An embryonic arrest shapes the Syngnathid body plan_ Insights from Seahorses, Pipefishes, and their Relatives

The Syngnathidae (seahorses, seadragons, pipefish, pipehorses) exhibit a remarkable, enigmatic body plan, challenging conventional explanations for their fused jaws, toothlessness, cartilaginous skeleton, fin loss, male pregnancy, and their distinctive morphology, which includes the acute head-trunk angle of seahorses and the family's unique curling, often prehensile, tail. We propose a unifying, parsimonious hypothesis, termed "<b>pharyngulation</b>," that the entire lineage originated from a profound paedomorphic arrest (retention of juvenile traits) during a specific embryonic pharyngula stage. This arrest, likely driven by ancestral Hox gene cluster disintegration, fundamentally halted morphological progression in a common teleost ancestor. This single event explains their entire suite of <b>primary characteristics</b>–including universal low body mass and volume and unique A-P locomotion. It also establishes a framework to differentiate these foundational <b>family-defining traits</b> from <b>ancestral features shared with the broader Syngnathiformes order</b> (such as the elongated snout, as exemplified by Trumpetfish) and from later adaptive refinements, such as the leaf-like appendages in seadragons. Our "pharyngulation" hypothesis offers a novel, testable model for macroevolutionary innovation, demonstrating how a singular, profound alteration to a conserved developmental program can rapidly forge a new, viable body plan. This concept, synthesizing evidence from genomics, the fossil record, and developmental biology, is of broad interest to evolutionary biologists and developmental biologists alike.