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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 skeleton of a small (approximately 2 m) armoured dinosaur from the late Cretaceous period of Magallanes in southernmost Chile, a region that is biogeographically related to West Antarctica5. Stegouros elengassen gen. et sp. nov. evolved a large tail weapon unlike any dinosaur: a flat, frond-like structure formed by seven pairs of laterally projecting osteoderms encasing the distal half of the tail. Stegouros shows ankylosaurian cranial characters, but a largely ancestral postcranial skeleton, with some stegosaur-like characters. Phylogenetic analyses placed Stegouros in Ankylosauria; specifically, it is related to Kunbarrasaurus from Australia6 and Antarctopelta from Antarctica7, forming a clade of Gondwanan ankylosaurs that split earliest from all other ankylosaurs. The large osteoderms and specialized tail vertebrae in Antarctopelta suggest that it had a tail weapon similar to Stegouros. We propose a new clade, the Parankylosauria, to include the first ancestor of Stegouros—but not Ankylosaurus—and all descendants of that ancestor.Stegouros elengassen, an ankylosaur from the late Cretaceous of Chile, has a large tail weapon, named a macuahuitl after the Aztec club, with a frond-like structure formed by seven pairs of laterally projecting osteoderms encasing the distal half of the tail. [ABSTRACT FROM AUTHOR]

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 meridiolestidan mammal, Orretherium tzen gen. et sp. nov., from the Late Cretaceous of southern Chile, based on a partial jaw with five cheek teeth in locis and an isolated upper premolar. Phylogenetic analysis places Orretherium as the earliest divergence within Mesungulatidae, before other forms such as the Late Cretaceous Mesungulatum and Coloniatherium, and the early Paleocene Peligrotherium. The in loco tooth sequence (last two premolars and three molars) is the first recovered for a Cretaceous taxon in this family and suggests that reconstructed tooth sequences for other Mesozoic mesungulatids may include more than one species. Tooth eruption and replacement show that molar eruption in mesungulatids is heterochronically delayed with regard to basal dryolestoids, with therian-like simultaneous eruption of the last premolar and last molar. Meridiolestidans seem endemic to Patagonia, but given their diversity and abundance, and the similarity of vertebrate faunas in other regions of Gondwana, they may yet be discovered in other continents. [ABSTRACT FROM AUTHOR]

Egg size and structure reflect important constraints on the reproductive and life-history characteristics of vertebrates1. More than two-thirds of all extant amniotes lay eggs2. 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 elephant bird3, which was roughly 66 million years younger than the last nonavian dinosaurs and giant marine reptiles. Here we report a new type of egg discovered in nearshore marine deposits from the Late Cretaceous period (roughly 68 million years ago) of Antarctica. It exceeds all nonavian dinosaur eggs in volume and differs from them in structure. Although the elephant bird egg is slightly larger, its eggshell is roughly five times thicker and shows a substantial prismatic layer and complex pore structure4. By contrast, the new fossil, visibly collapsed and folded, presents a thin eggshell with a layered structure that lacks a prismatic layer and distinct pores, and is similar to that of most extant lizards and snakes (Lepidosauria)5. The identity of the animal that laid the egg is unknown, but these preserved morphologies are consistent with the skeletal remains of mosasaurs (large marine lepidosaurs) found nearby. They are not consistent with described morphologies of dinosaur eggs of a similar size class. Phylogenetic analyses of traits for 259 lepidosaur species plus outgroups suggest that the egg belonged to an individual that was at least 7 metres long, hypothesized to be a giant marine reptile, all clades of which have previously been proposed to show live birth6. Such a large egg with a relatively thin eggshell may reflect derived constraints associated with body shape, reproductive investment linked with gigantism, and lepidosaurian viviparity, in which a 'vestigial' egg is laid and hatches immediately7. A fossil egg unearthed from Cretaceous deposits in Antarctica is more than 20 cm long, exceeds all known nonavian eggs in volume, is soft-shelled, and was perhaps laid by a giant marine lizard such as a mosasaur. [ABSTRACT FROM AUTHOR]

Specialized morphologies of bird feet have evolved several times independently as different groups have become zygodactyl, semi-zygodactyl, heterodactyl, pamprodactyl 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 molecular phylogenies, we compared the evolution of foot morphology and modes of development among extant avian families. Variations in the arrangement of toes with respect to the anisodactyl ancestral condition have occurred only in altricial groups. Those groups represent four independent events of super-altriciality and many independent transformations of toe arrangements (at least four zygodactyl, three semi-zygodactyl, one heterodactyl, one pamprodactyl group, and several syndactyl). We propose that delayed skeletal maturation due to altriciality facilitates the epigenetic influence of embryonic muscular activity over developing toes, allowing for repeated evolution of innovations in their morphology. [ABSTRACT FROM AUTHOR]

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). [ABSTRACT FROM AUTHOR]

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 twisting of metatarsal 1 (Mt1) and retroversion of the hallux. Pharmacologically paralyzed embryos do not fully retrovert the hallux and have a straight Mt1 shaft, phenocopying the morphology of early tetanuran dinosaurs. Molecular markers of cartilage maturation and ossification show that differentiation of Mt1 is significantly delayed compared to Mt2-4. We hypothesize on how delayed maturation may have increased plasticity, facilitating muscular twisting. Our experimental results emphasize the importance of embryonic muscular activity in the evolutionary origin of a crucial adaptation. [ABSTRACT FROM AUTHOR]