snakes lost their legs in several steps, and between 128 and 95 million years ago an early branch of snakes still had a pelvis and working hind legs while its body was already long and its skull was already losing bones.

Snakes are long-bodied reptiles that lost their legs in several steps. An entire early branch of snakes retained a pelvis and functional hind legs. In that branch the body was already elongated and the skull was already losing bones one by one. This loss of skull bones ends in the mobile jaw suspension, the loose attachment of the jaws to the skull that allows a snake to swallow prey wider than its own head. The kind of animal from which snakes arose remains debated. For Caldwell and Lee (1997), snakes with legs from seas of the Cretaceous period in the Middle East supported a swimming ancestor. Apesteguía and Zaher (2006) described a Patagonian snake with functional hind legs and a sacrum, the part of the spine to which the pelvis attaches. This snake supports a terrestrial ancestor, burrowing or ground-dwelling.

Why it matters

Vertebrates, the animals with a backbone, have lost their limbs on dozens of occasions. Almost every time, the limbless lineage amounts to a handful of species confined to a single habitat. Snakes are the exception: they number four thousand species and are predators that swallow animals larger than themselves. Only snakes, among limbless lineages, have a skull whose bones come apart.

Dating & uncertaintyDisputed

The origin of snakes remains debated. The snakes with legs found in marine deposits of the Cretaceous in the Middle East supported the hypothesis of a marine ancestor. The Patagonian fossils have a sacrum and functional hind legs. More recently, Garberoglio and colleagues (2019) described Patagonian skulls preserved in three dimensions. These fossils are currently the stronger evidence, and they place the origin of snakes on land, in a burrowing or ground-dwelling animal. If this hypothesis is right, snakes with hind legs were an early and lasting radiation, that is, a diversification into several lineages.

Earth history as one day

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