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Limaysaurus

Limaysaurus is a genus of rebbachisaurid sauropod dinosaur that lived during the Cretaceous period in what is now Patagonia, Argentina.^[1] The type and only known species is L. tessonei, originally described as Rebbachisaurus tessonei based on a well-preserved partial skeleton (holotype MUCPv-205) that includes the skull, several vertebrae, ribs, a partial pelvis, and most limb bones.^[2] It was renamed to its own genus in 2004 after cladistic analysis revealed significant differences from the North African Rebbachisaurus garasbae.^[1] The holotype of Limaysaurus tessonei was discovered in 1988 near El Chocón in the Río Limay Formation of Neuquén Province, dating to the Cenomanian stage (approximately 100–94 million years ago).^[2] Additional specimens, including elements from at least three individuals such as teeth, vertebrae, and limb bones, have been found in the underlying Lohan Cura Formation (Aptian-Albian, Puesto Quiroga Member) at Cerro Aguada del León, confirming its presence in northern Patagonia during the mid-Early Cretaceous.^[1] This material represents one of the most complete sauropod skeletons from South America's Early Cretaceous, providing key insights into rebbachisaurid diversity in the region.^[2] Limaysaurus was a medium-sized herbivorous dinosaur, with the holotype's femur measuring 144 cm in length and overall body size estimated at up to 15 meters long and 7,000 kg in mass.^[3] It possessed distinctive features typical of rebbachisaurids, including tall, elongated neural spines on the dorsal vertebrae that may have supported a low sail or fatty hump, pencil-shaped teeth adapted for cropping vegetation, and a relatively short neck compared to other sauropods.^[2]^[1] Phylogenetic analyses place it firmly within Rebbachisauridae, a clade of basal diplodocoids that shared synapomorphies with taxa like Nigersaurus and Rebbachisaurus, highlighting the group's Gondwanan origins and radiation during the Cretaceous.^[1] Fossils of Limaysaurus indicate it inhabited fluvial and floodplain environments alongside other dinosaurs, including theropods and ornithischians, in a warm, humid climate.^[1] Its specialized dentition and vertebral structure suggest a feeding strategy focused on low-lying plants, differing from the high-browsing habits of contemporaneous titanosaurs. Recent studies using CT scans of the braincase have revealed details of its endocranial anatomy, including a relatively large brain and inner ear morphology consistent with agile head movements for precise foraging.^[3] As one of the better-understood rebbachisaurids, Limaysaurus contributes to understanding the evolutionary success of diplodocoids in southern continents before their decline in the Late Cretaceous.^[1]

Taxonomy and discovery

Etymology and naming

The genus name Limaysaurus is derived from the Río Limay, the principal river bordering the region in Patagonia, Argentina, where the type locality is situated, combined with the Ancient Greek word sauros, meaning "lizard" or "reptile".^[1] The specific epithet tessonei honors the Argentine fossil collector Lieto Tessone, who discovered the holotype specimen.^[2] The taxon was initially described and named as a new species of the North African genus Rebbachisaurus—Rebbachisaurus tessonei—by José O. Calvo and Leonardo Salgado in 1995, based on the holotype (MUCPv-205), a partial skeleton from the Candeleros Formation.^[2] In 2004, Salgado, Rodolfo A. Coria, and Calvo recognized that the material exhibited distinct morphological features, such as unique vertebral laminae and neural arch configurations, warranting separation from Rebbachisaurus, and they erected the new genus Limaysaurus with L. tessonei as the type and only species. This reclassification was formally published by Salgado, Coria, and Calvo in the Journal of Vertebrate Paleontology (volume 24, issue 4, pages 903–913). No additional species have been assigned to the genus. Additional referred specimens of Limaysaurus tessonei have since been recovered from the Candeleros, Huincul, and Lohan Cura Formations in the same region. The Lohan Cura material, from the Puesto Quiroga Member at Cerro Aguada del León, includes elements from at least three individuals such as teeth, vertebrae, and limb bones, and dates to the Aptian-Albian stages of the Early Cretaceous.^[1]

Discovery history and specimens

The holotype specimen of Limaysaurus, MUCPv-205, was discovered in 1988 by amateur paleontologist Lieto Tessone during a field expedition approximately 15 km southwest of Villa El Chocón in Neuquén Province, Argentina.^[2] This nearly complete skeleton, representing about 80% of the individual, includes an articulated basicranium, disarticulated cervical vertebrae, posterior dorsal vertebrae, all caudal vertebrae, a partial pelvis, and substantial limb elements (lacking only the manus).^[2] Excavation and preparation of the material were conducted by teams from the Museo de Paleontología de la Universidad del Comahue, highlighting its status as one of the most complete Cretaceous sauropod skeletons recovered from South America and the first well-preserved rebbachisaurid from Patagonia.^[2] Originally named Rebbachisaurus tessonei in a 1995 description honoring its discoverer, the taxon was reclassified as Limaysaurus tessonei in 2004 based on comparative analysis with additional rebbachisaurid material, establishing it as the type species of the new genus.^[1] The holotype derives from the Candeleros Formation, a unit within the Neuquén Group dated to the Cenomanian stage of the Late Cretaceous (ca. 100–97 million years ago).^[2] Additional referred specimens include partial skeletons from the Candeleros and Huincul Formations, such as MUCPv-206 (a disarticulated skeleton) and MUCPv-153 (a partial articulated skeleton), along with isolated vertebrae and limb bones that support the generic assignment through shared diagnostic features like vertebral morphology.^[2] These remains, also housed at the Museo de Paleontología de la Universidad del Comahue, underscore the relative abundance of rebbachisaurids in the region and facilitate ongoing studies of their anatomical variation.^[1]

Anatomy

Skull and dentition

The skull of Limaysaurus tessonei is partially preserved in the holotype specimen MUCPv-205, featuring an elongated and slightly laterally deformed structure with the narial opening positioned far back above the orbit. The braincase is robust, with a large occipital condyle primarily formed by the basioccipital and a small quadrangular supraoccipital.^[2] A CT scan analysis of the holotype braincase revealed detailed endocranial features, including a cranial endocast approximately 130 mm long and 44.3 mm wide at its maximum, with poorly laterally projected cerebral hemispheres and a small flocculus. The estimated brain volume is about 125 cm³, corresponding to encephalization quotients ranging from 0.49 to 1.32 depending on body mass estimates of 7,000–11,688 kg. Large olfactory bulbs in the endocast suggest a well-developed sense of smell, while the inner ear exhibits slender semicircular canals with the anterior canal slightly larger than the posterior one and an angle of approximately 82° between them, indicating adaptations for agile head movements and balance suited to a long neck.^[3]^[4] The dentition is represented by a few isolated teeth in the holotype, which are long, curved, and slender with circular to slightly compressed cross-sections and pencil-like tips lacking wear. These teeth differ from the straight, peg-like forms seen in diplodocids and show features consistent with cropping vegetation, such as their curved profile. Replacement rates are inferred from wear patterns observed in related rebbachisaurids, though the preserved examples in L. tessonei remain unworn.^[2] The tooth morphology aligns with a herbivorous diet potentially aided by gastroliths associated with the holotype remains.^[2]

Postcranial skeleton

Limaysaurus tessonei exhibits a typical sauropod body plan in its postcranial skeleton, characterized by an elongated neck and tail that contributed to its overall length of approximately 12–15 meters and body mass of approximately 7 metric tons (7,000 kg), as estimated from femoral dimensions and volumetric reconstructions.^[2] The holotype specimen (MUCPv-205) preserves substantial portions of the axial and appendicular elements, revealing adaptations for quadrupedal locomotion and weight-bearing in a medium-sized herbivore.^[2] The vertebral column includes at least 13 cervical vertebrae, of which eight are preserved in the holotype; these are elongated and opisthocoelous, with neural spines about 1.5 times the height of the centrum and low neural arches facilitating neck flexibility.^[2] The dorsal series comprises 10–12 vertebrae with platycoelous centra and notably tall neural spines—reaching up to three times the centrum height in posterior dorsals—that are petal-shaped with a straight, I-like outline, contrasting with the V-shaped spines seen in more derived diplodocoids.^[2]^[5] Over 50 caudal vertebrae form a long tail, with the anterior ones featuring tall neural spines (twice the centrum height) and platycoelous centra; chevron elements articulate to create hemal arches along the series.^[2] Pneumaticity is evident throughout the vertebral column, particularly in the cervical and dorsal regions, where deep lateral fossae (pleurocoels) and ovoid cavities in the neural arches connect to internal chambers via foramina, indicating invasion by an avian-like air sac system that likely reduced skeletal mass. Recent CT studies on rebbachisaurid vertebrae confirm extensive pneumaticity similar to that in Limaysaurus, supporting avian-like air sac systems.^[6]^[7] This feature is consistent with broader rebbachisaurid adaptations for efficient respiration and thermoregulation.^[6] The pectoral girdle consists of a robust, paddle-shaped scapula with a broad blade, V-shaped acromion, and strong proximal expansions, paired with a coracoid that has a rounded anterior margin and thick glenoid region.^[2] Forelimb elements include a humerus measuring 90 cm in length—shorter than the femur, yielding a humerus-to-femur ratio of 0.62 that supports a pillar-erect posture—and a five-toed manus with a prominent claw on digit I, as evidenced by preserved metacarpals (e.g., metacarpal II at 20 cm).^[2] In the pelvic region, the ilium is wide and short with reduced processes, the pubis reaches 98 cm with an oval shaft, and the ischium measures 94 cm with a twisted shaft.^[2] Hindlimb bones are robust for weight support, featuring a femur up to 1.44 m long, a tibia of 85 cm, and a slender fibula of 88 cm; the pes is four-toed, aiding stability in potentially humid terrains.^[2] Distinctive postcranial traits include the absence of a hyposphene-hypantrum complex in the vertebrae and tall, petal-shaped neural spines in the posterior dorsals and anterior caudals, enhancing axial mobility through structures like the "U-eave and shelf complex."^[5]

Classification

Phylogenetic position

Limaysaurus tessonei is classified within the sauropod lineage as a member of Sauropodomorpha, specifically under Sauropoda, Diplodocoidea, and the family Rebbachisauridae, representing a basal clade of diplodocoid sauropods known from Cretaceous deposits.^[8]^[9] Its inclusion in Rebbachisauridae is supported by shared synapomorphies including tall neural spines exceeding twice the height of the vertebral centra in the mid-dorsal region, specialized pencil-shaped premaxillary and dentary teeth with asymmetrical enamel and subconical crowns, and extensive pneumatic foramina in presacral vertebrae.^[10] These features distinguish rebbachisaurids from more derived diplodocoids like diplodocids and dicraeosaurids, positioning the family as an early-diverging branch within Diplodocoidea.^[11] The original phylogenetic analysis by Salgado et al. (2004) utilized a modified character matrix with 49 characters across 20 sauropod taxa, recovering Limaysaurus as a basal rebbachisaurid more closely related to South American forms than to African or European taxa, and explicitly excluding it from Diplodocidae. This analysis highlighted Limaysaurus-specific traits such as straight (non-bifurcated) dorsal neural spines and a nearly complete caudal vertebral series, which provide diagnostic resolution amid the fragmentary nature of many rebbachisaurid relatives. Subsequent studies, including Gallina and Apesteguía (2005), expanded the taxon sampling to include additional South American rebbachisaurids like Cathartesaura, reinforcing Limaysaurus's basal position within the family while confirming its non-diplodocid affinities through broader cladistic parsimony analyses. In consensus phylogenies from multiple analyses, Limaysaurus occupies a position sister to other South American rebbachisaurids, such as Cathartesaura and Rayososaurus, forming a subclade (often termed Limaysaurinae) that branches early within Rebbachisauridae and underscores the group's Gondwanan diversification.^[12]^[13]

Evolutionary relationships

Limaysaurus tessonei is closely related to other rebbachisaurids such as Rebbachisaurus garasbae from North Africa and Nigersaurus taqueti from Niger, forming part of the clade Khebbashia, which unites these taxa based on shared derived features including expanded neural arches and specialized vertebral pneumatization.^[14] This grouping highlights potential faunal exchanges between Gondwana and Laurasia during the mid-Cretaceous via temporary land bridges.^[15] Within South America, Limaysaurus belongs to the subclade Limaysaurinae, which includes Cathartesaura australis, Rayososaurus agrioensis, and possibly Zapalasaurus bonapartei, characterized by enhanced pneumatization in the presacral vertebrae and a reduced hyposphene-hypantrum system.^[16]^[17] These shared anatomical synapomorphies, such as extensive pneumatic foramina on the dorsal vertebrae, suggest a regional radiation among South American rebbachisaurids during the Early Cretaceous.^[15] In the broader context of Diplodocimorpha, Rebbachisauridae, including Limaysaurus, occupies a basal position relative to Flagellicaudata—the clade encompassing whip-tailed sauropods like Diplodocus and Dicraeosaurus—representing a short-lived evolutionary radiation that flourished from the Barremian to the Turonian stages of the Cretaceous before declining.^[16] This positioning underscores rebbachisaurids as an early offshoot of diplodocoids, diverging after the Late Jurassic but achieving peak diversity in the mid-Cretaceous.^[15] Biogeographically, the distribution of Limaysaurus and its relatives supports a model of vicariance following the breakup of Pangaea, with Limaysaurus exemplifying post-Jurassic diversification in southern Gondwana, while the presence of sister taxa in Africa indicates dispersal across narrowing seaways in the Early Cretaceous.^[15] Recent phylogenetic analyses in the 2020s, such as those incorporating new South American specimens, have refined Rebbachisauridae as monophyletic, placing Limaysaurus near the base of more derived subclades like Limaysaurinae and confirming a South American origin for the family with subsequent global spread.^[15]

Paleobiology and paleoecology

Diet and gastroliths

Limaysaurus tessonei was a herbivorous sauropod, as evidenced by its peg-like, pencil-shaped teeth adapted for cropping vegetation rather than grinding, consistent with the feeding strategies of other diplodocoid dinosaurs.^[2] The relatively short neck of rebbachisaurids like Limaysaurus suggests it was a low-level browser, targeting ground or low-canopy plants such as ferns, cycads, and possibly horsetails in the floodplain environments of its habitat.^[18] Tooth morphology, featuring narrow, unworn crowns with thin tips, indicates minimal oral processing and a diet of tough, fibrous plant material that required post-ingestive digestion.^[2] Gastroliths provide direct evidence of mechanical digestion in Limaysaurus, with polished stones discovered in association with skeletal remains functioning as a gastric mill to grind ingested vegetation, akin to that in modern herbivorous birds. In the holotype specimen (MUCPv-205), six such stones were found in the abdominal region, two adhered to ribs, composed of igneous rocks like granites and quartzites with rounded, rugose surfaces; five weighed 350–375 g each, and one 850 g.^[2] A referred specimen (MUCPv-206) preserved another six stones (190–420 g each) positioned between ribs and dorsal vertebrae, supporting the interpretation of these as stomach contents rather than incidental inclusions.^[2] The gastroliths' composition matches local river gravels, indicating ingestion from nearby fluvial environments during feeding.^[2] These features imply a digestive system with a large gut capacity suited to the animal's estimated 7-tonne body mass, where gastroliths facilitated the breakdown of high-fiber plant matter poorly suited to the teeth's limited cropping function.^[2] Such adaptations highlight the efficiency of sauropod herbivory, relying on bulk ingestion and internal processing to support gigantism.^[19]

Habitat and associated fauna

Limaysaurus is known from the Lohan Cura Formation (Aptian-Albian stages of the Early Cretaceous) and the Candeleros Formation within the Río Limay Subgroup of the Neuquén Basin, northern Patagonia, Argentina. Specimens from the Lohan Cura Formation (Puesto Quiroga Member) at Cerro Aguada del León represent fluvial and floodplain environments during the mid-Early Cretaceous. The holotype and additional material from the Candeleros Formation date to the Cenomanian stage of the Late Cretaceous, approximately 100–94 million years ago. This formation records a continental paleoenvironment dominated by fluvial-lacustrine systems, including meandering rivers, seasonal streams, shallow lagoons, and vegetated floodplains developed amid the Cenomanian marine transgression in the foreland basin. The climate was subtropical, characterized by warm temperatures with alternating wet and dry periods that supported episodic fluvial activity.^[20]^[21]^[22]^[1] Sedimentological features of the Candeleros Formation include red-colored sandstones and mudstones deposited in low-energy channel and overbank settings, reflecting meandering fluvial systems with associated aeolian dunes and lacustrine intervals. These deposits contain fossilized wood fragments and leaf impressions, evidencing lush riparian vegetation composed primarily of conifers, ferns, and early angiosperms along the floodplains, which sustained a diverse herbivorous community. The presence of calcic paleosols and vertisols further indicates periodic wetting and drying cycles in the subtropical setting.^[23]^[24]^[25] In this ecosystem, Limaysaurus coexisted with a variety of vertebrates, including the titanosaur sauropod Andesaurus as a fellow large herbivore, theropods such as the dromaeosaurid Buitreraptor and carcharodontosaurids like Giganotosaurus (from the overlying Huincul Formation), and ornithischians including small ornithopods. Crocodylomorphs, such as peirosaurids and notosuchians, were also present, occupying aquatic and semi-aquatic niches in the riverine habitats. As a rebbachisaurid sauropod estimated at up to 15 meters in length, Limaysaurus likely functioned as a low- to mid-level browser, potentially engaging in niche partitioning with taller herbivores like Andesaurus by targeting understory vegetation in the floodplain forests.^[26]^[2] Taphonomic evidence suggests that Limaysaurus specimens were preserved in overbank mudstone and sandstone deposits, indicative of death in distal floodplain settings followed by rapid burial during seasonal flooding events. The bones show minimal transport, with little abrasion or disarticulation in some cases, and lack bite marks or other signs of predation, pointing to abiotic causes of mortality such as drowning or stranding in receding waters. This mode of preservation is consistent with the formation's fluvial facies, which favored the accumulation of vertebrate remains in low-energy environments.^[27]^[28]

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