Seahorse Placoderms
A speculative evolution exercise
In an alternate timeline two lineages of placoderms convergent with syngnathid fish evolved:
Longmaiformes were a clade of arthrodires of uncertain affinities as they more or less appear in the mid-Devonian fossil reccord as they are, with the general consensus leaning towards being part of Coccosteina. They developed long, tubular jaws similar to those of the Cretaceous turtle Ocepechelon, the upper jaw dental plates flanking it to keep the suction tube's shape and the lower jaw's dental plate reduced to a small hook keep the jaw closed, with only a small jaw motion required for a gap to be formed and prey sucked in. Like seahorses and trumpetfish a bent neck was developed, allowing a greater degree of motion and thus more efficient prey capture. In derived taxa the pectoral fins became the dominant propulsion mechanism while the tail lost its fin and became prehensile. Like in most placoderms they were viviparous, the females giving birth to a few large pups.
Jurakaniformes were a clade related to Entelognathus and Qilinyu, and as just converged more deeply with syngnathids, their small jaws at the end of a long tubular snout much their actinopterygian mimics. Derived taxa took a more serpentine apparence, relying on their dorsal and anal fins to swim, and like both Longmaiformes and syngnathids they developed a bent neck. Little is known about their reproductive habits, but the presence of claspers suggests internal fertilization at least.
Both clades survived the Devonian mass extinction, presumably due to more generalistic habits as well as the spread of coastoal swamps and other well vegetated aquatic biomes that favoured their unique lifestyle. Longmaiformes in particular were well successful in marine habitats, some reaching large sizes and becoming some of the largest marine vertebrates of the Paleozoic. Jurakaniformes, however, were the longer survivors: longmaiform diversity plummeted in the early Permian and eventually the group became extinct in the Capitanian Extinction event, while seahorse and pipefish like jurakaniforms made it into the earliest Triassic, before dying out, presumably due to predation from marine temnospondyls and reptiles.