Alexander O Vargas

University of Chile · Departamento de Biología

The early evolutionary and biogeographical history of Gondwanan iguanodontian dinosaurs is poorly understood due to their scarce Lower Cretaceous fossil record. In South America, the Lower Cretaceous iguanodontian osteological record is very fragmentary and most published reports cannot be used to discard or confirm hadrosauroid affinities. The sin...

The first dinosaur discovered in the Antarctic continent was the ankylosaur Antarctopelta oliveroi in the 1980s. Nevertheless, since then several hypotheses of phylogenetical relationships have been proposed because these have been depended on how the skeletal remains have been interpreted. The main obstacle for clarifying its phylogenetic position...

Gonkoken nanoi es un hadrosauroideo sudamericano representado por restos desarticulados procedentes de “Loma Koken”, un yacimiento monoespecífico en depósitos continentales de la Formación Dorotea (Cretácico Superior), en el Valle del Río de Las Chinas, Región de Magallanes, Chile. Estudios morfológicos y filogenéticos sugieren que Gonkoken sería...

In the dusk of the Mesozoic, advanced duck-billed dinosaurs (Hadrosauridae) were so successful that they likely outcompeted other herbivores, contributing to declines in dinosaur diversity. From Laurasia, hadrosaurids dispersed widely, colonizing Africa, South America, and, allegedly, Antarctica. Here, we present the first species of a duck-billed...

In the dusk of the dinosaur era, the advanced duck-billed dinosaurs (Family Hadrosauridae) are thought to have outcompeted other herbivores, making ecosystems less diverse and more vulnerable to the Cretaceous-Paleogene asteroid impact. They were also among the first terrestrial organisms to disperse from North America into South America. Here, we...

https://constructivist.info/18/1/101

The end Cretaceous mass extinction was marked by a dramatic change in biodiversity, and the extinction of all non-avian dinosaurs. To understand the diversity of dinosaur clades prior to this event, as well as recovery by avian dinosaurs (birds), we need a better understanding of the global fossil record. However, the fossil record from southern lo...

Durante mucho tiempo la principal fuente de información sobre mamíferos del Cretácico Superior de Gondwana estuvo dominada por los registros provenientes de la Patagonia argentina y Madagascar, en conjunto con registros más antiguos distribuidos en África continental y Australia. En el registro del Campaniano-Maastrichtiano predominan numerosas esp...

Las asociaciones de vertebrados del Cretácico Superior en altas latitudes de Sudamérica son relevantes para el entendimiento de la evolución y paleobiogeografía de las biotas australes. En particular, los afloramientos de la Formación Dorotea en el Valle del río de Las Chinas, ubicado en la Región de Magallanes, preservan un extenso registro geológ...

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...

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...

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...

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...

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...

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...

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-...

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...

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...

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...

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...

Se describe a Magallanodon baikashkenke gen. et. sp. nov., un nuevo mamífero gondwanaterio del Cretácico tardío de la Región de Magallanes, en el sur de Chile (Valle del Río de Las Chinas, Estancia Cerro Guido, norte de Puerto Natales, Provincia de Última Esperanza). Las capas portadoras se ubican entre los niveles del Campaniano tardío-Maastrichti...

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...

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...

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...

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...

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...

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...

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...

Reportamos una vértebra caudal anterior aislada de un saurópodo titanosaurio Lithostrotia. El material proviene de la Formación Hornitos en quebrada La Higuera, Región de Atacama, norte de Chile. Los registros locales previos corresponden exclusivamente a individuos juveniles. El nuevo hallazgo representa la primera evidencia de titanosaurios adult...

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...

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...

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...

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...

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...

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....

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...

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...

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.

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.

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)

Supplementary Figures 1-12 and Supplementary References

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.

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...

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...

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...

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...