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Macronaria

Macronaria is a clade of gigantic, herbivorous sauropod dinosaurs characterized by broad skulls with enlarged external nares, spoon-shaped teeth, and vertebral features such as opisthocoelous anterior dorsal centra, representing some of the largest terrestrial vertebrates in Earth's history.^[1] Named for the large diameter of their nasal openings (from Greek makros meaning "long" or "large" and naris meaning "nostril"), macronarians originated in the Middle Jurassic epoch around 166 million years ago and achieved a worldwide distribution by the Late Jurassic, persisting until the end of the Late Cretaceous approximately 66 million years ago.^[2]^[1] Phylogenetically, Macronaria forms one of the two major subclades of Neosauropoda (alongside Diplodocoidea), defined as a node-based group comprising all sauropods more closely related to Saltasaurus loricatus than to Diplodocus longus.^[3]^[2] This positioning reflects their evolutionary divergence from diplodocoids during the Jurassic, with macronarians exhibiting adaptations for high browsing, such as elongated necks and pillar-like limbs supporting massive body masses often exceeding 20 metric tons.^[1] Within Macronaria, key subgroups include the basal Camarasauromorpha—featuring robust, short-necked forms like Camarasaurus from the Late Jurassic of North America—and the more derived Titanosauriformes, which encompasses long-necked giants like Brachiosaurus from North America and Giraffatitan from Africa, as well as the highly diverse Titanosauria of the Cretaceous.^[2] Fossils of macronarians have been recovered from all seven continents, underscoring their ecological success as dominant megaherbivores in Mesozoic terrestrial ecosystems.^[1]^[4] Notable discoveries include early forms like Yuzhoulong qurenensis from the Middle Jurassic of China, which suggests rapid diversification of neosauropods in Asia, and Late Cretaceous titanosaurs such as Argentinosaurus from South America, renowned for their extreme size potentially reaching 30 meters in length, and Petrustitan hungaricus from Romania as of 2025.^[1]^[2]^[5] Their decline coincided with the end-Cretaceous mass extinction, leaving no descendants beyond the K-Pg boundary.^[3]

Etymology and taxonomy

Etymology

The clade name Macronaria derives from the Greek makros (μακρός), meaning "long" or "large", combined with the Latin naris, meaning "nostril", in reference to the notably large external nares or nasal openings observed in the skulls of macronarian sauropods.^[6] This naming highlights a key anatomical characteristic where the nasal aperture is significantly expanded compared to more basal sauropod groups. Macronaria was formally established in 1998 by paleontologists Jeffrey A. Wilson and Paul C. Sereno as part of their comprehensive phylogenetic analysis of sauropod dinosaurs, which reorganized the higher-level relationships within the group.^[6] The expanded nasal aperture serves as a diagnostic synapomorphy for the clade, distinguishing macronarians from their sister group Diplodocoidea within Neosauropoda by featuring a broad, elongate external naris that occupies a substantial portion of the anterior skull.^[6]

Taxonomic definition

Macronaria is a node-based clade within the larger sauropod group Sauropoda, formally defined as comprising the most recent common ancestor of Camarasaurus supremus and Saltasaurus loricatus, and all of that ancestor's descendants.^[6] This definition captures a diverse assemblage of long-necked, herbivorous dinosaurs characterized by their advanced neosauropod features, distinguishing them from more basal sauropods. The clade was established to reflect the phylogenetic structure of advanced sauropods, emphasizing shared evolutionary innovations beyond those of earlier forms.^[6] Within the sauropod hierarchy, Macronaria occupies a key position inside Neosauropoda, serving as the sister taxon to Diplodocoidea, which includes familiar forms like Diplodocus and Apatosaurus.^[6] Major subgroups within Macronaria include Camarasauridae (exemplified by Camarasaurus), Brachiosauridae (such as Brachiosaurus and Giraffatitan), and the more derived Titanosauriformes, which itself encompasses Titanosauria—a highly diverse lineage featuring armored and gigantic forms like Saltasaurus and Argentinosaurus.^[6] These subgroups highlight the clade's internal diversity, from robust mid-sized herbivores to colossal giants. The temporal span of Macronaria extends from the Middle Jurassic Bathonian stage, approximately 167 million years ago, to the Late Cretaceous Maastrichtian stage, about 66 million years ago, marking one of the longest durations among dinosaur clades.^[7] Geographically, macronarians originated on Laurasian continents (including North America, Europe, and Asia) but dispersed worldwide, with titanosaurs achieving particular dominance in Gondwanan regions such as South America, Africa, and India during the Cretaceous.^[7] This broad distribution underscores their adaptability across varied paleoenvironments.

Anatomy

Cranial and dental features

Macronarians exhibit distinctive cranial features that set them apart from other sauropod clades, particularly in the structure of the skull roof and nasal region. The clade name Macronaria derives from the prominent expansion of the external nares, which are large openings positioned dorsally and retracted toward the posterior portion of the skull, often as large as or larger than the orbits in basal forms.^[8] In advanced macronarians such as titanosauriforms, these nares are further displaced to the top of the skull, contributing to an elongated snout that projects anteriorly beyond the narial opening.^[8] This configuration contrasts with the more anteriorly placed nares in non-macronarian sauropods and reflects a simpler overall cranial architecture, retaining the standard diapsid pattern of fenestrae—including the antorbital, infratemporal, and supratemporal openings—without the additional subdivisions or accessory fenestrae characteristic of diplodocoid skulls.^[9] The dentition of macronarians is adapted for processing tough plant material through robust, broad-crowned teeth that are typically spatulate or spoon-shaped, with low crowns and thick enamel to withstand wear during cropping and shearing.^[10] These teeth feature a D-shaped cross-section, a pronounced cingulum at the base, and a rough enamel surface, enabling effective occlusion for grinding vegetation.^[11] Tooth replacement in macronarians occurs at a relatively slow rate compared to diplodocoids, averaging approximately 62 days per tooth position, as evidenced by histological analysis of Camarasaurus specimens where each tooth family supports up to three successive generations. This rate supports sustained herbivory but contrasts with the faster 35-day cycle in diplodocoids, aligning with the macronarian reliance on durable, low-crowned dentition rather than frequent replacement. Variations in dental morphology occur within macronarians, as seen in Camarasaurus, where anterior teeth are narrower and more symmetrical in outline, transitioning to broader, asymmetrical spoon-shaped forms in mid- and posterior positions along the dental row.^[12] Unlike the complex, interlocking jaw mechanics of diplodocoids that facilitate lateral tooth-to-tooth shearing with narrow-crowned teeth, macronarians employ a simpler occlusal mechanism involving precise vertical alignment for broad-crowned grinding, which underpins their efficiency in resource-poor environments.^[10]

Postcranial skeleton

The axial skeleton of macronarians exhibits distinctive features adapted to support their massive bodies and elongated necks. In brachiosaurids such as Brachiosaurus, the presacral vertebrae often feature tall neural spines, which in some specimens reach heights approaching or exceeding the length of the centra, contributing to a sail-like dorsal profile that may have aided in thermoregulation or display.^[13] Robust cervical ribs, elongated and ossified as tendons, extend along the ventral and lateral surfaces of the cervical vertebrae, providing structural reinforcement to the neck and limiting lateral flexibility while enabling vertical support for the head.^[14] These ribs are particularly pronounced in camarasaurids and titanosaurs, where they overlap multiple vertebrae to distribute muscular loads effectively. Additionally, dorsal vertebrae in macronarians commonly possess hyposphene-hypantrum articulations, accessory intervertebral joints that enhance sagittal stability by interlocking the postzygapophyses and prezygapophyses of adjacent vertebrae, a feature widespread across the clade but varying in robustness.^[15] The appendicular skeleton reflects adaptations for weight-bearing in a columnar posture, with notable variations among subgroups. In brachiosaurids, the forelimbs are disproportionately long relative to the hindlimbs, exemplified by the humerus exceeding the femur in length by up to 10-20%, which elevates the anterior body and inclines the trunk. Metacarpals and metatarsals are elongated and slender, forming pillar-like supports that minimize bending stress under immense body mass, as seen in camarasaurids.^[16] Pneumaticity in the vertebrae is generally present but less invasively complex than in diplodocoids, featuring camerate chambers rather than extensive camellar networks, which reduces skeletal mass while maintaining structural integrity.^[17] Pelvic and caudal elements further define macronarian morphology, with broad iliac blades providing expansive surfaces for sacral rib attachment and hindlimb musculature, as observed in basal forms like Lourinhasaurus. The distal ischial shafts are oriented transversely and coplanar, forming a reinforced puboischial complex that stabilizes the pelvic outlet during locomotion.^[18] Tail length varies across the clade, with titanosaurs generally exhibiting shorter, more robust caudal series compared to the elongated tails of more basal macronarians, reflecting adaptations to different ecological niches.^[16] Overall, camarasaurids display more compact vertebral proportions and shorter limbs relative to body size, contrasting with the elongated necks and trunks of titanosaurs, which emphasize reach and foraging efficiency.^[9]

Paleobiology

Body size, growth, and locomotion

Macronarians displayed remarkable variation in body size, reflecting their diverse evolutionary adaptations within the clade. The smallest known forms, such as the dwarf sauropod Europasaurus holgeri from the Late Jurassic of Germany, attained adult lengths of approximately 6–7 m and masses around 1 ton, representing an insular dwarfism likely driven by limited island resources.^[19]^[20] In contrast, the largest macronarians, including the titanosaur Argentinosaurus huinculensis from the Late Cretaceous of Argentina, reached lengths of 30–35 m and estimated masses of 70–100 tons, establishing them among the most massive terrestrial animals ever.^[21] More typical macronarians, like camarasaurids such as Camarasaurus lentus, averaged 15–18 m in length and 10–20 tons in mass, providing a benchmark for the clade's mid-sized representatives.^[22] These size disparities highlight the clade's capacity for both miniaturization and extreme gigantism, supported by fossil scaling analyses.^[23] Growth in macronarians followed patterns of rapid juvenile development, enabling quick attainment of large adult sizes despite their eventual massiveness. Histological studies of long bone cross-sections reveal fast deposition of fibrolamellar bone tissue with vascular canals oriented parallel to the growth axis, indicative of high metabolic rates comparable to those in modern mammals.^[24] Lines of arrested growth (LAGs), periodic pauses in bone formation visible as annuli, suggest individuals reached sexual maturity and full skeletal adulthood in 20–30 years, as modeled from specimens of camarasaurids and related forms; for instance, growth trajectories indicate Camarasaurus achieved near-maximum size by age 25.^[25] This accelerated ontogeny, absent in smaller dinosaur relatives, underscores the evolutionary innovations permitting macronarian gigantism.^[20] Locomotion in macronarians was adapted for efficient support of immense body masses, primarily through a stable quadrupedal gait with pillar-like limbs positioned directly beneath the body to reduce bending moments and energy costs during weight-bearing.^[26] Trackway evidence, including those from the Morrison Formation, documents a wide-gauge posture with limbs splayed outward for enhanced lateral stability, often showing pes prints with distinct claw impressions from digits I–III that reflect the robust, tridactyl foot morphology.^[27] Estimated maximum speeds ranged from 5–10 km/h, inferred from stride lengths and limb proportions in biomechanical models, prioritizing endurance over agility in these giants.^[28] Posture varied phylogenetically: brachiosaurids featured elevated shoulders relative to the hips, supporting upright forelimbs for accessing high vegetation, whereas most titanosaurs exhibited a more horizontal neck orientation aligned with a level-backed stance for ground-level foraging.^[29]^[30] These locomotor traits, corroborated by ichnological and osteological data, optimized macronarians for low-energy movement across vast habitats.^[31]

Diet and feeding

Macronarian sauropods were obligate herbivores that primarily browsed on vegetation within Mesozoic forested environments, targeting conifers, cycads, and ferns as dominant components of their diet.^[20] Browsing heights varied by subfamily: brachiosaurids, with their elongated necks and elevated shoulders, accessed foliage up to 10-15 meters above the ground, enabling high-level feeding on taller canopy elements, while camarasaurids typically foraged at lower strata of 4-6 meters, focusing on mid-level shrubs and understory plants.^[32]^[32] Feeding mechanics in macronarians centered on a simple shearing action facilitated by their spatulate, peg-like teeth, which were suited for stripping leaves and soft foliage rather than grinding tough material.^[33] Tooth microwear patterns reveal consumption of abrasive, fibrous vegetation, consistent with a diet of coarse plant matter that caused significant wear on enamel surfaces.^[34] Unlike some modern herbivores, there is no compelling evidence for gastrolith use in macronarians to aid mechanical breakdown in the stomach; instead, digestion likely relied on hindgut fermentation by symbiotic microbes to process ingested plant material efficiently.^[35] Tooth replacement rates were rapid, averaging around 62 days in taxa like Camarasaurus, allowing continuous feeding despite high wear and supporting their enormous metabolic demands.^[36] Carbon isotopic analysis of macronarian teeth and associated bones indicates a diet dominated by C3 plants, such as gymnosperms and ferns, with minimal seasonal variation in resource intake, suggesting stable access to preferred foliage throughout the year.^[37] In ecosystems like the Late Jurassic Morrison Formation, macronarians occupied distinct ecological niches through resource partitioning, with their higher browsing levels complementing the ground- or low-level foraging of sympatric diplodocoids, thereby reducing competition and supporting diverse sauropod assemblages.^[10] These adaptations, including robust cranial features for precise leaf cropping, underscore the evolutionary efficiency of macronarian feeding strategies in resource-limited environments.^[33]

Biogeography and paleoecology

Macronaria likely originated in the early Middle Jurassic, with evidence suggesting a Gondwanan cradle followed by rapid dispersal to Laurasia prior to the full separation of the supercontinents. Fossils such as Dashanpusaurus dongi from the Middle Jurassic Xiashaximiao Formation in China indicate an early global distribution of neosauropods, including basal macronarians, supporting a widespread dispersal event during this period. By the Late Jurassic, macronarians underwent significant radiation in Laurasia, particularly in North American floodplains like those of the Morrison Formation, where genera such as Camarasaurus and Brachiosaurus dominated as large herbivores. This dispersal is attributed to vicariance patterns initiated by the breakup of Pangaea, which fragmented dinosaur populations across emerging landmasses.^[38] In the Cretaceous, macronarian diversity shifted toward dominance in the Southern Hemisphere, with titanosaurs representing the clade's most successful lineage in isolated Gondwanan regions. Key localities include Patagonia in Argentina, where massive titanosaurs like Argentinosaurus huinculensis from the Late Cretaceous Neuquén Basin exemplify the group's adaptation to subtropical floodplain and coastal environments.^[39] In Asia, early macronarian relatives appear in Middle Jurassic deposits of China, such as the newly described Yuzhoulong qurenensis from the Lower Member of the Xiashaximiao Formation in Chongqing, highlighting continued presence in eastern Laurasia.^[7] Endemic forms evolved in vicariance-driven isolates like India and Madagascar, where titanosaurs such as Isisaurus from the Upper Cretaceous Lameta Formation in India show close affinities to South American and Madagascan taxa, reflecting post-Pangaea fragmentation.^[40] Paleoecologically, macronarians served as apex herbivores in Mesozoic terrestrial ecosystems, particularly in floodplain and coastal settings where their body fossils and trackways are abundant.^[41] They occupied niches in riverine and deltaic landscapes, coexisting with ornithopod herbivores through spatial partitioning and facing predation pressure from theropods in these dynamic environments.^[42] In Australia, no confirmed macronarian body fossils were known until the recent description of Australotitan cooperensis, a titanosaur from the Early Cretaceous Winton Formation, though titanosaur tracks from sites like Broome in Western Australia provide earlier evidence of their presence.^[43]^[44]

Phylogeny and classification

Historical development

The scientific study of sauropod dinosaurs, which include the clade Macronaria, began in the early 19th century with fragmentary discoveries, but gained momentum in the mid-1800s. The first recognized sauropod genera, such as the genus Cetiosaurus by Richard Owen in 1841 based on material from the Middle Jurassic of England, with the species C. oxoniensis named later by Phillips in 1871, marking the initial formal acknowledgment of these long-necked herbivores as distinct from other reptiles. By the late 1800s, more complete specimens emerged during the Bone Wars in North America, exemplified by Edward Drinker Cope's 1877 description of Camarasaurus supremus from the Late Jurassic Morrison Formation, which provided one of the earliest insights into the anatomy of non-diplodocid sauropods. Prior to the late 20th century, sauropod classifications were broad and often paraphyletic, with many taxa lumped into wastebasket groups like Cetiosauridae, erected by Richard Lydekker in 1888 to encompass basal forms such as Cetiosaurus and Patagosaurus. Harry Govier Seeley further advanced early taxonomy in 1887 by proposing the divisions Saurischia and Ornithischia within Dinosauria and introducing Brachiosauria as a subgroup for taxa with elevated external nares, a concept later exemplified by Elmer S. Riggs's 1903 naming of Brachiosaurus altithorax from the Morrison Formation.^[45] The recognition of Titanosauria as a distinct lineage began earlier with Lydekker's 1886 proposal, but its separation from other "cetiosaur" groups evolved gradually through the 20th century as more South American fossils were described. The clade Macronaria was not formalized until 1998, when Jeffrey A. Wilson and Paul C. Sereno defined it in their cladistic analysis of sauropod higher-level phylogeny as all neosauropods more closely related to Saltasaurus loricatus than to Diplodocus longus, using 12 synapomorphies—primarily cranial and dental features—to resolve the paraphyly of previous groupings like Brachiosauria.^[18] This work built on contemporaneous efforts, such as Paul Upchurch's 1998 phylogenetic analysis, which refined neosauropod interrelationships through a character matrix of 135 traits across 27 taxa, supporting Macronaria as a robust sister group to Diplodocoidea.^[46] Subsequent refinements in the early 2000s incorporated discoveries from understudied regions, such as the African sauropod Jobaria tiguidensis, described by Sereno et al. in 1999 from Middle Jurassic deposits in Niger, which clarified the basal positions of eusauropods within Macronaria through its mosaic of primitive and derived traits. However, challenges persisted due to incomplete fossil records, leading to initial misclassifications; for instance, Andesaurus delgadoi, named by José F. Bonaparte and Jorge O. Calvo in 1991 from the Cenomanian of Argentina, was originally placed as a primitive member of Titanosauridae but later reinterpreted as a basal titanosaur outside more derived subgroups like Lithostrotia based on vertebral morphology.

Phylogenetic relationships

Macronaria represents one of the two primary clades within Neosauropoda, the other being Diplodocoidea, a relationship consistently recovered in cladistic analyses of sauropod dinosaurs.^[9] This positioning is bolstered by numerous synapomorphies, including features of the vertebral column and limb elements, as identified in comprehensive character matrices scoring over 200 morphological traits across dozens of taxa.^[47] Within Macronaria, the internal phylogeny features a series of basal taxa such as Jobaria tiguidensis from the Middle Jurassic of Niger, Yuzhoulong qurenensis from the Middle Jurassic of China, and Haplocanthosaurus priscus from the Late Jurassic of North America, which branch off prior to more derived groups.^[1]^[48] These are succeeded by Camarasauromorpha, encompassing Camarasauridae (exemplified by Camarasaurus from the Late Jurassic Morrison Formation) alongside several basal titanosaurs, while advanced macronarians form the clade Titanosauriformes.^[49] Titanosauriformes further subdivides into Brachiosauridae (e.g., Brachiosaurus altithorax), Somphospondyli, and the derived Titanosauria, the latter achieving global dominance through the Cretaceous.^[50] Ongoing debates in macronarian phylogeny include the precise placement of Europasaurus holgeri, a diminutive Late Jurassic taxon from Germany, which some analyses position as a basal brachiosaurid potentially representing insular dwarfism, while others recover it nearer the base of Macronaria.^[51] Similarly, the position of the Middle Jurassic Chinese taxon Bellusaurus sui remains unresolved, with placements varying between basal Macronaria and outside Neosauropoda depending on the dataset employed.^[52] Cladistic studies of Macronaria employ both node-based and stem-based definitions to delineate clade boundaries, relying on extensive character matrices that incorporate traits such as the extent of vertebral pneumatization, relative limb proportions, and dental morphology.^[53] These matrices, often comprising 100–500 characters and 50–150 operational taxonomic units, utilize parsimony algorithms to resolve branching patterns, with sensitivity analyses testing the robustness of key synapomorphies like elongated humeri in brachiosaurids.^[54] Multiple phylogenetic datasets confirm Camarasaurus as the sister taxon to Titanosauriformes, a relationship upheld across analyses incorporating postcranial and cranial data from Jurassic macronarians.^[49]

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Jun 28, 2019 · The height of the axis is 27 cm, and its centrum measures about 9 cm in diameter caudally. Jensen (1987) posited that these vertebrae were too ...
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Jun 1, 2018 · Bellusaurus sui is an enigmatic sauropod dinosaur from the Middle-Late Jurassic Shishugou Formation of northwest China.
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Aug 9, 2025 · Request PDF | Sauropod dinosaur phylogeny: critique and cladistic analysis | Sauropoda is among the most diverse and widespread dinosaur ...
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The earliest known titanosauriform sauropod dinosaur and ... - PeerJ
May 2, 2017 · ... Titanosauriformes. Full preparation and description of this specimen ... This morphology is present in a range of eusauropod taxa, including ...