A Phylogenetic Paedomorphic Decanalization Event Shaped the Syngnathid Body Plan: Insights from Seahorses and Their Relatives

The Syngnathidae family (seahorses, pipefishes, pipehorses, and seadragons) presents a profound evolutionary anomaly: a clade defined by the simultaneous, family-wide reduction or loss of multiple, otherwise highly conserved teleost traits, including teeth, ribs, pelvic fins, and the stomach. This suite of reductions constitutes a severe evolutionary phenotype lacking a unifying adaptive explanation. We propose the Syngnathid Phylogenetic Paedomorphic Decanalization (PPD) hypothesis as the parsimonious solution. By PPD, we denote a lineage-defining (Phylogenetic) retention of embryonic characteristics (Paedomorphic) caused by a catastrophic loss of developmental robustness (Decanalization). We posit that the foundational Syngnathid body plan resulted from a single, upstream developmental disruption that occurred in a pioneer individual at the origin of the lineage (approx. 60 Mya). This pivotal event, centered around the pharyngula (phylotypic) stage, did not merely remove traits; it destabilized the temporal buffering of development, thereby eliminating the robust time-allocation mechanism necessary for the successful completion of complex, duration-dependent developmental programs. This systemic loss of duration caused the arrest of ”slow,” complex programs—specifically endochondral bone (ribs), complex digestion (stomach), and appendicular initiation (fins)—while preserving ”fast,” early-forming modules like dermal armor. Furthermore, we argue that the lineage did not adapt trait-by-trait, but persisted through survival by simplification. By collapsing the complex requirements of the standard teleost plan into a streamlined, low-energy phenotype, the ancestor bypassed the lethal constraints of the phylotypic stage. This hypothesis unifies the morphological, genomic, and paleontological record, demonstrating how a catastrophic loss of canalization—when functionally and structurally coherent—can rapidly generate a novel, stable, and highly evolvable body plan.