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Anamniotes
Trout spawning showing typical anamniote external fertilization
Trout spawning showing typical anamniote external fertilization
Anamniotes have a distinct larval stage, such as in the smooth newt.
Anamniotes have a distinct larval stage, such as in the smooth newt.
Scientific classificationEdit this classification
Kingdom: Animalia
Phylum: Chordata
Clade: Olfactores
Subphylum: Vertebrata
Groups included
Cladistically included but traditionally excluded taxa

The anamniotes are an informal group of craniates comprising all fish and amphibians, which lay their eggs in aquatic environments. They are distinguished from the amniotes (reptiles, birds and mammals), which can reproduce on dry land either by laying shelled eggs or by carrying fertilized eggs within the female. Older sources, particularly before the 20th century, may refer to anamniotes as "lower vertebrates" and amniotes as "higher vertebrates", based on the antiquated idea of the evolutionary great chain of being.

The name "anamniote" is a back-formation word created by adding the prefix an- to the word amniote, which in turn refers to the amnion, an extraembryonic membrane present during the amniotes' embryonic development which serves as a biochemical barrier that shields the embryo from environmental fluctuations by regulating the oxygen, carbon dioxide and metabolic waste exchanges and secreting a cushioning fluid. As the name suggests, anamniote embryos lack an amnion during embryonic development, and therefore rely on the presence of external water to provide oxygen and help dilute and excrete waste products (particularly ammonia) via diffusion in order for the embryo to complete development without being intoxicated by their own metabolites.[1] This means anamniotes are almost always dependent on an aqueous (or at least very moist) environment for reproduction[2] and are thus restricted to spawning in or near water bodies. They are also highly sensitive to chemical and temperature variation in the surrounding water, and are also more vulnerable to egg predation and parasitism.

During their life cycle, all anamniote classes pass through a completely aquatic egg stage, as well as an aquatic larval stage during which all hatchlings are gill-dependent and morphologically resemble tiny finless fish (known as a fry or a tadpole for fish and amphibians, respectively), before metamorphosizing into juvenile and adult forms (which might be aquatic, semiaquatic or even terrestrial), thus indicating their physiological homology.

Anamniote traits

[edit]
Anamniote eggs from a frog.

The group is characterized by retaining the primitive vertebrate condition in several traits:[3][4]

  • Absence of an amnion
  • Absence or rudimentary condition of the allantois
  • Permeable skin allowing diffusion of water and gases directly through the skin.
  • Presence at some period of life of gills.

History of discovery

[edit]

The features unifying the anamniotes was first noted by Thomas Henry Huxley in 1863, who coined the phrase Ichtioid or Ichthyopsida ("fish-face") for the group.[5][failed verification] It is a taxonomic classification just below the level of Vertebrata, though Huxley presented the Ichthyopsida as an informal unit and never ventured to forward a Linnaean rank for the group. The term Ichthyopsida means fish-face or fish-like as opposed to the Sauropsida or lizard-face animals (reptiles and birds) and the mammals.[6] The group representing an evolutionary grade rather than a clade, the term anamniote is now used as an informal way of denoting the physical property of the group, rather than as a systematic unit.

References

[edit]
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Anamniotes

View on Grokipedia
from Grokipedia
Anamniotes are a paraphyletic group of vertebrates characterized by the absence of an —a fluid-filled that surrounds the in amniotes—during embryonic development. This group encompasses all non-amniote vertebrates, including cyclostomes (such as lampreys and ), jawed fishes (chondrichthyans and osteichthyans), and amphibians (lissamphibians). Unlike amniotes, which include reptiles, birds, and mammals, anamniotes typically require aquatic environments for , as their eggs lack protective extra-embryonic membranes and are laid in water to prevent . Key characteristics of anamniotes include a reliance on gill-based or in early life stages, with somites primarily composed of tissue and minimal sclerotome development, reflecting their evolutionary adaptations to aquatic or semi-aquatic lifestyles. and waste removal in their embryos occur primarily through across thin membranes in moist environments, and many species exhibit larval stages that are morphologically distinct from adults, such as tadpoles in amphibians. These traits distinguish anamniotes from the more terrestrially adapted amniotes, which evolved waterproof eggs enabling away from water. Evolutionarily, anamniotes represent an ancestral grade from which amniotes diverged during the late era, specifically in the period, with the earliest evidence dating to approximately million years ago (as of 2025 discoveries), when amphibian-like tetrapods transitioned to fully terrestrial via the of the amniotic . As a paraphyletic assemblage, anamniotes do not form a single but include all lineages basal to Amniota, highlighting their role as a foundational group in diversification from aquatic origins to complex terrestrial ecosystems.

Definition and Overview

Definition

Anamniotes are a paraphyletic group of vertebrates characterized by the absence of the extraembryonic membranes—, , and —during embryonic development. These structures, present in amniotes, protect the and facilitate and waste storage within a self-contained environment. Without them, anamniote embryos are vulnerable to , necessitating in aquatic or consistently moist habitats to ensure successful development. The term "anamniote" originates from the Greek prefix "an-," meaning "without," combined with "amnion," the innermost membrane enveloping the fetus in higher vertebrates, derived from the Greek word for a fetal membrane. This nomenclature highlights the defining reproductive limitation relative to amniotes, which evolved these membranes to support terrestrial egg-laying. The scope of anamniotes includes all vertebrates outside the amniote clade, encompassing cyclostomes (lampreys and hagfish), jawed fishes (chondrichthyans and osteichthyans), and amphibians (lissamphibians).

Distinction from Amniotes

The primary distinction between anamniotes and lies in their reproductive strategies, particularly the structure and deposition of eggs. Anamniotes, encompassing fishes and amphibians, typically lay eggs in aquatic environments or utilize followed by oviposition without the protective extraembryonic membranes that define amniote reproduction. In contrast, —reptiles, birds, and mammals—produce amniotic eggs equipped with a leathery or calcified shell and internal membranes, enabling fully terrestrial development independent of external water sources. This difference underscores the evolutionary shift from water-dependent reproduction in anamniotes to land-adapted strategies in . Developmentally, anamniote embryos face significant constraints due to the absence of key extraembryonic structures, such as the , which in encloses the in a protective, fluid-filled sac. Without these membranes, anamniote rely on surrounding for essential through across permeable membranes and for the dilution and removal of metabolic wastes, limiting size and hatching success to moist or submerged conditions. , however, benefit from internalized support systems—the for waste storage and respiration, and the for gas permeability—allowing prolonged development in a stable, self-sustained environment that mitigates and osmotic stress. Evolutionarily, these reproductive and developmental differences highlight the basal position of anamniotes as vertebrates adapted primarily to aquatic or semi-aquatic niches, where environmental water availability dictates life cycles. The emergence of the amniotic in amniotes is traditionally estimated at 312–340 million years ago based on the earliest known fossils, but recent evidence from trackways dated to approximately 355 million years ago suggests an earlier origin. This transition not only expanded ecological opportunities but also marked a profound divide in independence from water-bound constraints.

Key Characteristics

Reproductive Adaptations

Anamniotes, lacking the amniotic membranes that enable terrestrial egg development in amniotes, exhibit reproductive strategies that are predominantly tied to aquatic or moist environments to ensure successful fertilization and embryonic survival. In most fishes and amphibians, external fertilization predominates, where females release eggs into water and males simultaneously discharge sperm over them, facilitating fusion in a hydrated medium. This process produces vast quantities of gametes to compensate for high mortality rates, but the eggs are unshelled and gelatinous, rendering them highly susceptible to desiccation if not submerged, thus confining reproduction to aquatic habitats. Certain anamniote groups have evolved internal fertilization as an adaptation to enhance reproductive efficiency in variable environments, though their offspring still lack amniotic protection and often require moisture. Cartilaginous fishes, such as sharks and rays, utilize specialized pelvic claspers in males to deposit sperm directly into the female's reproductive tract, leading to either egg-laying in leathery cases or viviparity, ranging from aplacental (yolk-dependent) to forms with limited maternal nutrient provision via placental-like structures. Similarly, caecilians, a limbless amphibian order, employ an eversible phallodeum for internal fertilization, resulting in either oviparity with gelatinous eggs laid in moist burrows or viviparity in some species, where embryos receive limited nourishment from uterine secretions but not through a complex placental interface. Additionally, internal fertilization has evolved in some bony fishes, such as live-bearing species in the family Poeciliidae, where sperm is transferred via gonopodia and embryos develop internally within the female. These mechanisms allow limited independence from open water compared to external fertilization but do not fully liberate anamniotes from desiccation risks, in contrast to the shelled, terrestrial-capable eggs of amniotes. Amphibians further demonstrate reproductive adaptations through distinct larval stages that bridge aquatic and terrestrial phases, optimizing development in water-dependent early life. In frogs and toads, for instance, eggs hatch into tadpoles—free-living aquatic larvae with gills, tails for propulsion, and herbivorous or filter-feeding mouthparts suited to pond or ecosystems—allowing growth and organ maturation before into air-breathing adults. This larval period, regulated by , serves as an evolutionary compromise for transitioning between habitats, enabling juveniles to exploit abundant aquatic resources while avoiding the vulnerabilities of direct development. Salamanders exhibit similar gilled larvae in many cases, though some bypass the free-living stage via direct development in moist terrestrial nests guarded by parents.

Structural and Physiological Features

Anamniotes display a range of structures tailored to their aquatic or semi-aquatic habitats, emphasizing protection, , and . In fishes, the is typically covered by scales that overlap to form a protective barrier, minimizing influx in freshwater or efflux in marine ones, thus aiding . A mucous layer secreted by epidermal glands further enhances this impermeability while preventing invasion and reducing friction during swimming. Amphibians, by contrast, possess thin, glandular that is highly permeable to and gases, enabling where up to 50% of oxygen uptake occurs directly through the in many , a necessity for their moist-dependent . This permeability, supported by mucus-producing glands, maintains hydration but limits prolonged terrestrial exposure. Respiratory adaptations in anamniotes prioritize efficient oxygen extraction from water, with gills serving as the primary organ in fishes and amphibian larvae. Fish gills facilitate countercurrent exchange, allowing nearly complete oxygen diffusion from water to blood across thin lamellae, supporting high metabolic demands in oxygenated aquatic environments. In adult amphibians, lungs supplement gill-independent respiration, but their simple, sac-like structure provides lower efficiency on land due to limited surface area and reliance on buccal pumping, often complemented by cutaneous exchange for up to 90% of carbon dioxide release in some species. Certain fishes, such as lungfish, possess paired lungs derived from the swim bladder that enable aerial breathing in hypoxic waters, though these organs are less effective outside submersion due to inadequate vascularization for sustained terrestrial use. Circulatory systems in anamniotes reflect their respiratory demands, with fishes featuring a single-circuit arrangement via a two-chambered heart that directs deoxygenated blood sequentially to gills and then the body, optimizing aquatic oxygen loading but limiting pressure for distant tissues. Amphibians exhibit a three-chambered heart with partial ventricular separation, enabling a dual pulmocutaneous circuit that mixes oxygenated blood from lungs and skin with deoxygenated systemic return, sufficient for variable aquatic-terrestrial transitions but less efficient than fully separated systems. These configurations support moderate metabolic rates without the high-pressure demands of terrestrial life. Skeletal features in anamniotes emphasize lightness for in water, with fishes exhibiting either cartilaginous endoskeletons, as in , or bony endoskeletons, as in , both reducing density to facilitate and energy-efficient locomotion. skeletons, while more ossified, retain lightweight elements like elongated limb bones adapted for flexible movement in viscous media, aiding semi-aquatic propulsion. The lack of amniotic support structures underscores their physiological tether to moist environments for overall .

Taxonomy and Phylogeny

Major Groups

Anamniotes encompass a diverse array of vertebrates that lack the amniotic egg characteristic of amniotes, forming a paraphyletic group that includes jawless fishes, cartilaginous fishes, bony fishes, and amphibians. These major taxonomic divisions highlight the of aquatic and semi-aquatic vertebrates adapted to various environments through distinct skeletal, reproductive, and fin structures. The , or jawless fishes, represent the most basal extant group of vertebrates and include the lampreys and hagfishes. Lampreys are often parasitic, attaching to host fishes with a suctorial disc and rasping mouth to feed on blood and tissues, while hagfishes are primarily that burrow into decaying carcasses and produce copious slime as a defense mechanism. Both groups possess cartilaginous skeletons, lack true jaws, and have no paired fins, relying instead on a continuous fold for locomotion; their persists as a primary axial support throughout life. Chondrichthyes, the cartilaginous fishes, comprise , rays, skates, and chimaeras, distinguished by their lightweight cartilaginous endoskeletons that enhance and agility in marine habitats. These fishes are covered in placoid scales, small tooth-like dermal denticles that reduce drag and contribute to a rough texture, and they exhibit via claspers in males, leading to either egg-laying or live birth in many species. The bony fishes, or , are divided into two subclasses: (ray-finned fishes) and (lobe-finned fishes), both featuring ossified skeletons, swim bladders for buoyancy, and bony gill covers. dominate modern aquatic ecosystems, encompassing over 30,000 species including teleosts such as , , and , characterized by fins supported by flexible lepidotrichia rays that allow precise maneuvering. , though less diverse today with only a few living representatives like coelacanths and lungfishes, possess fleshy lobe-like fins with internal bones that foreshadowed the limb structure in . Lissamphibia includes all modern amphibians, subdivided into three orders: Anura (frogs and toads), (salamanders and newts), and Gymnophiona (caecilians). These limbless or limbed tetrapods share moist, permeable skin for and typically exhibit a biphasic life cycle with aquatic larval stages and terrestrial or semi-aquatic adults, though some show direct development without free-living larvae. Anurans are adapted for jumping with elongated hindlimbs, caudates retain tails and exhibit regenerative abilities, and gymnophiones are burrowing worm-like forms with reduced eyes and sensory tentacles.

Evolutionary Relationships

Anamniota represents a grade rather than a monophyletic within vertebrates, comprising all non- lineages that diverged basal to the origin of Amniota, including cyclostomes, fishes, and amphibians, but excluding the of reptiles, birds, and mammals. This paraphyly arises because amniotes evolved from within the lineage, rendering anamniotes a sequential series of branches leading to more derived terrestrial adaptations, without forming an exclusive common ancestor shared only among themselves. In the broader vertebrate phylogeny, key nodes establish the basal positioning of anamniotes. Cyclostomes (lampreys and hagfishes) form a monophyletic to gnathostomes (jawed vertebrates), supported by molecular evidence from reconstructions showing shared ancestral tetraploidization events but distinct subsequent chromosomal fusions unique to gnathostomes. Within gnathostomes, chondrichthyans (cartilaginous fishes) and osteichthyans (bony fishes) diverge as monophyletic , with fossil and molecular data indicating their split around 423 million years ago, positioning chondrichthyans as the basal gnathostome branch relative to the osteichthyan radiation. Sarcopterygians, a subgroup of osteichthyans, represent the lineage leading to tetrapods, with tetrapodomorph sarcopterygians giving rise to limbed vertebrates, including amphibians as the basal tetrapod group. Lissamphibia, the crown-group amphibians encompassing modern frogs, salamanders, and , exhibits strong supported by both molecular and morphological evidence. Molecular analyses of complete mitochondrial genomes reveal high bootstrap support for lissamphibian unity, with frogs and salamanders () as sisters to caecilians, rejecting alternative polyphyletic arrangements. Morphologically, shared derived traits such as pedicellate teeth and bifold tongue structure reinforce this , with phylogenetic placements favoring origins within temnospondyl dissorophoids as stem relatives, though lepospondyl affinities remain debated in some fossil-inclusive trees. These relationships underscore the nested position of lissamphibians within sarcopterygian-derived tetrapods, bridging aquatic anamniote grades to amniote innovations.

Evolutionary History

Origins and Early Development

The earliest anamniotes trace their origins to the late Cambrian period, approximately 500 million years ago, with the appearance of jawless vertebrates such as , which possessed tooth-like phosphatic elements and represented an early innovation in vertebrate mineralized structures. These primitive forms were eel-like and likely filter-fed in marine environments, marking the initial diversification of vertebrates from chordate ancestors. By the period around 480 million years ago, ostracoderms emerged as a diverse group of armored jawless fishes, characterized by heavy bony head shields composed of enamel, dentine, and , which provided protection against predators while lacking paired fins or jaws. Ostracoderms, including orders like Pteraspidomorphi and Cephalaspidomorphi, evolved filter-feeding adaptations and thrived in shallow marine and freshwater habitats, exemplifying the foundational role of anamniotes in vertebrate evolution. The period (419–358 million years ago), often termed the "Age of Fishes," witnessed a major radiation of anamniotic lineages with the rise of jawed vertebrates, or gnathostomes, beginning in the around 419 million years ago. Placoderms, the earliest jawed fishes, featured primitive jaws derived from arches and robust dermal armor, enabling predatory lifestyles in marine and freshwater ecosystems; notable examples include the massive Dunkleosteus, which reached lengths of up to 10 meters. Concurrently, acanthodians—small, shark-like fishes with spiny fins and lightweight scales—diversified rapidly, adapting to a range of ecological niches through enhanced maneuverability and sensory capabilities. This " explosion" of jawed anamniotes, driven by innovations in predation and locomotion, significantly increased diversity and ecological impact. A critical phase in anamniote development occurred in the Late Devonian, around 375 million years ago, as lobe-finned fishes (sarcopterygians) evolved limb-like fins for navigating shallow, vegetated waters. Species such as , a predatory fish from the Miguasha Formation in , exhibited robust pectoral and pelvic fins supported by bony elements homologous to limbs, including a humerus-like that facilitated and in marginal habitats. These adaptations, including strengthened fin skeletons and improved lung-like swim bladders for air gulping, positioned lobe-finned anamniotes as direct precursors to early tetrapods without crossing into fully terrestrial forms.

Transition to Amniotic Vertebrates

The period, spanning approximately 359 to 299 million years ago, continued the evolution of anamniote tetrapods from Late forms like , which remained largely aquatic, relying on fish-like traits such as an anterior and paddle-like limbs for swimming in shallow waters. Over this era, however, selective pressures drove the development of semi-terrestrial adaptations in some lineages, including stronger limb girdles and improved , allowing limited forays onto swampy land environments amid the period's vast . These changes represented incremental steps toward terrestriality but still tethered reproduction to moist habitats. The defining transition to amniotic vertebrates occurred with the evolution of the amniotic egg, a shelled structure enclosing protective extraembryonic membranes that prevented and enabled development on dry land. This innovation likely arose in reptiliomorph stem groups during the early , with evidence from tracks dating to approximately 356 million years ago and body fossils around 312 million years ago, from lineages distinct from the temnospondyl and lepospondyl clades that gave rise to modern amphibians. By the onset of the Permian period approximately 299 million years ago, the amniotic egg facilitated fully independent terrestrial reproduction, decisively shifting ecological dominance from water-dependent anamniotes to more versatile amniotes. Environmental pressures, particularly the aridification following the around 305 million years ago, accelerated the decline of anamniote amphibians by diminishing habitats crucial for their larval stages and egg-laying. This climatic shift, coupled with competition from more adaptable forms, favored reptiliomorphs that possessed proto-amniotic traits, paving the way for the radiation of true amniotes and effectively ending anamniote prevalence on land. Living amphibians today represent relics of these transitional anamniote groups.

Diversity and Ecology

Modern Representatives

Anamniotes are represented today primarily by the vast diversity of fishes—including jawless cyclostomes such as lampreys and —and the more limited but ecologically significant amphibians, which together illustrate the persistence of non-amniotic reproductive strategies in modern vertebrates. Fishes encompass over 36,000 described as of 2025, with ray-finned teleosts () comprising the majority and dominating both freshwater and marine ecosystems through their and morphological innovations. Cyclostomes, numbering around 120 , exhibit unique ecological roles, such as the parasitic feeding of lampreys on host fishes and the scavenging behavior of in deep-sea environments. These teleosts exhibit remarkable behavioral adaptations, such as the anadromous migration of like Oncorhynchus spp., which hatch in freshwater, mature in the ocean, and return to natal rivers to spawn, supporting nutrient cycling across ecosystems. In contrast, lungfishes such as Protopterus spp. demonstrate survival strategies like , burrowing into mud cocoons during seasonal droughts to endure prolonged periods without water by reducing metabolic rates and relying on air-breathing. Amphibians number approximately 8,973 described species as of late 2025, with frogs and toads (Anura) forming the largest group and showcasing extreme physiological tolerances to environmental stressors. For instance, the wood frog (Rana sylvatica) can survive freezing temperatures where up to 70% of its body water turns to ice, halting heart function and breathing while protecting vital organs through cryoprotectant accumulation like glucose. , a limbless order, are specialized burrowing forms adapted to subterranean life, featuring reinforced skulls and annulated bodies that facilitate forceful wedging through soil in tropical habitats. Despite their biodiversity, many anamniote species face severe conservation challenges, with habitat loss and degradation affecting the majority of threatened populations across both groups. In amphibians, the chytrid fungus (Batrachochytrium dendrobatidis) has driven declines in over 500 species and contributed to at least 90 extinctions, exacerbating vulnerabilities in moist environments. For fishes, overfishing has depleted stocks in key species, leading to an 81% decline in global migratory freshwater populations over the past 50 years, while habitat fragmentation from dams and pollution further imperils their survival. Overall, around 41% of amphibian species and 26% of freshwater fish species are now threatened with extinction, underscoring the urgent need for targeted protection.

Habitats and Adaptations

Anamniotes, comprising fishes and amphibians, predominantly occupy aquatic and semi-aquatic environments due to their physiological dependence on water for respiration, , and reproduction. Fishes, the most diverse group, thrive in oceans, rivers, and lakes worldwide, where adaptations such as —darker dorsal coloration blending with the ocean floor from above and lighter ventral surfaces matching the sky from below—provide against predators. Additionally, many bony fishes possess a , a gas-filled organ that adjusts to maintain neutral floatation without constant , enabling efficient energy use in varied water depths. Amphibians exhibit semi-terrestrial lifestyles, often confined to riparian zones near , , and wetlands that offer high and access to breeding sites. These habitats mitigate the risk of through permeable skin, with behavioral adaptations like nocturnal activity reducing exposure to daytime and while facilitating moisture retention. Such reliance on moist environments underscores their physiological constraints, tying reproductive strategies closely to nearby water bodies for egg-laying and larval development. Anamniotes display a global distribution spanning extreme climates, from polar to tropical regions, supported by specialized adaptations. In polar waters, notothenioid fishes survive subzero temperatures through antifreeze glycoproteins in their blood, which bind to ice crystals to prevent internal freezing and allow dominance in the . Conversely, in tropical settings like the , poison dart frogs inhabit humid leaf litter and vegetation, where their vibrant aposematic coloration warns predators of skin toxins derived from diet, aiding survival in predator-rich, moist ecosystems. These examples illustrate how anamniotes' ecological niches are shaped by environmental pressures and physiological limits.

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