How the Snake Lost Its Legs

By Carl Zimmer|Tuesday, July 01, 1997
All vertebrates that live on land--from humans to alligators to birds--are collectively known as tetrapods, meaning four feet. The name sticks even though the legs to which two of these feet are attached have become arms in humans and wings in birds. In snakes, the change was even more radical: they lost all four of their limbs. One of the few remaining signs of their limbed heritage is the presence of vestigial hips imprisoned in the rib cage.

How did snakes come to be? The distinctiveness of the animals obscures their ancestry. Their scales, eggs, and subtle features of their skulls show them to be descended from lizards, but it’s been difficult to link them to any specific group. Unable to pin them down taxonomically, paleontologists have been able to construct only the flimsiest of scenarios for how snakes lost their limbs. But this confusion may now dissolve, thanks to a 100-million-year-old fossil of a snake with legs.

The three-foot-long creature, Pachyrachis problematicus (meaning problematic thick-ribbed animal), was discovered in the late 1970s by quarry workers 12 miles north of Jerusalem. After a preliminary study, Hebrew University herpetologist George Haas suggested that while the fossil looked serpentine, there was no evidence that it was closely related to snakes. There matters rested until 1996, when Michael Lee of the University of Sydney in Australia and Michael Caldwell of the Field Museum in Chicago came to Jerusalem to study the fossil more thoroughly. They exposed more of the fossil from its limestone slab and carefully compared its skeleton with those of snakes and lizards. The first thing you’ve got to do is look at every possible animal it could be related to, and Haas didn’t have access to lots of primitive snake material, says Lee.

He and Caldwell conclude that Pachyrachis possesses many characteristics unique to snakes. Its body, for example, is long and sinuous: it has 140 vertebrae in its trunk; most lizards have just 25. And while lizards have open braincases, Pachyrachis, like snakes, has a completely sealed one. Its jaws are extraordinarily flexible: the lower jaw doesn’t fuse at the chin, so the two halves can bend out to the sides to swallow big prey. Many hinges lie along the length of both the upper and lower jaws to expand the gape even more. They’re snakes, no doubt about it, says Lee.

Yet Pachyrachis still held on to some primitive bits of anatomy, and in them Lee and Caldwell glimpse the genealogy of snakes. Most obvious, of course, are the legs. The fossil of Pachyrachis bears two hind legs, each about an inch long, that lack only feet. It’s possible that Pachyrachis’s feet were washed away after it died, but Lee suspects they would have been vestigial at best, perhaps with a few toes. Less obvious but just as significant are its hips, which were outside its rib cage rather than within, and a number of diagnostic details of its spine and skull.

Taken together, Pachyrachis’s traits point to a surprising animal as the closest relative of snakes: the extinct gigantic marine lizard known as a mosasaur. The two lineages probably diverged at least 110 million years ago. Mosasaurs began as four-legged lizards on land, looking much like the closely related Komodo dragons. Their bodies then elongated and their limbs dwindled as they took to the water. By 100 million years ago mosasaurs were up to 45 feet long and swam by beating their powerful tails and paddlelike legs. Along with the dinosaurs on land, they vanished 65 million years ago, perhaps as a result of the impact of an asteroid or comet.

Lee and Caldwell believe that this kinship with mosasaurs argues against the most popular scenario of snake evolution. Although snakes live today in habitats ranging from trees to deserts to rivers, the most primitive snakes all burrow underground. That fact led researchers to propose that snakes descended from a terrestrial lizard--maybe one resembling a skink--that adapted itself for subterranean life. Many characteristics of snakes today could have evolved as they became suited to a burrowing lifestyle. Elongating their bodies would have given them more muscular force to drive their heads through soil and slip through crevices. Nearly all reptiles except snakes have an eardrum. It would have made sense for an animal worming through the ground to seal over its ears, since it would hear few airborne sounds.

Lee and Caldwell, however, doubt that snakes evolved from burrowing lizards. I wasn’t particularly happy with that, because it’s not sufficient to just look at the most primitive living snakes, says Lee. You have to look at the closest relatives of the snakes as well. He and Caldwell now wonder if it’s time to revive an old, dismissed theory: namely, that snakes actually got their start in the water.

Mosasaurs, their closest relatives, lived in the ocean, and Pachyrachis, the most primitive snake, lived there as well. Its fossil shows that it was well adapted to a marine life--its tail, for example, was flattened like an eel’s to help propel it forward through the water. And to counteract the buoyancy of its air-filled lungs, Pachyrachis had heavy ribs. If you’ve ever gone scuba diving, you know you have to carry several kilos of weights, says Lee. Many of the adaptations of living snakes would make just as much sense for swimming as for burrowing. Eardrums (which Pachyrachis lacks) would have been just as useless underwater as underground, and an elongated, limbless body would have been useful for swimming. You’ve got features of snakes that are conventionally interpreted as burrowing adaptations, but if you stop and think about it, they could be aquatic as well, says Lee.

It’s a compelling yet strange scenario Lee and Caldwell are now contemplating: the animals that produced today’s snakes leaped into the water, lost their legs, and then slithered out. After snakes had become adapted to swimming in the ocean, they discovered that they had a body well suited for life on land, as burrowers and in other habitats. To make the tale even more dizzying, some snakes then went back into the water, becoming today’s sea snakes. Before this description of Pachyrachis, nobody had seriously proposed an aquatic origin of snakes for a century, says Lee. It’s a big change.
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