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Diapsid

Diapsids are a major of sauropsid reptiles distinguished by the presence of two pairs of temporal fenestrae—openings in the behind each eye that accommodate jaw muscles—originating in the late period around 300 million years ago. This configuration, which enhances cranial strength and muscle attachment for powerful bites, defines the group and includes a vast array of extant and extinct forms such as , , , crocodilians, , , and numerous lineages like pterosaurs and dinosaurs. Diapsids represent the dominant radiation of reptiles, achieving unparalleled diversity in terrestrial, aquatic, and aerial environments throughout the era and into the present. The evolutionary history of diapsids traces back to early amniotes, with the earliest known member being from the Late Carboniferous of , exemplifying the primitive diapsid skull with fully open fenestrae. Over time, diapsids diverged into two primary lineages during the Permian and periods: Lepidosauromorpha, which includes squamates ( and , ~12,000 species as of 2025) and sphenodontians (); and , encompassing archosaurs such as crocodilians, extinct non-avian dinosaurs, pterosaurs, and (the only surviving dinosaur lineage). Turtles, long debated for their anapsid-like skulls lacking visible fenestrae, are now firmly placed within Diapsida as the to Archosauria, based on molecular phylogenomic analyses of genes showing their divergence around 255 million years ago and secondary closure of the temporal openings during shell evolution. Diapsids exhibit remarkable , with subgroups exploiting diverse niches: lepidosaurs dominate modern terrestrial and subterranean habitats with limbless forms like and specialized climbers like geckos, while archosaurs include the apex predators of the , such as theropod dinosaurs leading to flight; other diapsids include specialists like ichthyosaurs (extinct forms convergent on fish-like body plans). Their success is underscored by physiological innovations, including ectothermy in most forms (with endothermy in and some dinosaurs), amniotic eggs for terrestrial reproduction, and scaly for water retention. Today, diapsids comprise approximately 23,500 living species as of , far outnumbering other clades, and continue to shape ecosystems as pollinators, predators, and herbivores.

Anatomy

Skull Structure

Diapsids are defined as a of amniotes characterized by the presence of two pairs of e in the : the upper temporal fenestra (supratemporal fenestra) and the lower temporal fenestra (infratemporal fenestra), located behind the orbits. These openings allow for the expansion and increased attachment area of the adductor muscles, such as the temporalis and pterygoideus, thereby enhancing bite force while reducing overall weight compared to more solid crania in basal amniotes. The infratemporal bar, formed by the jugal and quadratojugal bones, separates the upper and lower fenestrae, maintaining structural integrity while permitting muscular expansion. In some derived diapsids, additional fenestrae appear, including the antorbital fenestra anterior to the orbit in archosauromorphs, which lightens the snout and may house pneumatic diverticula, and the mandibular fenestra in the lower jaw of archosaurs, accommodating the intramandibularis muscle for improved jaw mechanics. These features contribute to the functional diversity of diapsid crania, adapting to varied feeding and sensory demands across the clade. Evolutionary modifications to the temporal fenestrae occur in advanced lineages; for instance, in (lepidosaurs), the fenestrae are reduced or fused due to kinetic adaptations for swallowing large prey, with the loss of the infratemporal bar exposing the braincase. In archosaurs, the fenestrae often expand, as seen in where modifications support lightweight construction and integrate with structures for enhanced aerial hearing via the elongated . Primitive , such as the Permian Youngina capensis, exhibit fully open upper and lower temporal fenestrae bordered by distinct bony arches, representing the ancestral condition with minimal secondary closure. In contrast, derived forms like crocodilians retain the diapsid pattern with prominent supratemporal and infratemporal fenestrae, though the orbits lack sclerotic rings unlike in many and ; these fenestrae support powerful jaw adduction suited to ambush predation.

Postcranial Features

Diapsids exhibit a characterized by distinct regionalization into , dorsal (trunk), and caudal (tail) segments, with variations in centrum shape reflecting locomotor and structural adaptations across the . In early diapsid forms, such as those from the Permian and , vertebral centra are typically amphicoelous, featuring concave anterior and posterior faces that facilitate flexibility in the . Later diapsids, including some lepidosaurs and archosauromorphs, show procoelous centra in certain regions, with a concave anterior face and convex posterior, enhancing shock absorption during movement. This vertebral morphology supports diverse gaits, from the flexible undulation in squamates to the more rigid support in archosaurs. Many diapsids possess , paired dermal ossifications forming a ventral along the that provides structural support to the belly and aids in by stabilizing the trunk during . These belly are particularly prominent in basal diapsids, crocodilians, and some dinosaurs, where they articulate with the and to reinforce the ventral body wall against gravitational and torsional stresses. In contrast, are reduced or absent in and many snakes, reflecting secondary losses associated with flight or limblessness. The postcranial skeleton of diapsids includes specialized limb girdles adapted to sprawling or erect postures, with the pectoral girdle featuring a scapulocoracoid that varies markedly between major subclades. In archosauromorphs, the scapulocoracoid is robust and often fused, providing strong anchorage for muscles that support semi-erect to fully erect gaits in forms like dinosaurs and crocodilians. Conversely, in lepidosaurs such as and tuataras, the scapulocoracoid is reduced and more loosely articulated, accommodating the sprawling posture typical of these groups. Pelvic girdles similarly reflect postural diversity, with expanded ilia and robust pubes-ischia in archosauromorphs enabling upright limb positioning, while lepidosaur pelves feature broader, more flexible acetabula for lateral limb excursion in sprawling locomotion. Forelimbs in diapsids often feature elongated and bones, which enhance pronation-supination and extend reach for or predation. This elongation contributes to agile maneuvers in sprawling taxa like , where the parallel shafts allow for effective substrate contact. Specific dermal specializations include osteoderms, polygonal bony plates in , present in crocodilians for armor-like and in some dinosaurs such as ankylosaurs and stegosaurs for defensive reinforcement along the and caudal regions. Additionally, tail represents a derived postcranial trait in (squamates), where fracture planes in caudal vertebrae enable voluntary tail shedding as an anti-predator mechanism, followed by regeneration of a cartilaginous . The lightweight cranial architecture enabled by temporal fenestrae complements these postcranial traits by reducing overall mass for enhanced agility.

Evolutionary Origins

Fossil Record

The fossil record of diapsids begins in the Late Carboniferous to Early Permian, approximately 310–300 million years ago (Ma), marking their initial diversification among early amniotes. Key early localities include the Karoo Basin in South Africa, which has yielded Permian specimens preserving characteristic diapsid skull fenestrae, and the Texas Red Beds in the United States, where Lower Permian deposits have produced early amniote remains. Throughout the Permian, diapsid fossils appear in several major horizons, including the Tropidostoma-Dicynodon zones of the Karoo Basin, exemplified by taxa like Youngina from . Diapsids remained relatively rare during this period but underwent explosive radiation in the following the end-Permian mass extinction, around 252 Ma, leading to their ecological dominance across terrestrial, marine, and aerial environments. This diversification continued into the and , with diapsids persisting and adapting through the to the present day. The record is exceptionally rich, encompassing thousands of extinct species across diverse clades, with major Lagerstätten providing exceptional preservation. The in has yielded well-preserved pterosaurs and early avialan birds, while the in has produced abundant skeletons, highlighting diapsid dominance in ecosystems. Recent discoveries from 2021–2025, including new stem saurian reptiles from the late Permian of , have refined timelines for early diapsid evolution by filling gaps in the Gondwanan record.

Earliest Forms

The earliest known diapsids appeared in the Late Carboniferous period, approximately 305 to 300 million years ago, during the Pennsylvanian epoch. These primitive forms were small, agile reptiles that bridged the gap between earlier reptiliomorphs and the more diverse diapsid radiation. kansensis, from the Upper Pennsylvanian of , , represents a classic early example, with fossils dated to around 302 million years ago. Araeoscelidians, such as Araeoscelis from the Early Permian of (approximately 290 million years ago), have also been proposed as basal diapsids, though their exact position remains debated. Morphologically, these early diapsids were lizard-like in build, typically measuring 20 to 60 centimeters in length, with slender bodies adapted for terrestrial life. Their skulls exhibited the defining diapsid condition: two pairs of temporal fenestrae (upper and lower temporal openings) that were fully open and unobstructed by bony bars, facilitating jaw muscle attachments for efficient feeding. Postcranially, they possessed a sprawling gait, with limbs positioned laterally to the body, enabling scuttling movement similar to modern lizards. Dental structures, including sharp, conical teeth of varying lengths without significant crushing adaptations, indicate an insectivorous diet, consistent with their small size and predatory niche in Paleozoic ecosystems. These taxa display transitional features linking them to earlier reptiliomorphs, such as the captorhinids, through shared primitive traits like simple, homodont and lightweight skeletal construction suited to insectivory. Protorothyridids, intermediate forms between captorhinids and , further illustrate this progression, with phylogenetic analyses placing them closer to based on cranial and postcranial similarities. This morphological continuity underscores the gradual of the from basal eureptilian ancestors. Recent phylogenetic studies have challenged the inclusion of araeoscelidians within crown Diapsida, proposing instead that they represent stem sauropsids outside the true diapsid clade. For instance, Simões et al. (2022) analyzed an expanded dataset of early fossils and recovered araeoscelidians as more basal, shifting the origin of crown diapsids later into the Permian and emphasizing as a more reliable basal representative. This debate highlights the ongoing refinement of early diapsid origins, with implications for understanding the clade's diversification amid environmental changes.

Phylogeny and Classification

Relationships to Other Amniotes

Diapsida represents one of the two primary clades within , the sauropsid branch of Amniota, with the other being the reduced Anapsida, which traditionally encompassed turtles and various extinct lineages but now excludes turtles based on phylogenetic evidence. as a whole forms the to Synapsida—the lineage leading to mammals—within the broader Amniota , which originated from ancestors and is defined by the synapomorphy of the amniotic egg, an enabling terrestrial reproduction through a shelled egg containing extraembryonic membranes. All amniotes, including diapsids, synapsids, and anapsids, share this amniotic innovation, which protects the embryo from desiccation. A defining synapomorphy of Diapsida is the presence of two temporal in the , paired openings that accommodate adductor muscles and enhance cranial strength, contrasting with the single fenestra in synapsids and the absence of fenestrae in anapsids. This diapsid configuration distinguishes the from its relatives and supports diverse feeding adaptations across its members. Historically, early 20th-century classifications divided reptiles into three groups based on temporal : Anapsida (no , including ), Synapsida (one , leading to mammals), and (two , encompassing , , crocodilians, birds, and dinosaurs). Modern cladistic analyses, however, have revised this framework, repositioning within as the to Archosauria within , supported by genomic studies in the 2010s revealing shared molecular markers with archosaurs and lepidosaurs, as well as fossil evidence of transitional forms like Pappochelys showing incipient diapsid features. Further resolving debates on early relationships, a phylogenetic by and integrated morphological and molecular data from over 200 taxa, nesting the traditionally separate and within Diapsida as the to crown-group diapsids (Neodiapsida), thereby eliminating as a distinct outside and simplifying the tree. This study also confirmed that temporal evolved fewer times than previously thought, with diapsid-like openings appearing convergently in some synapsids but ancestrally in the sauropsid lineage.

Internal Phylogenetic Structure

The internal phylogenetic structure of Diapsida reflects a hierarchical branching that originated in the late , with the encompassing a diverse array of reptiles characterized by two temporal fenestrae in the . At its base, Diapsida comprises as a group sister to Neodiapsida, the latter representing group that emerged around 290 million years ago (Ma) during the early Permian and excluding more primitive forms. Neodiapsida forms the core of modern reptile diversity, splitting into two major subclades: Lepidosauromorpha, which includes , , and , and , encompassing crocodilians, birds, and various extinct lineages. Sauria is recognized as the total group uniting lepidosaurs (crown ) and archosaurs (crown ), capturing their and all descendants, thus framing the evolutionary radiation of these dominant diapsid lineages from the Permian onward. This structure is supported by cladistic analyses emphasizing shared anatomical synapomorphies, such as modifications in the temporal region and postcranial adaptations, though these are briefly noted here as they underpin the branching without defining it exclusively. Recent phylogenetic revisions, particularly from a 2022 analysis incorporating stratigraphic and morphological data, and further supported by 2025 studies on late relatives of Neodiapsida, have refined this topology by excluding araeoscelidians from —the broader crown assemblage—and positioning them instead as early stem diapsids outside the saurian radiation. This study also affirms the inclusion of marine s like ichthyosaurs and sauropterygians within Diapsida, nesting them proximally to and highlighting convergent aquatic adaptations within the clade. Such updates underscore short internodal durations in early diapsid , around 1.9 , which contribute to ongoing topological instability. Debates persist regarding the placement of certain taxa within this framework, notably thalattosaurs, whose semiaquatic forms show uncertain affinities within , with analyses variably positioning them near lepidosauromorphs or as basal neodiapsids due to incomplete fossil material. Similarly, parareptiles are increasingly viewed as a paraphyletic assemblage rather than a cohesive , scattered across the diapsid stem and challenging traditional boundaries between parareptilian and eureptilian reptiles. , however, are consistently nested within in molecular and combined analyses, forming the as sister to Archosauria (crocodilians and ), with divergence estimated around 255 Ma.

Major Clades

Lepidosauromorpha

Lepidosauromorpha is a stem-based clade of reptiles defined as the of Sphenodon (the ) and squamates ( and ), and all descendants of that ancestor. This clade encompasses the crown group , which includes the orders and Sphenodontia (formerly ), as well as various extinct stem lineages such as the gliding kuehneosaurids from the period. Lepidosauromorpha forms the to within the larger saurian clade. Key subgroups within Lepidosauromorpha include , which comprises approximately 11,000 extant species of lizards, snakes, and amphisbaenians, representing the most diverse reptilian order. Sphenodontia is restricted to a single living genus, Sphenodon, with two species of endemic to , though the group was more widespread in the . Extinct forms, such as the (e.g., Kuehneosaurus and Kuehneosuchus), were small, lizard-like reptiles adapted for gliding via elongated, airfoil-like ribs, known from deposits in and . These subgroups highlight the clade's range from highly speciose modern lineages to specialized fossil taxa. Synapomorphies diagnosing Lepidosauromorpha include a transversely oriented , which facilitates the eversion of hemipenes in males of derived members like . In specifically, additional shared traits encompass flexible kinetics, enabling wide gape and enhanced feeding capabilities through streptostylic and mesokinetic joints. Other features, such as the fusion of the atlantal and axial neural centra, further support the clade's . The evolutionary trajectory of Lepidosauromorpha began with divergence from archosauromorphs in the mid-Permian, allowing survival through the end-Permian mass extinction. Diversification accelerated in the Early to , with fossils indicating global distribution by the , including early squamates and rhynchocephalians. The crown group originated around 242 million years ago in the , followed by Jurassic radiation of . Modern endemism is exemplified by the , whose lineage has persisted with minimal morphological change since the , confined to offshore islands in due to historical biogeographic isolation.

Archosauromorpha

is a major of reptiles comprising all taxa more closely related to archosaurs than to lepidosaurs, including the crown group Archosauria (encompassing crocodilians, , non-avian dinosaurs, and pterosaurs) as well as various stem-archosauromorphs such as proterosuchids, which represent early archosauriforms with transitional features. This clade originated in the middle to late Permian period, with the earliest known members including Archosaurus rossicus from the Late and Eorasaurus olsoni from the –Wuchiapingian stages, marking the initial diversification of diapsids following the rainforests collapse. Key subgroups within include , which comprises crocodylians and their extinct relatives such as phytosaurs, rauisuchids, and aetosaurs, and , which includes dinosaurs, , and pterosaurs as ornithodirans. Recent phylogenetic analyses have placed (Testudines) within as the to crown-group Archosauria, supported by molecular data and shared features including a modified diapsid pattern with secondarily closed temporal fenestrae and certain limb specializations. Diagnostic synapomorphies of and its subclades include the , an opening in the anterior to the orbit that lightens the and accommodates jaw musculature, observed in basal forms like Proterosuchus and Prolacerta; thecodont , where teeth are deeply socketed in the jaw bones for enhanced anchorage, seen in taxa such as Tasmaniosaurus triassicus; and, in more derived archosaurs, a four-chambered heart that improves circulatory efficiency, a trait retained in extant crocodilians and . Evolutionarily, underwent significant radiation following the end-Permian mass extinction around 252 million years ago, achieving dominance in terrestrial and aerial ecosystems throughout the Mesozoic Era, with archosaurs supplanting other diapsid lineages in predatory and herbivorous niches. Non-avian dinosaurs and many pseudosuchians went extinct at the – (K-Pg) boundary approximately 66 million years ago due to the Chicxulub impact and associated environmental catastrophes, while dinosaurs (birds) survived and underwent a major in the , diversifying into over 10,000 extant species today. This post-extinction expansion highlights the clade's resilience and ongoing evolutionary success.

Diversity and Adaptations

Extant Representatives

Extant diapsids represent one of the most species-rich lineages of vertebrates, with over 23,500 known distributed across and reptiles as of 2025. Approximately 11,100 species belong to Aves (), while reptiles comprise around 12,500 species, including approximately 12,000 squamates (lizards, snakes, and amphisbaenians), 360 turtles (Testudines), 26 crocodilians, and the single extant rhynchocephalian, the (Sphenodon punctatus). This diversity underscores the evolutionary success of diapsids, which dominate modern terrestrial, aquatic, and aerial ecosystems. Diapsids exhibit a global distribution, inhabiting virtually every on . Birds range from polar regions, exemplified by (Spheniscidae) in waters, to equatorial rainforests and high-altitude mountains. Reptiles occupy tropical forests, arid deserts (e.g., various lizard genera like Phrynosoma in North American deserts), freshwater systems, and marine environments, with sea turtles () traversing oceans worldwide. This broad ecological occupancy reflects adaptations to extreme conditions, from the heat of Saharan dunes to the cold depths of subpolar seas. Biologically, extant diapsids display key physiological contrasts that influence their . Reptiles are predominantly ectothermic, regulating body temperature primarily through behavioral adjustments to environmental conditions, which enables efficient energy use in varied habitats but limits activity in cold climates. In contrast, birds are endothermic, generating internal heat via high metabolic rates to sustain activity across extremes. Reproductive strategies are similarly diverse: predominates, with most species laying shelled eggs that develop externally, but —live birth with internal embryonic nourishment—has arisen independently over 100 times in squamates, facilitating reproduction in unstable or cold environments. Conservation challenges threaten this diversity, with habitat loss from , , and identified as the primary driver of decline across groups. According to IUCN assessments, at least 21% of species are threatened with , a figure rising to 30% for forest-dwellers. The , confined to offshore islands in , holds Least Concern status overall but faces risks from invasive predators and , necessitating protected reserves and predator control. Among crocodilians, seven of the 26 are , largely due to historical overhunting and ongoing degradation.

Extinct Lineages and Ecological Roles

Diapsids encompass several major extinct lineages that dominated Mesozoic ecosystems before their ultimate demise. Pterosaurs, flying relatives of dinosaurs within the archosaur group, represent one such lineage, with over 200 known species that thrived from the Late Triassic to the end of the Cretaceous. These aerial reptiles went extinct during the Cretaceous-Paleogene (K-Pg) boundary event approximately 66 million years ago. Non-avian dinosaurs, another prominent archosaurian group, included roughly 1,000 described genera that radiated across terrestrial environments from the Late Triassic to the Late Cretaceous. Marine reptiles such as ichthyosaurs, dolphin-like diapsids adapted to fully aquatic life, persisted from the Early Triassic to the mid-Cretaceous, filling oceanic predatory niches. These extinct diapsids occupied diverse ecological roles, shaping food webs. Large theropod dinosaurs like Tyrannosaurus rex served as apex predators in North American floodplains, preying on other vertebrates with powerful jaws and keen senses. Giant sauropods such as Brachiosaurus functioned as primary herbivores, consuming vast quantities of vegetation and influencing plant community structure through their browsing habits. Pterosaurs may have contributed to and potentially of early angiosperms via frugivory, aiding the diversification of flowering plants as endozoochorous agents. In the Late Permian, gliding weigeltisaurids like Weigeltisaurus filled arboreal niches in forested environments, using dermal sails for aerial locomotion among high canopy trees, representing early experimentation with . Extinction events profoundly impacted diapsid diversity. The Permian-Triassic mass extinction around 252 million years ago created a bottleneck for early diapsids, drastically reducing their abundance and limiting them to rare, low-diversity forms in the immediate aftermath, which delayed their full radiation until the Early Triassic. The K-Pg extinction event eradicated non-avian archosaurs, including all pterosaurs and non-avian dinosaurs, due to the combined effects of an asteroid impact, volcanism, and environmental collapse, thereby opening ecological opportunities for surviving lepidosaurs like lizards and snakes, as well as avian dinosaurs (birds). Notable adaptations in these lineages facilitated their ecological success. Mosasaurs, late-diving lepidosaurs of the , evolved paddle-like flippers from their limbs and a flattened, bilobed for efficient undulatory in open habitats, enabling them to pursue fast prey like and ammonites. Pterosaurs achieved aerial prowess with wingspans reaching up to 12 meters in species like , supported by elongated finger bones bearing leathery membranes for powered flight and gliding over vast distances. Recent analyses of thalattosaurs, enigmatic diapsids, reveal transitional adaptations to fully aquatic lifestyles, including elongated snouts for piscivory and limb modifications for paddling, as evidenced by bone histology indicating rapid growth and marine specialization during the post-Permian recovery.