Sealacanth

As the sun sets and the waters calm down, a strange creature hauls itself onto a beach. Its limbs end in fins, yet they already have joints which foreshadow elbows and knees. Its two dorsal fins now hang to the sides, useless outside the sea. It breathes heavily, almost strained, but it is doing better than the other water-dwellers that have been swept here, dead ammonites, clams and fish, which have been crushed by the gravity and heat. It succeeds where others fail. Beyond the beach lies a lush rainforest, filled with big invertebrates. To the victor will go the spoils.

Only the presence of scaly, bipedal creatures on the beach and the distant cries of seabirds betray that this is not an ancient scene from the middle of the Devonian period. This is the very modern life of a particular fish on the island of Ryl Madol. The sealacanth (Erpetolepis littoralis) looks like an ancient holdover but is in many ways also something quite new, perhaps among the most advanced of the walking fish that live on Earth.

Despite the name and appearance, Erpetolepis is not a close relative of the coelacanths which also swim around in the Indian Ocean. The presence of internal nostrils and labyrinthodont teeth identify it not as an actinistian but as a tetrapodomorph, though closer to basal forms such as Eusthenopteron than to more famous fossils like Tiktaalik or the true tetrapods. Its many adaptations towards littoral life seem to be parallel developments. Unlike archaic lobe-finned fish, the sealacanth has clearly defined limb-joints, though not yet true fingers, instead just a flexible fan of lepidotrichia like other fish. It is a very subtle difference, but the shoulder-girdle is also not fully connected anymore to the skull, allowing it to move its head independently of the body, an important development which in true tetrapods led to the evolution of a true neck. It also sports fully developed lungs in addition to gills.

Actual seals are largely absent from the coastlines of Ryl Madol, on account of the presence of the toxic spores. While there is no shortage of aquatic and semiaquatic amphibians and reptiles on these beaches, very few of them have seemingly managed to utilized the resulting opportunity quite as successfully as these clumsy-looking fish, earning them their name. Like pinnipeds, sealacanths hunt in the shallow and open waters for food but when they want to sleep and rest, they come onto land and sometimes even sleep in groups. Though gregarious, they are not exactly social, as the lack of ears limits their ability to communicate. Towards human explorers they are largely apathetic, though some degree of curiosity is reported on occasion.

The prey of sealacanths consists of slow-swimming or crawling invertebrates and small vertebrates, largely fish, lizard-sized marine reptiles and washed-up carcasses. Sometimes sealacanths can be observed digging up the rocky nests of Spheniscodapedon, the penguin-like reptile seen here in the background, in order to feed on their eggs. This does not always go well, as these “rhynchoguins” tend to guard their nests and have pretty mean hooked beaks, more than capable of ripping chunks of flesh out of even a large fish such as this. Though it is larger and stronger, if a flock of these reptiles gangs up on a sealacanth, they can very easily rip it to shreds. The further the fish crawls into the island, the more dangerous the threats become and so too the farther out into the sea it swims. Perhaps being stuck, ecologically and evolutionarily, on the water’s edge is good enough, at least for now.