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

A Phylogenetic Paedomorphic Decanalization Event Shaped the Syngnathid Body Plan: Insights from Seahorses and Their Relatives | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL This is a preprint and has not been peer reviewed. Data may be preliminary. 16 December 2025 V1 Latest version Share on A Phylogenetic Paedomorphic Decanalization Event Shaped the Syngnathid Body Plan: Insights from Seahorses and Their Relatives Author : Giora Pasca 0009-0008-1517-1642 [email protected] Authors Info & Affiliations https://doi.org/10.22541/au.176590833.32813479/v1 243 views 107 downloads Contents Abstract Introduction 4. The Syngnathid PPD Hypothesis: Developmental Timing and Genetic Mechanism 4.3 Possible Molecular Origin of the Syngnathid PPD Event 6. Conclusion 7. Potential Falsification and Testable Predictions Acknowledgments Data Availability Statement Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract 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. Giora Pasca Independent Researcher [email protected] 1 Abstract 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. 1 Abstract Introduction The teleost fish (Actinopterygii) represent the largest radiation of vertebrates, characterized by remarkable morphological diversity and the retention of a highly conserved, robust developmental program across key stages, particularly the pharyngula stage (the phylotypic stage). Within this vast diversity, the family Syngnathidae (seahorses, pipefishes, pipehorses and seadragons) presents a profound evolutionary anomaly. Syngnathids are uniquely defined not by a single novel trait, but by the simultaneous loss or extreme reduction of multiple highly conserved teleost features: the absence of teeth, the loss of pelvic fins, the reduction (or loss) of caudal fins, the loss of the stomach and pyloric caeca, and the abbreviated or skipped larval stage leading, we claim, to almost universal small adult size. This syndrome also includes the partial retention of cartilage in the adult form, indicating a family-wide failure of complete ossification. Furthermore, while the fusion of dermal ossifications into a rigid, protective body ring structure is the widely accepted cause for the constraint on locomotion, we suggest a more profound, underlying developmental deficit: the disruption of neuromuscular maturation before Central Pattern Generators (CPGs) for lateral undulation can develop. This leaves syngnathids with only the primitive anterior–posterior (A-P) flexion, lacking the side-to-side undulation typical of most teleosts. This collection of morphological reductions and kinetic constraints, which lacks a unifying functional or ecological driver, constitutes an evolutionary syndrome unique among teleosts (Wilson & Orr, 2011). A central challenge in evolutionary biology is explaining how macroevolutionary change—the rapid generation of novel body plans—occurs. While other specialized teleosts demonstrate radical change in a single developmental module—such as the fused jaw ”beak” of Pufferfish (Tetraodontidae) or the profound cranial asymmetry of Flatfish (Pleuroniformes)—the Syngnathid phenotype is unique. It is not one specialization, but a systemic ”laundry list” of simultaneous, reductive failures across disconnected systems: skeletal, visceral, appendicular, respiratory, and neuromuscular. This failure is striking because, in typical teleost development, these disconnected systems would all normally undergo differentiation and maturation in a tightly clustered temporal window during the post-pharyngula stages. For decades, the Syngnathid condition has been explained by diverse adaptive scenarios addressing each loss independently (e.g., loss of teeth due to suction feeding, loss of the stomach due to rapid throughput), which fails the test of parsimony. The full Syngnathid body plan must instead be understood through the framework of Developmental System Drift (DSD), the neutral evolution of the underlying developmental mechanisms that produce a robust (canalized) phenotype. We contend that the Syngnathid body plan is the product of an abrupt shift in this developmental system. This shift is best framed as a severe instance of Paedomorphism—the retention of juvenile or embryonic characteristics in the adult—suggesting a shared, abbreviated developmental trajectory. This is consistent with genomic evidence pointing to the early loss of key genes governing structure initiation, such as e.g., the SCPP gene cluster and the tbx4 gene (Lin et al., 2016). We thus propose that a single Syngnathid Phylogenetic Paedomorphic Decanalization (PPD) event, occurring at or near the conserved pharyngula (phylotypic) stage approximately 60 million years ago, established the family’s defining traits. By PPD we denote: lineage-defining ( Phylogenetic ); retention of embryonic traits ( Paedomorphic ); via the breakdown of developmental canalization ( Decanalization ). We hypothesize that this family-defining phenotype originated from a single, upstream developmental failure—a dramatic Decanalization event that occurred in a pioneer individual at the foundation of the Syngnathidae lineage. Canalization (Waddington, 1942), the ability of a system to consistently produce the same phenotype, was lost in this ancestral form, resulting in the concurrent failure of multiple developmental programs to complete. This loss of constraint released cryptic phenotypic variation, allowing a rapid shift in the derived syngnathid body plan. Such developmental plasticity is widely recognized as a major catalyst for the origin of evolutionary novelty (West-Eberhard, 2003). Seahorses offer a particularly striking case. Their adult morphology preserves features otherwise restricted to the pharyngula stage—an acute head–trunk angle and a curled tail. Among all known teleosts, only seahorses exhibit these traits in the adult form. This observation, coupled with their late emergence in the fossil record (approximately 20–25 Mya, nearly 40 million years after the lineage foundation), constitutes the apparent ”Seahorse Paradox.” This secondary, extreme expression of Paedomorphism can be explained within the Syngnathid PPD framework, which will be provided in detail in the Discussion (Section 5). The morphological correspondence between the adult seahorse and the embryonic pharyngula—specifically the preservation of the primary cranial flexure (acute head–trunk angle) and the finless, coiled tail—is not the result of evolutionary convergence (analogy). Rather, it represents a developmental homology: the direct retention of a conserved embryonic blueprint. To resolve the core evolutionary anomaly of the Syngnathid family and the subsequent Seahorse Paradox, the Syngnathid Phylogenetic Paedomorphic Decanalization (PPD) Hypothesis provides a unifying, parsimonious explanation for the origin of the Syngnathid body plan, rooting the family’s defining features in a single, catastrophic loss of developmental robustness that became genetically assimilated. This paper will evaluate the morphological, developmental, genomic and paleontological evidence for the Syngnathid PPD event, demonstrating its explanatory power across the Syngnathid family tree and confirming that the defining Syngnathid body plan was fully established at the time of the earliest known fossil representatives. 1 2. Theoretical Framework: Developmental System Drift (DSD), Decanalization, and Paedomorphism The Syngnathid Phylogenetic Paedomorphic Decanalization (PPD) hypothesis operates at the intersection of three fundamental concepts in evolutionary developmental biology (Evo-Devo): Developmental System Drift (DSD), Developmental Canalization, and Paedomorphism. Before anchoring these concepts in established literature, we first place the Syngnathidae lineage within its ancestral evolutionary context. 1.1 2.1 Syngnathiformes as the ancestral canvas Placing Syngnathidae within Syngnathiformes (trumpetfishes: Aulostomidae; cornetfishes: Fistulariidae; ghost pipefishes: Solenostomidae; snipefishes: Centriscidae) highlights the distinctiveness of the syngnathids. Fossils from the early Paleocene (approx. 66 Mya) indicate these lineages diverged earlier, with forms resembling trumpetfish and cornetfish present at that time (Bannikov, 2004; Friedman, 2010), well before the origin of Syngnathidae (approx. 60 Mya). The extant relatives share elongated bodies and tubular snouts and may possess dermal armor or exhibit limited undulatory swimming, yet retain the conventional teleost condition: fully ossified skeletons, ribs, functional—albeit reduced—dentition associated with specialized suction feeding, a pelagic larval stage, as well as the potential for large adult size. Even where male care occurs, it is sporadic and far less elaborate than in syngnathids.We therefore argue that Syngnathidae can be seen as an incremental exaggeration of syngnathiform trends and also a qualitatively different outcome of developmental decanalization. Recent phylogenetic re-evaluations of the Danian fossil Eekaulostomus (approx. 63 Mya) as a transitional form possessing ’clustered scutes’ further illustrate this pre-existing trend toward dermal armoring immediately preceding the origin of the Syngnathidae. At the same time, the viability of such a shift may have been facilitated by pre-existing tendencies—elongate morphologies (reducing reliance on a rib-supported trunk), pipette-like snouts, a reliance on fin-based propulsion (amiiform swimming) rather than axial undulation, and incipient male care—a suite of precursors that Syngnathid PPD magnified and locked into a novel body plan.This ”permissive ancestral canvas” is crucial, as it provides the context for the Syngnathid PPD event’s viability. Had this same decanalization—with its resulting loss of propulsive CPGs, abbreviated larval stage, simplified lophobranchial gills, cartilaginous skeleton, and loss of the stomach—occurred in a generalist, pelagic ancestor like a Tuna, the outcome would have been immediately lethal. Such an ancestor is metabolically dependent on a high-oxygen-uptake gill system for a high-energy lifestyle, a pelagic larval growth phase to reach adult size, a stomach for acidic digestion and storage, and a powerful, ossified axial skeleton (including ribs) for propulsive undulation. The Syngnathidae lineage, therefore, likely represents an extreme case of ”survivorship bias,” where a catastrophic mutation was viable only because it occurred in an ancestor already behaviorally and morphologically predisposed to a low-energy, structurally-rigid, and elongated body plan. 1.2 2.2 Developmental System Drift (DSD) and Canalization Developmental Canalization is the capacity of a developmental pathway to produce a constant, robust phenotype despite genetic or environmental perturbation (Waddington, 1942). The integrity of this buffering system ensures that homologous traits remain stable across evolutionary time. Conversely, Developmental System Drift (DSD)—defined as the divergence in the genetic basis of a conserved phenotype over evolutionary time (True & Haag, 2001; McColgan & DiFrisco, 2024)—is typically seen as a consequence of this system robustness (Wagner, 2005), allowing neutral genetic changes to accumulate. This divergence often involves changes to non-coding, cis-regulatory elements rather than the protein-coding genes themselves, making regulatory evolution the critical source of morphological change and subsequent system drift (Wray et al., 2003). The PPD hypothesis proposes a release from this process: a dramatic Decanalization event—the breakdown of developmental buffering—that caused an abrupt shift in phenotype. This singular event must have occurred in a pioneer individual by overwhelming the developmental system’s capacity for robustness (Wagner, 2005) at a highly sensitive time in ontogeny. While such mutations constantly probe and attempt new possibilities, the overwhelming result is non-viability or abortion at this conserved stage. The Syngnathidae lineage thus represents an extremely rare instance of developmental ’survivorship bias’ that fixed a novel, functional body plan, reinforcing the difficulty of reconciling homology with evolutionary innovation (Wagner, 2014). This failure released a reservoir of latent or cryptic genetic variation (CGV) that was previously masked by canalization (Flatt, 2005). The resulting novel phenotype (the Syngnathid body plan) was viable because the developmental pathway stabilized in a simplified, truncated state, which was then subjected to purifying selection—the process that eliminates harmful mutations to preserve function—and rapid genetic assimilation (Lin et al., 2016). This loss of developmental robustness consequently initiated long-term DSD, leading to the permanent erosion of underlying regulatory programs across the clade. This release of constraint and subsequent specialization is precisely what drives adaptive phenotypic variation patterns and evolvability (Pavlicev et al., 2011). 1.3 1.4 1.5 2.3 Paedomorphism and the Truncated Body Plan Paedomorphism is a form of heterochrony (change in the timing of developmental events) that results in the retention of ancestral juvenile or embryonic characteristics in the adult form (Gould, 1977; McKinney & McNamara, 1991). The Syngnathid syndrome—the simultaneous loss of ribs, teeth, pelvic fins, and the partial retention of cartilage—aligns perfectly with paedomorphic outcomes observed in other miniature or derived fish lineages (Moore, 1993). Crucially, this Paedomorphic pattern is mechanistically driven by the widespread occurrence of direct development across the family, where the fast-growth larval phase is drastically abbreviated or skipped entirely. This acceleration of sexual maturity relative to somatic growth—a form of paedomorphism known as progenesis—explains the family’s general constraint toward small adult size (miniaturization), a feature often correlated with reductive evolution (Moore, 1993). The PPD hypothesis posits that the founding Decanalization event was inherently paedomorphic, manifesting as a collective truncation or premature arrest of multiple post-pharyngula developmental programs. This systemic failure includes: the abrupt halting of skeletal ossification, resulting in cartilage retention (SCPP gene loss); the suppressed differentiation in appendage formation (such as fin bud differentiation and tbx4 gene loss); and the failure of visceral maturation (loss of the complex digestive tract and stomach). By parsimonious inference, we contend that this systemic developmental arrest also applies to locomotion. We hypothesize a corresponding arrest of neuromuscular development, where the formation of Central Pattern Generators (CPGs) is prevented. The inability of the CPGs to initiate necessitates reliance on simplified A-P flexion, which is the primitive, earliest mode of teleost movement. We expand on that in section 5.2. This complete suite of failures indicates a systemic developmental abbreviation—a Paedomorphic outcome—driven by the Decanalization of the robust post-pharyngula stage. This framework allows us to view all the defining Syngnathid reductions as the single consequence of a single, ancestral, and Phylogenetic event. 1.6 2.4 Foundational (Primary) vs. Derived (Secondary) Consequences The Syngnathid PPD hypothesis establishes a crucial distinction between the resulting traits. Foundational (Primary) Consequences are those that arise directly from the upstream Decanalization event as a collective failure or truncation of development. These include family-wide traits like complete toothlessness, absence of ribs, partial cartilage retention, digestive simplification, lophobranchial gills, loss of propulsive undulation (due to neuromuscular arrest) and the preservation of the early-forming dermal plates, and small adult size, which is a direct paedomorphic outcome (progenesis) driven by the abbreviated or skipped larval phase and the subsequent removal of the primary fast-growth period common to most teleosts. This initial size limit was then reinforced by other systemic failures: the simplified lophobranchial gills established a low ceiling on oxygen uptake and metabolic rate, while the absence of ribs removed the internal scaffolding required for a larger body. We also hypothesize that the potential for male pregnancy is a primary consequence, enabled by the retained developmental plasticity (preserving a state of pluripotency) in ventral tail area that were later co-opted into specialized brood pouch structures (Whittington & Friesen, 2020) and will be discussed in detail in section 5.3.Conversely, Derived (Secondary) Adaptations arose later from the interaction between the Foundational body plan and subsequent environmental selection acting on the released developmental potential (evolvability). These include lineage-specific traits such as upright posture, the co-option of the finless, curled tail into a prehensile organ, and a fully enclosed brood pouch in Hippocampus (seahorses), the specialization and rigid fusion of the dermal ossification that structurally reinforces the new trunk, and the leaf-like appendages in seadragons. We further hypothesize that the dichotomy between the reduced internal skeleton and the hypertrophied external armor is mechanistically driven by developmental speed. The Syngnathid PPD event imposes a severe temporal constraint: by truncating the larval phase, the organism must reach a functional, protected juvenile state rapidly. Standard teleost endoskeletons form via endochondral ossification, a temporally expensive ”two-step” process requiring the formation of a cartilage precursor which is only later replaced by bone. In a lineage defined by developmental acceleration (progenesis), this slow pathway is a liability. This systemic ”rush” explains the family-wide partial retention of cartilage (chondrodystrophy) in the cranium and vertebrae—structures that are essential and thus cannot be discarded, only underdeveloped. However, the rib module—which relies on this same slow cartilage maturation but is not strictly essential for viability—was effectively deleted.In contrast, the external bony armor develops via intramembranous ossification. This is a ”direct-deposit” mechanism where mesenchymal cells differentiate immediately into bone, bypassing the slow cartilaginous stage entirely. We propose that the expansion of this faster, direct ossification pathway was facilitated by the ancestral predisposition toward dermal armoring (as noted in Section 2.1). With the temporal window for ”slow” endochondral ossification effectively closed, the organism’s metabolic budget for biomineralization was effectively re-routed into this faster, pre-existing channel. Thus, the Syngnathid body plan represents a survivor bias: it persists because the rapid formation of the external skeleton mechanically compensated for the developmental deletion of the slow, internal one. Generally speaking, this distinction, where Foundational consequences are largely shared family traits and Derived adaptations are lineage-specific, is observable without the PPD framework. However, the added value of the PPD hypothesis is that it provides a causal explanation for this distribution, framing the shared traits as direct consequences of a single developmental breakdown, and the lineage-specific traits as later exploitation of the reduced, low-constraint template. 1 2. Theoretical Framework: Developmental System Drift (DSD), Decanalization, and Paedomorphism 1 3. Phylogenetic Evidence for the PPD Founder Event A major requirement for testing the Syngnathid PPD hypothesis —which argues for a single, lineage-founding event—is to place that event onto a reliable, dated family tree (a time-calibrated phylogeny). Recent phylogenomic and morphological analyses of Syngnathoidei (the clade containing Syngnathidae and its relatives) (e.g., Brownstein, 2023; Stiller et al., 2022) provide the necessary temporal and evolutionary context. Their findings on the timing of the lineage’s origin and the abrupt nature of its morphological shift are strongly consistent with the Syngnathid PPD hypothesis . 1.1 3.1 Timing the Origin and Deep Split of Syngnathidae Phylogenetic studies consistently place the Syngnathidae family within the highly morphologically diverse Syngnatharia, which underwent a major diversification following the Cretaceous–Paleogene mass extinction (Alfaro et al., 2018). High-resolution phylogenomic analysis confirms a deep, singular origin for the lineage (Stiller et al., 2022). The foundation of the family (pan-Syngnathidae ), which includes the most ancient fossil relatives, is consistently dated to the Paleocene or early Eocene (between approximately 63 and 52 Ma, depending on the analysis) (Brownstein, 2023; Stiller et al., 2022). This timing anchors the Syngnathid PPD Decanalization event to a single, deep node at the very root of the lineage approx 60 Ma. Immediately following this event, molecular phylogenies confirm that the earliest and deepest divergence within Syngnathidae separates the tail-brooding lineages (Urophori ) from the trunk-brooding lineages (Gastrophori ) (Hamilton et al., 2017). This fundamental division, based on the primary developmental consequence of male brooding structures, strongly indicates that the initial PPD event established one foundational, yet highly unstable developmental template (the pioneer), which then almost immediately diverged into the two major, genetically distinct brooding strategies (Gastrophori and Urophori ). The analysis of this lineage also highlights the ”Seahorse Paradox”: the extreme paedomorphic body plan of true seahorses (Hippocampini) is a derived character that emerged much later, roughly 40 million years after the Syngnathid foundation (Brownstein, 2023; Stiller et al., 2022). The finding that the sister group to seahorses is the pygmy pipehorses (Acentronura complex) supports the idea that the extreme seahorse phenotype was generated rapidly. We explicitly distinguish these pygmy pipehorses (the sister lineage) from the larger ’pipehorses’ (e.g., Solegnathus ), which appear similar but are distantly related and evolved the bent-neck morphology convergently. While pygmy pipehorses share the prehensile tail, the divergence times suggest the full ”seahorse” suite of traits appeared as a distinct event (Stiller et al., 2022). 1.2 3.2 The Morphological and Diversification Rate Shift: The Signature of Decanalization The Syngnathid PPD hypothesis predicts that this singular Decanalization event should be seen not only as an abrupt shift in body plan but also as a significant increase in evolutionary potential (evolvability) unmasked at the lineage’s origin. This prediction is empirically supported by recent quantitative analyses of phenotypic evolution. Brownstein (2023) identified a major morphological rate shift concentrated at the base of the Syngnathoidei and the Solenostomidae + Syngnathidae node. This statistically significant increase in the rate of character state changes—measuring the magnitude of physical divergence rather than the number of species—confirms the sudden, non-gradual nature of the founder event, consistent with a release from canalization. The PPD framework provides a specific explanation for the timing of these shifts. First, the ancestral decanalization event (ca. 60 Ma) established a low-constraint developmental template (the ”floodplain”), which unmasked a vast reservoir of cryptic genetic variation (CGV). Second, analyses of net diversification rates confirm that speciation rates remained low immediately after this event (Stiller et al., 2022). This creates a distinct ”Lag Phase” between the morphological innovation and the ecological radiation. We attribute this lag to the PPD-driven loss of the pelagic larval stage (detailed in Section 5.8), which forced a transition to direct, benthic development (where offspring hatch and remain associated with the seafloor rather than drifting in the water column) and thus drastically reduced gene flow and dispersal by confining populations to highly localized habitats but did not immediately trigger radiation. Finally, when global ecological change (the Middle Miocene Climate Transition, ca. 17–14 Ma) acted as a major environmental stressor nearly 40 million years later, it provided the necessary ecological trigger. This stressor acted upon the ancestral genetic potential (CGV) and the population-level mechanism (low dispersal), leading to an abrupt peak in speciation rates. This resulted in the rapid emergence of highly derived forms like Hippocampus (seahorses) and Acentronura (pygmy pipehorses), alongside the explosive radiation of the Syngnathus pipefish clade (Stiller et al., 2022). Thus, the PPD event provided the developmental capacity for change, while the Miocene environment provided the ecological opportunity to utilize it. 1 3. Phylogenetic Evidence for the PPD Founder Event 4. The Syngnathid PPD Hypothesis: Developmental Timing and Genetic Mechanism We posit that the Syngnathid Phylogenetic Paedomorphic Decanalization (PPD) event was a singular developmental disruption that occurred in a pioneer individual near the highly conserved pharyngula (phylotypic) stage. This stage, typically spanning the transition from early to mid-embryogenesis, represents a crucial bottleneck in vertebrate development where the fundamental body plan is established, and minor disruptions are typically lethal. The subsequent survival and fixation of this highly modified ancestral form hinged on its ability to bypass this bottleneck with a viable, albeit drastically reduced, phenotype. We propose that viability was maintained through survival by simplification. In standard teleosts, the high lethality at this stage is due to ”pleiotropic constraints”—the tight interconnection of complex developmental modules (e.g., the signaling networks that build ribs, teeth, and stomach). A disruption in one usually catastrophically crashes the others. However, the PPD event did not merely tweak these modules; it uncoupled them. By effectively truncating the ”slow,” complex programs (endochondral ossification, gastric differentiation), the lineage removed the primary sources of developmental conflict. The embryo survived not despite the disruption, but because the resulting phenotype—armored, simple, and direct—was metabolically and structurally simpler to build. Thus, the PPD event essentially ”broke” the hourglass, allowing a streamlined, low-complexity morph to slip through a typically rigid developmental gate. The Syngnathid PPD event is hypothesized to have occurred immediately before or during the molecular handoff that initiates post-pharyngular programs in the ancestral teleost. This timing is critical because it explains the systemic cascade of absences and reductions across multiple, simultaneous pathways, rather than the less parsimonious, independent loss of individual traits over time. To provide a clear temporal ”ruler” for these events, we can compare the rapid development of the zebrafish ( Danio rerio ) which serves as the most thoroughly studied teleost model organism and thus offers the most detailed temporal blueprint for teleost development, with a slower-developing, more closely related Acanthopterygian (spiny-rayed fish) model, Fundulus heteroclitus (Ballard et al., 1993). The conserved teleost stages are clear in both, but the absolute timing differs dramatically. In Zebrafish: Gastrulation (~5-10 hpf), neurulation (~10-24 hpf), and the pharyngula stage (~24-48 hpf), with hatching around 72 hpf (Kimmel et al., 1995). While the slower developing Fundulus develops as follows: Gastrulation (~1-2 days), neurulation (~2-3 days), and the pharyngula stage (~4-6 days), with hatching around day 12-14 (Ballard et al., 1993). This comparison validates our approach. While the absolute timing (hpf, hours post fertilization or days post fertilization) is highly variable, the relative order of these stages is a deeply conserved feature of teleost development. For the Syngnathid lineage, the consequence was a synchronized failure of programs that typically manifest after this conserved embryonic window. This evidence is observable in the striking temporal alignment between the features lost in Syngnathids and the timing of their normal onset in ancestral teleost models (Kimmel et al., 1995; Ballard et al., 1993). This comparison relies not on absolute timing, but on the conserved temporal hierarchy of developmental events across teleosts (Table 1). Table 1. Developmental timing of absent or simplified syngnathid traits relative to the Pharyngula Stage Syngnathid Trait (Absent / Modified) Normal Onset in Zebrafish (Danio rerio) (Post-Pharyngula) Implication for Syngnathid PPD Hypothesis Complete Rib Absence 96-120 hpf Arrest of the rib developmental program (sclerotome pathway) → complete rib absence. Systemic Cartilage Retention (Chondrodystrophy) 96-168 hpf Systemic failure of skeletal maturation → widespread retention of embryonic cartilage (cranium, vertebrae). Caudal Fin Rays (Lepidotrichia) ~14–21 dpf (Flexion Stage) Deletion of the fin-ray specification module → Complete absence of caudal fin (leaving tail tip flexible). Stomach & Pyloric Caeca Differentiation 96-168 hpf Destabilization of digestive-tract regionalization→ no acidic stomach or caeca. Pelvic-fin Buds / Hind-limb Homologues 96-120 hpf Matches tbx4 gene loss phenotype →absence of pelvic fins. Tooth Formation 48-72 hpf Failure to complete odontogenesis →toothless snout. Central Pattern Generators (CPGs) for Undulation > 72 hpf Decanalization of neuromuscular maturation → retention of only A–P flexion mode. Extended Pelagic Larval Phase approx 72 hpf (Hatching) Severe truncation or complete loss of larval phase → abbreviated development and direct (or near-direct) juvenile form. This well-defined temporal clustering supports the view that a single developmental decanalization event immediately following the most conserved stage accounts for the family’s defining morphology, reinforcing its parsimony. The widespread morphological truncations detailed in Table 1—such as the absence of ribs, pelvic fins, and teeth—are not just developmental observations; they have deep genomic correlates that are interpreted through the PPD framework as secondary genetic assimilation. This is the genetic consolidation of a phenotype initially caused by an upstream regulatory failure. Genomic studies provide molecular confirmation that the systems truncated developmentally exhibit corresponding genetic lesions. For instance, Syngnathids show a clear loss of the tbx4 gene, a master regulator for hind-limb development (Lin et al., 2016). The loss of this gene is viewed as a genetic assimilation that stabilizes the already truncated, finless state. Similarly, the near-complete loss or massive reduction of P/Q-rich SCPP genes—which encode the secretory calcium-binding phosphoproteins crucial for dentin and enamel—aligns with the developmental truncation of odontogenesis (Lin et al., 2016; Schneider et al., 2022). This confirms that the genetic machinery for complex tooth construction was not only arrested but genetically erased. Finally, this process is evident in the extensive erosion of Conserved Non-coding Elements (CNEs) near major patterning clusters (Lin et al., 2016). This CNE erosion is a clear signature of Developmental System Drift (DSD) that stabilizes the new, simpler body plan via genetic simplification (Healey et al., 2024). Beyond these confirmed losses, it is an intrinsic expectation of molecular evolution—independent of any specific origin hypothesis—that the long-term absence of phenotypic traits leads to the decay of their underlying genetic machinery. It goes without saying that just as tbx4 and SCPP losses mirror the appendicular and dental absences, future genomic analyses will almost certainly reveal corresponding erosions in the gene networks governing other lost structures, such as the sclerotome induction pathways for ribs or the pepsinogen and proton pump genes for the stomach. These ”genomic scars” reflect the progressive genetic erosion accompanying the loss of the structures. 4.3 Possible Molecular Origin of the Syngnathid PPD Event The precise molecular lesion that triggered the ancestral Syngnathid PPD event needs to be determined. Observed gene losses and regulatory erosions in living syngnathids likely represent the long-term consequences of that event rather than its root cause. After been overwritten by secondary changes; accordingly, the initiating disruption is best sought upstream of individual trait loci. Probable mistakes include incorrect gradients of maternal factors that set up the embryo’s first axes (Gilbert & Barresi, 2016), or errors in the timing of the maternal-to-zygotic transition—the handoff where the embryo first begins to control its own genome (Kimmel et al., 1995). Failures of epigenetic buffering, such as Hsp90 collapse (Lindquist, 2009) could have unmasked hidden variation, while distorted gradients of key morphogens—retinoic acid, Nodal, BMP, Wnt, or FGF—might have shifted or blurred the boundaries that normally organize the pharyngula stage (Burke et al., 1995; Duboule, 2007). Likewise, a perturbation of the Notch/FGF-based segmentation clock (Pourquié, 2003) could explain the loss of ribs and altered axial proportions seen across the family. Finally, disruptions to neural crest deployment—the embryonic source of much of the craniofacial skeleton and gills—could have compounded these effects by truncating jaws, teeth, and sensory organs (Couly & Le Douarin, 1988; Hall, 2005). Together, these factors outline the kind of early, system-wide disturbance that could have triggered a decanalization event. Each of these control points ultimately interacts with Hox positional codes, but in this view, Hox disintegration would be a downstream, not the initiating cause (Krumlauf, 1994; Davidson, 2006). 1 5. Discussion and Synthesis The Syngnathid Phylogenetic Paedomorphic Decanalization (PPD) event, which defined the Syngnathidae lineage in the Paleocene (ca. 60 Ma), established the novel evolutionary landscape for the entire family. It created an ancestral developmental template stripped of complexity and genetic buffering—a ”developmental floodplain.” The subsequent evolutionary history of Syngnathidae, including the emergence of its most extreme forms, reflects the exploitation of this released developmental potential (evolvability) under changing ecological pressures. 1.1 5.1 The Seahorse Paradox, Developmental Decanalization, and the Miocene as the Trigger for the Resolution If the Syngnathid PPD event occurred ~60 million years ago (ca. 60 Ma), why do seahorses—the clearest expression of paedomorphic decanalization, retaining the embryonic angled head-trunk and finless curled tail seen only in the vertebrate embryo, on top of all the other traits characteristic of the family—only appear in the fossil record To answer this, we can think of development as a system of dams and channels. In a typical teleost embryo, buffering mechanisms keep development flowing in an orderly way. Retinoic acid gradients ensure that cranial and vertebral boundaries form at the right place and time. Hox genes sharpen these boundaries and specify ribs and head–trunk relationships. The segmentation clock paces out somites and elongates the body axis. Crucially, Thyroid Hormone (TH) pulses trigger the transition from larva to juvenile, dictating the straightening of the body axis and the resorption of embryonic fin folds. These metamorphic changes are orchestrated by interacting gene regulatory networks (GRNs) that normally canalize development, ensuring reliable adult outcomes—teeth, ribs, lamellar gills, ossified vertebrae, pelvic fins. This functional timing is resolved by the high evolutionary conservation of the Thyroid Hormone Receptor (TR), which is often maternally supplied and actively expressed early in teleost embryos (approx. 24 hpf), ensuring the genetic machinery for metamorphic signaling is present even in the accelerated, embryonic-like Syngnathid state (Kimmel et al., 1995; Yamamuro et al., 2004). The Syngnathid PPD event effectively removed the intermediary larval phase, ensuring that any subsequent TH signaling triggers a direct, abbreviated transition from an embryonic-like state to a juvenile form. Together, these systems act like a dam with controlled spillways: water flows into established channels, producing predictable forms. The Syngnathid PPD event was the molecular rupture of these highly canalized GRNs. Conceptually, this was the rupture of the GRN dam. A lesion upstream—whether in cis-regulatory control of RA metabolism, Hox timing, or Thyroid Hormone (TH) sensitivity—collapsed buffering across GRNs. Once the dam gave way, water (developmental flow) no longer traveled exclusively through its old, robust channels. This specific rupture created the developmental instability that eventually permitted Hippocampus to bypass the TH-driven metamorphic program entirely, skipping the phase where the body axis straightens. This failure of canalization was not uniform across all systems; instead, it resulted in a mosaic of failure modes. Some channels dried up completely (e.g., pelvic fins, ribs), leading to immediate initiation truncation. Others began to flow but then collapsed (e.g., pectoral fins that initiate then fuse, caudal fins that start but regress), leading to partial differentiation and regression. A few channels stayed strong (the dorsal fin, armored plates). Still others narrowed rather than dried, carrying only a reduced trickle of flow. The clearest case is the gills: lamellar gills were lost, but functional lophobranchial tufts developed instead, a narrowed channel that supports life but imposes an oxygen ceiling. Cartilage in place of full ossification, and vestigial anal fins likewise represent channels where reduced flow produces reduced instead of absent outcomes. The sensory system offers a similar example: syngnathids show relatively small cranial volume with reduced olfactory sense, yet compensate with enlarged eyes (that uniquely, move independently) and vision-dominant behavior. Here again, the channel is narrowed rather than absent, redirecting capacity toward sight at the expense of smell. This mosaic of outcomes represents a typical outcome of GRN decanalization: a patchwork of modules—some failing at initiation, some failing after partial differentiation, some persisting in narrowed form, and some surviving intact. This rupture also left developmental modules, such as those governing final body shape, in a low-threshold, highly plastic state, creating a developmental ”floodplain” of exposed cryptic genetic variation (CGV). The most extreme paedomorphic features (like the angled body and coiled tail) were not immediately favored by selection but were left in a ”ready but suppressed” state, held back for millions of years. When new ecological conditions of the Miocene—specifically the expansion of seagrass habitats (Teske & Beheregaray, 2009)—favored an upright posture in structurally complex environments, natural selection rapidly leveraged this pre-conditioned developmental potential. This required only minor genetic shifts (e.g., altered Hox/RA timing) to recruit the embryonic angled body plan into the adult form, resulting in the explosive, rapid speciation observed in Hippocampus (Stiller et al., 2022). This explains the 40-million-year delay. The PPD event lowered the developmental threshold for extreme paedomorphism across the entire lineage, confirming that the ancestral failure of canalization provided the evolvability, while Miocene ecology determined the timing of the successful morphological innovation. 1.2 5.2 Absence of Propulsive Undulation and Neuromuscular Arrest The lack of lateral undulatory swimming in Syngnathidae is typically attributed solely to the rigid external bony armor, which mechanically prevents lateral flexion. However, the Syngnathid PPD hypothesis suggests a more profound, primary consequence involving the neuromuscular system. In model teleosts, efficient undulatory swimming is controlled by complex Central Pattern Generators (CPGs)—spinal neural circuits that coordinate motor function and mature post-pharyngula. We hypothesize that the ancestral Decanalization event disrupted neuromuscular maturation before the CPGs could properly develop or fully mature, leaving the entire family’s basal motor capacity restricted to the simplified anterior–posterior (A-P) flexion. This rudimentary, reflex-driven curling is the default state of early embryos (pre-hatching), but is normally superseded by lateral undulation once the spinal circuits mature in the free-swimming larval stage. This makes the lack of undulation a direct, primary consequence of developmental arrest (Paedomorphism), rather than merely a secondary mechanical constraint. This provides a clear and testable prediction: Comparative developmental neurobiology should reveal a consistent pattern of absent or mis-timed expression of the genes known to drive CPG maturation (e.g., evx1, vsx2 ) in syngnathid embryos relative to their teleost outgroups. This would provide the first neurological evidence for a primary Paedomorphic constraint on adult locomotion in the family. 1.3 5.3 The Origin of Male Pregnancy and The Division of Reproductive Energetic Costs 1.4 The evolution of male pregnancy is a spectacular example of adaptive novelty, and it represents a Primary Consequence of the PPD event because it arose from the unique developmental potential created at the lineage’s foundation. We propose that the PPD event provided both the structural opportunity and the physiological pressure that drove this sexual role reversal.We propose that the PPD event provided both the structural opportunity and the physiological pressure that favored this sexual role reversal. The pressure arose from the physiological constraint imposed by the ancestral PPD event. The simplified, underdeveloped lophobranchial gills, in place of complex lamellar gills, established a low ceiling on oxygen uptake. This aligns with the Gill-Oxygen Limitation Theory (GOLT), which posits that the 2D scaling of gill surface area often cannot keep up with the 3D volumetric growth of the body (Pauly, 1981). While GOLT is typically used to explain an organism’s maximum size with normal lamellar gills, the PPD event creates a ”pre-constrained” state: the gills are already underdeveloped, imposing a low metabolic ceiling from the start. This constraint made it energetically challenging and physiologically stressful for a single sex to handle both oogenesis (egg production) and brood care (high oxygen demand for developing embryos), two of the most metabolically intensive investments in fish life history. Splitting these costs thus offered a powerful adaptive advantage. This metabolic ceiling was paralleled by a severe anatomical constraint. The PPD-driven loss of ribs and the consequent reduction of the coelomic cavity (the body trunk) left the ancestor with limited internal volume. The ancestral Urophori lineage resolved this by shifting brooding to the post-anal tail, a structural platform supported by haemal arches that did not compete with digestive organs. However, Hippocampus took this innovation a step further. While active, horizontally swimming pipefish require streamlined brooding structures to minimize drag, the seahorse’s transition to a sedentary, upright posture—a consequence of caudal fin loss and the tail becoming prehensile—removed this hydrodynamic requirement. This release allowed the rapid elaboration of the brood patch into a fully enclosed, placenta-like pouch, a structure that would have been prohibitive in a free-swimming ancestor. Simultaneously, the structural foundation was provided by a developmental opportunity inherent in the Decanalization event. By interrupting normal canalization, the PPD event likely left many tissues, including ventral tissues, in a low-threshold, highly plastic, and partly pluripotent state. This retained embryonic flexibility meant these tissues were uniquely susceptible to endocrine cues, allowing them to be co-opted into the complex structures required for true male pregnancy, including specialized vasculature and immune tolerance interfaces (Whittington & Friesen, 2020). The evolutionary closure of the brood pouch effectively transformed an external surface into a pseudo-internal organ, necessitating a profound immunological tradeoff. To tolerate semi-allogeneic embryos within this vascularized ”self” environment, these lineages underwent specific loss and downregulation of MHC Class II pathway genes (Roth et al., 2020), a molecular strategy that parallels the establishment of the maternal-fetal interface in mammals. Thus, PPD created a modular, less constrained template—a generic caudal tissue that could assume a de novo viviparous function. In sum, the metabolic limits of the gills and the spatial constraints of the abdomen provided the evolutionary why, while the retained plasticity of the tail tissue supplied the developmental how . 1.5 5.4 Mosaic Fin Outcomes Across the family, fins provide some of the clearest examples of how a decanalization event could play out in different developmental programs or modules. The outcomes are not uniform: some fin systems appear robust enough to complete differentiation, others initiate but fail to progress and are later resorbed, and still others never develop beyond an early rudiment. Comparative ontogeny data (Schneider et al., 2022) document this variation in detail. In pectoral fins, for instance, both Hippocampus and Syngnathus form an endoskeletal disk early in development, but this disk fails to partition into separate elements. Instead, the structures fuse and later become integrated with the dermal armor, producing the unusual adult condition. In Nerophis, the pectoral fins truncate even earlier, halting at the endoskeletal disk stage. These rudimentary larval pectorals are briefly functional but are ultimately resorbed at metamorphosis, leaving the juvenile and adult nearly finless. The caudal fin shows a similar spectrum. In Hippocampus, a hypural cartilage and a few rays begin to form but their growth stalls and they regress before adulthood. In Syngnathus, the hypural cartilage does differentiate and support fin rays, though in a simplified configuration compared to other teleosts. In many taxa, the entire caudal fin is lost altogether, often replaced functionally by a prehensile tail. By contrast, dorsal and anal fins tend to be more robust. In seahorses, the dorsal fin completes relatively early and is strongly developed, consistent with its central role in locomotion. Anal fins are more variable, with some species retaining a simplified but functional version.Taken together, these mosaics illustrate the central expectation of the Syngnathid PPD hypothesis: decanalization does not produce identical outcomes across all systems. Instead, modules differ in their thresholds of robustness. Some complete their developmental program, others initiate but collapse into resorption, and still others never progress beyond a rudiment. This spectrum of outcomes provides an internal consistency check for the hypothesis, showing how a single upstream disruption could yield diverse but patterned effects across the fin systems of syngnathids. 1.6 5.5 The Temporal Bottleneck: Speed as the Universal Filter The collection of traits brought about by the Syngnathid PPD event are the results of a coherent governing variable: Developmental Speed. As detailed in Section 2.4, the shift from internal to external ossification is driven by the specific temporal costs of the endochondral pathway. We contend that this ”Temporal Bottleneck” explains the PPD syndrome in its entirety. The simultaneous arrest of the skeletal, digestive, and neuromuscular systems indicates that the ancestral PPD event imposed a strict ”functional deadline” on the developing embryo. By eliminating the extended pelagic larval phase (direct development), the lineage effectively truncated the temporal window required for complex, multi-stage developmental programs. The PPD event consequently acted as a high-speed developmental filter: ”Slow” Modules Truncated: Systems requiring intermediate steps—such as ribs (cartilage replacement), the stomach (acidic differentiation), and undulating motor circuits (CPG maturation)—were effectively ”timed out.” ”Fast” Modules Prioritized: Systems capable of direct differentiation—such as dermal armoring (intramembranous ossification) and simple flexion—were retained and expanded. Thus, the Syngnathid body plan represents the survivors of a temporal bottleneck: only the most direct morphogenetic programs could keep pace with the accelerated schedule of the PPD event. 1.7 5.6 The Loss of the Larval Scaffold as a Morphological Constraint Our analysis suggests that the unique morphology of the Syngnathid tail is not merely the result of random, independent mutation resulting in localized structural defects, but rather a systemic shift in developmental timing. In standard teleost models (e.g., Danio rerio ), the transition from embryo to juvenile is punctuated by a distinct, plastic post-larval phase (metamorphosis). It is during this specific window that key ”adult” modules—such as the Pyloric caeca (structures that, in other teleosts, collaborate closely with the stomach for complex mechanical digestion), dermal squamation (the multi-stage process of forming overlapping scales), and adaptive immunity (the maturation of specialized B- and T-cell systems)—are constructed. However, the Syngnathidae display a phenomenon of ”Whole-Organism Acceleration” (Direct Development). By extending the embryonic period (through male brood pouch gestation, which provides unparalleled protection and developmental control), they effectively bypass the free-living larval stage. This creates a critical temporal disconnect: the PPD event occurs early in development, but, its morphological consequences are clustered in the post-larval window. By closing the developmental door early, the organism skips the specific timeframe required to build these complex systems. 1.7.1 Evolutionary vs. Ontogenetic Implications It is important to distinguish the evolutionary origin of the PPD event from its developmental execution. Phylogenetically, the PPD event likely arose in the Syngnathid ancestor as a heterochronic shift (a change in timing). Ontogenetically, however, this shift is recapitulated in every developing embryo. The distinction is crucial: the PPD event, a single evolutionary novelty, provides a system where its developmental consequences—the impact of accelerated timing on morphological modules—can be studied within the lifespan of every single developing Syngnathid. As the Syngnathid embryo undergoes direct development within the pouch, the ”larval” genetic programs are rendered redundant and are effectively silenced by the acceleration toward the juvenile form. 1.7.2 The Mechanism: Absence of the Scaffold This temporal shift explains the specific absence of the caudal fin. In teleosts, the caudal fin is not a primary embryonic structure (i.e., a structure formed in the early stages of cleavage and gastrulation); it is constructed later by remodeling a transient larval structure—the median fin-fold. This fin-fold serves as the necessary biological ”scaffold.” Under normal conditions, genetic markers such as sonic hedgehog (shh) must be re-expressed in the distal fold to trigger the formation and branching of bony rays (lepidotrichia). In Syngnathids, the direct development effectively deletes this ”scaffolding phase.” Consequently, even if the genetic toolkit for fin formation remains intact, the physical substrate required to execute those instructions (the fin-fold) is absent. The lack of a caudal fin, therefore, fails to materialize not necessarily because specific genes were individually purged from the genome, but because the ontogenetic window and physical scaffold required for their activation were bypassed entirely. Crucially, while the PPD event made the caudal fin vulnerable, its complete loss is a defining feature of the Hippocampus genus and is a result of the secondary saltational event detailed in Section 5.7. 1.8 5.7 The Spectrum of Paedomorphosis and the ”Two-Step” Saltation To encompass the full morphological diversity of the Syngnathidae, we propose a cumulative, two-step saltational model. This framework clarifies that the family’s evolution was not a single continuum, but rather two distinct punctuations in developmental timing. 1. The Foundational Step (The PPD Event): This primary saltation is the sufficient condition for the majority of the family. It established the ancestral ”Syngnathid” template—armored, ribless, and stomachless—alongside a significant abbreviation of the larval phase. This results in the ”mosaic” phenotype (e.g. detailed in Section 5.4) characteristic of pipefishes and seadragons, which retain specific larval traits (like fin folds) while accelerating others (armor). 2. The Secondary Step (Pharyngulation): This secondary saltation is an additive requirement exclusive to the Hippocampus lineage. While seahorses share the foundational PPD traits, their unique morphology requires this second event—the complete elimination of the larval phase. This event ”locked in” the two defining features of the vertebrate pharyngula: the primary cranial flexure (angled head) and the finless, curled tail. Thus, while the first saltation defined the family, the second saltation defined the seahorse. Crucially, while this saltational event instantly established the morphology of the curled tail, the refinement of this feature into a fully functional prehensile organ likely followed as a rapid adaptive response to the newly available seagrass habitat. To account for the full morphological range of the family, Table 2 maps these developmental modes against the proposed events. 1 5. Discussion and Synthesis Table 2: The Spectrum of Syngnathid Paedomorphosis & Metamorphic-Truncation Progression Group & Est. Origin Developmental Mode Relation to Hypothesized Events Trunk-Brooding Pipefishese.g., Nerophis(~58 Mya / Paleocene) Indirect: Hatch with larval fin folds; prolonged planktonic drift phase. Ancestral Condition (Step 1): Exhibits primary PPD traits but retains larval anatomy (fin folds) and planktonic ecology. Seadragonse.g., Phycodurus(~45–50 Mya / Eocene) Intermediate: Large yolk-sac hatchlings; prolonged planktonic drifting phase. Independent Radiation: A successful, divergent exploitation of the Step 1 platform, adding secondary complexity (appendages) while retaining the planktonic phase. Tail-Brooding Pipefishese.g., Syngnathus, Halicampus(~40–45 Mya / Eocene) Direct (Elongating): Hatch as miniature adults (no fin folds) but undergo distinct axial elongation. The Substrate for Step 2: Complete loss of larval fin folds and metamorphic stages, but retains the ancestral vertebrate elongation program. Pygmy Pipehorsese.g., Acentronura(~25 Mya / Oligocene) Transitional / Convergent: Hatch with prehensile tail but distinct planktonic drifting phase; horizontal swimming. Parallel Evolution: Demonstrates that the ”curled tail” module is a latent potential (Cryptic Genetic Variation) accessible to multiple lineages, distinct from the full Pharyngulation event. Seahorsese.g., Hippocampus(~20–25 Mya / Miocene) Saltational (Pharyngulated): Hatch as fully curled miniature adults; immediate benthic settling. The Pharyngulation Event (Step 2): Total bypass of planktonic phase and axial straightening, locking the adult into the embryonic pharyngula profile. Interpretation of the Developmental Spectrum The data in Table 2 illustrates a progressive loss of the larval program. Step 1: The Syngnathid PPD Event (~60 Mya) This foundational event created the armored, ribless phenotype of the family but retained the ancestral larval developmental program. Basal lineages like Nerophis still undergo a ”Narrowed Channel” development, retaining anatomical vestiges such as the larval fin fold and a planktonic drift phase. Thus, the ancestor was not a ”miniature adult,” but a novel, armored form that still required a distinct metamorphic transition (resorption of fin folds) to reach maturity. Step 2: The Pharyngulation Event (~20–25 Mya) The emergence of Hippocampus represents the ”Dry Channel.” In this secondary saltation, the lineage completely deleted the final two vestiges of the larval program: the planktonic drift and the axial straightening. Crucially, the existence of Pygmy Pipehorses (Acentronura )—which possess a prehensile tail but swim horizontally—demonstrates that the ”curled tail” morphology was a latent potential (Cryptic Genetic Variation) released by the initial PPD event. Phylogenetically, pygmy pipehorses are the sister group to seahorses (Stiller et al., 2022). However, their retention of horizontal swimming suggests that the common ancestor of the Hippocampus + Acentronura clade likely resembled a straight-bodied pipefish. This confirms that the seahorse did not evolve gradually through a ”semi-curled” phase. Instead, it was a singular, saltational leap from a straight ancestor to a fully upright, curled form—a profound shift achieved by the complete elimination of the larval stage and the clear retention of pharyngula-stage embryonic features. 0.1 5.8 Ecological Consequences: Passive Dispersal and Global Distribution The PPD event not only reshaped the syngnathid body but also dictated their ecological destiny. The defining constraint was not merely anatomical, but neuromuscular: the developmental arrest prevented the maturation of the complex Central Pattern Generators (CPGs) required for axial undulation. Without the neural circuitry to drive a rhythmic body wave, the lineage was stripped of the propulsive power required for active migration. Whether they retained a vestigial caudal fin (as in some pipefishes) or lost it entirely (as in seahorses), the entire family was effectively ”frozen” in a weak-swimming state. This physiological limitation aligns with recent phylogeographic analyses by Stiller et al. (2022), which suggest that the global spread of syngnathids, particularly seahorses, was driven by passive rafting rather than active swimming. The PPD hypothesis provides the mechanistic reason for this behavior: since they were incapable of generating the thrust to gradually cross oceans on their own power, they relied on ocean currents instead. This passive drifting allowed them to colonize distant habitats, eventually leading to the widespread distribution observed today across tropical and subtropical waters. Thus, the ecological habit of rafting is not a behavioral choice, but a secondary consequence of the primary neuromuscular constraints imposed by the PPD event. 0.1 5.8 Ecological Consequences: Passive Dispersal and Global Distribution 6. Conclusion The Syngnathid Phylogenetic Paedomorphic Decanalization (PPD) hypothesis frames the syngnathid body plan as the aftermath of a single, lineage-founding developmental disruption rather than the gradual accumulation of independent adaptations. We propose that the initial Decanalization event, situated near the pharyngula stage, was fundamentally a loss of developmental duration. This collapse of buffering served as a universal selective filter. Mechanisms requiring prolonged formation periods—such as the endochondral ossification of ribs, the complex differentiation of the stomach, and the maturation of propulsive CPGs—were destabilized and arrested. In their place, the system defaulted to ”fast” modules—like dermal armoring—that could reach functional competency without sustained canalization. This systemic fallout created a developmental template stripped of constraint and possessing untapped evolvability. This framework resolves the ”Seahorse Paradox” through a Two-Step Saltation model: The PPD Event (ca. 60 Ma): The foundational decanalization that created the armored, ribless, and stomachless template by destabilizing ”slow” post-pharyngular programs. The Pharyngulation Event (ca. 25 Ma): A secondary heterochronic shift exclusive to the Hippocampus lineage that deleted the final vestige of the larval program (axial straightening), locking the adult into the highly paedomorphic, embryonic profile (acute head-trunk angle and curled tail). Comparative ontogeny and molecular data support this view, showing that Decanalization created a mosaic of effects—some programs failing to initiate, others failing after partial differentiation—consistent with the upstream rupture of highly interconnected Gene Regulatory Networks (GRNs). If correct, the PPD hypothesis illustrates that macroevolution does not always proceed through the gradual accumulation of fitness increments. Instead, the Syngnathidae represent a unique case of survival by simplification. The PPD event was a failure of developmental robustness that, by stripping away the most time-intensive modules, collapsed the organism into a novel, stable, and surprisingly successful evolutionary basin, demonstrating how extreme novelty can wait 40 million years for the right ecological circumstances to be fully realized. 7. Potential Falsification and Testable Predictions The Syngnathid PPD framework is powerful because it generates concrete, falsifiable predictions that can be tested using modern Evo-Devo and comparative neurobiology. The hypothesis would be weakened or falsified if: • Temporal Alignment Failure: Syngnathid embryos were found to match typical teleost development through the pharyngula stage before diverging, rather than showing simultaneous truncations immediately post-pharyngula. • Independent Trait Loss: The fossil record revealed that key Syngnathid reductions (e.g., teeth, ribs, pelvic fins) did not appear together in the ancestral form but were lost independently in different lineages at widely separated geological times. • Lack of GRN Plasticity: Gene Regulatory Networks (GRN) responsible for the lost traits were found to be completely erased in the Syngnathid ancestor, indicating that the PPD event resulted in a total genomic loss rather than leaving a ”ready but suppressed” state capable of later Paedomorphic recruitment. • Evidence of Axial Straightening in Hippocampus : If detailed kinematic analysis of seahorse embryogenesis reveals a transient phase of axial straightening that is subsequently reversed (secondary curling), it would falsify the ”Pharyngulation” claim that the straightening program was deleted via saltation. 1. Neuromuscular Arrest (CPG Failure): Hypothesis: The lack of undulatory swimming is a primary consequence of Syngnathid PPD due to an arrested maturation of the Central Pattern Generators (CPG) for lateral flexion. Experiment: Comparative developmental neurobiology can test this by assessing the developmental timeline of spinal motor circuits. We predict a consistent pattern of absent or mis-timed expression of the genes known to drive CPG maturation (e.g., evx1 or vsx2) in syngnathid embryos relative to a closely related teleost outgroup. A neurological deficit would provide primary evidence for a Paedomorphic constraint on adult locomotion. 2. Rescue-by-Timing and TH-Modulation Experiments: Hypothesis: The Syngnathid PPD event involved upstream timing errors, while the secondary Pharyngulation event (Step 2) specifically involved the truncation of Thyroid Hormone (TH) driven metamorphic straightening. Experiment: This can be tested by manipulating TH signaling. If the straight-bodied pipefish phenotype relies on a metamorphic straightening phase (Step 1), blocking TH signaling in developing Syngnathus embryos should arrest axial elongation and induce a curled, seahorse-like phenocopy. Conversely, exposing Hippocampus embryos to exogenous TH pulses might attempt to re-activate the suppressed straightening program. Failure to respond to TH in Hippocampus would support the hypothesis that the genomic module for straightening has been permanently deleted (Decanalized) rather than merely silenced. 3. Rescue by Caudal Fin Signaling Factor (Scaffolding/Morphogenesis): Hypothesis: The failure of caudal fin development is due to a physical constraint (the absence of the median fin-fold scaffold), despite the retention of the underlying genetic toolkit for bony ray formation. Experiment: Introduce the Caudal Fin Signaling Factor (e.g., an shh pathway agonist) locally to the distal tip of the developing Hippocampus tail. The presence of the signaling factor, bypassing the missing scaffold, should trigger the retained genomic machinery for ray formation, providing unambiguous evidence that the developmental constraint is morphological. 4. Chromatin Logic and CNE Erosion: Hypothesis: The Syngnathid PPD founder event was a global upstream disturbance that caused extensive DSD and genomic simplification. Experiment: Comparative ATAC-seq/ChIP-seq analysis contrasting Syngnathid and outgroup embryos should reveal broad enhancer erosion (Conserved Non-coding Elements) around post-pharyngular GRNs (RA/Hox/FGF), providing molecular support for the systemic Decanalization and genetic assimilation event. 5. Paleontological Constraint Prediction: Hypothesis: Low body mass and the absence of key ancestral teleost traits (e.g., pelvic fins) are primary, ancestral consequences of the PPD event. Prediction: Comprehensive analysis of all Syngnathid fossils will show a consistent constraint on maximum body size and will reveal the presence of the full suite of PPD-related morphological losses (ribs, stomach, pelvic girdle) together in the earliest forms, supporting their origin as a single, co-occurring saltational event. Acknowledgments The author thanks Professor Günter Wagner (Yale University/University of Vienna) for his critical feedback and supportive comments on the plausibility of the Phylogenetic Paedomorphic Decanalization hypothesis in an early draft of this work. Data Availability Statement The conclusions and framework presented in this conceptual study are based entirely on published data. All genomic, phylogenetic, and paleontological evidence cited herein were sourced from publicly accessible databases and peer-reviewed literature, and are explicitly detailed and referenced within the manuscript’s main text. 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Molecular Biology and Evolution, 20, 1377–1419. 1 References Information & Authors Information Version history V1 Version 1 16 December 2025 Copyright This work is licensed under a Non Exclusive No Reuse License. Keywords comparative ecosystem evolutionary ecology laboratory marine molecular genetics selection analysis theoretical theory vertebrate Authors Affiliations Giora Pasca 0009-0008-1517-1642 [email protected] n/a View all articles by this author Metrics & Citations Metrics Article Usage 243 views 107 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Giora Pasca. A Phylogenetic Paedomorphic Decanalization Event Shaped the Syngnathid Body Plan: Insights from Seahorses and Their Relatives. Authorea . 16 December 2025. DOI: https://doi.org/10.22541/au.176590833.32813479/v1 If you have the appropriate software installed, you can download article citation data to the citation manager of your choice. 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