Fact-checked by Grok 2 weeks ago

Synapsida

Synapsida is a diverse clade of amniote vertebrates defined by the presence of a single temporal fenestra—a large opening in the skull behind each eye socket that facilitates jaw muscle attachment—encompassing all living mammals and their extinct relatives, which originated around 320 million years ago in the Late Carboniferous period.^[1]^[2]^[3] This group represents one of the two primary lineages diverging from early amniotes, the other being Sauropsida (which includes reptiles and birds), and synapsids were the dominant large terrestrial vertebrates during the Late Carboniferous and Permian periods, occupying diverse ecological niches from carnivorous predators to herbivorous grazers.^[4]^[5] Early synapsids, often referred to as pelycosaurs, featured sprawling gaits, ectothermic physiologies, and sprawling postures similar to modern reptiles, exemplified by sail-backed forms like Dimetrodon.^[6] Later subgroups, known as therapsids, emerged in the Permian and exhibited progressive mammal-like traits, including more upright postures, differentiated teeth, and evidence of endothermy, with advanced cynodonts in the Triassic evolving features such as secondary palates and mammalian jaw joints that contributed to the origin of true mammals by the Late Triassic or Early Jurassic.^[1]^[7] Synapsids survived the Permian-Triassic mass extinction— the most severe in Earth's history—through adaptable therapsid lineages, leading to multiple radiations and the eventual dominance of mammals in the Mesozoic and Cenozoic eras, with over 6,000 extant species today displaying traits like fur, lactation, and warm-blooded metabolism that trace back to synapsid innovations.^[1]^[7] Despite their "mammal-like reptile" moniker, synapsids are not reptiles but a separate amniote branch, with evolutionary transitions such as the reconfiguration of jaw bones into middle ear ossicles (forming the malleus and incus) marking key adaptations for enhanced hearing and mastication in mammalian descendants.^[7]^[8]

Taxonomy and Classification

Cladistic Definition and Phylogeny

Synapsida is defined cladistically as the monophyletic clade consisting of the last common ancestor of Mammaliaformes and all of its descendants, encompassing all mammals and their extinct relatives closer to mammals than to sauropsids. This clade is diagnosed by key synapomorphies in the skull, including a single lateral temporal fenestra positioned low behind the orbit, with the postorbital and squamosal bones excluded from the temporal roof, and the reflected lamina of the angular bone forming the ventral margin of the fenestra.^[9] These features distinguish synapsids from other amniotes and mark the origin of the group around 318 million years ago during the Late Carboniferous, when the synapsid-sauropsid divergence occurred near the base of Amniota.^[10] The phylogenetic framework of Synapsida features basal stem synapsids, often referred to as pelycosauromorphs or non-therapsid synapsids, which include groups like Ophiacodontidae, Varanopidae, and Caseidae, representing the earliest diverging lineages after the split from sauropsids. More derived within Synapsida is the clade Therapsida, which encompasses all synapsids more closely related to mammals, including Anomodontia, Dinocephalia, and the mammalian stem group Cynodontia leading to Mammaliaformes. This structure reflects a progressive nesting toward mammalian traits, with therapsids emerging in the Early Permian and dominating terrestrial ecosystems thereafter.^[11] The modern consensus on synapsid phylogeny emerged from the cladistic revolution of the 1980s, pioneered by analyses like Reisz's 1986 systematic revision of basal synapsids, which established monophyly and basic branching patterns using morphological characters from fossils. Subsequent refinements integrated additional fossil discoveries, such as those from the Karoo Basin, and molecular data from living mammals to calibrate divergence times and resolve deeper nodes, confirming Synapsida's position as the sister group to Sauropsida within Amniota.^[12]

Historical Linnaean Approaches

In traditional Linnaean taxonomy, synapsids were classified as a subgroup of reptiles under the class Reptilia, commonly termed "mammal-like reptiles" owing to their intermediate skeletal and dental features between typical reptiles and mammals. This placement reflected early 19th-century views that emphasized shared reptilian traits like sprawling limbs and ectothermy while noting progressive mammalian affinities in later forms.^[13] Early hierarchical schemes distinguished basal and advanced groups, with Edward Drinker Cope establishing the order Pelycosauria in 1878 for primitive synapsids, including sail-backed sphenacodonts like Dimetrodon, characterized by robust skulls and predatory adaptations but lacking advanced mammalian traits. In 1889, Harry Govier Seeley proposed the order Theromorpha to group more derived forms with mammal-like skull structures, such as differentiated teeth and enlarged temporal openings, positioning them as a transitional category within Reptilia. These orders exemplified the Linnaean focus on morphological grades rather than strict ancestry, treating pelycosaurs as basal "reptilian" stock and theromorphs as progressively "mammalian."^[14]^[15] Robert Broom formalized Therapsida as a subclass in 1912, elevating advanced mammal-like forms (including gorgonopsians and cynodonts) above pelycosaurs in the hierarchy and underscoring an evolutionary sequence from primitive to sophisticated morphologies. This subclass encompassed taxa with features like a secondary palate and upright posture, critiqued later for rendering Pelycosauria paraphyletic since therapsids arose from within pelycosaur-grade synapsids. Broom's framework, built on South African Permian fossils, dominated early 20th-century classifications but faced challenges for implying linear progression over branching evolution.^[16]^[17] The shift from Linnaean to cladistic approaches accelerated in the 1970s and 1980s, with seminal works like those of Robert R. Reisz (1980) and Thomas S. Kemp (1982) demonstrating the monophyly of Synapsida as a distinct amniote lineage separate from Reptilia, rendering "mammal-like reptiles" obsolete. In contemporary paleontological practice, "synapsid" denotes the full clade encompassing all non-reptilian amniotes including mammals, while "therapsid" persists informally for mammal-proximate non-mammalian forms to retain descriptive utility in fossil descriptions, despite ongoing debates over nomenclatural precision.^[4]^[18]

Major Subgroups and Nomenclature

Synapsida encompasses a diverse array of extinct and extant clades, with mammals as the sole surviving lineage. The major subgroups are organized cladistically into basal forms and more derived branches, reflecting successive outgroups to the mammalian stem. Basal synapsids include Ophiacodontidae, a clade of primarily aquatic or semi-aquatic carnivores from the Late Carboniferous, with elongate skulls and paddle-like limbs, represented by genera like Ophiacodon from North American deposits. Caseasauria, another basal clade from the Late Carboniferous to Middle Permian, consists of primarily herbivorous forms characterized by robust skulls, leaf-shaped teeth for grinding vegetation, and barrel-shaped bodies adapted for a sedentary lifestyle; representative genera include Casea and Cotylorhynchus, known from North American deposits.^[19] The clade Pelycosauromorpha comprises more advanced non-therapsid synapsids, with Eupelycosauria as its principal subgroup, including Sphenacodontia, Edaphosauridae, and the derived Therapsida. Edaphosauridae, within Eupelycosauria, features distinctive sail-like dorsal spines supported by elongated neural spines, possibly for thermoregulation or display, as seen in Edaphosaurus from the Early Permian of Texas and Oklahoma. Sphenacodontia, known from the Late Carboniferous to Middle Permian, consists of carnivorous forms with deep skulls, serrated teeth, and robust limbs suited for terrestrial predation; Dimetrodon, famous for its neural spine sail, exemplifies this group as an apex predator in Permian swamp ecosystems. Therapsida represents the mammalian stem lineage, diversifying in the Permian and Triassic, and is divided into several major subclades: Biarmosuchia (basal carnivores with primitive jaw mechanics), Dinocephalia (thick-skulled herbivores and carnivores like Tapinocephalus), Anomodontia (predominantly herbivorous, including dicynodonts such as Lystrosaurus with beak-like jaws and tusks), Gorgonopsia (sabre-toothed carnivores akin to big cats), Therocephalia (small to medium predators with mammalian-like ear structures), and Cynodontia (leading to mammals, featuring advanced dentition and endothermy indicators in forms like Thrinaxodon).^[18] Nomenclature within Synapsida has seen revisions, particularly regarding varanopids (Varanopidae), a group of agile, lizard-like forms from the Late Carboniferous to Middle Permian. Traditionally placed as basal synapsids or stem-amniotes, 2010s phylogenetic analyses debated their affinity, with some studies positioning them outside Synapsida due to diapsid-like features, while others, incorporating cranial and postcranial data, confirmed them within Eupelycosauria as successive outgroups to Sphenacodontia and Therapsida; a 2019 analysis of new Oklahoma material reinforced their synapsid status, emphasizing the need for additional fossils to resolve uncertainties.^[20] In Anomodontia, nomenclature controversies surround dicynodont extinction timelines; while long thought to end in the Late Carnian, 2018-2019 revisions based on Polish faunas (e.g., Lisowicia) extended survival into the Late Triassic (Rhaetian), challenging but not overturning end-Triassic extinction models; a controversial 2002 report of a possible relic in Australian Cretaceous rocks remains unconfirmed by recent studies. These debates underscore ongoing refinements in synapsid phylogeny, driven by integrated morphological and stratigraphic data.^[21]

Anatomy and Physiology

Skull Structure and Temporal Fenestra

The synapsid skull is defined by a single temporal opening, the infratemporal fenestra, which represents a key synapomorphy distinguishing Synapsida from other amniote clades such as diapsids with their dual fenestrae.^[22] This fenestra originates in the Late Carboniferous and persists throughout synapsid evolution, including in modern mammals, where it accommodates the temporalis muscle.^[22] In basal synapsids, the overall skull architecture resembles the anapsid condition of early amniotes, featuring a relatively solid temporal region with fewer perforations, but the infratemporal fenestra emerges as a novel opening bounded dorsally by the postorbital and squamosal bones and ventrally by the jugal and quadratojugal.^[23] Over evolutionary time, synapsid skulls underwent progressive simplification, including reduction in the number of dermal bones and partial loss of exposure for elements like the quadrate, which became more internalized compared to the superficial positioning in basal forms.^[23] The infratemporal fenestra itself forms through the separation of dermatocranial sutures rather than within a single bone, facilitating biomechanical adaptations for terrestrial feeding.^[24] Functionally, this opening enables the expansion and low-angle insertion of jaw adductor muscles, such as the temporalis and masseter precursors, thereby increasing bite force without requiring excessive skull doming.^[24] This structural innovation correlates with enhanced anterior biting capabilities, as seen in the stress distributions modeled for early synapsid crania.^[22] In more derived synapsids like therapsids, the temporal fenestra often enlarges to support further muscle hypertrophy, with pronounced temporal arches developing to counter tensile stresses during mastication.^[22] For instance, in the Permian sphenacodontid Dimetrodon, the robust skull features a laterally expansive infratemporal fenestra that underscores the shift toward dominant anterior bite mechanics in predatory lifestyles.^[22] These variations highlight the fenestra's role in accommodating diverse feeding strategies while maintaining the core synapsid cranial plan.^[24]

Dentition and Feeding Adaptations

Early synapsids exhibited a homodont dentition characterized by uniform, conical, unicuspid teeth adapted primarily for grasping and piercing prey, as seen in basal forms like sphenacodontids such as Dimetrodon.^[25]^[26] These teeth were simple in morphology, with minimal size or shape variation along the jaw, reflecting a faunivorous lifestyle in Carboniferous and early Permian environments.^[26] The evolution of heterodonty marked a significant advancement in synapsid dentition, beginning in the Permian with therapsids that developed differentiated tooth types, including incisor-like anterior teeth, enlarged canines, and multicuspid postcanines resembling premolars and molars.^[25] This progression allowed for more versatile feeding strategies, with therapsids independently evolving additional cusps on teeth up to three times across lineages.^[25] In advanced cynodonts, heterodonty further refined, featuring occluding molars with complex shearing surfaces that enhanced processing of tougher foods, approaching the mammalian condition.^[25] Specialized dental adaptations reflected dietary diversification among therapsids; for instance, herbivorous dicynodonts often reduced marginal teeth in favor of a keratinous beak, retaining prominent tusks for cropping vegetation or defense, as evidenced by histological studies of tusk development in taxa like Lystrosaurus.^[27] In contrast, carnivorous gorgonopsians possessed serrated canines and postcanines with dentine-enamel serrations for efficient tearing of flesh, enabling them to dominate as apex predators in Permian ecosystems.^[28] Tooth replacement patterns also evolved progressively, with basal synapsids displaying polyphyodonty—continuous replacement throughout life—via ankylosed attachment to the jawbone, as observed in sphenacodonts like Dimetrodon.^[29] Therapsids showed transitional states with prolonged ligamentous (gomphotic) attachment, reducing replacement frequency, while advanced cynodonts such as Thrinaxodon and Diademodon approached the diphyodont condition of mammals, featuring two generations of teeth and permanent gomphosis for durability.^[29] This shift, driven by heterochronic changes like neoteny, supported more efficient long-term feeding in increasingly mammalian-like forms.^[29]

Jaw Mechanics and Musculature

In basal synapsids such as pelycosaurs, the jaw joint consisted of the quadrate and articular bones, enabling a wide gape suitable for capturing prey but providing limited occlusion for precise biting. This configuration supported primarily vertical jaw motion, with adductor musculature dominated by a single temporalis-like muscle inserting on a modest coronoid process of the dentary, resulting in relatively weak bite forces compared to later forms.^[30] Therapsids exhibited significant advancements in jaw mechanics, including enlargement of the temporal fenestra, which accommodated expanded temporalis and emerging masseter muscles for greater leverage and force generation. In advanced therapsids, the raised coronoid process enhanced the moment arm of the adductor muscles, allowing for improved stabilization of prey during feeding, while the masseter began to develop as a distinct layer inserting laterally on the angular bone.^[30] These changes facilitated a shift toward more efficient mastication, though the primary joint remained quadrate-articular. The transition to cynodonts marked the emergence of the dentary-squamosal joint, which progressively replaced the quadrate-articular articulation, with the quadrate reducing to form the incus and the articular becoming the malleus in the mammalian middle ear.^[31] This reconfiguration, evident in forms like Thrinaxodon, allowed for transverse jaw motion and precise occlusion, supported by further expansion of the temporalis and masseter muscles via the enlarged temporal fenestra.^[32] Functionally, these adaptations increased bite efficiency and enabled complex chewing, benefiting from heterodont dentition for varied food processing.^[31]

Secondary Palate Development

The secondary palate in synapsids is a bony structure that separates the nasal and oral cavities, enabling independent function of respiration and feeding. This feature is absent in basal synapsids, such as pelycosaurs and early therapsids, where the nasal and oral passages remain connected. Its origin traces to the late Permian in non-mammalian therapsids, with the earliest evidence appearing in forms like the biarmosuchian Promoschorhynchus platyrhinus, where incipient choanal crests and medial bony processes begin to form a partial barrier.^[33] The development involves contributions from the premaxilla, maxilla, and palatine bones, which project medially to create a shelf-like extension, often supplemented by vomerine elements in early stages.^[34] In early therapsids, such as some dinocephalians and dicynodonts, the secondary palate remains incomplete, consisting of short, perforated shelves with prominent choanae (posterior nasal openings) positioned anteriorly. For instance, in certain dicynodonts like Kawingasaurus fossilis, the palatine portion features large foramina, allowing limited communication between cavities while providing structural support.^[35] By the early Triassic, this structure becomes more robust and elongated in cynodonts, such as Procynosuchus delaharpe, where the maxilla and palatine fuse to form a near-complete bony partition, marking a key transition toward mammalian conditions.^[34] This progressive closure is linked to enhanced skull reinforcement, as the palate distributes masticatory stresses across the snout.^[36] The primary function of the secondary palate is to permit breathing during feeding or drinking, as air can flow through the nasal passages uninterrupted by oral contents—a critical adaptation for active terrestrial lifestyles. In advanced forms, it also facilitates the warming and humidification of inhaled air via contact with vascularized nasal turbinals, aiding physiological efficiency.^[37] These developments in non-mammalian therapsids represent a precursor to the fully ossified hard palate of mammals, where the structure supports additional behaviors like suckling in neonates.^[36]

Integument, Skin, and Sensory Features

The integument of basal synapsids, such as those in the Carboniferous and Early Permian, consisted primarily of scaly skin akin to that of reptiles, with epidermal scales providing protection and possibly aiding in thermoregulation. Skin impressions preserved in Early Permian trackways, including those attributed to synapsids like Dimetropus makers, reveal rectangular or keeled scales on the limbs, trunk, and tail, indicating a non-overlapping, tuberculate texture.00574-3) In caseids, the earliest detailed soft-tissue preservation from the Early Permian of Germany shows a similar scaly integument without osteoderms, though dermal bony plates may have occurred sporadically in related basal forms for armor-like reinforcement. Evidence for glandular structures in basal synapsid skin is sparse, but impressions suggest the presence of simple apocrine-like glands associated with scales, precursors to more complex mammalian skin derivatives.^[38] Fur evolved as a transformative feature in synapsids, marking the shift toward mammalian insulation and sensory capabilities, with the transition from scaly to pilose integument occurring gradually in therapsids. Hair-like filaments reported in Late Permian coprolites from Russian therapsid localities were initially interpreted as evidence of early fur, but subsequent analyses identified them as probable plant debris or fungal hyphae rather than true hair.^[39] Definitive fur first appears in the fossil record during the Late Triassic to Early Jurassic among advanced cynodonts, where skin impressions show a mix of scales and hair follicles; for instance, the Early Triassic Thrinaxodon exhibits dermal patterns with small pits suggestive of whisker-like vibrissae emerging amid scales.^[40] By the Middle Jurassic, mammaliaforms such as Castorocauda display clear impressions of dense underfur and guard hairs across the body, confirming pelage as a fully developed insulating cover. Sensory adaptations in synapsid skin evolved alongside fur, enhancing tactile and environmental perception in nocturnal or burrowing lifestyles. In advanced therapsids, including therocephalians and probainognathian cynodonts, enlarged infraorbital foramina and associated nerve canals in the snout indicate the presence of whisker follicles, enabling mechanoreception for navigation in low-light conditions.^[39] These vibrissae likely originated as specialized hairs rooted in deep dermal follicles, with the trigeminal nerve expansions providing neural support for heightened sensitivity. Mammary glands, as specialized skin-derived structures from ectodermal tissues linked to hair follicles, are inferred to have arisen in early Jurassic mammaliaforms, though direct fossils are lacking; indirect evidence includes correlated soft-tissue impressions and genetic markers like MSX2 expression in related lineages, suggesting nutrient-secreting glands for offspring care.^[38] Certain Mesozoic synapsids developed specialized skin extensions for locomotion, exemplified by patagia in haramiyidan mammaliaforms. Jurassic euharamiyidans such as Vilevolodon and Maiopatagium possessed broad gliding membranes of furred skin stretched between elongated limbs and the body, supported by a furcula and styliform bones, facilitating arboreal gliding over forest canopies. These patagia, covered in fine guard hairs, represent convergent adaptations with modern gliding mammals and highlight the diversity of skin-based locomotor structures in early mammaliaforms. Color patterns in synapsid pelage, inferred from melanosome morphology in Jurassic and Cretaceous fossils, were predominantly dark and monotonous, with eumelanosomes indicating gray-brown hues for crypsis amid leaf litter.^[41] Phaeomelanosomes, responsible for reddish tones, appear rarely, suggesting limited pigmentation diversity until the Cenozoic radiation of crown mammals.^[41]

Metabolic and Thermoregulatory Traits

Synapsids exhibit a progressive evolutionary shift from ectothermic to endothermic metabolic strategies, marking a key physiological divergence from other amniotes. Basal synapsids, such as pelycosaurs, are inferred to have been ectothermic based on their sprawling limb posture, which limited efficient heat retention, and evidence of slow bone growth rates from histological analysis of long bones showing laminar or parallel-fibered bone tissue with low vascularity. This ectothermy likely supported a low metabolic rate, akin to modern reptiles, allowing energy conservation in the variable climates of the late Carboniferous and early Permian periods. In contrast, advanced synapsids, particularly therapsids, display anatomical adaptations indicative of emerging endothermy. Therapsids possessed nasal turbinates—complex bony or cartilaginous structures within the nasal cavity—that facilitated warming and humidifying inhaled air, a trait essential for conserving body heat and minimizing respiratory water loss in endotherms. Similarly, cynodont therapsids, closer to mammals, show high vascularity in their long bones, with extensive Haversian canals suggesting rapid blood flow to support elevated metabolic demands. These features collectively point to a higher basal metabolic rate, enabling sustained activity and potentially nocturnal or high-latitude lifestyles. Bone histology further underscores this metabolic transition through shifts in growth patterns. Early synapsids like pelycosaurs formed slow-growing, avascular bone, but by the late Permian, non-mammalian therapsids developed fibrolamellar bone—a highly vascularized tissue associated with rapid, uninterrupted growth and high metabolism in modern endotherms. In advanced cynodonts, such as those from the Triassic, evidence from oxygen isotopes and inner ear biomechanics suggests elevated body temperatures of around 30–34°C, indicating emerging endothermy with internal heat generation supplementing environmental sources.^[42]^[43] A 2022 study using inner ear biomechanics estimated body temperatures of approximately 34°C in non-mammalian mammaliamorphs from the Late Triassic, supporting a stepwise evolution toward full endothermy in mammals.^[43] This thermoregulatory evolution unfolded gradually, with initial signs of elevated metabolism appearing in Permian therapsids through enhanced respiratory efficiency and bone remodeling, culminating in fully endothermic, mammalian-like physiology by the Jurassic period in true mammals. Ancillary traits, such as insulating fur in cynodonts and the secondary palate in therapsids, complemented these metabolic advancements by aiding heat retention and efficient breathing, respectively.

Evolutionary History

Origins and Early Diversification in the Carboniferous

Synapsids originated through the divergence of the amniote lineage from sauropsids during the Late Carboniferous, approximately 310–319 million years ago (mya), marking the initial split within crown-group amniotes.^[44] This event occurred amid the diversification of early tetrapods following Romer's Gap, a period of sparse fossil preservation from about 360 to 345 mya that obscured initial amniote evolution.^[45] The earliest known synapsid body fossils date to around 312 mya, represented by small, lizard-like forms such as Protoclepsydrops haplous from the Joggins Formation in Nova Scotia, Canada, which exhibit primitive features including a single lower temporal fenestra in the skull for enhanced jaw musculature.^[46] These basal synapsids were adapted to swampy, coal-forming environments of the equatorial Euramerica paleocontinent, preying on insects and smaller tetrapods in floodplain ecosystems.^[47] The most basal synapsid clade, Ophiacodontidae, emerged in the Late Carboniferous and persisted into the Early Permian, characterized by elongated snouts, robust limbs, and piscivorous or insectivorous diets suited to wetland habitats.^[48] Genera like Ophiacodon and Archaeothyris (the latter known primarily from Early Permian deposits but with roots in Late Carboniferous ancestry) were typically under 2 meters in length, with slender bodies resembling modern lizards and teeth adapted for grasping slippery prey.^[49] Fossil evidence from sites such as the Joggins Formation includes not only Protoclepsydrops but also trackways attributable to basal synapsids, indicating quadrupedal locomotion in humid, forested lowlands.^[50] These early forms established the synapsid clade through key innovations like the temporal fenestra, which supported stronger bite forces compared to contemporary anapsid amniotes.^[51] Early diversification of synapsids was driven by the post-Romer's Gap radiation of terrestrial ecosystems, where increased oxygen levels and abundant arthropod prey facilitated the occupation of carnivorous niches previously dominated by stem-amniotes.^[52] By the late Westphalian stage (around 315–311 mya), synapsids had begun to split into lineages like ophiacodontids and the more herbivorous-leaning caseids, such as Eocasea martini from the Late Pennsylvanian of Kansas, reflecting adaptive shifts toward varied feeding strategies in carboniferous swamps.^[49] This initial radiation laid the groundwork for synapsid dominance in later periods, with fossil assemblages from Joggins preserving a snapshot of this transitional fauna alongside early diadectomorphs and other amniotes.^[53]

Permian Dominance and Key Lineages

The Permian period marked a peak in synapsid diversification, with pelycosaurian-grade forms radiating prominently in the Early Permian (Cisuralian) across equatorial regions of Pangea, achieving up to approximately 40 genera by the Kungurian stage.^[54] Pelycosaurs occupied diverse ecological niches, including apex predation and herbivory; for instance, Dimetrodon (Sphenacodontidae), a macro-carnivore reaching lengths of 3-4 meters, featured elongated neural spines forming a dorsal sail likely used for thermoregulation through solar absorption and radiative cooling.^[55] Similarly, Edaphosaurus (Edaphosauridae), an early terrestrial herbivore with comparable sail structures, utilized specialized grinding dentition for processing vegetation, exemplifying the group's adaptation to plant-based diets.^[54] Therapsids emerged in the late Kungurian and underwent rapid radiation following Olson's Extinction at the Kungurian-Roadian boundary, supplanting pelycosaurs as the dominant synapsids by the Middle Permian (Guadalupian).^[54] Early therapsid lineages included dinocephalians, robust forms such as Titanophoneus potens (Anteosauridae), which attained sizes up to 6 meters and served as top carnivores with powerful jaws for bone-crushing predation.^[56] Concurrently, dicynodonts within Anomodontia diversified as herbivores, evolving reduced dentition with keratinous beaks for cropping vegetation, which supported their proliferation into over 60 species by the early Capitanian and enabled exploitation of varied plant resources.^[57] Synapsids exerted profound ecological influence as the predominant terrestrial vertebrates in both Gondwanan and Laurasian faunas, numerically and taxonomically dominating Permian tetrapod assemblages, with therapsids alone comprising a substantial portion—often exceeding 50 species in peak assemblages—of known genera.^[58] This dominance underscored their roles in trophic webs, from predation to herbivory, shaping community structures amid increasing aridity and floral shifts.^[59] The Capitanian extinction event, approximately 260 million years ago, imposed selective pressures that curtailed pelycosaurian diversity, reducing species counts from 59 in the early Capitanian to just 2 by its close, effectively ending their prominence while sparing higher therapsid clades.^[54] Therapsids experienced an initial diversity dip but rebounded swiftly, with anomodonts like dicynodonts proving resilient and setting the stage for their continued success.^[57]

Triassic Transitions to Mammal-like Forms

The Permian-Triassic extinction event, occurring approximately 252 million years ago, devastated terrestrial vertebrate communities, with around 70% of families lost, yet certain synapsids rapidly recolonized post-extinction ecosystems. Lystrosaurus, a dicynodont therapsid, emerged as a quintessential "disaster taxon," dominating Early Triassic (Induan stage, ~252–251 Ma) faunas across Pangea, including sites in South Africa, Antarctica, and China.^[60] Its success stemmed from ecological versatility, including a generalist herbivorous diet, burrowing behavior, and developmental plasticity that enabled rapid growth in harsh, fluctuating environments, as evidenced by bone histology showing woven and parallel-fibered tissues indicative of fast life histories.^[60] By the mid-Early Triassic (Olenekian, ~251–247 Ma), Lystrosaurus assemblages comprised up to 95% of vertebrate fossils in some localities, facilitating initial ecosystem recovery before declining as diversity rebounded.^[61] Amid this recovery, therapsid lineages streamlined, with gorgonopsians and therocephalians representing the final non-cynodont therapsids. Gorgonopsians, apex predators of the Late Permian, persisted sparingly into the Early Triassic (Induan–Early Anisian, ~252–242 Ma) in regions like the Karoo Basin of South Africa and the Buntsandstein of Europe, but their populations dwindled due to competition and environmental instability.^[61] Similarly, therocephalians, small carnivorous forms, survived into the Middle Triassic (Anisian–Ladinian, ~242–237 Ma) in Gondwanan deposits from South Africa and Brazil, exhibiting skeletal features like reduced postcranial ossification that hinted at transitional physiologies, though they ultimately failed to diversify further.^[61] In contrast, cynodonts began emerging prominently from the mid-Triassic onward, with basal forms like Galesaurus appearing in the Early Triassic (~250 Ma) and radiating into diverse ecological roles by the Anisian.^[61] Advanced cynodonts, such as Thrinaxodon from the Early Triassic Lystrosaurus Assemblage Zone (~248 Ma), displayed early signs of endothermy, inferred from stable oxygen isotope analyses (δ¹⁸Oₚ values ranging from -3.9‰ to +2.1‰ in co-occurring taxa) indicating elevated body temperatures and improved thermoregulation compared to Permian ancestors.^[42] These transitions coincided with notable shifts in body size and habitat preferences among cynodonts, driven by competitive pressures from rising archosaur groups. Advanced cynodonts underwent a pattern of size reduction, with body mass estimates for 29 Triassic eucynodont species spanning 0.009 kg (e.g., Kuehneotherium) to over 123 kg (e.g., Aleodon), but showing a shift toward lower evolutionary rates of size increase in prozostrodontian lineages by the Carnian (~237–227 Ma).^[62] This miniaturization, peaking in disparity during the Carnian before declining due to extinctions of larger forms, likely represented a plesiomorphic small-bodied state rather than paedomorphic evolution, allowing persistence in underutilized niches.^[62] Many occupied nocturnal or fossorial habitats to evade diurnal archosaurs, supported by brain endocast reconstructions of Thrinaxodon revealing enlarged olfactory bulbs (18–26% of endocast length) and moderate encephalization quotients (0.16–0.38), enhancements suited for low-light sensory reliance on smell and touch via whisker-like structures.^[63] Such adaptations, including a secondary palate and refined jaw mechanics, underscored their proto-mammalian trajectory.^[61] By the Late Triassic (~237–201 Ma), non-mammalian synapsid lineages had largely vanished, leaving only mammaliaforms as persistent survivors. Dicynodonts like Lystrosaurus faded by the Middle Triassic, though other dicynodonts persisted until the Late Triassic, while gorgonopsians and therocephalians disappeared entirely by the Ladinian, and many non-mammaliaform cynodont lineages succumbed to ecological replacement by dinosaurs around 225 Ma, though specialized groups like tritylodontids endured into the Jurassic as the sole non-mammaliaform cynodont legacy alongside morganucodontids.^[61] This pruning set the stage for the cynodont-mammal continuum, with small-bodied forms like tritylodontids and morganucodontids enduring into the Jurassic.^[61]

Mesozoic Survival and Cenozoic Mammalian Radiation

During the Mesozoic Era, non-mammalian synapsids persisted beyond the Triassic, with lineages such as tritylodontid cynodonts surviving into the Late Jurassic, as evidenced by fossils like Bienotheroides wucaiensis from the Shishugou Formation in China, representing one of the last known non-mammaliaform synapsids.^[64] However, the fossil record of cynodonts exhibits significant gaps during the Jurassic and Cretaceous periods, particularly around the Jurassic-Cretaceous transition, where non-mammaliaform remains are scarce and largely restricted to Asia, underscoring the need for further paleontological exploration to clarify their decline.^[65] Early mammaliaforms, such as those in the family Morganucodontidae, bridged this interval, appearing in the Late Triassic and extending into the Middle Jurassic, with specimens like a newly described morganucodontan from the Upper Jurassic Morrison Formation displaying dental variations that highlight their adaptive persistence.^[66] Amidst dinosaur dominance, Mesozoic mammals and mammaliaforms remained small and ecologically marginalized, typically shrew-like in size—often under 100 grams—and occupying understory niches as insectivores or omnivores, which limited their body size expansion until the Cretaceous-Paleogene (K-Pg) boundary approximately 66 million years ago. Groups like multituberculates originated in the Middle Jurassic around 160 million years ago and diversified through the Cretaceous, achieving notable success with over 150 genera and adapting to herbivorous diets using specialized teeth.^[67] Similarly, monotremes began diversifying in the Early Cretaceous, with fossils from Australia and a 2023 discovery of the first South American monotreme, Patagorhynchus pascuali, from the Late Cretaceous indicating an extensive Gondwanan radiation into semiaquatic and burrowing lifestyles before the K-Pg extinction.^[68] These traits, including intermediate metabolic rates between reptiles and modern mammals as seen in Morganucodon, likely aided their survival in niche roles alongside larger reptiles.^[69] Recent 2025 studies using adaptive landscapes and fossil analyses indicate that the transition to upright posture in synapsids occurred later than previously thought, with early forms retaining sprawling gaits similar to reptiles until advanced therapsids.^[70] The K-Pg mass extinction event, which eradicated non-avian dinosaurs, triggered a profound radiation of surviving therian mammals in the Cenozoic, with placentals and marsupials rapidly achieving dominance through adaptive expansions into vacant ecological niches.^[71] Placentals underwent explosive diversification in the Paleocene and Eocene, exemplified by radiations into aquatic forms like cetaceans (whales) and aerial specialists like chiropterans (bats), filling marine, terrestrial, and volant habitats previously unoccupied by synapsids. Marsupials similarly radiated, particularly in South America and Australia, evolving diverse morphologies such as gliding possums and herbivorous wombats, though they were later outcompeted by placentals in many regions. This post-extinction burst increased mammalian species richness fourfold within the first million years, establishing the modern mammalian fauna. Recent discoveries have illuminated these Mesozoic transitions, with the 2011 description of Juramaia sinensis from the Late Jurassic of China (~160 million years ago) identifying it as the earliest known eutherian mammal and pushing back the placental-marsupial divergence. In the 2020s, finds such as two new Jurassic mammaliaforms (Shenshoua and Sinobaatar) from China have revealed early dental and middle ear innovations, while a 2024 shuotheriid species and a 2025 tritylodontid from the Jurassic further fill gaps in mammaliaform and non-mammalian synapsid diversity, refining our understanding of pre-Cretaceous evolution.^[72]^[73]

Phylogenetic Relationships

Position Within Amniota

Synapsida and Sauropsida together comprise the crown group Amniota, defined as the most recent common ancestor of these two clades and all of its descendants. Molecular clock analyses using genome-wide datasets estimate the common ancestor of crown-group Amniota lived approximately 319 million years ago (Ma; 95% confidence interval: 308.5–334.7 Ma) during the Carboniferous, though recent fossil trackway evidence suggests an earlier origin by ~359–354 Ma in the Early Carboniferous.^[74]^[75] This divergence marks the basal split within Amniota, with Synapsida leading to mammals and Sauropsida encompassing reptiles and birds.^[74] Evidence for Synapsida as the sister clade to Sauropsida includes shared synapomorphies of Amniota, such as the amniotic egg, which encloses the embryo in a protective fluid-filled sac and allows fully terrestrial reproduction independent of aquatic environments.^[76] Molecular clock estimates further support this relationship, with a median age of ~319 Ma, consistent with the earliest fossil records of derived amniotes now extended into the Early Carboniferous by trackway discoveries.^[74]^[75] Controversies surrounding parareptiles, once considered a separate amniote clade, have reinforced Synapsida's position as the sister to a more inclusive Sauropsida. For instance, recent phylogenetic analyses place families like Millerettidae as stem-sauropsids or close sisters to crown-group diapsids such as Neodiapsida, integrating former parareptiles into Sauropsida and excluding them from a basal position relative to Synapsida.^[77] Fossil calibrations for the amniote tree often draw on early taxa like Westlothiana, a Viséan-stage (approximately 338 Ma) tetrapod from the Early Carboniferous that exhibits amniote-like features and is positioned as potentially basal to Synapsida or the broader amniote stem in some analyses.^[78] This placement helps constrain the minimum age of the Synapsida-Sauropsida divergence and aligns with the Carboniferous origins of amniote diversification, now supported by even earlier trackway evidence.^[78]^[75]

Interrelationships Among Synapsid Groups

The interrelationships among synapsid groups are primarily resolved through cladistic analyses of morphological characters, revealing a basal grade of non-therapsid synapsids ("pelycosaurs") succeeded by the more mammal-like therapsids. Basal branching patterns position Ophiacodontidae as the sister group to a clade comprising Eupelycosauria and Therapsida, with Caseasauria representing an early offshoot within this structure. This topology is supported by comprehensive meta-analyses integrating hundreds of morphological matrices, which confirm the monophyly of Eupelycosauria (including sphenacodontians and edaphosaurids) as more closely related to Therapsida than to ophiacodontids or caseasaurs. Within Eupelycosauria, sphenacodontians form a basal subclade sister to Therapsida, as evidenced by shared cranial features like enlarged temporal fenestrae in early therapsids.^[79] Within Therapsida, Biarmosuchia occupies the basalmost position, followed by a branching pattern that unites Anomodontia and Theriodontia as successive sister clades. Theriodontia itself comprises Gorgonopsia as the basal subclade, with Cynodontia (including therocephalians in some resolutions) as its sister group, a relationship bolstered by shared postcranial traits such as more erect limb postures.^[80] Anomodontia, encompassing herbivorous forms like dicynodonts, branches off prior to the diversification of theriodonts, supported by dental specializations absent in gorgonopsians. These interrelationships are derived from large-scale phylogenetic datasets emphasizing skull and skeletal morphology, with fossil integrations from Permian localities providing critical calibration. Cynodontia exhibits a progressive hierarchy toward mammalian features, with Epicynodontia (including taxa like Thrinaxodon and Galesaurus) forming a basal grade leading to Eucynodontia.^[80] Within Eucynodontia, Probainognathia represents an intermediate clade characterized by advanced auditory structures, directly ancestral to Mammaliaformes, which includes the earliest near-mammalian forms with differentiated teeth and secondary palate elements.^[80] Haramiyidans, known from Late Triassic and Jurassic fossils, are positioned as early members of crown Mammalia, potentially within or sister to Allotheria, based on multituberculate-like dentition and inferred arboreal adaptations. This cynodont phylogeny is reinforced by high-resolution 3D imaging of cranial fossils, highlighting evolutionary trends in jaw mechanics and ear ossification.^[80] These resolved topologies stem from morphological matrices, such as those compiled in meta-phylogenetic syntheses incorporating over 260 datasets, which prioritize cranial and postcranial characters while accounting for stratigraphic and biogeographic constraints. For instance, a 2017 study on anomodont interrelationships utilized expanded matrices to refine placements within Therapsida, integrating new fossil data to resolve polytomies in earlier analyses. Cladograms generated from such methods consistently depict synapsid evolution as a series of nested monophyletic groups, with fossil discoveries periodically refining branch supports but upholding the core structure.

Uncertainties and Recent Discoveries

One persistent uncertainty in synapsid phylogeny concerns the placement of varanopids, which have been debated as either basal synapsids or stem-sauropsids (diapsids).^[81] Recent analyses using computed tomography (CT) scans of cranial endocasts from multiple varanopid specimens, including Varanops brevirostris and Heleosaurus scholtzi, reveal neurosensory features such as an ossified inner ear and maxillary canal morphology that align closely with other early synapsids rather than sauropsids, supporting their position as the earliest-diverging synapsids.^[81] However, a 2020 morphological study proposed a diapsid affinity based on postcranial traits, and preliminary 2024 analyses suggest some varanopid taxa may nest within sauropsids, highlighting ongoing topological instability at the base of Amniota.^[81]^[82] Advances in dicynodont paleobiology have revised timelines for their extinction, addressing gaps in Late Triassic diversity. Prior estimates from 2003 posited dicynodont survival into the Cretaceous based on Australian fragments, but reexamination identified these as Cenozoic mammal bones, limiting their range to the Triassic.^[83] The 2019 discovery of Lisowicia bojani in Norian-Rhaetian deposits of Poland extends undisputed dicynodont persistence into the Late Triassic (~210-201 Ma), representing the youngest known member of the group and contradicting earlier views of their decline by the Middle Triassic (~240 Ma).^[84] This ~9-ton herbivore with erect limbs indicates dicynodonts coexisted with early dinosaurs as large megaherbivores, filling ecological roles previously attributed solely to archosaurs.^[84] Jurassic cynodont records remain sparse, particularly for non-mammalian forms, creating gaps in understanding the transition to mammals. The Yanliao Biota (Middle-Late Jurassic, ~164-157 Ma) in northeastern China has yielded new mammaliaform fossils since 2023, including the euharamiyidan Mirusodens caii, a skeletal specimen preserving soft tissues that elucidates allotherian dental and locomotor evolution. Updated radiometric dating of Yanliao strata in 2023 constrains the biota to 164-157 Ma, aligning it with the diversification of crown-group mammals and highlighting rapid ecological shifts among cynodonts toward insectivory and arboreality. These finds underscore the underrepresentation of Mesozoic non-mammalian cynodonts outside Laurasia, with few post-Triassic Gondwanan examples, emphasizing the need for broader sampling to resolve mammal origins. A major 2025 discovery has recalibrated the timeline of early amniote evolution: trackways attributed to crown-group amniotes from the Snowy Plains Formation in Australia (~359–354 Ma, Early Tournaisian) and Silesia, Poland (~330–323 Ma), push back the inferred origin of the Synapsida-Sauropsida split by at least 35–40 million years compared to prior fossil records. This evidence suggests crown Amniota existed much earlier in the Carboniferous, prompting reconciliation with molecular clock estimates around 319 Ma and highlighting potential ghost lineages or calibration needs in phylogenetic models.^[75] Methodological innovations have begun resolving basal synapsid ambiguities. High-resolution CT and synchrotron imaging of Permian specimens, such as those from Orobates and Dimetrodon, have revealed neurosensory adaptations like enlarged trigeminal canals, informing phylogenetic placements and behavioral inferences since the early 2020s.^[85] Integrated morphological phylogenies incorporating postcranial data, as in 2024 analyses of cynodont origins, recover more stable trees for basal groups but highlight paraphyly in therocephalians, advancing beyond earlier partition-based conflicts. Although phylogenomic approaches remain limited for extinct basal taxa due to DNA preservation issues, calls persist for intensified excavations in Gondwanan deposits, where synapsid diversity is poorly documented compared to Laurasia, to test Laurasian-origin hypotheses for therapsids.^[86]

References

[1]
Phylogeny, function and ecology in the deep evolutionary history of ...
Synapsida, the amniote clade that includes all living mammals and their extinct forerunners, spans an estimated 320 Myr of evolutionary history [3,4], three ...
[2]
Mammalian vs. Reptilian Skull Morphology | Writing in Biology
Feb 1, 2019 · Mammal skulls are synapsid: they have a single, large opening called a temporal fenestra behind each eye that allows for more attachment of jaw muscles.Missing: definition | Show results with:definition
[3]
[PDF] A Description of the Cranial Anatomy of a Specimen of the ...
Synapsida, the clade that includes mammals and their ancestors, represents one of the two principal branches of amniote vertebrates. Consequently, studying the ...
[4]
Synapsida - Tolweb.org
Aug 14, 2011 · Synapsids include mammals and all extinct amniotes more closely related to mammals than to reptiles. Synapsids are the dominant large ...
[5]
GEOL 104 Four on the Floor: Tetrapod Colonization of the Land
Aug 15, 2025 · The name "synapsid" refers to a condition where the skull had a complete infratemporal fenestra with a lowerbar but no supratemporal fenestra.Missing: temporal | Show results with:temporal
[6]
Lecture 8 - Tetrapods
Even within Dimetrodon, which is a relatively primitive synapsid, we can see two of the features that characterize synapsid evolution as a whole. 1. ...Missing: definition | Show results with:definition
[7]
Jaws to ears in the ancestors of mammals - Understanding Evolution
All the animals you see on this evogram are synapsids, the group that gave rise to the mammals. Sometimes synapsids are called “mammal-like reptiles;” ...Missing: definition | Show results with:definition
[8]
Biology, Biological Diversity, Vertebrates, Reptiles - OERTX
The initial split was into synapsids and sauropsids. Synapsids include all mammals, including extinct mammalian species. Synapsids also include therapsids, ...Missing: Synapsida | Show results with:Synapsida<|control11|><|separator|>
[9]
Evolution of the mammal-like reptiles - Oxford Academic
Reisz (1986) noted the synapomorphies of the Synapsida, which are therefore the characters that distinguish pelycosaurs from non-synapsid amniotes: temporal ...
[10]
Early Origins of Divergent Patterns of Morphological Evolution on ...
Their earliest fossils occur around 318 million years ago and already include representatives of the two major subgroups that persist to the present day ( ...
[11]
The early evolution of synapsids, and the influence of sampling on ...
Aug 6, 2025 · Synapsids dominated the terrestrial realm between the late Pennsylvanian and the Triassic. Their early evolution includes some of the first ...
[12]
[PDF] A reevaluation of early amniote phylogeny
The phylogeny suggested by Reisz (1986) was used to optimize the states present in the four most basal families on a tree of the Synapsida. The condition at the ...
[13]
Synapsida (Extinct Mammal Relatives) - Sues - Wiley Online Library
Sep 30, 2020 · Based in part on the previous version of this eLS article “ Synapsida ('Mammal-Like Reptiles')” (2001) by Hans-Dieter Sues. Read the full ...Missing: Linnaean classification
[14]
The structure of the Columella auris in the Pelycosauria
... Pelycosauria / by E. D. Cope. By. Cope, E. D. (Edward Drinker), 1840-1897. Publication Details. [Washington, 1885]. Year. 1885. DOI. https://doi.org/10.5962/bhl ...
[15]
https://digitallibrary.amnh.org/server/api/core/bi...
... 1889 and 1895. His most important work on the ... Theromorpha, afterwards changed to Theromora, for all the mammal-like forms. Between 1889 and 1897 Zittel, ...
[16]
[PDF] A review of Robert Broom's therapsid holotypes - CORE
In a resulting paper (Broom 1910), described by Watson (1952) as 'the most important he ever wrote', Broom resuscitated the theory, first propounded by Cope ...
[17]
The origin and early radiation of the therapsid mammal‐like reptiles ...
Feb 16, 2006 · Therapsida is actually nested cladistically within the Pelycosauria, and in turn Mammalia is nested within Therapsida, so that both these mammal ...Missing: critiques | Show results with:critiques
[18]
The origin and evolution of Cynodontia (Synapsida, Therapsida ...
Mar 5, 2024 · Cynodonts are a highly distinctive clade and can be recognized by nearly 30 synapomorphies, mostly craniomandibular, including homologs of many ...
[19]
Cladistic analysis of Caseidae (Caseasauria, Synapsida): using the ...
Sep 23, 2015 · In the case of non-therapsid synapsids (analysed synapsids ... 1B) constitutes the sister group of the clade Edaphosauridae + Sphenacodontia.Missing: therapsida subgroups
[20]
Morphology and Evolutionary Significance of the Atlas-Axis ...
Dec 1, 2011 · —The main systematic and phylogenetic debate in varanopids centres on the diversity of South African specimens and the phylogenetic position ...
[21]
Evolution of the temporal skull openings in land vertebrates: A ...
Jan 10, 2024 · Synapsida is characterized by one infratemporal opening (monapsidy sensu Werneburg, 2019). One opening is also seen in many Permian parareptiles ...
[22]
Interrelationships of basal synapsids: Cranial and postcranial ...
Aug 6, 2025 · Perhaps the most conspicuous potential synapomorphy of Synapsida is a single lateral temporal fenestra in the skull, primitively bounded on the ...
[23]
The Origin of Temporal Fenestrae - ResearchGate
Aug 6, 2025 · The evolution of temporal fenestrae is correlated with the expansion of the jaw adductor muscles and the doming of the reptile skull through time.
[24]
Evolution of tooth morphological complexity and its association with ...
Aug 12, 2024 · Our phylogenetic comparative analyses showed a significant correlation between an increase in tooth complexity and a posterior shift in the dentition position.
[25]
A Carboniferous synapsid with caniniform teeth and a reappraisal of ...
Dec 15, 2021 · Nonetheless, the early evolution of heterodonty in basal synapsids is poorly understood, especially in the mandibular dentition.
[26]
The evolution of the synapsid tusk: insights from dicynodont ... - NIH
Oct 27, 2021 · Dicynodont tusks are the earliest iteration of this morphological novelty, evolving well before the definitive mammalian patterns of tooth ...
[27]
Convergent dental adaptations in the serrations of hypercarnivorous ...
Dec 16, 2020 · 3. Description · (a). Gorgonopsian serrations. Gorgonopsian serrations in NHCC LB248 and LB334 are formed by both enamel and dentine (figure 1d).
[28]
Dental ontogeny in extinct synapsids reveals a complex evolutionary ...
Nov 7, 2018 · This study shows for the first time that the ligamentous tooth attachment system is not unique to crown mammals within Synapsida.
[29]
[PDF] Postilla - EliScholar - Yale University
This paper is the initial report of a comprehensive reconstruc- tion of adductor jaw musculature in Dimetrodon, which represents a pretherapsid stage of ...
[30]
https://elischolar.library.yale.edu/cgi/viewcontent.cgi?article=1115&context=peabody_museum_natural_history_postilla
[31]
https://doi.org/10.1111/ede.12426
[32]
New therapsid specimens and the origin of the secondary hard and ...
Aug 10, 2025 · ... Early crocodylomorphs exhibited open palates with anteriorly positioned choanae, while later forms developed ossified secondary palates and ...
[33]
Craniodental anatomy in Permian–Jurassic Cynodontia and ... - NIH
May 15, 2023 · We review recent works that illustrate how evolutionary changes concentrated in the cranial and dental morphology of mammalian ancestorsMissing: characteristics | Show results with:characteristics
[34]
Kembawacela yajuwayeyi n. sp., a new cistecephalid species ...
... secondary palate of K. yajuwayeyi, being pierced by a large palatine foramen ... Therefore, the quadrate foramen in dicynodonts can only be compared with ...
[35]
Developmental palaeontology in synapsids: the fossil record of ...
Jan 13, 2010 · The earliest synapsids, including sphenacodontians, preserved the characteristic indeterminate growth of basal amniotes. In therapsids growth ...Missing: nomenclature debates paleontology
[36]
[PDF] The Origin and Evolution of Mammals - RERO DOC
This book arose from twin aims: to update my 1982 book on the origin of mammals in relation to the steady stream of new fossils that have been found or re- ...
[37]
[PDF] The Mammary Gland and Its Origin During Synapsid Evolution
The mammary gland likely evolved from an ancestral apocrine-like gland associated with hair follicles, with early synapsids possibly secreting some nutrients.
[38]
Palaeoneurological clues to the evolution of defining mammalian ...
May 9, 2016 · Here we report the paleoneurology of a variety of non-mammaliaform therapsids (NMT) using in silico studies and X-ray microtomography (μCT).
[39]
What fossils reveal about the hairy history of mammals' ancestors
Jun 23, 2016 · The oldest skin impression preserved with hair is 160 million years old and belongs to a mammal. Then you have skin impressions of therapsids ...Missing: follicles | Show results with:follicles
[40]
Mesozoic mammaliaforms illuminate the origins of pelage coloration
Mar 13, 2025 · Our results suggest that the melanosome variation and color expansion seen in extant mammals may have occurred during their rapid radiation and diversification.
[41]
Phylogeny of Synapsida-Diapsida with divergence time constrained ...
Thus, the preliminary range based on maximum-minimum ages for the synapsid-diapsid divergence is 332.3-306.1 MYA. The upper bound (306.1 MYA) is probably the ...
[42]
Can We Reliably Calibrate Deep Nodes in the Tetrapod Tree? Case ...
Oct 16, 2020 · The former is generally referred to as Romer's Gap and likely coincides with the origin of the tetrapod crown group, whereas the second, ...
[43]
[PDF] Synapsida (Extinct Mammal Relatives)
Sep 30, 2020 · Early synapsids were insectivorous or carnivorous, but her- bivory ... phylogenetic classifications (Angielczyk and Kammerer, 2018).
[44]
[PDF] Early Evolutionary History of the Synapsida
Synapsids appear to dominate the earliest stages of terrestrial vertebrate evolution, with abundant fossil remains and numerous taxa (Reisz. 1986). They ...
[45]
Long bone histology of Ophiacodon reveals the geologically earliest ...
Ophiacodon (Marsh, 1878) is a basal synapsid that belongs to the clade Ophiacodontidae (Nopcsa, 1923) that existed between the Late Carboniferous and the Early ...
[46]
The Oldest Caseid Synapsid from the Late Pennsylvanian of Kansas ...
Apr 16, 2014 · Conclusions. Eocasea martini represents the earliest and most basal known caseid synapsid, extending the fossil record of Caseasauria into the ...
[47]
(PDF) The Joggins Fossil Cliffs UNESCO World Heritage site
Aug 6, 2025 · The upper Joggins Formation has previously yielded body fossils of Pygocephalus shrimp preserved in organic-rich limestones and sideritic ...
[48]
Introduction to the Pelycosaurs
Pelycosaurs are the earliest, most primitive synapsids, a group characterized by a single dermal opening in the skull permitting muscle attachment to the jaw.Missing: sphenacodontia | Show results with:sphenacodontia
[49]
Confirmation of Romer's Gap as a low oxygen interval constraining ...
Romer's Gap ends with a significant radiation of new land plant, arthropod, and stegocephalian taxa beginning ≈345 MYBP but undergoing its highest rate of new ...Missing: synapsid | Show results with:synapsid<|control11|><|separator|>
[50]
The earliest reptiles - ResearchGate
Aug 9, 2025 · Our results demonstrate that synapsids and reptiles underwent an early divergence of macroevolutionary patterns. This laid the foundation ...
[51]
The Early Evolution of Synapsida (Vertebrata, Amniota) and the ...
... Synapsids. 42. Hypotheses of the Relationships of Pelycosaurian-grade Synapsids. 43. Expanding Phylogenetic Analysis of Pelycosaurian-grade Synapsids. 46.<|control11|><|separator|>
[52]
Comparative osteohistology of hyperelongate neural spines in the ...
Mar 29, 2011 · ... (Reisz 1986; Modesto and Reisz 1990). ... Numerous authors have advocated a thermoregulatory function for the sail of the sphenacodontid Dimetrodon ...
[53]
Carnivorous dinocephalian from the Middle Permian of Brazil and ...
We present evidence for the presence of a terrestrial carnivorous vertebrate from the Middle Permian of South America based on a complete skull.
[54]
Global Taxonomic Diversity of Anomodonts (Tetrapoda, Therapsida ...
This paper investigates diversity patterns in Anomodontia, an extinct group of therapsid synapsids ('mammal-like reptiles'), through time and in particular ...Missing: percentage | Show results with:percentage
[55]
A comparison of Late Permian Gondwanan and Laurasian amniote ...
Aug 7, 2025 · ... Permian and Triassic anomodont-bearing tetrapod faunas ... fauna that is dominated both numerically and taxonomically by therapsid synapsids.
[56]
Recovery from the most profound mass extinction of all time - NIH
The end-Permian mass extinction, 251 million years (Myr) ago, was the most devastating ecological event of all time, and it was exacerbated by two earlier ...
[57]
Living fast in the Triassic: New data on life history in Lystrosaurus ...
Lystrosaurus was one of the few tetrapods to survive the Permo-Triassic mass extinction, the most profound biotic crisis in Earth's history.
[58]
The Triassic: A pivotal period in tetrapod evolution - Sues - 2024
Feb 9, 2024 · It is noteworthy that Owen already noted the close similarity between the Early Triassic cynodont synapsids such as Galesaurus and mammals (Owen ...
[59]
Oxygen isotopes suggest elevated thermometabolism within ... - eLife
Jul 18, 2017 · Two therapsid clades, dicynodonts and cynodonts, independently acquired and developed an endotherm-like thermoregulation during the Late ...
[60]
Weighing in on miniaturization: New body mass estimates for ...
Jan 16, 2024 · We estimated the body masses of 29 species, ranging from Theriocephalia to Mammaliaformes, providing the largest collection of Triassic cynodont ...Missing: nocturnal | Show results with:nocturnal
[61]
[PDF] BRAIN MORPHOLOGY OF THE EARLY TRIASSIC CYNODONT ...
Studies have suggested that basal cynodonts may have been nocturnal, fossorial and could have cared for young. Reconstructed morphological changes to ...
[62]
Bienotheroides wucaiensis sp. nov., a new tritylodontid (Cynodontia ...
Feb 6, 2025 · Bienotheroides is a genus of highly specialized tritylodontids, characterized by a short and wide snout, ventrally expanded zygomatic process, ...
[63]
Enigmatic teeth from the Jurassic–Cretaceous transition of Morocco ...
Thus, the non-mammaliaform cynodont fossil record is really poor around the Jurassic–Cretaceous transition, and it is restricted to Asia. Hence, in the current ...
[64]
[PDF] A morganucodontan mammaliaform from the Upper Jurassic ...
Mar 30, 2022 · Morganucodontans are a rare component of Late Jurassic faunas but display surprising dental diversity through variations in a tooth count and ...
[65]
Earliest Evolution of Multituberculate Mammals Revealed by a New ...
Aug 16, 2013 · Early Multi Multituberculate mammals (multis) first arose in the Jurassic and became extinct in the Oligocene, a span of over 100 million years ...
[66]
First monotreme from the Late Cretaceous of South America - Nature
Feb 16, 2023 · The diversification of monotremes towards the end of the Mesozoic suggested by the present discovery implies that an extensive and still ...
[67]
Reptile-like physiology in Early Jurassic stem-mammals - PMC - NIH
Oct 12, 2020 · Measurements of femoral nutrient foramina show Morganucodon had blood flow rates intermediate between living mammals and reptiles, suggesting ...Missing: Morganucodontidae | Show results with:Morganucodontidae
[68]
Evolutionary Models for the Diversification of Placental Mammals ...
The Long Fuse Model predicts the occurrence of placental fossils deep in the Cretaceous. Possible eutherian forms are recognized in the fossil record as far ...Missing: cynodont | Show results with:cynodont
[69]
Two Jurassic mammaliaforms from China shed light on mammalian ...
Apr 3, 2024 · Two Jurassic mammaliaforms from China, revealing the earliest dental diversification, mandibular middle ears, and articular-quadrate joint transformation of ...Missing: 2020s synapsids<|separator|>
[70]
New Jurassic Shuotheriid Species Sheds Light on Early Evolution of ...
Apr 3, 2024 · This discovery highlights the significance of pseudotribosphenic characteristics in elucidating the first diversification of mammaliaforms.Missing: 2020s Cretaceous synapsids
[71]
Successive climate crises in the deep past drove the early evolution ...
Aug 19, 2022 · Fig. 1. Revised evolutionary tree and divergence times for the major groups of early amniotes, early synapsids, and reptiles.
[72]
Amniote - an overview | ScienceDirect Topics
Synapsida is the clade represented today by mammals; they are commonly called the mammal-like reptiles, an inappropriate and misleading name. Pelycosaurs were ...
[73]
Evolutionary assembly of crown reptile anatomy clarified by late ...
Aug 28, 2025 · We present a substantially revised phylogenetic hypothesis, informed by new anatomical data from high-resolution synchrotron tomography of ...
[74]
A Mississippian (early Carboniferous) tetrapod showing early ...
Apr 14, 2022 · ... amniote-like Westlothiana. In some recent studies, these taxa have been placed phylogenetically as the earliest known members of stem ...
[75]
https://www.nature.com/articles/s41586-025-08884-5
[76]
https://www.sciencedirect.com/topics/immunology-and-microbiology/amniote
[77]
https://peercommunityjournal.org/articles/10.24072/pcjournal.620/
[78]
The rise and fall of Varanopidae† (Amniota, Synapsida) - Frontiers
Our results suggest that this clade diversified in the Pennsylvanian but dwindled in diversity in the Cisuralian.Introduction · Methods · Results · DiscussionMissing: modern | Show results with:modern<|control11|><|separator|>
[79]
Osteology and relationships of the Late Triassic giant dicynodont ...
Sep 14, 2024 · Scientific Reports 2019;9:1–9. Racki G, Lucas SG. Timing of dicynodont extinction in light of an un- usual Late Triassic Polish fauna and ...
[80]
https://doi.org/10.1002/ar.25394
[81]
(PDF) Early synapsids neurosensory diversity revealed by CT and ...
Apr 12, 2024 · Here, using CT and Synchrotron scanning, we document for the first time three extreme cases of neurosensory and behavioral adaptations that ...Missing: phylogenomics resolving
[82]
New Basal Synapsid Supports Laurasian Origin for Therapsids
Sep 1, 2009 · A rich, nearly continuous 315 million year fossil record documents the evolutionary history of a diverse clade of synapsid amniotes that ...Systematic Palaeontology · Discussion · Appendices