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Euryapsida

Euryapsida is a polyphyletic assemblage of extinct sauropsid reptiles distinguished by a euryapsid skull condition, characterized by a single supratemporal fenestra as the sole temporal opening behind the orbit, typically resulting from the secondary closure or reduction of the infratemporal fenestra in ancestors that possessed the diapsid configuration of two fenestrae.^[1] This group, which flourished primarily in marine environments during the Mesozoic era from the Early Triassic to the Late Cretaceous (approximately 252 to 66 million years ago), includes diverse lineages adapted for aquatic life, such as the dolphin-like ichthyosaurs (Ichthyosauria), long-necked plesiosaurs and their kin (Sauropterygia), as well as other forms like thalattosaurs (Thalattosauria) and placodonts (Placodontia).^[2] Historically, Euryapsida was proposed as a formal taxonomic category in the mid-20th century by paleontologists like Edwin H. Colbert (1945), who grouped these reptiles based on their shared temporal skull morphology, viewing them as a distinct subclass alongside anapsids, synapsids, and diapsids.^[1] However, subsequent cladistic analyses have demonstrated that this classification is artificial and convergent, driven by similar selective pressures for underwater predation and locomotion rather than shared ancestry; for instance, ichthyosaurs are now placed as a basal or early-branching diapsid lineage, while sauropterygians nest within Neodiapsida.^[3] The euryapsid condition thus represents a derived trait in multiple independent diapsid radiations, with no evidence for monophyly.^[1] Key anatomical adaptations unifying euryapsids under this outdated umbrella include streamlined bodies, modified limbs into flippers for propulsion, and specialized skulls with large orbits for enhanced underwater vision and robust jaws for capturing prey, enabling them to occupy top predator and piscivorous niches in ancient oceans.^[2] Ichthyosaurs, appearing shortly after the end-Permian mass extinction, evolved fish-like forms with dorsal fins and viviparity, reaching sizes from 1 meter to over 20 meters in length (e.g., Shastasaurus).^[3] Sauropterygians, another major component, displayed remarkable neck elongation in some plesiosauroids (up to 72 cervical vertebrae in elasmosaurs) and short-necked, crocodile-like forms in pliosaurs, with evidence suggesting possible endothermy in certain taxa.^[2] These reptiles played a pivotal ecological role in Mesozoic marine ecosystems, filling niches left vacant by the extinction of pre-existing aquatic vertebrates and contributing to the "reptilian invasion of the seas."^[2] Fossil discoveries, first documented in the early 19th century (e.g., Plesiosaurus in 1821), span global deposits like the Posidonia Shale of Germany and the Holzmaden Lagerstätte, revealing exceptional preservation that highlights soft tissues, embryos, and stomach contents.^[3] Despite their extinction at or before the Cretaceous-Paleogene boundary—likely due to environmental changes and competition—their evolutionary innovations underscore the versatility of diapsid reptiles in adapting to fully aquatic lifestyles.^[2]

Definition and History

Etymology and Definition

The term Euryapsida derives from the Ancient Greek words eurys (εὐρύς), meaning "broad," and apsis (ἀψίς), meaning "arch" or "vault," alluding to the broad upper temporal arch observed in the skulls of reptiles classified within this group.^[4]^[1] The term was proposed in 1945 by paleontologist Edwin H. Colbert as a more descriptive replacement for the earlier designation Synaptosauria, introduced by Edward Drinker Cope in the late 19th century.^[5]^[1] Euryapsida was initially established as a subclass of reptiles, positioned alongside Anapsida (no temporal fenestrae), Synapsida (single lower temporal fenestra), and Diapsida (two temporal fenestrae), based on skull architecture as a primary classificatory criterion.^[5]^[1] In anatomical terms, Euryapsida encompasses a polyphyletic assemblage of sauropsids distinguished by a single upper temporal fenestra located behind the orbit and positioned high on the skull roof.^[6]^[7] This fenestra, which accommodated jaw adductor musculature, represents a derived condition arising from the evolutionary fusion or complete loss of the lower (infratemporal) fenestra in ancestors that originally possessed the diapsid skull configuration of two openings.^[6]^[2]

Historical Classification

The concept of Euryapsida arose from 19th-century efforts to classify marine reptiles based on shared cranial and postcranial features. In 1869, Edward Drinker Cope proposed the term Streptosauria for a group encompassing plesiosaurs, interpreting their vertebral structure as indicating a reversed orientation relative to other reptiles.^[8] This early grouping highlighted the distinctiveness of these aquatic forms but was limited in scope. Subsequently, in 1874, Harry Govier Seeley introduced Enaliosauria to unite a broader array of Mesozoic marine reptiles, including ichthyosaurs, plesiosaurs, and related taxa, emphasizing their adaptations for marine life as a unifying theme.^[9] By the early 20th century, classifications shifted toward skull fenestration as a key diagnostic trait. Samuel Wendell Williston, in 1925, applied Synaptosauria for reptiles exhibiting a single upper temporal fenestra, incorporating sauropterygians, ichthyosaurs, and placodonts while excluding synapsids with their lower fenestra.^[10] This term reflected a growing recognition of temporal arch patterns as evolutionary markers, building on earlier work by Cope and Seeley. Edwin H. Colbert formalized Euryapsida in 1945 as a subclass defined by the presence of a single upper temporal fenestra bounded laterally by the postorbital and squamosal bones, explicitly substituting it for Synaptosauria (a suggestion originally from A.S. Romer via personal correspondence).^[11] Colbert's framework included core groups such as Sauropterygia, Ichthyosauria, and Placodontia, positioning Euryapsida as a parallel lineage to Anapsida, Synapsida, and Diapsida. Mid-20th-century refinements expanded Euryapsida to encompass additional fossil taxa tentatively linked by the euryapsid skull condition. Groups like Araeoscelidia and Mesosauria were briefly incorporated, with Araeoscelidia viewed as potential basal members due to their fenestration and Mesosauria assigned based on aquatic affinities and primitive features.^[12] However, these inclusions were provisional, as ongoing discoveries revealed inconsistencies in their affinities. By the 1980s, accumulating evidence from cladistic analyses indicated that Euryapsida represented an artificial assemblage resulting from convergent evolution in temporal fenestration among unrelated lineages, prompting proposals for revised groupings that avoided the term in favor of more precise phylogenetic categories.^[13]

Taxonomy and Phylogeny

Included Taxa

Traditionally, Euryapsida encompassed several major clades of extinct Mesozoic marine reptiles characterized by the euryapsid skull condition, featuring a single upper temporal fenestra formed by the fusion or loss of the lower temporal bar, which provided structural support for jaw musculature adapted to aquatic predation.^[2] This grouping, proposed as a subclass by early paleontologists, primarily included Sauropterygia and Ichthyosauria, with occasional minor inclusions like Thalattosauria based on superficial similarities in temporal morphology.^[14] Sauropterygia formed the largest component of traditional Euryapsida, comprising diverse semi-aquatic to fully pelagic reptiles that dominated marine ecosystems from the Early Triassic to the Late Cretaceous. This clade subdivided into several subgroups, including Placodontia, which consisted of heavily armored, durophagous forms like Placodus with turtle-like bodies and crushing dentition suited for feeding on mollusks and crustaceans in shallow coastal waters.^[15] Nothosauria, another key subgroup, included semi-aquatic predators such as Nothosaurus, characterized by elongated snouts, robust limbs for near-shore ambulation, and piscivorous diets during the Middle to Late Triassic.^[2] Plesiosauria, the most iconic sauropterygian lineage, encompassed long-necked plesiosaurs (e.g., Elasmosaurus) for pursuit predation and short-necked pliosaurs (e.g., Liopleurodon) as apex ambush hunters, both with modified limbs as flippers for efficient swimming across Jurassic and Cretaceous seas.^[15] Ichthyosauria, another core euryapsid group, comprised dolphin-like fully aquatic reptiles ranging from the Early Triassic to the Late Cretaceous, with streamlined bodies, large eyes for low-light hunting, and viviparous reproduction. Their skulls exhibited a euryapsid-like condition, though debated as potentially derived from a diapsid ancestor via fusion of temporal elements, enabling powerful bites on fish and cephalopods in open oceans.^[2] Thalattosauria represented a minor historical inclusion in Euryapsida, consisting of Middle to Late Triassic marine reptiles like Thalattosaurus with elongated snouts, paddle-like limbs, and varied diets from piscivory to scavenging along continental margins.^[2] Early classifications also erroneously placed certain diapsid groups, such as marine turtles, within Euryapsida due to convergent skull modifications.^[2]

Modern Phylogenetic Position

Modern cladistic analyses, incorporating both morphological and molecular data, have established that Euryapsida is a polyphyletic assemblage rather than a natural clade, with the defining euryapsid skull condition—a single upper temporal fenestra—arising through convergent evolution in disparate diapsid lineages adapted to aquatic environments. Studies from the 2000s onward, including comprehensive phylogenetic matrices of amniote relationships, demonstrate that this fenestration pattern evolved independently at least twice among marine reptiles, reflecting parallel adaptations for jaw musculature and cranial streamlining rather than shared ancestry.^[1]^[3] Recent analyses (e.g., Simões et al., 2022) further suggest these groups form a monophyletic clade of basal archosauromorphs specialized for marine life.^[16] Within this framework, Sauropterygia, encompassing the diverse marine reptiles such as plesiosaurs and their relatives, is positioned within Diapsida, often as the sister group to Archosauromorpha or closely related to Testudines within Archosauromorpha, based on shared cranial and postcranial synapomorphies.^[16] Placodontia, the armored durophagous forms traditionally included in Euryapsida, is resolved as the sister group to Eosauropterygia (the remaining sauropterygians), forming the basalmost subclade of Sauropterygia and confirming their nested position within diapsids. In contrast, Ichthyosauria occupies a more basal position among diapsids, often as stem-ichthyopterygians near the base of Archosauromorpha or as its immediate sister taxon, supported by shared derived features such as elongated snouts and vertebral modifications despite the superficial similarity in temporal fenestration.^[17]^[18]^[3] The inclusion of turtles (Testudines) in historical euryapsid classifications has also been refuted by modern phylogenies, which place them firmly within Diapsida despite their anapsid-like skull lacking temporal fenestrae; this condition is now interpreted as a secondary loss, with molecular evidence strongly supporting turtles as the sister group to Archosauria or more broadly within crown-group diapsids. Key contributions to these resolutions include Motani's (1999) cladistic analysis of ichthyopterygian osteology, which affirmed their diapsid affinities and basal archosauromorph position, and Neenan et al.'s (2013) examination of placodont cranial anatomy, which bolstered the sister-group relationship of Placodontia to other sauropterygians through shared synapomorphies in dentition and osteoderm structure.^[19]^[20]^[18]^[17]

Morphology

Cranial Characteristics

The euryapsid skull is defined by a single supratemporal fenestra, an opening in the temporal region that is laterally bounded by the postorbital and squamosal bones, with the lower temporal fenestra either absent or fused due to secondary closure.^[1] This configuration results in a broad postorbital-squamosal bar that separates the fenestra from the jaw region, providing structural support while accommodating jaw musculature.^[1] The supratemporal fenestra is typically bordered dorsally by the parietal and sometimes the supratemporal bone, contributing to a reinforced skull roof adapted for marine predation.^[1] In sauropterygians, the supratemporal bone is particularly prominent, forming a key part of the fenestral margin and enhancing the attachment area for adductor muscles essential for powerful bites in aquatic environments.^[1] Ichthyosaurs exhibit a modified euryapsid condition, often termed metapsid, where the lower temporal elements are reduced and the infratemporal fenestra is closed, resulting in a streamlined temporal region with the fenestra primarily bounded by the postfrontal, parietal, and squamosal.^[3] This arrangement reflects convergence from diapsid ancestors, emphasizing efficiency in hydrodynamic skull design.^[3] Variations among euryapsid groups highlight adaptations to specific ecological niches. In placodonts, the temporal fenestra is notably reduced in size relative to the orbit, with thickened surrounding bones supporting a robust cranium suited for durophagous feeding on hard-shelled prey.^[21] Plesiosaurs exhibit a supratemporal fenestra typically similar in size to the orbit, contributing to their overall streamlined skull. Thalattosaurs, with their elongate snouts and single temporal fenestra bordered by postorbital, squamosal, and parietal bones, show adaptations for piscivory in coastal environments. Functionally, the euryapsid fenestra primarily housed the jaw adductor muscles, such as the pterygoideus and temporalis, enabling strong closing forces for capturing elusive marine prey despite the challenges of underwater biting.^[1] This muscular accommodation, combined with the fenestra's position, optimized bite mechanics in a high-drag medium, distinguishing euryapsid crania from terrestrial amniote counterparts.^[1]

Postcranial Adaptations

Euryapsids displayed a range of postcranial modifications that facilitated adaptation to fully or partially aquatic lifestyles, with a predominant emphasis on streamlined fusiform body shapes for reduced drag during swimming. These forms often featured pachyostotic bones—increased density and thickness in the skeleton—for buoyancy regulation, particularly evident in early sauropterygian ancestors like Hanosaurus, which possessed an elongate trunk supported by dense ribs.^[15] Dermal ossicles provided protective armor in some taxa, such as placodonts, where osteoderms formed dorsal and pelvic shields to safeguard against predators while foraging on the seafloor.^[22] The vertebral column in ichthyosaurs was characterized by numerous elongated, disc-like centra that enhanced lateral flexibility for thunniform (tail-driven) propulsion, allowing efficient cruising in open water.^[23] In contrast, sauropterygians showed an ancestral pattern of an extended trunk with approximately 25 dorsal vertebrae, often accompanied by pachyostotic ribs for stability and buoyancy, though derived plesiosaurs shortened the trunk while elongating the neck.^[15] Gastralia, or belly ribs, were present but exhibited varying degrees of reduction in sauropterygians, with microstructural analyses revealing fibrolamellar bone formation indicative of aquatic adaptations, differing from the more robust gastralia of terrestrial diapsids.^[24] Limb modifications were diverse, reflecting degrees of aquatic specialization. In ichthyosaurs and plesiosaurs, hyperphalangy—the linear addition of phalanges beyond the ancestral count—produced rigid, paddle-like flippers optimized for hydrodynamic lift and thrust, a trait linked to modified perichondral ossification that minimized long bone shaft development.^[25] Placodonts, however, retained pentadactyl limbs without significant elongation or hyperphalangy, featuring short, robust elements suited for bottom-walking rather than sustained swimming, as seen in taxa like Placodus with no specialized propulsion adaptations.^[22] Nothosaurs represented transitional semi-aquatic forms, with dorsoventrally flattened humeri forming hydrofoil-like structures for paraxial rowing motions and inferred webbing between elongated digits to enhance paddling efficiency.^[26] Pectoral and pelvic girdles were typically expanded to accommodate powerful flipper strokes in more derived euryapsids. Ancestral sauropterygians like Hanosaurus had disc-like coracoids, rounded pubes, and reduced ilia, providing broad attachment surfaces for limb muscles while supporting an axial swimming style.^[15] In nothosaurs, massive, flat pectoral girdles anchored forelimb musculature for primary propulsion, with thin-walled limb cortices in larger species reducing inertia for agile maneuvering in coastal habitats.^[26] Placodont girdles remained compact and unexpanded, aligning with their less pelagic lifestyles and emphasis on armored, benthic mobility.^[22]

Evolutionary History

Origins and Early Evolution

The earliest records of euryapsid-like reptiles trace back to the aftermath of the Permian-Triassic mass extinction, with possible precursors in Permian aquatic forms such as mesosaurs, though modern phylogenies reclassify mesosaurs as basal sauropsids rather than direct ancestors of euryapsids.^[27] True euryapsids, characterized by their distinctive temporal fenestration, emerged in the Early Triassic, approximately 252 million years ago (Ma), coinciding with the recovery of marine ecosystems.^[28] The oldest known sauropterygian, a key euryapsid clade, is represented by basal forms like Prosaurosphargis yingzishanensis from South China, dated to the late Olenekian stage (~251 Ma), marking the initial radiation of these aquatic adapters. Recent discoveries include the oldest Southern Hemisphere sauropterygian, a nothosaur from New Zealand dated to ~246 Ma (as of 2024), and Lijiangosaurus yongshengensis, the earliest known long-necked form with 42 cervical vertebrae from the Middle Triassic (as of 2025).^[29]^[30]^[28] Similarly, early ichthyopterygians, another euryapsid group, appeared around the same time in near-shore environments, indicating a rapid colonization of marine habitats by diapsid reptiles.^[31] Transitional forms between terrestrial diapsids and fully aquatic euryapsids include basal sauropterygians such as nothosaurs and pachypleurosaurs, which exhibit intermediate adaptations like elongated bodies and paddle-like limbs for axial undulatory swimming.^[32] These taxa, originating from diapsid stock, represent early aquatic adapters; for instance, Lariosaurus sanxiaensis from the Early Triassic of South China is identified as a basal nothosaur linking to more derived pachypleurosaurs and nothosaurs.^[33] Phylogenetic analyses place these forms at the base of Sauropterygia, highlighting their role in bridging terrestrial origins to marine lifestyles through gradual modifications in locomotion and osmoregulation.^[33] The post-Permian extinction recovery provided key environmental drivers for euryapsid origins, as depleted marine communities allowed opportunistic invasion by reptiles into shallow coastal and lagoonal niches.^[34] This radiation occurred primarily in the Tethys Sea and associated epicontinental basins, where warming climates and increased nutrient availability post-extinction facilitated the establishment of simple food webs dominated by these predators and herbivores.^[31] Fossils from these deposits reveal a top-down ecosystem restructuring, with euryapsids filling apex roles by the Middle Triassic.^[33] Key fossil localities underscore this early evolution, including Alpine Triassic sites like Monte San Giorgio (Switzerland-Italy border) for placodonts, which preserve articulated specimens from Middle Triassic Lagerstätten revealing basal durophagous forms.^[35] In the Germanic Basin, the Muschelkalk Formation of Germany, particularly around Bayreuth and Winterswijk (extending into the Netherlands), yields abundant nothosaur remains, such as Nothosaurus mirabilis, documenting the diversification of these transitional aquatic reptiles in shallow marine settings.^[36] These sites highlight the Tethyan distribution of early euryapsids, rooted within the broader Diapsida clade.^[27]

Diversification and Extinction

The euryapsids, encompassing groups such as ichthyosaurs and sauropterygians, achieved their peak diversity during the Mesozoic era, particularly from the Jurassic through the Cretaceous periods. Plesiosaurs underwent a notable radiation in the Early Jurassic, with diversification continuing across the first 20 million years of the period and resulting in nearly 120 recognized species that occupied diverse marine habitats.^[37] Ichthyosaurs, meanwhile, maintained a limited presence with some morphological variation in the Early Cretaceous, particularly in European faunas, but overall diversity was low compared to earlier periods, prior to their extinction around the Cenomanian-Turonian boundary.^[38] This expansion involved niche partitioning among euryapsids, exemplified by pliosaurs such as Liopleurodon, which served as apex predators preying on large marine vertebrates in Jurassic seas.^[39] Ecologically, euryapsids filled a range of roles in Mesozoic marine food webs, comparable in overall diversity to modern cetaceans.^[40] Many acted as top predators, with robust skulls and conical teeth adapted for capturing fish and other vertebrates, while placodonts functioned as durophagous feeders, crushing hard-shelled mollusks and invertebrates in nearshore environments.^[41] Some late Cretaceous elasmosaurs, such as Morturneria, developed tooth arrangements suggestive of filter-feeding on small prey, showing convergence with baleen whale adaptations.^[42] These groups displayed remarkable convergence with cetaceans in body plan, locomotion, and ecological niches, including streamlined forms and blubber-like insulation in ichthyosaurs.^[43] Extinction events profoundly shaped the euryapsid record. Ichthyosaurs underwent a two-phase extinction culminating at the Cenomanian-Turonian boundary around 94 million years ago, linked to oceanic anoxic events that reduced productivity and evolutionary rates in marine ecosystems.^[44] Sauropterygians, including plesiosaurs, persisted until the Cretaceous-Paleogene boundary approximately 66 million years ago, succumbing alongside non-avian dinosaurs due to the combined effects of asteroid impact and Deccan Traps volcanism, which triggered global cooling and habitat disruption.^[45]^[46] Although euryapsids left no direct descendants, their evolutionary history has informed studies of convergent evolution, highlighting how unrelated lineages independently adapted to similar aquatic pressures.^[47]

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