Before there were venomous snakes, there were venomous snakes. Sort of. Caecilians are secretive burrowing amphibians that somewhat resemble snakes, but they couldn’t be more different from the reptiles they look like.
These creatures are more closely related to frogs and toads and salamanders. The only thing they really have in common with snakes is a venomous bite. Scientists who studied them up close now believe that bite is an evolutionary stage frozen in time from about 250 million years ago — before venomous snakes ever sank their fangs into anything.
"These oral glands develop from the dental tissue, and this is the same developmental origin we find in the venom glands of reptiles," said Pedro Luiz Mailho-Fontana, whose colleague Edmund Brodie added, "If we can verify the secretions are toxic, these glands could indicate an early evolutionary design of oral venom organs, they may have evolved in caecilians earlier than in snakes.”
Mailho-Fontana and Brodie, who recently led a study recently published in Cell, literally had to dig deep to find specimens. Most caecilians burrow deep in the ground to avoid predators (and plug the hole of the burrow with poisonous secretions from their tail just in case). Poisonous skin secretions are definitely an amphibian thing. This is probably why things like eye of newt and wart of toad ended up as ingredients in mythical potions. Caecilians also secrete a poisonous mucousy substance from their skin, but this and the tail secretions are both separate from what has been found in their mouths.
While these amphibians have no fangs to flash, and no grooves or slits on their teeth through which to pump venom as snakes do, they still have poisonous glands at the base of their sharp teeth. However there is something unexpected that they have in common with snakes (besides the poisonous bite itself). Further analysis revealed that their poison glands spawn from the same tissue from which teeth originate. This is something common in venomous snakes, but was never thought to occur in any amphibian.
“Caecilians might have independently developed mechanisms of production and injection of toxins early in their evolutionary history,” Mailho-Fontana and Brodie said in the study.
This could be a case of convergent evolution, meaning that while snakes and caecilians evolved on their own, they still appear related. Caecilians are obviously not reptiles and also bite differently from snakes. Most snakes inject venom by biting and allowing it to enter their prey’s bloodstream through the grooves in their fangs that caecilians lack. Only rattlesnakes and a few other species have hollow fangs which it flows through. By studying a caecilian up close, the scientists found that the potentially lethal secretions are already there when the animal bites into its prey. It is a groove in its upper jaw, not its teeth, that seems to help distribute the poison.
When the team examined the chemical composition of this slimy substance, they discovered a cell type which was chemically similar to the venom glands of the poisonous Texas alligator lizard. There is also a possibility that it contains additional substances found in the skin secretions of other amphibians, though that has not yet been proven. While its chemical composition needs further study, one more thing makes it almost certain that caecilians were the first venomous “snakes.”
“Some lineages of snakes and lizards, however, have functional glands in both jaws,” Mailho-Fontana and Brodie said. "Thus, the presence of dental glands both in upper and lower jaws as seen in caecilians [is ancestral] in relation to the reptile venomous system,” adding that it is likely “caecilians developed the ability to actively inoculate toxins through their teeth early in their evolutionary history, probably representing one of the first terrestrial vertebrates having an oral venom system.”
You probably have nothing to worry about, since caecilians hide so deep underground they are nearly impossible to find. It is still unknown whether their secretions are poisonous to humans—let a microscope figure that one out.