You could call me an old snake man. I don’t mean I’m old (I’m only 30!), nor have I been studying snakes for a long time (18 years), but I work on old snakes – fossils, millions of years old.
I began research on fossil snakes at the University of New South Wales after a long interest in the living snakes of Australia. Our continent is blessed with well over a hundred species, some of which are common in the suburbs where I grew up and places I went on holidays. After getting to know these snakes, I wanted to understand their similarities and differences; that means their evolution.
Let me explain. Imagine being part of a large family and noticing similarities and differences among your relatives in hair and eye colour, nose shape, the sound of their laugh, or whatever. Often you can trace these traits to one grandparent or another, or even further back if you have old photographs or surviving great-grandparents. With snakes, I was looking at a large number of ‘children’, but hardly any of their ancestors. I knew that sooner or later I would have to go looking for the fossil bones of their forebears.
The last common ancestor of two related species of snake would have died at least tens of thousands (or more likely millions) of years ago. The five living families in Australia, each including many species, have been separate for at least forty million years. We can try to reconstruct relationships among living animals, but only fossils can tell us where and when their ancestors lived.
Several years ago, we found the oldest Australian snakes yet known; they lived over fifty million years ago, near Murgon in southern Queensland. The oldest known snake fossils in the world are from north Africa and they lived about a hundred million years ago. Snakes evolved from lizards some time earlier, about the same time that birds evolved from meat-eating dinosaurs. Goannas are the nearest living relatives of the snakes and both are more distantly related to other lizards. One of the things that snakes and goannas have in common, that no other lizards have, is a long, narrow, forked tongue which they flick in and out to pick up traces of scent.
Most kinds of lizards can swim, burrow in soft soil, or slither through thick vegetation, and many of them fold their legs to their sides when they do; even fat blue-tongues tuck their feet away when they push through long grass. Over thousands of generations lizards that spend nearly all their time doing these things will eventually loose their legs. Legs have been lost in this way many times in different families of lizards; snakes are one of these groups. There are nearly three thousand species of snakes living now; they are very successful animals.
I have recently returned from Riversleigh in the far north-west of Queensland where a team of volunteers, staff and students from the University of New South Wales make an annual trip to survey and quarry for fossils. Parts of the limestone hills near the Gregory River at Riversleigh are so full of fossils that they turn up when you least expect them, as well as when you do. Some of the richest sites have been discovered when somebody stooped to tie a shoelace, got lost, walked the other side of a tree or simply sat down for a rest in the shade. But there can also be days of searching without finding any new fossils. Almost always, ‘sites’ occur as patches of fossil-bearing rock a few metres across, surrounded by bone-free but otherwise very similar rocks.
When these rocks are dissolved in dilute acetic acid the bones of hundreds of kinds of animals are revealed. Such high numbers of animals indicate an environment similar to rainforest, but today Riversleigh is a dry place; if there was a rainforest there, it must have covered much of the continent instead of the few patches we see now. The Riversleigh fossils are from a time about 25 million years ago through to about 12 million years ago. There are also a few younger deposits in the area from about 4 million years ago and 20-30 thousand years ago. Nearly half of the sites have at least some snake bones, and some have hundreds. Snakes do have a lot of bones!
One morning as we searched a hillside for new sites, Bronwyn, one of the student volunteers, stepped up on a rock, then turned and calmly asked “John, what’s a snake with a black head and yellow bands on its neck?”. I thought at first she was describing a black-headed python, but quickly changed my mind when I caught sight of it. Behind the small dark head and bright-banded neck was a slender purplish-grey body, thin as a pencil and half a metre long; it could only be one species, a kind of whipsnake previously found only around Mt Isa, nearly three hundred kilometres to the south. These snakes roam rocky hills with their heads raised off the ground, their large red eyes scanning for movement, and chase the slippery skink lizards which run too fast for most predators to catch. This species (Demansia flagellatio) is the slenderest and ‘whippiest’ of the group, and one of Australia’s rarest snakes; it still isn’t in the field guides, and the Queensland Museum doesn’t have a single specimen.
Whipsnakes are members of the family Elapidae, the group of venomous snakes including cobras, mambas, coral snakes and sea snakes as well as our dangerous brown snakes, taipans, death adders and many smaller, relatively harmless species like the whips. In the fossil deposits at Riversleigh, we have found the bones of several kinds of elapid snakes: vertebrae, ribs, and skull bones including an upper jaw with the fang. Some of these may be the oldest known fossils of this family; elapids from France and the United States are about the same age. The Riversleigh elapids were mostly small, about the same size as the whipsnakes which live there now, and probably, like them, fed on small lizards; similarly the skinks and geckoes there today are related to those we find as fossils, but adapted to the harsher climate.
Riversleigh has changed a lot in 25 million years. The climate has altered, the forests have fragmented and retreated to the water’s edge but the ancient families of lizards and the snakes have adapted to these changes. The pursuit of life and death among the limestone boulders continues today in bright, hot sunlight and the spiky domes of spinifex, where once the same drama was played in the mossy gloom among the buttress roots of rainforest trees.
For more information about the excavations at Riversleigh contact the Riversleigh Society, P.O. Box 281, Gordon, N.S.W. – Ed.
University of New South Wales