Tuesday, January 7, 2020
150 Million Years of Snake Evolution
Considering how diverse they are today--nearly 500 genera comprising almost 3,000 named species--we still know surprisingly little about the ultimate origin of snakes. Clearly, these cold-blooded, slithering, legless creatures evolved from four-legged reptilian ancestors, either small, burrowing, landbound lizards (the prevailing theory) or, just possibly, the family ofÃ marine reptiles called mosasaurs that appeared in the earths seas around 100 million years ago. Piecing Together the Evolution of Snakes Why is snake evolution such an enduring mystery? A big part of the problem is that the vast majority of snakes are small, relatively fragile creatures, and their even smaller, even more fragile ancestors are represented in the fossil record by incomplete remains, mostly consisting of scattered vertebrae. Paleontologists have discovered putative snake fossils dating as far back as 150 million years, to the late Jurassic period, but the traces are so evanescent as to be practically useless. (Further complicating matters, snake-like amphibians called aistopods appear in the fossil record over 300 million years ago, the most notable genus being Ophiderpeton; these were completely unrelated to modern snakes.) Recently, though, solid fossil evidence has emerged for Eophis, a 10-inch-long middle Jurassic snake native to England. The Early Snakes of the Cretaceous Period Needless to say, the key event in snake evolution was the gradual withering away of these reptiles front and hind limbs. Creationists like to claim that there are no such transitional forms in the fossil record, but in the case of prehistoric snakes theyre dead wrong: paleontologists have identified no less than four separate genera, dating back to the Cretaceous period, that was equipped with stubby, vestigial hind legs. Oddly enough, three of these snakes--Eupodophis, Haasiophis, and Pachyrhachis--were discovered in the Middle East, not otherwise a hotbed of fossil activity, while a fourth, Najash, lived on the other side of the world, in South America. What do these two-legged ancestors reveal about snake evolution? Well, that answer is complicated by the fact that the Middle Eastern genera were discovered first--and, since they were found in geologic strata that were submerged in water a hundred million years ago, paleontologists took that as evidenceÃ that snakes as a whole evolved from water-dwelling reptiles, most likely the sleek, fierce mosasaurs of the late Cretaceous period. Unfortunately, the South American Najash throws a monkey wrench into that theory: this two-legged snake was clearly terrestrial, and appears in the fossil record at roughly the same time as its Middle Eastern cousins. Today, the prevailing view is that snakes evolved from an as-yet-unidentified land-dwelling (and probably burrowing) lizard of the early Cretaceous period, most likely a type of lizard known as a varanid. Today, varanids are represented by monitor lizards (genus Varanus), the largest living lizards on earth. Oddly enough, then, prehistoric snakes may have been kissing cousins of the giant prehistoric monitor lizard Megalania, which measured about 25 feet from head to tail and weighed over two tons! The Giant Prehistoric Snakes of the Cenozoic Era Speaking of giant monitor lizards, some prehistoric snakes also attained gigantic sizes, though once again the fossil evidence can be frustratingly inconclusive. Until recently, the biggest prehistoric snake in the fossil record was the appropriately named Gigantophis, a late Eocene monster that measured about 33 feet from head to tail and weighed as much as half a ton. Technically, Gigantophis is classified as a madtsoiid snake, meaning it was closely related to the widespread genus Madtsoia. Unfortunately for Gigantophis fans, this prehistoric snake has been eclipsed in the record books by an even bigger genus with an even cooler name: the South American Titanoboa, which measured over 50 feet long and conceivably weighed as much as a ton. Oddly enough, Titanoboa dates from the middle Paleocene epoch, about five million years after the dinosaurs went extinct but millions of years before mammals evolved into giant sizes. The only logical conclusion is that this prehistoric snake preyed on equally huge prehistoric crocodiles, a scenario you can expect to see computer-simulated in some future TV special; it may also have occasionally crossed paths with the equally giant prehistoric turtle Carbonemys.