Paleontologists have pulled a jaw-dropper (get it?) from the Carboniferous. A 310-million-year-old ray-finned fish, Platysomus parvulus, shows the now oldest known instance of a “tongue bite”. It has extra tooth plates on the roof and floor of its mouth, which work together to crush prey.
This discovery, published in Biology Letters last month, by pushes back the known origin of this feeding innovation by over 150 million years.
3D fossils, 3D scans
Thanks to a relatively rare three-dimensional preservation of the fossil, scientists were able to perform CT scanning on the fossil. This in-turn allowed researchers to reconstruct internal anatomy.
Cranial anatomy of Platysomus parvulus (NHMUK PV P11697) based on µCT scanning. (a) Bones of skull as preserved, in left lateral view; grey lines indicate external skull bones. Inset shows reconstruction of body shape inferred from complete specimens, with maxilla and dentary in blue. (b) As in (a), but with dermal and jaw bones removed and elements repositioned. (c) Basibranchial tooth plate in dorsal view. (d) Upper jaw bones and parasphenoid in ventral view, with maxilla and palatal bones mirrored. (e) As in (d), but with basibranchial tooth plate shown in articulation. (f) Left mandible in lateral view and (g) medial view. Abbreviations: add, adductor fossa; ang, angular; art, articular; asp, ascending process of parasphenoid; ax, axial skeleton; bc, braincase; bhc, buccohypophysial canal; btp, basibranchial toothplate; cb, ceratobranchial; chy, ceratohyal; cor, coronoids; de, dentary; dpal, dermopalatine; ent, entopterygoid; entp, entopterygoid teeth; hmd, hyomandibula; mx, maxilla; orn, ornament; part, prearticular; pb, pharyngobranchial; pop, preoperculum; pp, posterior process of parasphenoid; psp, parasphenoid; quad, quadrate; sang, surangular; scl, sclerotic ossicle; te, teeth; vom, vomer. Colour coding: blue, cheek and outer jaw bones; light purple, palate and inner jaw bones; dark purple, braincase; turquoise, hyoid arch; yellow, branchial skeleton; brown, axial skeleton; red, lateral-line sensory canal pores. Image Credit: Sam Giles et al
They found opposing tooth plates, a narrow upper plate on the roof of the mouth and a multipart lower plate. The scientists noted that the fish also retained the teeth in its jaw teeth. This means it likely combined both systems, jaws and tongue biting, for eating.
This would have helped Platysomus expand its diet to harder prey, such as with shells or exoskeletons. “These upper (palatal) and lower (branchial) components interact to reduce or manipulate prey”, the researchers explained. “Tongue bites and modified pharyngeal jaws serve as additional zones for prey processing, with their decoupling from mandibular jaws hypothesized to increase functional versatility.“
The timeline of evolution is pushed back
Before now, the earliest known fish with a tongue bite appeared about 150 million years later. The discovery also suggests that fish were experimenting with new ways to eat hard prey (shells, insect parts) much earlier than thought.
The discovery could also be a possible example of convergent evolution, as tongue-bite mechanism appears to have evolved multiple times across ray-finned fish lineages. Platysomus parvulus is not related to any extant tongue-biters.
Multiple origins of tongue-bite mechanisms in actinopterygians. Stratigraphic range (left), schematic drawing of tongue bite components (middle) and body profile (right) of actinopterygian lineages with tongue bites. Ranges for extinct taxa are based on fossil occurrences; ranges for extant taxa are based on molecular clock estimates of total-group ages. Colour coding: grey, midline supporting skeletal elements; purple, upper dentigerous region; yellow, lower dentigerous region. Image Credit: Sam Giles et al
Most importantly, this fossil fills a gap in how fish evolved after the End-Devonian Mass Extinction, when fish began diversifying new body forms and strategies. Of course, older tongue-biters may still be found, as the researchers note that the fossil record is patchy and durophagy (eating hard shelled prey) had already evolved). But this is currently oldest documented fossil of its kind.
What is the evolutionary advantage?
The fact that Platysomus used both jawbite and tongue-bite means early fish could be flexible in what they ate, and were not restricted to one method of feeding. This versatility could have given them evolutionary advantages during times of ecological change. Successful modern fish still employ tongue-bite systems.
“Living ray-finned fishes display a remarkable range of specializations associated with feeding”, the researchers wrote. “Among the most celebrated of these adaptations are modifications to the branchial apparatus in teleosts, including so-called ‘tongue bites’, which involve opposing dentition on the ventral surface of the braincase and palate and tooth plates on the dorsal surface of the median gill skeleton.”
“Extant teleost lineages with tongue bites are relatively young, with records extending only to the Mesozoic.”