The combination of fangs and venom makes for fearsome predators. However, the widespread presence of fangs in snakes poses an evolutionary puzzle – snakes have evolved fangs several times, including with major differences such as front and rear placement in the jaw. A new study has explained why snakes have been so suited to evolving effective venom-delivery systems, while other creatures struggle on without.
Researchers studied the teeth of many snake species, both living and extinct, to seek a feature known as plicidentine. In Proceedings of the Royal Society B, they report plicidentine's presence in every snake they studied, other than Anilios (Ramphotyphlops) bicolor, a burrowing blind snake, and argue this is the key.
"It's always been a mystery why fangs have evolved so many times in snakes, but rarely in other reptiles. Our study answers this, showing how easy it is for normal snake teeth to turn into hypodermic needles," lead author Dr Alessandro Palci, from Flinders University, said in a statement.
Plicidentine are wrinkles near the tooth's base. They apparently provide a start for venom delivery systems, which natural selection improves when this is useful to the snake – ie any time they use venom to subdue prey or ward off predators.
In some venomous snakes, these plicidentine folds extend into grooves running the full length of the fang, along which the venom flows. Others have gone even further, closing over the grooves to make hollowed fangs and venom sacks that in combination act like hypodermic needles.
In recent years, many lizards have also been found to produce venom, some with medical potential. However, not only are these venoms relatively weak compared to those of most snakes, few lizards have the mouthparts to deliver them efficiently, making for a much-reduced threat.
Dr Palci told IFLScience that, lacking any seed for evolution to work on, the lizards have had to rely on including venom in their saliva and hoping some gets to their prey. This may be why they never developed really potent venoms.
The most obvious question arising from this is why plicidentine evolved in the first place. The authors conclude they help join tooth to the jawbone. “We propose that snakes and ‘varanoid’ lizards share teeth that are, relative to other squamates, relatively tall, slender and with little bony support at their bases; the increased area of attachment provided by plicidentine might be the evolutionary solution for this potential weakness,” the paper notes.
It was pure coincidence this feature ended up having another use. It seems the development of plicidentine goes back to close to the beginnings of snakedom, since they are seen in Yurlunggur, a giant extinct Australian snake genus from up to 20 million years ago.
Although the blind snakes may have lost this feature, Palci told IFLScience the apparent absence; “Could also be because their teeth are very small. In pythons we can only see plicidentine in large teeth.”
Snakes were not the only animals to develop plicidentine – it was also found in some fishes and early amphibians, among others. The paper also notes the possible presence of mosasaurs, as if these marine monsters were not terrifying enough without the possibility of venom. However, Palci told IFLScience the mosasaur plicidentine were so fine it's debated if they existed at all.