Pelvis morphology suggests that early Mesozoic birds were too heavy to contact incubate their egg

Numerous new fossils have driven an interest in reproduction of early birds, but direct evidence remains elusive. No Mesozoic avian eggs can be unambiguously assigned to a species, which hampers our understanding of the evolution of contact incubation, which is a defining feature of extant birds. Compared to living species, eggs of Mesozoic birds are relatively small, but whether the eggs of Mesozoic birds could actually have borne the weight of a breeding adult has not yet been investigated. We estimated maximal egg breadth for a range of Mesozoic avian taxa from the width of the pelvic canal defined by the pubic symphysis. Known elongation ratios of Mesozoic bird eggs allowed us to predict egg mass and hence the load mass an egg could endure before cracking. These values were compared to the predicted body masses of the adult birds based on skeletal remains. Based on 21 fossil species, we show that for nonornithothoracine birds body mass was 187% of the load mass of the eggs. For Enantiornithes, body mass was 127% greater than the egg load mass, but some early Cretaceous ornithuromorphs were 179% heavier than their eggs could support. Our indirect approach provides the best evidence yet that early birds could not have sat on their eggs without running the risk of causing damage. We suggest that contact incubation evolved comparatively late in birds

Paleobiology of titanosaurs: reproduction, development, histology, pneumaticity, locomotion and neuroanatomy from the South American fossil record

Fil: García, Rodolfo A.. Instituto de Investigación en Paleobiología y Geología. Museo Provincial Carlos Ameghino. Cipolletti; ArgentinaFil: Salgado, Leonardo. Instituto de Investigación en Paleobiología y Geología. General Roca. Río Negro; ArgentinaFil: Fernández, Mariela. Inibioma-Centro Regional Universitario Bariloche. Bariloche. Río Negro; ArgentinaFil: Cerda, Ignacio A.. Instituto de Investigación en Paleobiología y Geología. Museo Provincial Carlos Ameghino. Cipolletti; ArgentinaFil: Carabajal, Ariana Paulina. Museo Carmen Funes. Plaza Huincul. Neuquén; ArgentinaFil: Otero, Alejandro. Museo de La Plata. Universidad Nacional de La Plata; ArgentinaFil: Coria, Rodolfo A.. Instituto de Paleobiología y Geología. Universidad Nacional de Río Negro. Neuquén; ArgentinaFil: Fiorelli, Lucas E.. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica. Anillaco. La Rioja; Argentin

The Mesozoic radiation of birds

Until recently, most knowledge of the early history of birds and the evolution of their unique specializations was based on just a handful of diverse Mesozoic taxa widely separated in time and restricted to marine environments. Although Archaeopteryx is still the oldest and only Jurassic bird, a wealth of recent discoveries combined with new phylogenetic analyses have documented the divergence of a number of lineages by the beginning of the Cretaceous. These and younger Cretaceous fossils have filled much of the morphological chasm that existed between Archaeopteryx and its living counterparts, providing insights into the evolutionary development of feathers and other important features of the avian flight system. Dramatic new perceptions of the life history, growth and development of early birds have also been made possible by the latest data. Although no primitive birds are known to have survived beyond the end of the Cretaceous, the present fossil record provides no evidence for a sudden disappearance. Likewise, a Mesozoic origin for extant birds remains controversial

Osteology of the sauropod embryos from the Upper Cretaceous of Patagonia

Exceptionally well−preserved embryonic skulls of Upper Cretaceous (Campanian Anacleto Formation) sauropods from Auca Mahuevo (Neuquén Province, Argentina) provide important insights into the ontogeny and evolution of titanosaurian neosauropods. The most important cranial modifications occurring during titanosaurian ontogeny appear to be centered on the infraorbital and narial regions, which exhibit a substantial degree of “mosaic” evolution. On one hand, the Auca Mahuevo embryos show a large jugal that forms part of the lower margin of the skull and unretracted external nares, as indicated by the position and orientation of the lacrimals as well as the anterior extension of the frontals. Both of these features are ancestral for neosauropods, being present in prosauropods. On the other hand, the embryonic skull exhibits a large ventral notch, tentatively interpreted as homologous to the neosauropod preantorbital fenestra, that opens ventral to the jugal and between the maxilla and the quadratojugal, and a temporal region that closely resembles the adult neosauropod condition. This mosaic of character states indicates that different regions of the skull of titanosaurian neosauropods acquired their characteristic morphology at substantially different rates during their ontogenetic development

The marine bird Asiahesperornis from the Cretaceous of northern Kazakhstan

Fossil bird material is exceptionally rare in the Mesozoic of western Asia. With the notable exception of the Upper Cretaceous foot-propelled diving bird Asiahesperornis bazhanovi from northern Kazakhstan, the only bird fossils described from this region have been bone fragments from the Upper Cretaceous of Uzbekistan. In this paper, all material referred to date to the hesperornithiform Asiahesperornis is reviewed and re-described. New postcranial elements of this taxon from the holotype Kushmurun locality (Maastrichtian Zhuravlovskaya Svita) are presented, and the taxonomic history and likely affinities of this ocean-going bird are discussed.<br/

Cretaceous fossil birds from Hornby Island (British Columbia)

We present the first records of Mesozoic fossil birds to be described from British Columbia. New fossil avians from the Campanian Northumberland Formation on Hornby Island (Strait of Georgia) add to the known distributions of two groups of fossil birds during the latter stage of the Mesozoic. New specimens referred to the clades Ornithurae and Enantiornithes demonstrate the presence of a diverse marine avifauna in Canadian Pacific marine sediments prior to the Cretaceous–Tertiary (K–T) boundary. These new fossil bird remains from coastal rocks on the west coast of British Columbia lend further support to suggestions that ocean-going birds were important constituents of marine ecosystems in the terminal stages of the Mesozoic