Written by T.K. Sivgin, illustrated by Bob Guan
On Earth, jawless fish are a rare sight today. The only ones left are the somewhat common lamprey and the more elusive hagfish of the deep sea. But back in the good old days, jawless fish were the height of vertebrate engineering, coming in all sorts of creative forms, be it eel-like slitherers, armored tadpoles and even to discus-shaped creatures which prefigured some of the jawed teleosts that would come millions of years later. While they have also become marginalised here, millions of years of isolation have nonetheless allowed the Rhynia to preserve some of this diversity and even innovate on the most ancient vertebrate Bauplan.
Here we see Dome 5’s Gladioglossus chamaeleoformis, better known as the daggertongue. It is a tuna-sized relative of the elf fish, whom we have already met while discussing the Icebreaker. Both seem to belong to an ancient group called Osteostraci, of which extinct Cephalaspis was probably the most well-known member. These were the jawless fish most closely related to us jawed ones, already possessing traits such as paired pectoral fins, but apart from that they looked mostly like armored tadpoles. The daggertongue and the elf fish have however deviated from this anatomy greatly, which makes sense, as temporally they are as far removed from Cephalaspis as we humans are from an Eusthenopteron. The most notable change is that the sides of the head-shield, which formerly resembled a flying saucer, have been folded upwards like the wings of an origami swan and now look like big, comical bunny-ears. In the fossils of their Earth-ancestors we can find in their homologous regions nervatures in the bone texture on the underside of the headshield. It is hypothesized that these likely housed some sort of sensory organ, either sensing pressure-changes in the water or weak electrical fields. Now that we have actual living osteostracans we might finally have the opportunity to test this idea. These pseudo-ears definitely do seem like they are derived sensory organs, though we still do not know of what kind. If they are mainly there to sense pressure-changes or vibrations, they funnily might be called actual ears, but if they instead serve to sense electrical fields from the muscular action of other fish, they might be more analogous to the eyestalks of hammerhead shark.
Of course the more notable aspect about the daggertongue is its mouth-apparatus. When it and the elf fish close their mouth by bending their gill-arches it does actually look like it might have a jawline, but that is just an illusion. Their bendable gill-arches only help with breathing or protecting the mouth from accidentally ingesting pebbles and dirt. They are not involved with processing food and do not have teeth. Even if their ancestors were close to the origin of Gnathostomata, these fish evidently did not bother to convergently evolve a jaw similar to ours. The elf fish seems to instead have taken some cues from the molluscs and processes its food with a barbed rasping tongue and a hard plate on the mouth-roof. The daggertongue is more creative. In a fraction of a second it can dart out its long tongue like a chameleon to then stab its soft-bodied prey with a spear-tip formed from dentine and enamel (essentially making it one large tooth). After a successful stab, the fish swallows and digests the prey whole like a snake might. While the radula and the tooth-tongue of these two fish are obviously homologous in some way, the actual nature of these tongues is even more intriguing. Muscular tongues as we commonly know them only evolved in tetrapods. Some fish might have flaps of skin in their lower jaws (which infamously get replaced sometimes by parasitic isopods), but these do not have any muscular support and are only called tongues in an informal sense. These Rhynian osteostracans must therefore have evolved their tongues independently from ours, though we have no idea yet out of what previous structure. The radula of the elf fish does interestingly share some similarities with the mouth-apparatus of Earth’s extant hagfish, but this similarity might only be superficial.
The prey the daggertongue was seen here spearing is also interesting in its own right. Originally this free-swimming mollusc, Zanclocanthus lyelli, was thought to be a gastropod, a marine snail similar or maybe even related to the extant nudibranchs of Earth. Closer analysis revealed the little facehugger to instead be a polyplacophoran, a wholly different class of mollusc, today mostly represented by the chiton.
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