First appearing during the Late Ordovician, thelodonts were once one of the most successful groups of jawless fish, their distinctive scales being one of the most common index fossils for the Silurian. But the fate of any group that is successful is that, at some point, they simple are not anymore. Thelodonts vanish from the fossil record by the Late Devonian, along with most other jawless fish.
Such is not the case on the lost world of Ryl Madol though, where small thelodonts still swarm the waters of the coasts and rivers. Their success story seems to have continued here, where, despite still being jawless, they have managed to expand into all kinds of ecological niches.
Original monochrome
While Thelodonti as a whole may have been a paraphyletic group, perhaps a transitional grade between Heterostraci or Anaspida on one side and Gnathostomata on the other, most or all of the Rylian thelodonts seem to descend from the monophyletic group Furcacaudata. As the name implies, they are all unified by their characteristic tailfin. It is composed of a forked bone, from which sprout multiple, rigid rods, each suspending a membrane, a bauplan not seen in any other fish. Apart from this trait, Neothelodonti, as this island group has been named, varies a lot in morphology and appearance, making their internal classification unclear. A few have paired fins, though it is unclear if these are homologous with the pectoral fins of other fish. It is also unknown if those that lack these fins completely lost them secondarily or never had them in the first place. The same goes for dorsal fins. A few, in lieu of true jaws, have evolved a radula-like tongue, though it is again unclear if this is a shared or convergent trait.
The perhaps most unchanged of the neothelodonts is Aureocauda rylensis, which looks virtually identical to relatives like Furcacauda from the Devonian, except for the fact that its mysterious paired fins are more developed. It can be found suspension-feeding at various water-levels off the coastal reefs.
One of the largest neothelodonts is Biceratopsis albus, a hand-sized, triangular fish with two rhinoceros-like horns growing from its head. It is unclear what these are used for, as they do not seem to be a sexually dimorphic trait. This rhino-fish sports one of the least unusual mouths of the neothelodonts and as such it is most often encountered suspension-feeding in the open water of the reefs.
This is in contrast to the smaller Proboscichthys hippotigris, who is another reef-dweller, but one that mostly swims close to the ground. Its mouth forms a downturned nozzle or trunk, which it uses to sift the soft mud for any detritus that has fallen down from the upper water layers.
Highest in the water-column feeds Gleptodus ridens, in which the furcoid tailfin makes up over half the bodylength. This seems to make the body rather stiff and spiny and in turn also harder to swallow for predators. Also unique for a thelodont is that it has evolved a sort of expanded lower lip, giving it the appearance of a fake jaw. This seems to be an adaptation towards filter-feeding on planktonic algae floating close to the water’s surface.
Swimming at about the same height is Neothelodus homocercus. It is a fast swimmer which mainly feeds on zooplankton, such as copepods and brachiopod larvae. Due to its elongated shape, strongly contrasting with the other neothelodonts, it has been proposed that it actually falls outside of that clade, not being a furcacaudatan but instead being more closely related with the other major Palaeozoic clade Thelodontiformes.
Levicercus rufus is an adorable little fellow that shares some behavioural traits in common with clownfish, mainly the fact that it lives in symbiosis with local sessile cnidarians, including not just anemones but also rugose horn corals (another Palaeozoic survivor of the island). The fish is immune to the their nettle-stings and uses their tentacle-crown as a sheltered home. In turn, it feeds on small radiodont and trilobite parasites that might attack the anemone. On occasion it has even been observed guarding its home against marine dicynodonts, who, just like sea turtles, can feed on the cnidarians with their hardened beaks. Being jawless, Levicercus’ main method of attack against these giants mainly lies in being as annoying as possible, harassing the eyes and face of the synapsid. Unlike clownfish, Levicercus are not social and do not form hierarchical family groups. Each horn coral instead seems to be inhabited by only one or two of the fish, indicating some territoriality.
Hypersubcercus aureus shares a similar association with corals, though in this case living between them rather than in them. It is also more commonly found among tabulate corals, as well as strophomenid brachiopods. With its radula, it mainly feeds on the algae that grow on the shells of these invertebrates. The fish are surprisingly territorial over their grazing grounds, using their hyperelongated lower tail-lobe as striking weapons against unwelcome intruders.
Echineidion ramjeti is comparable to a remora, using its torpedo-like shape to follow and trail behind larger marine animals, be they sharks, acanthodians or mesosaurs, in order to feed on their parasites and loose dead skin with its downturned mouth. It is one of many finless thelodonts, though it is uncertain if these form a unified group. Its shark-like, pointy nose is unique among thelodonts.
Another commensal feeder is Capronia arcuata, which is also finless. Instead of chasing other animals for their waste, it waits for them to come to it. A mated pair of Capronia usually “set up shop” at their own territory in the reef and wait for larger animals to stop by and rest, letting themselves get cleaned by the little fish.
Adontus homocercus is a freshwater-dweller, inhabiting most of the vast inland waterways that innerve the island. Being small, living in murky waters patrolled by far more dangerous creatures, it is not any wonder that very little is known about this fish. The first descriptor of the animal originally wanted to name it after its distinctive three-humped dorsal fins, bafflingly calling it Triceratops, until he had to be informed that there was already a genus of dinosaur with that name, leading to a minor éclat with the ICZN and a subsequent broken nose when unable to accept that fact.
Monocauda is another freshwater thelodont genus, but much more widespread than Adontus and encompassing at least seven different species. It is most closely associated with swamps and mangroves and feeds on the abundant algae and detritus there, itself being prey to the various tadpoles and efts of the anamniotes. It is unique for having lost the distinctive fork-shape of the tail, having only the upper lobe left, with the little finlets extending all the way down to below the head and moving the animal forward in an undulating motion. It is perhaps this adaptation which allows it to be so successful, making it far more flexible, mobile and better able to hide and shelter in the various nooks and crannies of its environment.
