Oscar A Alcober

National University of San Juan, Ciudad de San Juan, San Juan, Argentina

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Publications (20)75.29 Total impact

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    ABSTRACT: A measured magnetozone sequence defined by 24 sampling sites with normal polarity and 28 sites with reverse polarity characteristic magnetizations was established for the heretofore poorly age-constrained Los Colorados Formation and its dinosaur-bearing vertebrate fauna in the Ischigualasto–Villa Union continental rift basin of Argentina. The polarity pattern in this ∼600-m-thick red-bed section can be correlated to Chrons E7r to E15n of the Newark astrochronological polarity time scale. This represents a time interval from 227 to 213 Ma, indicating that the Los Colorados Formation is predominantly Norian in age, ending more than 11 My before the onset of the Jurassic. The magnetochronology confirms that the underlying Ischigualasto Formation and its vertebrate assemblages including some of the earliest known dinosaurs are of Carnian age. The oldest dated occurrences of vertebrate assemblages with dinosaurs in North America (Chinle Formation) are younger (Norian), and thus the rise of dinosaurs was diachronous across the Americas. Paleogeography of the Ischigualasto and Los Colorados Formations indicates prolonged residence in the austral temperate humid belt where a provincial vertebrate fauna with early dinosaurs may have incubated. Faunal dispersal across the Pangean supercontinent in the development of more cosmopolitan vertebrate assemblages later in the Norian may have been in response to reduced contrasts between climate zones and lowered barriers resulting from decreasing atmospheric pCO2 levels.
    Proceedings of the National Academy of Sciences 05/2014; · 9.81 Impact Factor
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    ABSTRACT: The record of non-mammaliaformes eucynodonts from the Carnian-Norian Ischigualasto Formation is diverse and abundant, including a medium to large size herbivore and small carnivores. Here is described a new small eucynodont from the Ischigualasto Formation, on the basis of a partial skull. The new taxon is characterized by palatal process of the premaxilla extending posterior to the level of the fi rst postcanine; deep and large maxillary laterodorsal fossa that opens at the level of the root of the upper canine; and postorbital bar diverging posterolaterally at very low angle (35.6°) from the anteroposterior axis of the skull. Results from a phylogenetic analysis supports the new genus placement as a probainognathian eucynodont, more derived than Probainognathus Romer, and more closely related to Ecteninion Martinez, May & Forster and Trucidocynodon Oliveira, Soares & Schultz than to any other eucynodont. Ecteniniidae is proposed as a new clade including the new genus, Ecteninion and Trucidocynodon, and in the phylogenetic hypothesis represents the sister-group of Prozostrodontia (Prozostrodon Bonaparte & Barberena, Tritylodontidae and Mammaliaformes). Additionally, the new taxon from the Ischigualasto Formation shows that the Scaphonyx-Exaeretodon Herrerasaurus biozone has similar cynodont diversity than the supposedly contemporaneous Hyperodapedon Assemblage Zone of Santa Maria 2 Sequence, in Southern Brazil.
    Revista Brasileira de Paleontologia 05/2013; 16(1):61-76. · 0.58 Impact Factor
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    V Simposio Argentino del Jurásico; 04/2013
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    ABSTRACT: Sphenodontians were a successful group of rhynchocephalian reptiles that dominated the fossil record of Lepidosauria during the Triassic and Jurassic. Although evidence of extinction is seen at the end of the Laurasian Early Cretaceous, they appeared to remain numerically abundant in South America until the end of the period. Most of the known Late Cretaceous record in South America is composed of opisthodontians, the herbivorous branch of Sphenodontia, whose oldest members were until recently reported to be from the Kimmeridgian-Tithonian (Late Jurassic). Here, we report a new sphenodontian, Sphenotitan leyesi gen. et sp. nov., collected from the Upper Triassic Quebrada del Barro Formation of northwestern Argentina. Phylogenetic analysis identifies Sphenotitan as a basal member of Opisthodontia, extending the known record of opisthodontians and the origin of herbivory in this group by 50 Myr.
    Proceedings of the Royal Society B: Biological Sciences 01/2013; 280(1772):20132057. · 5.68 Impact Factor
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    ABSTRACT: The proterochampsids are a group of crocodile-like archosauri-forms usually considered as one of the potential successive sister taxa of the crown group Archosauria (e.g., Sereno and Ar-cucci, 1990; Sereno, 1991; Dilkes and Sues, 2009; Ezcurra et al., 2010). The clade is currently endemic to the late Middle and early Late Triassic of South America. The best-known proterochampsids come from the Chã nares and Ischigualasto formations, which belong to the Ischigualasto-Villa Uní on Basin (Fig. 1), which formed during the breakup of Gondwana (Uliana and Biddle, 1988; Ramos and Kay, 1991). The tetrapod assemblage from the Chã nares Formation (Anisian–early Car-nian; Desojo et al., 2011) has been usually interpreted to differ from that of the Ischigualasto Formation (late Carnian–earliest Norian; Martínez et al., 2011), and this traditional view also applies for proterochampsids. In the Chã nares Formation, the proterochampsid genera Chanaresuchus, Gualosuchus, and Tropidosuchus (Romer, 1971, 1972; Arcucci, 1990) are docu-mented, whereas in the Ischigualasto Formation only the genus Proterochampsa has been reported (Reig, 1959). The latter highlighted a complete replacement at a generic level among these proterochampsid assemblages in southwestern Pangaea. However, an almost complete proterochampsid skeleton from the Late Triassic Ischigualasto Formation is given a preliminarily description here and assigned to the traditionally Middle Triassic genus Chanaresuchus (cf. Sill et al., 1994). Accordingly, the new proterochampsid record reported here changes the pattern of macroevolutionary history of the group during the Middle to Late Triassic in southwestern Pangaea.
    Journal of Vertebrate Paleontology 04/2012; 32(2-32):485-489. · 2.08 Impact Factor
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    Journal of Vertebrate Paleontology 01/2012; 32:83-179. · 2.08 Impact Factor
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    ABSTRACT: The Upper Triassic (Carnian–Norian) Ischigualasto Formation has yielded a diverse vertebrate fauna that records the initial phase of dinosaur evolution. Radioisotopic dates from ash layers within the formation provide a chronostratigraphic framework, and stratigraphic and sedimetological studies have subdivided the formation into four members and three abundance-based biozones. We describe two new basal dinosauromorphs, an unnamed lagerpetid and a new silesaurid, Ignotosaurus fragilis, gen. et sp. nov., which increase to 29 the number of vertebrates in the Ischigualasto fauna. We provide a census of 848 fossil specimens representing 26 vertebrate taxa logged to stratigraphic intervals of 50 m. This temporally calibrated census shows that abundance and taxonomic diversity within the Ischigualasto Formation does not change suddenly but rather appears to gradually decline in response to climatic deterioration. The only abrupt shift in faunal composition occurs at the end of the second of three biozones, when the abundant cynodont Exaeretodon is replaced by the rare dicynodont Jachaleria.
    Journal of Vertebrate Paleontology 01/2012; 32(sup1):10-30. · 2.08 Impact Factor
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    ABSTRACT: Large-diameter ichnofossils comprising three morphotypes have been identified in the Upper Triassic Ischigualasto and Los Colorados formations of northwestern Argentina. These burrows add to the global record of the early appearance of fossorial behavior during early Mesozoic time. Morphotypes 1 and 2 are characterized by a network of tunnels and shafts that can be assigned to tetrapod burrows given similarities with previously described forms. However, differences in diameter, overall morphology, and stratigraphic occurrence allow their independent classification. Morphotype 3 forms a complex network of straight branches that intersect at oblique angles. Their calcareous composition and surface morphology indicate these structures have a composite biogenic origin likely developed due to combined plant/animal interactions. The association of Morphotypes 1 and 2 with fluvial overbank lithologies deposited under an extremely seasonal arid climate confirms interpretations that the early appearance of burrowing behavior was employed by vertebrates in response to both temperature and moisture-stress associated with seasonally or perpetually dry Pangean paleoclimates. Comparisons of burrow morphology and biomechanical attributes of the abundant paleovertebrate fauna preserved in both formations permit interpretations regarding the possible burrow architects for Morphotypes 1 and 2. In the case of the Morphotype 1, the burrow constructor could be one of the small carnivorous cynodonts, Ecteninion or Probelesodon. Assigning an architect for Morphotype 2 is more problematic due to mismatches between the observed burrow morphology and the size of the known Los Colorados vertebrates.
    PLoS ONE 01/2012; 7(12):e50662. · 3.73 Impact Factor
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    ABSTRACT: Upper Triassic rocks in northwestern Argentina preserve the most complete record of dinosaurs before their rise to dominance in the Early Jurassic. Here, we describe a previously unidentified basal theropod, reassess its contemporary Eoraptor as a basal sauropodomorph, divide the faunal record of the Ischigualasto Formation with biozones, and bracket the formation with (40)Ar/(39)Ar ages. Some 230 million years ago in the Late Triassic (mid Carnian), the earliest dinosaurs were the dominant terrestrial carnivores and small herbivores in southwestern Pangaea. The extinction of nondinosaurian herbivores is sequential and is not linked to an increase in dinosaurian diversity, which weakens the predominant scenario for dinosaurian ascendancy as opportunistic replacement.
    Science 01/2011; 331(6014):206-10. · 31.20 Impact Factor
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    ABSTRACT: Argentinean basal sauropodomorphs are known by several specimens from different basins; Ischigualasto, El Tranquilo, and Mogna. The Argentinean record is diverse and includes some of the most primitive known sauropodomorphs such as Panphagia and Chromogisaurus, as well as more derived forms, including several massospondylids. Until now, the Massospondylidae were the group of basal sauropodomorphs most widely spread around Pangea with a record in almost all continents, mostly from the southern hemisphere, including the only record from Antarctica. We describe here a new basal sauropodomorph, Leyesaurus marayensis gen. et sp. nov., from the Quebrada del Barro Formation, an Upper Triassic-Lower Jurassic unit that crops out in northwestern Argentina. The new taxon is represented by a partial articulated skeleton that includes the skull, vertebral column, scapular and pelvic girdles, and hindlimb. Leyesaurus is diagnosed by a set of unique features, such as a sharply acute angle (50 degrees) formed by the ascending process of the maxilla and the alveolar margin, a straight ascending process of the maxilla with a longitudinal ridge on its lateral surface, noticeably bulging labial side of the maxillary teeth, greatly elongated cervical vertebrae, and proximal articular surface of metatarsal III that is shelf-like and medially deflected. Phylogenetic analysis recovers Leyesaurus as a basal sauropodomorph, sister taxon of Adeopapposaurus within the Massospondylidae. Moreover, the results suggest that massospondylids achieved a higher diversity than previously thought. Our phylogenetic results differ with respect to previous analyses by rejecting the massospondylid affinities of some taxa from the northern hemisphere (e.g., Seitaad, Sarahsaurus). As a result, the new taxon Leyesaurus, coupled with other recent discoveries, suggests that the diversity of massospondylids in the southern hemisphere was higher than in other regions of Pangea. Finally, the close affinities of Leyesaurus with the Lower Jurassic Massospondylus suggest a younger age for the Quebrada del Barro Formation than previously postulated.
    PLoS ONE 01/2011; 6(11):e26964. · 3.73 Impact Factor
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    Oscar A Alcober, Ricardo N Martinez
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    ABSTRACT: Herrerasauridae comprises a basal clade of dinosaurs best known from the Upper Triassic of Argentina and Brazil, which have yielded remains of Herrerasaurus ischigualastensis and Staurikosaurus pricei, respectively. Systematic opinion regarding the position of Herrerasauridae at the base of Dinosauria has varied. Here we describe a new herrerasaurid, Sanjuansaurus gordilloi gen. n., sp. n., based on a partial skeleton from Carnian-age strata of the the Upper Triassic Ischigualasto Formation of northwestern Argentina. The new taxon is diagnosed by numerous features, including long, band-shaped and posterolaterally oriented transverse process on the posterior cervical vertebrae; neural spines of the sixth to eighth dorsal vertebrae, at least, bearing acute anterior and posterior processes; scapula and coracoid with everted lateral margins of the glenoid; and short pubis (63% of the femoral length). Phylogenetic analysis placed Sanjuansaurus within a monophyletic Herrerasauridae, at the base of Theropoda and including Herrerasaurus and Staurikosaurus. The presence of Sanjuansaurus at the base of the Ischigualasto Formation, along with other dinosaurs such as Herrerasaurus, Eoraptor, Panphagia, and Chromogisaurus suggests that saurischian dinosaurs in southwestern Pangea were already widely diversified by the late Carnian rather than increasing in diversity across the Carnian-Norian boundary.
    ZooKeys 01/2010; · 0.92 Impact Factor
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    Ricardo N Martinez, Oscar A Alcober
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    ABSTRACT: The earliest dinosaurs are from the early Late Triassic (Carnian) of South America. By the Carnian the main clades Saurischia and Ornithischia were already established, and the presence of the most primitive known sauropodomorph Saturnalia suggests also that Saurischia had already diverged into Theropoda and Sauropodomorpha. Knowledge of Carnian sauropodomorphs has been restricted to this single species. We describe a new small sauropodomorph dinosaur from the Ischigualsto Formation (Carnian) in northwest Argentina, Panphagia protos gen. et sp. nov., on the basis of a partial skeleton. The genus and species are characterized by an anteroposteriorly elongated fossa on the base of the anteroventral process of the nasal; wide lateral flange on the quadrate with a large foramen; deep groove on the lateral surface of the lower jaw surrounded by prominent dorsal and ventral ridges; bifurcated posteroventral process of the dentary; long retroarticular process transversally wider than the articular area for the quadrate; oval scars on the lateral surface of the posterior border of the centra of cervical vertebrae; distinct prominences on the neural arc of the anterior cervical vertebra; distal end of the scapular blade nearly three times wider than the neck; scapular blade with an expanded posterodistal corner; and medial lamina of brevis fossa twice as wide as the iliac spine. We regard Panphagia as the most basal sauropodomorph, which shares the following apomorphies with Saturnalia and more derived sauropodomorphs: basally constricted crowns; lanceolate crowns; teeth of the anterior quarter of the dentary higher than the others; and short posterolateral flange of distal tibia. The presence of Panphagia at the base of the early Carnian Ischigualasto Formation suggests an earlier origin of Sauropodomorpha during the Middle Triassic.
    PLoS ONE 02/2009; 4(2):e4397. · 3.73 Impact Factor
  • OSCAR ALCOBER
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    ABSTRACT: A new skull of Saurosuchus galilei is described from a specimen collected at the base of the Upper Triassic Ischigualasto Formation in San Juan Province, Argentina. The skull of this taxon is described in detail and compared to those of other suchians. The poor ossification of the articular end of the quadrate, disarticulation of the braincase from the dermatocranium, and poor development of the suture between exoccipital and basioccipital in the skull of Saurosuchus indicate that the specimen was a juvenile. Possible cranial autapomorphies of Saurosuchus include: a thick, sculptured skull roof; reduced postfrontal; slender ventral projection of the lacrimal; lateral process of the posterolateral frontal; development of a crista on the dorsal supraoccipital; development of large lateral processes of the laterosphenoid; basal tuber wider than occipital condyle; and elongate anterior process of the coronoid.
    Journal of Vertebrate Paleontology 01/2009; · 2.08 Impact Factor
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    ABSTRACT: Living birds possess a unique heterogeneous pulmonary system composed of a rigid, dorsally-anchored lung and several compliant air sacs that operate as bellows, driving inspired air through the lung. Evidence from the fossil record for the origin and evolution of this system is extremely limited, because lungs do not fossilize and because the bellow-like air sacs in living birds only rarely penetrate (pneumatize) skeletal bone and thus leave a record of their presence. We describe a new predatory dinosaur from Upper Cretaceous rocks in Argentina, Aerosteon riocoloradensis gen. et sp. nov., that exhibits extreme pneumatization of skeletal bone, including pneumatic hollowing of the furcula and ilium. In living birds, these two bones are pneumatized by diverticulae of air sacs (clavicular, abdominal) that are involved in pulmonary ventilation. We also describe several pneumatized gastralia ("stomach ribs"), which suggest that diverticulae of the air sac system were present in surface tissues of the thorax. We present a four-phase model for the evolution of avian air sacs and costosternal-driven lung ventilation based on the known fossil record of theropod dinosaurs and osteological correlates in extant birds: (1) Phase I-Elaboration of paraxial cervical air sacs in basal theropods no later than the earliest Late Triassic. (2) Phase II-Differentiation of avian ventilatory air sacs, including both cranial (clavicular air sac) and caudal (abdominal air sac) divisions, in basal tetanurans during the Jurassic. A heterogeneous respiratory tract with compliant air sacs, in turn, suggests the presence of rigid, dorsally attached lungs with flow-through ventilation. (3) Phase III-Evolution of a primitive costosternal pump in maniraptoriform theropods before the close of the Jurassic. (4) Phase IV-Evolution of an advanced costosternal pump in maniraptoran theropods before the close of the Jurassic. In addition, we conclude: (5) The advent of avian unidirectional lung ventilation is not possible to pinpoint, as osteological correlates have yet to be identified for uni- or bidirectional lung ventilation. (6) The origin and evolution of avian air sacs may have been driven by one or more of the following three factors: flow-through lung ventilation, locomotory balance, and/or thermal regulation.
    PLoS ONE 02/2008; 3(9):e3303. · 3.73 Impact Factor
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    ABSTRACT: Coprolites were collected 74 m above the base of the Upper Triassic Ischigualasto Formation of northwestern Argenti-na and their chemistry, mineralogy, and textures were stud-ied to infer their biological source and taphonomy. They were found to contain a few fragments of bone and much crystalline apatite, and so can be assigned to carnivores able to digest bone. Primary minerals derived from endo-genous materials were apatite and pyrite, and secondary minerals derived from ground water were chiefly calcite and glauconite in one coprolite and glauconite in another. Primary apatite, presumed originally to have been mostly dahllite (a carbonate hydroxyapatite) precipitated from di-gested bone, was converted to francolite (a carbonate fluor-apatite) during diagenesis through the introduction of fluo-ride from ground water. The chemistry, mineralogy, and structure of the coprolites suggest an early onset of miner-alization, early anaerobic burial, and a recent anaerobic-to-aerobic transition during weathering. The chemistry of the coprolites is consistent with the stratigraphic identification of the coprolite horizon as an ancient flood plain.
    Palaios 01/2003; 20:51-63. · 1.79 Impact Factor
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    01/2001: pages 23-54;
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    Journal of Vertebrate Paleontology 10/1999; 19(3-19):591-594. · 2.08 Impact Factor
  • 01/1999: pages 377-398;
  • Oscar Alcober, J. Michael Parrish
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    ABSTRACT: The Poposauridae are a group of Middle to Late Triassic rauisuchian archosaurs that are the sister-group of the Crocodylomorpha. A new specimen from the Ischigualasto Formation (Carnian) of Argentina (PVSJ-85) is the first clear record of this group from Gondwana, and is designated the holotype of a new genus and species, Sillosuchus longicervix. The specimen consists of most of the vertebral column, both femora, pubes, and ischia, a partial right ilium, and a single paramedian plate. A distinctive feature shared with Chatterjeea, a poposaurid known from the Dockum Formation of Texas, is the presence of elongate cervical vertebrae with deep, dorsoventrally foreshortened excavations in the sides of the centra. The ilium of Sillosuchus has a prominent overhang that makes the upper surface of the acetabulum concave and partially encloses its dorsolateral edge. The pubes are elongate and narrow with a modest distal expansion forming a small foot. At least four and possibly as many as six sacral vertebrae are present.A number of other poposaurids are known from North America and Europe, although all are currently represented by fragmentary material. Sillosuchus has derived characters (e.g., increased number of sacral vertebrae, cervical morphology, acetabular shape) in common with more derived poposaurids such as Poposaurus and Chatterjeea.
    Journal of Vertebrate Paleontology - J VERTEBRATE PALEONTOL. 01/1997; 17(3):548-556.