Browsing by Author "Barta, Daniel E."

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A Cladistic Approach to Understanding Dinosaur Egg Diversity and the Evolution of Reproductive Traits Within Dinosauria: Preliminary Results
(2013-03) Barta, Daniel E.; Varricchio, David
Only a small percentage of fossil eggs contain identifiable embryonic remains. Consequently, knowledge of eggshell structure and reproductive strategies remains incomplete for many dinosaur clades. Most previous cladistic analyses of dinosaur eggs and eggshell focus on dinosaur egg types (ootaxa) with identified embryos and aim towards understanding the evolution of avian reproductive traits. To provide a broader phylogenetic framework for dinosaur ootaxa by which gain and loss of eggshell and reproductive characters might be better understood across the entire clade, a comprehensive cladistic analysis of representatives of each major dinosaur oofamily was undertaken. We utilize a greater number of characters than most prior studies and subject three oofamilies (Faveoloolithidae, Dendroolithidae, and Arriagadoolithidae) to cladistic analysis for the first time. Cladistic analyses of eggshell remain complicated by apparent homoplasy between some ootaxa, as evidenced by the polytomy of Faveoloolithidae (Sauropoda), Dendroolithidae (Therizinosauria), and Dictyoolithidae (Theropoda) consistently recovered in strict and majority-rule consensus trees in this study. Strong support is also found for a clade of derived theropod and avian eggshell. Future work includes adding taxonomically unassigned ootaxa to a dinosaur skeletal data matrix to form hypotheses (constrained by eggs with embryos) of likely assignment to parent dinosaur clades for unidentified ootaxa.
Revisiting Sabath's "Larger Avian Eggs" from the Gobi Cretaceous
(2015-06) Varricchio, David J.; Barta, Daniel E.
In 1991, Sabath described "larger avian eggs" from the Upper Cretaceous Barun Goyot and Djadokhta Formations of Mongolia. These were later included in the ootaxon Gobioolithus major. Here we recognize the larger avian eggs of Sabath as a distinct ootaxon, Styloolithus sabathi, oogen. et oosp. nov. These eggs differ from those of Gobioolithus in being larger (70 by 32 mm) and more elongate. Microscopically, the shell bears a third layer (possible external zone) thicker than the mammillary layer and nearly as thick as the second layer (possible squamatic zone); the continuous layer (including layers two and three) to mammillary layer thickness ratio is 3.1:1. Within the clutch, the tightly spaced eggs stand with their long axes steeply inclined. Adult remains are associated with two clutches, suggesting an incubation mode similar to that of troodontid maniraptorans, where adults sat atop largely buried eggs. S. sabathi provides evidence that relative egg size in Mesozoic non-ornithuromorph birds had increased markedly from the non-avian theropod condition in oviraptorids and troodontids, but had not yet reached the modern egg-adult proportions of Neornithes. Sediment-bound upright eggs appear common to Enantiornithes and more basal avians, suggesting that like non-avian theropods, these birds lacked chalazae, the chords of albumen allowing egg rotation in modern birds. Absence of this simple structure may have restricted these basal birds to ground nesting in areas with appropriate substrates and not permitted the type of nesting diversity found in Neornithes. Neornithes are the only Mesozoic clade of Dinosauria to nest completely free of sediment; this may have played a crucial role in their surviving the K-Pg mass extinction event.