The oldest tadpole reveals evolutionary stability of the anuran life cycle.

Anurans are characterized by a biphasic life cycle, with an aquatic larval (tadpole) stage followed by an adult (frog) stage, both connected through the metamorphic period in which drastic morphological and physiological changes occur1. Extant tadpoles exhibit great morphological diversity and ecological relevance2, but their absence in the pre-Cretaceous fossil record (older than 145 million years) makes their origins and early evolution enigmatic. This contrasts with the postmetamorphic anuran fossil record that dates back to the Early Jurassic and with closely related species in the Late Triassic (around 217-213 million years ago (Ma))3. Here we report a late-stage tadpole of the stem-anuran Notobatrachus degiustoi from the Middle Jurassic of Patagonia (around 168-161 Ma). This finding has dual importance because it represents the oldest-known tadpole and, to our knowledge, the first stem-anuran larva. Its exquisite preservation, including soft tissues, shows features associated with the filter-feeding mechanism characteristic of extant tadpoles4,5. Notably, both N. degiustoi tadpole and adult reached a large size, demonstrating that tadpole gigantism occurred among stem-anurans. This new discovery reveals that a biphasic life cycle, with filter-feeding tadpoles inhabiting aquatic ephemeral environments, was already present in the early evolutionary history of stem-anurans and has remained stable for at least 161 million years.

New reptile shows dinosaurs and pterosaurs evolved among diverse precursors.

Dinosaurs and pterosaurs have remarkable diversity and disparity through most of the Mesozoic Era1-3. Soon after their origins, these reptiles diversified into a number of long-lived lineages, evolved unprecedented ecologies (for example, flying, large herbivorous forms) and spread across Pangaea4,5. Recent discoveries of dinosaur and pterosaur precursors6-10 demonstrated that these animals were also speciose and widespread, but those precursors have few if any well-preserved skulls, hands and associated skeletons11,12. Here we present a well-preserved partial skeleton (Upper Triassic, Brazil) of the new lagerpetid Venetoraptor gassenae gen. et sp. nov. that offers a more comprehensive look into the skull and ecology of one of these precursors. Its skull has a sharp, raptorial-like beak, preceding that of dinosaurs by around 80 million years, and a large hand with long, trenchant claws that firmly establishes the loss of obligatory quadrupedalism in these precursor lineages. Combining anatomical information of the new species with other dinosaur and pterosaur precursors shows that morphological disparity of precursors resembles that of Triassic pterosaurs and exceeds that of Triassic dinosaurs. Thus, the 'success' of pterosaurs and dinosaurs was a result of differential survival among a broader pool of ecomorphological variation. Our results show that the morphological diversity of ornithodirans started to flourish among early-diverging lineages and not only after the origins of dinosaurs and pterosaurs.

Enigmatic dinosaur precursors bridge the gap to the origin of Pterosauria.

Pterosaurs were the first vertebrates to evolve powered flight1 and comprised one of the main evolutionary radiations in terrestrial ecosystems of the Mesozoic era (approximately 252-66 million years ago), but their origin has remained an unresolved enigma in palaeontology since the nineteenth century2-4. These flying reptiles have been hypothesized to be the close relatives of a wide variety of reptilian clades, including dinosaur relatives2-8, and there is still a major morphological gap between those forms and the oldest, unambiguous pterosaurs from the Upper Triassic series. Here, using recent discoveries of well-preserved cranial remains, microcomputed tomography scans of fragile skull bones (jaws, skull roofs and braincases) and reliably associated postcrania, we demonstrate that lagerpetids-a group of cursorial, non-volant dinosaur precursors-are the sister group of pterosaurs, sharing numerous synapomorphies across the entire skeleton. This finding substantially shortens the temporal and morphological gap between the oldest pterosaurs and their closest relatives and simultaneously strengthens the evidence that pterosaurs belong to the avian line of archosaurs. Neuroanatomical features related to the enhanced sensory abilities of pterosaurs9 are already present in lagerpetids, which indicates that these features evolved before flight. Our evidence illuminates the first steps of the assembly of the pterosaur body plan, whose conquest of aerial space represents a remarkable morphofunctional innovation in vertebrate evolution.

A new carcharodontosaurid specimen sheds light on the anatomy of South American giant predatory dinosaurs.

Carcharodontosaurids were gigantic terrestrial dinosaurs and top predators of dinosaur faunas in Gondwanan landmasses during the "Mid"-Cretaceous Period. Despite their wide geographical and stratigraphical distribution, essential parts of their anatomy are still poorly known. The present contribution aims to describe a new partial skeleton of the carcharodontosaurid Taurovenator violantei, which was previously known only by an isolated postorbital bone coming from Cenomanian-Turonian beds of northern Patagonia, Argentina. The neck of Taurovenator is composed of notably high anterior cervicals, bearing neural spines with expanded, flange-like dorsal tips which are successively imbricated. This condition has been reported previously in the carcharodontosaurid Acrocanthosaurus, but its occurrence in Taurovenator and other members of the clade suggests it may represent a synapomorphy of this theropod family. This unique neck morphology was probably related to strong modifications in musculature and restriction in the range of movements within the neck, but not with the head. The new specimen also affords valuable anatomical information on the forelimb of Patagonian carcharodontosaurids. As in other giganotosaurines, Taurovenator shows strongly reduced forelimbs, particularly the forearm, showing hand elements with elongated non-ungual phalanges, and well-marked articular surfaces and muscular insertions, suggesting highly movable digits. This new specimen of Taurovenator allows us to expand anatomical and morpho-functional discussions about the carcharodontosaurid clade.

An enigmatic plant-eating theropod from the Late Jurassic period of Chile.

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 coelurosaurian theropods. A new, bizarre herbivorous basal tetanuran from the Upper Jurassic of Chile challenges this conception. The new dinosaur was discovered at Aysén, a fossil locality in the Upper Jurassic Toqui Formation of southern Chile (General Carrera Lake). The site yielded abundant and exquisitely preserved three-dimensional skeletons of small archosaurs. Several articulated individuals of Chilesaurus at different ontogenetic stages have been collected, as well as less abundant basal crocodyliforms, and fragmentary remains of sauropod dinosaurs (diplodocids and titanosaurians).

Fossil fishes and anurans from the Miocene of Rio Chico and Cerro Zeballos, Chubut Province, Argentina.

The fossil record of freshwater fishes and anurans from the Miocene in Patagonia is relatively patchy, a large number of specimens remaining undescribed. The aim of the present contribution is to describe a fossil association of percomorphacean fishes and calyptocephalellid anurans from the early to late Miocene Collón Curá Formation, at Chubut province, Patagonia, Argentina. In spite of being represented by several specimens, both anurans and fishes show a very low taxonomic diversity. This pattern matches with other fossil sites from the Cenozoic of Patagonia, as well as with the extant Patagonian batrachofaunas and ichthyofaunas. The fossil record of frogs and fishes in Patagonia is represented by few lineages that have a large evolutionary history in the area, and occasionally can be traced up to the Late Mesozoic.

New theropod dinosaur from the Upper Cretaceous of Patagonia sheds light on the paravian radiation in Gondwana.

The fossil record of basal paravians in Gondwana is still poorly known, being limited to the Cretaceous unenlagiids from South America and the problematic Rahonavis from Madagascar. Here we report on a new paravian from the Cenomanian-Turonian (Late Cretaceous) of Río Negro province, NW Patagonia, Argentina. The new taxon exhibits a derived bird-like morphology of the forelimbs (e.g., robust ulna with prominent, anteriorly oriented, and proximally saddle-shaped radial cotyle and wide medial flange on metacarpal I) and a plesiomorphic foot with a raptorial pedal digit II. Phylogenetic analysis recovers the new taxon in a monophyletic clade with Rahonavis, being the sister group of the remaining Avialae and more derived than other non-avian dinosaurs. Both exhibit derived forelimb traits in opposition with their plesiomorphic hind limbs. The position of the new taxon and Rahonavis as stem avialans indicates that Gondwanan basal paravians are represented by two different clades, at least. The new taxon probably constitutes a previously unknown grade in the avian-line theropods in which some flight-related adaptations of the forelimbs are present in cursorial taxa. The present discovery sheds light on the acquisition of flight-related traits in non-avian dinosaurs and on the still poorly known paravian radiation in Gondwana.

New record of a Mesozoic gondwanatherian mammaliaform from Southern Patagonia.

Gondwanatheria is an enigmatic mammaliaform clade distributed in the Cretaceous and Paleogene of South America, Africa, Madagascar, India and Antarctica. The Mesozoic record in South America is restricted to the Latest Cretaceous of Río Negro and Chubut provinces, Argentina and Magallanes Region of southern Chile. The aim of the present contribution is to describe a new specimen of gondwanatherian mammaliaforms from beds belonging to the Maastrichtian Chorrillo Formation, cropping out 30 km SW of El Calafate, Santa Cruz Province, Argentina. It is represented by a single molariform referable to the species Magallanodon baikashkenke with which it shares a unique combination of characters. Analysis of the unique combination of characters exhibited by Magallanodon shed doubts on the monophyly of Ferugliotheriidae and suggest that South American taxa may be closely related to each other. The wide geographical distribution and occurrence of gondwanatherians on geological units of diverse origins suggest that they were capable of facing disparate environmental conditions.

Vegaviidae, a new clade of southern diving birds that survived the K/T boundary.

The fossil record of Late Cretaceous-Paleogene modern birds in the Southern Hemisphere includes the Maastrichtian Neogaeornis wetzeli from Chile, Polarornis gregorii and Vegavis iaai from Antarctica, and Australornis lovei from the Paleogene of New Zealand. The recent finding of a new and nearly complete Vegavis skeleton constitutes the most informative source for anatomical comparisons among Australornis, Polarornis, and Vegavis. The present contribution includes, for the first time, Vegavis, Polarornis, and Australornis in a comprehensive phylogenetic analysis. This analysis resulted in the recognition of these taxa as a clade of basal Anseriformes that we call Vegaviidae. Vegaviids share a combination of characters related to diving adaptations, including compact and thickened cortex of hindlimb bones, femur with anteroposteriorly compressed and bowed shaft, deep and wide popliteal fossa delimited by a medial ridge, tibiotarsus showing notably proximally expanded cnemial crests, expanded fibular crest, anteroposterior compression of the tibial shaft, and a tarsometatarsus with a strong transverse compression of the shaft. Isolated bones coming from the Cretaceous and Paleogene of South America, Antarctica, and New Zealand are also referred to here to Vegaviidae and support the view that these basal anseriforms were abundant and diverse at high southern latitudes. Moreover, vegaviids represent the first avian lineage to have definitely crossed the K-Pg boundary, supporting the idea that some avian clades were not affected by the end Mesozoic mass extinction event, countering previous interpretations. Recognition of Vegaviidae indicates that modern birds were diversified in southern continents by the Cretaceous and reinforces the hypothesis indicating the important role of Gondwana for the evolutionary history of Anseriformes and Neornithes as a whole.

A large therian mammal from the Late Cretaceous of South America.

Theria represent an extant clade that comprises placental and marsupial mammals. Here we report on the discovery of a new Late Cretaceous mammal from southern Patagonia, Patagomaia chainko gen. et sp. nov., represented by hindlimb and pelvic elements with unambiguous therian features. We estimate Patagomaia chainko attained a body mass of 14 kg, which is considerably greater than the 5 kg maximum body mass of coeval Laurasian therians. This new discovery demonstrates that Gondwanan therian mammals acquired large body size by the Late Cretaceous, preceding their Laurasian relatives, which remained small-bodied until the beginning of the Cenozoic. Patagomaia supports the view that the Southern Hemisphere was a cradle for the evolution of modern mammalian clades, alongside non-therian extinct groups such as meridiolestidans, gondwanatherians and monotremes.

Comments on the pelvic girdle anatomy of Lagerpeton chanarensis Romer, 1971 (Archosauria) and its implications on the posture and gait of early pterosauromorphs.

Lagerpeton chanarensis is an early avemetatarsalian from the lower Carnian (lowermost Upper Triassic) levels of the Chañares Formation, La Rioja Province, Argentina. Lagerpeton and its kin were traditionally interpreted as dinosaur precursors of cursorial habits, with a bipedal posture and parasagittal gait. Some authors also speculated saltatorial capabilities for this genus. Recent analyses indicate that lagerpetids are early-diverging pterosauromorphs, a hypothesis that invites a review of most aspects of their anatomy and function. A revision of available specimens and additional preparation of previously known individuals indicate that Lagerpeton lacked a parasagittal gait and was probably a sprawling archosaur. This latter inference is based on the femoral head articulation with the acetabulum. The acetabular rim has a strongly laterally projected posteroventral antitrochanteric corner, which results in a position of the legs that recalls that of sprawling living reptiles, such as lizards, and departs from the parasagittally positioned limbs of dinosaurs. This may indicate that early pterosauromorphs had a sprawling posture of their hindlegs, casting doubts on the significance of bipedal posture and parasagittal gait for the radiation of early ornithodirans, given that both traits have been regarded as key features that triggered the ecological and evolutionary success of the clade. Our results bolster recent claims of a high ecomorphological diversity among early avemetatarsalians.

A new global palaeobiogeographical model for the late Mesozoic and early Tertiary.

Late Mesozoic palaeobiogeography has been characterized by a distinction between the northern territories of Laurasia and the southern landmasses of Gondwana. The repeated discovery of Gondwanan lineages in Laurasia has led to the proposal of alternative scenarios to explain these anomalous occurrences. A new biogeographical model for late Mesozoic terrestrial ecosystems is here proposed in which Europe and "Gondwanan" territories possessed a common Eurogondwanan fauna during the earliest Cretaceous. Subsequently, following the Hauterivian, the European territories severed from Africa and then connected to Asiamerica resulting in a faunal interchange. This model explains the presence of Gondwanan taxa in Laurasia and the absence of Laurasian forms in the southern territories during the Cretaceous. In order to test this new palaeobiogeographical model, tree reconciliation analyses (TRAs) were performed based on biogeographical signals provided by a supertree of late Mesozoic archosaurs. The TRAs found significant evidence for the presence of an earliest Cretaceous Eurogondwanan fauna followed by a relatively short-term Gondwana-Laurasia dichotomy. The analysis recovered evidence for a biogeographical reconnection of the European territories with Africa and South America-Antarctica during the Campanian to Maastrichtian time-slice. This biogeographical scenario appears to continue through the early Tertiary and sheds light on the trans-Atlantic disjunct distributions of several extant plant and animal groups.

First monotreme from the Late Cretaceous of South America.

Monotremata is a clade of egg-lying mammals, represented by the living platypus and echidnas, which is endemic to Australia, and adjacent islands. Occurrence of basal monotremes in the Early Cretaceous of Australia has led to the consensus that this clade originated on that continent, arriving later to South America. Here we report on the discovery of a Late Cretaceous monotreme from southern Argentina, demonstrating that monotremes were present in circumpolar regions by the end of the Mesozoic, and that their distinctive anatomical features were probably present in these ancient forms as well.

A Maastrichtian insect assemblage from Patagonia sheds light on arthropod diversity previous to the K/Pg event.

Insect faunas from the latest Cretaceous are poorly known worldwide. Particularly, in the Southern Hemisphere, there is a gap regarding insect assemblages in the Campanian-Maastrichtian interval. Here we present an insect assemblage from the Maastrichtian Chorrillo Formation, southern Argentina, represented by well-preserved and non-deformed, chitinous microscopic remains including head capsules, wings and scales. Identified clades include Chironomidae dipterans, Coelolepida lepidopterans, and Ephemeroptera. The assemblage taxonomically resembles those of Cenozoic age, rather than other Mesozoic assemblages, in being composed by diverse chironomids and lepidopterans. To the best of our knowledge, present discovery constitutes the first insect body fossils for the Maastrichtian in the Southern Hemisphere, thus filling the gap between well-known Early Cretaceous entomofaunas and those of Paleogene age. The presented evidence shows that modern clades of chironomids were already dominant and diversified by the end of the Cretaceous, in concert with the parallel radiation of aquatic angiosperms which became dominant in freshwater habitats. This exceptional finding encourages the active search of microscopic remains of fossil arthropods in other geological units, which could provide a unique way of enhancing our knowledge on the past diversity of the clade.

Wing osteology, myology, and function of Rhea americana (Aves, Rheidae).

The Greater Rhea (Rhea americana, Rheidae) is a flightless paleognath with a wide geographical distribution in South America. The morphology of its shoulder girdle and wings are different from those of flying birds and some characteristics are similar to basal birds and paravian theropods. We present a detailed osteological, myological, and functional study of the shoulder and the wing of the Greater Rhea. Particular features of the anatomy of the pectoral girdle and wing of Rhea include the lack of triosseal canal, reduced origin area of the mm. pectoralis p. thoracica and supracoracoideus and the lack of a propatagium. The wing muscle mass is markedly reduced, reaching only 0.89% of total body mass (BM). Forelimb muscles mass values are low compared to those of flying birds and are congruent with the non-use of wings for active locomotion movements. R. americana does not flap the wings dorso-ventral as typical for flying birds, but predominantly in cranio-caudal direction, following a craniolateral to caudomedial abduction-adduction arc. When the wings are fully abducted, they are inverted L-shaped, with the inner surface caudally faced, and when the wings are folded against the body, they do not perform the complete automatic wing folding nor the circumduction of the manus, a movement performed by extant volant birds. This study complements our knowledge of the axial musculature of the flightless paleognaths and highlights the use of the Greater Rhea as a model, which may help understand the evolution of Palaeognathae, as well as the origin of flapping flight among paravian theropods.

A large Megaraptoridae (Theropoda: Coelurosauria) from Upper Cretaceous (Maastrichtian) of Patagonia, Argentina.

Megaraptora is a theropod clade known from former Gondwana landmasses and Asia. Most members of the clade are known from the Early to Late Cretaceous (Barremian-Santonian), with Maastrichtian megaraptorans known only from isolated and poorly informative remains. The aim of the present contribution is to describe a partial skeleton of a megaraptorid from Maastrichtian beds in Santa Cruz Province, Argentina. This new specimen is the most informative megaraptoran known from Maastrichtian age, and is herein described as a new taxon. Phylogenetic analysis nested the new taxon together with other South American megaraptorans in a monophyletic clade, whereas Australian and Asian members constitute successive stem groups. South American forms differ from more basal megaraptorans in several anatomical features and in being much larger and more robustly built.

The earliest dromaeosaurid theropod from South America.

The evolutionary history of Maniraptora, the clade of carnivorous dinosaurs that includes birds and the sickle-clawed Dromaeosauridae, has hitherto been largely restricted to Late Jurassic and Cretaceous deposits on northern continents. The stunning Early Cretaceous diversity of maniraptorans from Liaoning, China, coupled with a longevity implied by derived Late Jurassic forms such as Archaeopteryx, pushes the origins of maniraptoran lineages back to Pangaean times and engenders the possibility that such lineages existed in Gondwana. A few intriguing, but incomplete, maniraptoran specimens have been reported from South America, Africa and Madagascar. Their affinities remain contested, however, and they have been interpreted as biogeographic anomalies relative to other faunal components of these land-masses. Here we describe a near-complete, small dromaeosaurid that is both the most complete and the earliest member of the Maniraptora from South America, and which provides new evidence for a unique Gondwanan lineage of Dromaeosauridae with an origin predating the separation between northern and southern landmasses.

Unenlagiid theropods: are they members of the Dromaeosauridae (Theropoda, Maniraptora)?

In the present paper we analyze the phylogenetic position of the derived Gondwanan theropod clade Unenlagiidae. Although this group has been frequently considered as deeply nested within Deinonychosauria and Dromaeosauridae, most of the features supporting this interpretation are conflictive, at least. Modification of integrative databases, such as that recently published by Hu et al. (2009), produces significant changes in the topological distribution of taxa within Deinonychosauria, depicting unenlagiids outside this clade. Our analysis retrieves, in contrast, a monophyletic Avialae formed by Unenlagiidae plus Aves.

Osteology of Unenlagia comahuensis (Theropoda, Paraves, Unenlagiidae) from the Late Cretaceous of Patagonia.

Unenlagia comahuensis was originally described as a phylogenetic link between nonavian dinosaurs and birds. Later it was interpreted by some authors as belonging to the deinonychosaurian clade Dromaeosauridae, and more recently as phylogenetically closer to birds than to dromaeosaurids. The only known specimen is represented by an incomplete skeleton, including vertebrae, incomplete scapular girdle, pelvis, and limbs, coming from Upper Cretaceous beds of Neuquén province, Patagonia, Argentina. The aim of the present paper is to include a detailed anatomical description of Unenlagia (currently only known by preliminary descriptions). Detailed analysis of Unenlagia anatomy resulted in the recognition of one possible additional Unenlagiidae synapomorphy (i.e., the presence of cup-like iliac articulation on ischium). We recognize derived anatomical traits that Unenlagia and kin share with birds, lending support to the interpretation that unenlagiids are stem-Avialae. Particularly, some appendicular features (e.g., scapula with subtriangular and relatively reduced acromion, poor outward projection of the glenoid and glenoidal lips on the scapula, lateral orientation of scapular glenoid, craniolaterally oriented deltopectoral crest of humerus) may be related to the acquisition of anatomical novelties that in birds are associated with flight. The present contribution on Unenlagia provides new data regarding the early evolution of avian features.

New transitional fossil from late Jurassic of Chile sheds light on the origin of modern crocodiles.

We describe the basal mesoeucrocodylian Burkesuchus mallingrandensis nov. gen. et sp., from the Upper Jurassic (Tithonian) Toqui Formation of southern Chile. The new taxon constitutes one of the few records of non-pelagic Jurassic crocodyliforms for the entire South American continent. Burkesuchus was found on the same levels that yielded titanosauriform and diplodocoid sauropods and the herbivore theropod Chilesaurus diegosuarezi, thus expanding the taxonomic composition of currently poorly known Jurassic reptilian faunas from Patagonia. Burkesuchus was a small-sized crocodyliform (estimated length 70 cm), with a cranium that is dorsoventrally depressed and transversely wide posteriorly and distinguished by a posteroventrally flexed wing-like squamosal. A well-defined longitudinal groove runs along the lateral edge of the postorbital and squamosal, indicative of a anteroposteriorly extensive upper earlid. Phylogenetic analysis supports Burkesuchus as a basal member of Mesoeucrocodylia. This new discovery expands the meagre record of non-pelagic representatives of this clade for the Jurassic Period, and together with Batrachomimus, from Upper Jurassic beds of Brazil, supports the idea that South America represented a cradle for the evolution of derived crocodyliforms during the Late Jurassic.