Cancrichthys

Ryl Madol teems with all types of amphibious life in its many waterways. This includes actual amphibians, armoured proto-reptiles, various arthropods and even some derived sarcopterygian fish. But out of the swamps can sometimes also crawl a quite peculiar creature, which seems like a wild mix of all of the above. Cancrichthys zemani, whose name literally means “crabfish” is a placoderm, a member of an ancient group of armour-encased fish which went extinct elsewhere on Earth during the end of the Devonian period. More precisely, it seems to be an antiarch, unmistakenly related in some form to ancient fossil genera like Pterichthyodes or Bothriolepis.

What earns it its name are its unique limbs. Although, when viewed from the side, it appears as if it has six crab-like legs jutting out from the body, these are actually all attached to a main “fin”, a stylopodium. It therefore technically only has two legs, but this does not diminish the fact that no other type of vertebrate has such limbs, as it would correspond to a single upper arm/humerus out of which multiple lower arms sprout.

When on land, Cancrichthys breathes using a simple lung, which gives credit to the controversial idea that fish like Bothriolepis already evolved this trait independently of the ancestors of tetrapods. It seems therefore possible that these ancient placoderms were already somewhat amphibious creatures sifting through mud in lakes which regularly turned oxygen-poor. Evading extinction on what would become Ryl Madol, time could have put more selection pressure on the limbs to become more capable of terrestrial locomotion, in order to travel between ponds and lakes more easily. As antiarchs like this had no autopodium (what becomes the hand in tetrapods), evolution had to make-do with what was available. Even if unique, the mutation that leads to Cancrichthys’ odd limbs may be similar to the hox-genes that split the tetrapod autopodium into fingers, but instead applied one level higher to the zeugopodium. As Placodermi are a paraphyletic grade from which all later bony fish, including us, likely descended, it does not seem implausible that the genetic tool-set for this could have already been present or re-evolved in these fish.

Cancrichthys leads an unassuming life, most of it spent plying mud for invertebrate prey, such as worms, snails or freshwater trilobites. On land, its carapace gives it perfect protection against most predators. If that is not enough, the fish has sometimes been seen striking at threats with its tail. Like many of the fossil placoderms, Cancrichthys reproduces through internal fertilization and gives birth to live young. This is in a somewhat humorous contrast to many of the more terrestrial, reptile-like creatures of the island that have to lay their eggs in water.