Aistoconstrictor

Click to enlarge.

There are no snakes on Ryl Madol. In their stead slither other, more ancient creatures through the underbrush. Most numerous among these legless vertebrates are aistopods, an ancient lineage of stegocephalians. These were the very first tetrapods to completely lose all of their limbs and in these primeval jungles they have further converged on the serpents that replaced them elsewhere in the world.

Largest among them is Aistoconstrictor latagnathus, which, it has been reported, can grow up to nine or ten metres long, larger than any anaconda. Most individuals are smaller though, usually maxing out at around six or seven metres. Despite technically being an amphibian (in the classic paraphyletic sense), Aistoconstrictor shares many characteristics with actual constricting snakes, such as boas. It kills and captures small prey, such as lystrodos, by biting their head and then ensnaring them with its body, crushing the poor victim under its weight.

Click to enlarge.

Looking at the skull we can see a combination of both old and new. Overall, the cranium still bears great resemblance to ancient aistopods such as Phlegethontia, with large orbits, large fenestrae and a light construction at the back of the skull that gives the jaw-joints a larger range of movement. Differing from ancient aistopods, the “aistoboids” evolved an extra jaw-joint in their lower jaw, allowing the dentary to articulate with the surangular bone. Like in constricting snakes this allows the predator to “shove” prey down its gullet with its teeth by moving the lower jaw back and forth. Unlike in snakes, the mandibles are still connected at the tips, so they cannot open as widely.

Click to enlarge. Life stages are not drawn to scale.

Being a stegocephalian, likely of the reptiliomorph variety, Aistoconstrictor develops from an aquatic larval stage. It does not raise its young on land in burrows like a giant caecilian, as older textbooks have often wrongly stated. After internal fertilization, the eggs are instead laid into a breeding pond, which the mother often guards until hatching, as some toads are known to do. Upon hatching, the larvae emerge as little eel-like creatures with surprisingly long external gills. At this stage they bear a great resemblance to the larvae of caecilians, though this is surely a coincidence. These aistoboid larvae feed on aquatic insects and algal scum before they grow in size and enter the next life stage, the “spade eel”, named after the shape of its snout. Its gills have shrunk and become covered by a soft skin-flap, while its swim-bladder has expanded into a simple lung, allowing it to breathe both in and out of water. The spade eel lives much like a predatory fish, feeding on many smaller vertebrates by use of ambush attacks. They are also surprisingly gregarious, often swimming in small swarms for protection. Unlike the sub-adult stages of some other Rylian reptiliomorphs, spade eels cannot become reproductively active and are always destined to grow into fully adult “aistoboids” once they lose their gills and live on land.

As a human can in some ways resemble other bipedal animals on Ryl Madol like the lystrodos or eubolosaurs, it may not come as a surprise that stalkers and other explorers are frequently attacked by Aistoconstrictor and relatives, whose coloration conceals it behind the underbrush or lianas. While the creatures are capable of killing people through constriction, human shoulders are usually too wide to fit through the jaws, so the beasts tend to give up after the head and just leave behind a mangled corpse for the scavengers.