Tuesday, 19 July 2016

How did legless worm-lizards cross the Atlantic? – via Herp Digest

How did a tiny, reclusive, underground animal colonize so much of the world? The answer to this and other unexpected animal migrations could be “rafting”
by Elsa Panciroli, The Guardian, 7/13/16

Biogeography is the study of the geographic distribution of living things. As you can imagine, when you stick “palaeo” in front of it and apply it to fossils, not only will you win at scrabble, but produce a science often confounded by a patchy fossil record, as well as the changing configuration of the continents, natural disasters, and many other biological and geographical phenomena. So how do palaeobiogeographers explain how an animal that is tiny, burrows underground, and has no legs, lives on multiple continents separated by the Atlantic Ocean?

When two or more populations of a terrestrial animal group are found in vastly separated parts of the world, two explanations are most often proposed. Firstly, that they spread by land from one place to the other, then the populations in-between went extinct, leaving two geographically distant populations. This is called dispersal. Another explanation is plate tectonics: as the continents split they carried animals with them, physically separating populations. This is an example of vicariance. Dispersal and vicariance are the common and over-arching explanations that account for the patterns of life on earth that take place over vast periods of time.

Dispersal and vicariance are sometimes seen to be at odds. Scientists can tend to favour one explanation over the other as being the most important and frequent cause of animal distribution patterns. For palaeontologists, there could be a tendency to view life in terms of vicariance, particularly driven by the ancient distribution of continents and plate-tectonic shifts. It makes sense to favour this when you spend your career thinking in millions of years.

However, there are instances of animal occurrence that continental drift can’t explain. If you look at terrestrial life on earth and assume it could only disperse by walking from one place to the next when the continents were connected, you get some pretty silly estimates for splits between major animal groups. To take advantage of land bridges to explain their dispersal, the first land animals would have to have evolved millions of years before the Cambrian. As we know from fossils, the first complex life on earth was only just evolving in our seas at that time. Cambrian animals didn’t even have backbones, let alone a leg to stand on.

What if land animals frequently “rafted” across seas and oceans to their new destinations?

Sound far-fetched? Well this proposal is broadly accepted as the mechanism for several animal dispersals. For example the distribution of mammals across the islands of the Caribbean from mainland South America, such as monkeys and rodents. This series of small jumps is also called island-hopping, and occurs when animals find themselves on rafts of floating tangled vegetation, swept out to sea. Flyers fly, swimmers swim, and it appears that land-lubbers that cannot swim across oceans, raft.

It’s all very well to imagine this may happen between island chains; it is amazing but not impossible. Could the same thing happen across an entire ocean? What about between Africa and the Americas? That’s exactly what the worm-lizards have done. Not just once, but twice.

The worm-lizards, Amphisbaenia, are mostly legless squamates related to other lizards and snakes. Amphisbaenians lost their legs independently of the snake lineage and belong to their own distinct part of the squamate tree. Most species of worm-lizard still retain remnants of their pelvis and pectoral girdle (hips and shoulders), but only four species from Mexico out of the 180 currently known around the world still have functioning forelimbs. The group as a whole gets its name from Amphisbaena, the Greek serpent that had a head at each end. This is because the first specimens to become known scientifically had a very similar tail and head, ending bluntly and somewhat worm-like. They are vertebrates, and so have skulls with varying numbers of teeth (they are carnivorous and insectivorous) in a jawed mouth. They also have deeply recessed eyes that can discern light from dark. They may look like earthworms, but inside they are still every bit reptilian, with two lungs, one of which is squeezed and elongated while the other is greatly reduced. 

If you want to go looking for these strange legless lizards, you’ll have your work cut out. Although they are found all across South America, most of Africa, parts of the Middle East, North America and Europe, they live underground, rarely surfacing from their burrows. Most are less than 15cm long. This begs the question: how does such a reclusive underground animal colonise so much of the world?

When both the DNA and fossil evidence are studied together, it turns out that the ancestral subterranean legless lizards first began splitting into different families around 109 million years ago in North America, where they are likely to have originated. There was another major diversification between North American and European amphisbaenians 40-56 million years ago. Finally the African and South American forms split around 40 million years ago. These dates matter because the Atlantic Ocean had fully formed by the time all of these splits took place. To get from North America to Europe, then Africa to South America, these animals must have crossed the ocean.

Worm-lizards therefore must have “rafted” across the Atlantic. Natural erosion and storm events can loosen large sections of soil, often bound together with tree roots, and wash it out to sea. Some unsuspecting worm-lizards were set adrift this way, and the natural direction of ocean currents and prevailing winds carried their life rafts to shores new. Being able to survive without food for weeks, some of these sailors became colonists. 

Worm-lizards are not the only group that have dispersed this way. But you have to wonder: what are the chances? How could enough of any one species make it to new shores to breed and survive in the long term? It just seems so improbable. For an animal like us, who lives to around 100 years of age if they are lucky, we struggle to grasp probability over longer time spans.

You often hear the phrase “a once in a lifetime event” being used to describe huge natural disasters, such as the tragic Boxing Day tsunami in 2004. After this event, natural and man-made debris washed across the Pacific Ocean, reaching American coastlines. For arguments sake, let’s assume something on that scale happens once in a hundred years. Multiply that by the span of all modern humans, around 140,000 years. That means humans have experienced this kind of natural disaster around one and a half thousand times. Multiply this by geological time, and you have millions of disasters befalling populations of humans and other animals.

Japanese tsunami sweeps dock to Oregon beach
A cheery thought, no? The point is that as unlikely as it seems for a legless lizard to be washed unharmed to sea, float across the Atlantic for weeks, and make it to the next continent only to find another legless lizard waiting for them to mate (either from a previous journey, or from the same raft of earth as them), the chances are actually pretty good given enough time. Of course, nature doesn’t evenly space these events out, so two or three “once in a lifetime” events may happen within the life-span of a single legless lizard.

So when it comes to the question of dispersal versus vicariance, it would seem that the answer is a complex interplay of both. No one explanation is “better” than the other, we must instead look at the fossil and genetic evidence together, and work out for each group of animals what the most likely explanation is for their distribution across our little globe. In doing so, let’s not underestimate the role of vast amounts of time on the likelihood of seemingly improbably events. Call it serendipity if you will. If the chances are a million to one, on a geological timescale I’ll take those odds.


Longrich, N. R., Vinter, J., Pyron, A., Pisani, D. and Gauthier, J. A. (2015) Biogeography of worm lizards (Amphisbaenia) driven by end-Cretaceous mass extinction. Proceedings of the Royal Society B, 282, http://dx.doi.org/10.1098/rspb.2014.3034

Naish, D. (2014) Worm Lizards: Lifestyles of the Limbless. Tetrapod zoology blog for Scientific American http://blogs.scientificamerican.com/tetrapod-zoology/worm-lizards-lifestyles-of-the-limbless/

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