Alexander O Vargas

Alexander O Vargas
University of Chile · Departamento de Biología

PhD

About

116
Publications
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Publications

Publications (116)
Article
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We describe a new locality with ctenochasmatid pterosaurs found in a tidal estuarine paleoenvironment of the Quebrada Monardes Formation (Lower Cretaceous). The new locality, which is named "Cerro Tormento", is in Cerros Bravos in the northeast Atacama region, Northern Chile. Here, we describe four cervical vertebrae, one of them belonging to a sma...
Article
Full-text available
Armoured dinosaurs are well known for their evolution of specialized tail weapons—paired tail spikes in stegosaurs and heavy tail clubs in advanced ankylosaurs1. Armoured dinosaurs from southern Gondwana are rare and enigmatic, but probably include the earliest branches of Ankylosauria2–4. Here we describe a mostly complete, semi-articulated skelet...
Conference Paper
Full-text available
Hasta recientemente, la principal fuente de información sobre mamíferos mesozoicos sudamericanos provenía de la Patagonia argentina, con numerosas especies de mamíferos no-Theria, incluidos en los clados Gondwanatheria/Allotheria, Meridiolestida y Driolestidae. Sin embargo, hallazgos recientes en rocas del Cretácico Superior de la Formación Dorotea...
Article
Full-text available
We describe partial remains of a non-pterodactyloid pterosaur from Upper Jurassic levels of the Atacama Desert in northern Chile. The material includes a left humerus, a possible dorsal vertebra, and the shaft of a wing phalanx, all preserved in three dimensions and likely belonging to a single individual. The humerus has a hatchet-shaped deltopect...
Preprint
Full-text available
Armoured dinosaurs are well known for forms that evolved specialized tail weapons: paired tail spikes in stegosaurs, and heavy tail clubs in advanced ankylosaurs1. Armoured dinosaurs from southern Gondwana are rare and enigmatic, but likely include the earliest branches of Ankylosauria2-4. Here, we describe a mostly complete, semiarticulated skelet...
Article
Full-text available
In the last decades, several discoveries have uncovered the complexity of mammalian evolution during the Mesozoic Era, including important Gondwanan lineages: the australosphenidans, gondwanatherians, and meridiolestidans (Dryolestoidea). Most often, their presence and diversity is documented by isolated teeth and jaws. Here, we describe a new meri...
Article
A new lithostrotian sauropod, Arackar licanantay gen. et sp. nov. is described based on a partial skeleton from the Upper Cretaceous (CampanianeMaastrichtian) beds of the Hornitos Formation, Atacama Region, northern Chile. The holotype consists of axial (cervical and dorsal vertebrae) and appendicular (humerus, femur and ischium) elements of a sub-...
Article
Living beings are autopoietic systems with highly context-dependent structural dynamics and interactions, that determine whether a disturbance in the genotype or environment will lead or not to phenotypic change. The concept of epigenesis entails how a change in the phenotype may not correspond to a change in the structure of an earlier development...
Article
The adult ankle of early reptiles had five distal tarsal (dt) bones, but in Dinosauria, these were reduced to only two: dt3 and dt4, articulated to metatarsals (mt) mt3 and mt4. Birds have a single distal tarsal ossification center that fuses to the proximal metatarsals to form a new adult skeletal structure: the composite tarsometatarsus. This oss...
Preprint
Full-text available
A recent study by Norell et al. (2020) described new egg specimens for two dinosaur species, identified as the first soft-shelled dinosaur eggs. The authors used phylogenetic comparative methods to reconstruct eggshell type in a sample of reptiles, and identified the eggs of dinosaurs and archosaurs as ancestrally soft-shelled, with three independe...
Article
Full-text available
Egg size and structure reflect important constraints on the reproductive and life-history characteristics of vertebrates. More than two-thirds of all extant amniotes lay eggs. During the Mesozoic era (around 250 million to 65 million years ago), body sizes reached extremes; nevertheless, the largest known egg belongs to the only recently extinct el...
Article
Full-text available
We describe Magallanodon baikashkenke gen. et. sp. nov., a new gondwanatherian mammal from the Late Cretaceous of the Magallanes Region in southern Chile (Río de Las Chinas Valley, Estancia Cerro Guido, north of Puerto Natales city, Última Esperanza Province). The mammal-bearing layer is placed within the Late Campanian-Early Maastrichtian levels o...
Article
We describe remains of freshwater turtles from the Upper Cretaceous of Chilean Patagonia. The fossils, which comprise isolated shell fragments and incomplete appendicular bones, were recovered from meandering fluvial deposits of the Dorotea Formation (upper Campanian–Danian), in the Río de Las Chinas Valley, Magallanes region. These remains represe...
Article
Full-text available
Background: The origin of birds is marked by a significant decrease in body size along with an increase in relative forelimb size. However, before the evolution of flight, both traits may have already been related: It has been proposed that an evolutionary trend of negative forelimb allometry existed in non-avian Theropoda, such that larger specie...
Chapter
Full-text available
El tema del origen de las aves es uno de los más dinámicos en la paleontología de vertebrados, producto especialmente de la creciente evidencia del registro fósil (procedente mayormente de China y Argentina), y la tendencia a la integración interdisciplinaria con otras áreas como la biología del desarrollo (Chiappe & Vargas 2003). Las metodologías...
Article
Full-text available
Radical transformation of the skull characterizes bird evolution. An increase in the relative size of the brain and eyes was presumably related to the loss of two bones surrounding the eye, the prefrontal and postorbital. We report that ossification centres of the prefrontal and postorbital are still formed in bird embryos, which then fuse seamless...
Article
Full-text available
Many dinosaur skeletons show evidence of behavior, including feeding, predation, nesting, and parental care. The resting posture of the forelimbs has been studied in some theropod species, in relation to the acquisition of flight in advanced maniraptoran theropods. Chilesaurus diegosuarezi is a bizarre tetanuran recently described from the Toqui Fo...
Article
Synopsis Embryonic muscular activity (EMA) is involved in the development of several distinctive traits of birds. Modern avian diversity and the fossil record of the dinosaur-bird transition allow special insight into their evolution. Traits shaped by EMA result from mechanical forces acting at post-morphogenetic stages, such that genes often play...
Article
Full-text available
Paul Kammerer was the most outstanding neo-Lamarckian experimentalist of the early 20th century. He reported spectacular results in the midwife toad, including crosses of environmentally modified toads with normal toads, where acquired traits were inherited in Mendelian fashion. Accusations of fraud generated a great scandal, ending with Kammerer's...
Article
Full-text available
We report here an isolated anterior caudal vertebra belonging to a lithostrotian titanosaur sauropod. The material comes from the Hornitos Formation (Late Cretaceous) at quebrada La Higuera, Atacama Region, North of Chile. Previous local records only included juvenile individuals. The new find represents the first evidence of adult titanosaurs in t...
Article
Full-text available
We report here an isolated anterior caudal vertebra belonging to a lithostrotian titanosaur sauropod. The material comes from the Hornitos Formation (Late Cretaceous) at quebrada La Higuera, Atacama Region, North of Chile. Previous local records only included juvenile individuals. The new find represents the first evidence of adult titanosaurs in t...
Poster
Full-text available
Resumen. El registro óseo de dinosaurios no avianos en Chile está principalmente representado por saurópodos, los cuales son registrados desde el Titoniano hasta el Maastrichtiano. Dentro de este grupo, los hallazgos más abundantes corresponden a titanosaurios (Sauropoda, Lithostrotia), clado abundante en el Cretácico de Sudamérica y bien represent...
Article
In early theropod dinosaurs—the ancestors of birds—the hallux (digit 1) had an elevated position within the foot and had lost the proximal portion of its metatarsal. It no longer articulated with the ankle, but was attached at about mid-length of metatarsal 2 (mt2). In adult birds, the hallux is articulated closer to the distal end of mt2 at ground...
Data
Figure S1. Pharmacological inhibition of Ihh pathway. Figure S2. The fibula is as large as the tibia at early development in species with relatively short fibulae. Figure S3. The fibulare develops closer to the distal fibula in birds than in other extant reptiles. Figure S4. Fibular reduction in species with different adult fibulo‐tibial ratios....
Article
Full-text available
Birds have a distally reduced, splinter-like fibula that is shorter than the tibia. In embryonic development, both skeletal elements start out with similar lengths. We examined molecular markers of cartilage differentiation in chicken embryos. We found that the distal end of the fibula expresses Indian Hedgehog (IHH), undergoing terminal cartilage...
Article
The anklebone (astragalus) of dinosaurs presents a characteristic upward projection, the ‘ascending process’ (ASC). The ASC is present in modern birds, but develops a separate ossification centre, and projects from the calcaneum in most species. These differences have been argued to make it non-comparable to dinosaurs. We studied ASC development in...
Data
Whole mount Col9 immunostaining of Quail, stage HH33. Distinct domains of expression confirm the ASC is derived from the intermedium. 3D reconstruction of the embryonic ankle obtained from a spin-disc confocal microscope.
Data
Whole mount Col9 immunostaining of Quail, stage HH36. At late stages, all proximal elements of the ankle coalesce into a single large cartilage that forms a "distal cap" to the tibia. 3D reconstruction of the embryonic ankle obtained from a spin-disc confocal microscope.
Data
Astragalus of the basal theropod Dilophosaurus. Alleged "suture lines" are actually indistinguishable from various fracture lines in the specimen. (video horizontally flipped to match orientation of other figures in this article)
Data
Whole mount Col9 immunostaining of Quail, stage HH29-30. Distinct domains of expression confirm the intermedium is present at stages HH29-30. 3D reconstruction of the embryonic ankle obtained from a spin-disc confocal microscope.
Research
Por décadas, el registro fósil de vertebrados en Chile fue exiguo, restringido a pocos hallazgos, muchos de ellos fortuitos. En contraposición a esta situación, grandes figuras de la ciencia, entre ellos Charles Darwin, Rudolph Philippi y Claude Gay, entre otros, notaron tempranamente el valor de este registro, a la vez que dieron a conocer desde e...
Conference Paper
El tobillo de las aves modernas (Neornithes) está profundamente modificado respecto de su condición ancestral, debido al alto grado de fusión de elementos esqueléticos. En ancestros terópodos existen 2 osificaciones distales en la región mesotarsal. El desarrollo embrionario en aves revela una única masa de cartílago que se fusiona con las epífisis...
Article
Full-text available
A description is provided of the first sauropod remains (i.e., isolated vertebrae and appendicular bones) from the Late Jurassic of Aysén, in Chilean Patagonia (Toqui Formation, late Tithonian). Although the bones found are fragmentary, they still allow the recognition of an unsuspected sauropod diversity for this period in South America. The mater...
Article
Full-text available
Specialized morphologies of bird feet have evolved several times independently as different groups have become zygodactyl, semi-zygodactyl, heterodactyl, pam-prodactyl or syndactyl. Birds have also convergently evolved similar modes of development, in a spectrum that goes from precocial to altricial. Using the new context provided by recent molecul...
Article
Full-text available
Most birds have an opposable digit 1 (hallux) allowing the foot to grasp, which evolved from the non-opposable hallux of early theropod dinosaurs. An important morphological difference with early theropods is the twisting of the long axis of its metatarsal. Here, we show how embryonic musculature and the onset of its activity are required for twist...
Article
Full-text available
Theropod dinosaurs were the dominant predators in most Mesozoic era terrestrial ecosystems. Early theropod evolution is currently interpreted as the diversification of various carnivorous and cursorial taxa, whereas the acquisition of herbivorism, together with the secondary loss of cursorial adaptations, occurred much later among advanced coeluros...
Chapter
Full-text available
Se presenta a continuación una síntesis de los hallazgos de arcosaurios no avianos (Diapsida: Archosauria) en Chile. Estos registros cubren un rango temporal desde el Triásico medio-tardío hasta el Neógeno y se encuentran mayormente concentrados en la zona norte del país. Los materiales provenientes de niveles triásicos de los Estratos El Bordo est...
Conference Paper
El tarso proximal amnioto está conformado por dos huesos, el astrágalo y el calcáneo. El as- trágalo de amniotos actuales se forma a partir de la fusión de cartílagos tempranos, presentan- do solo un centro de osificación (Peabody, 1951). En tetrápodos no-amniotos y stem-amniotos existen tres huesos en la misma posición anatómica: tibiale, interme...
Article
Full-text available
From early dinosaurs with as many as nine wrist bones, modern birds evolved to develop only four ossifications. Their identity is uncertain, with different labels used in palaeontology and developmental biology. We examined embryos of several species and studied chicken embryos in detail through a new technique allowing whole-mount immunofluorescen...
Poster
Full-text available
The early Late Cretaceous (Cenomanian-early Turonian) was characterized by extreme greenhouse conditions. Polar Regions were free of ice and sea surface temperatures may have reached 35°C, or more. Late Cretaceous times are still considered to have been greenhouse-dominated, but a major gradual decline in global temperatures is seen during the Late...
Article
Full-text available
The zygodactyl orientation of toes (digits II and III pointing forwards, digits I and IV pointing backwards) evolved independently in different extant bird taxa. To understand the origin of this trait in modern birds, we investigated the development of the zygodactyl foot of the budgerigar (Psittaciformes). We compared its muscular development with...