Browsing by Author "Zelenitsky, Darla Karen"

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    A fossilized turtle egg clutch with embryos from the upper cretaceous Oldman formation, southeastern Alberta: description, taxonomic identity, and embryonic staging
    (2012) McGee, Amanda Rea; Zelenitsky, Darla Karen
    Fossilized turtle clutches containing embryos are heretofore undescribed. Here the description of such a specimen from the Upper Cretaceous of Alberta sheds light on the taxonomic identity and paleobiology of the turtle that laid the clutch. Examination of the clutch reveals 33 rigid-shelled, spherical eggs arranged in a layered cluster. Clutch size and the paleoenvironment of the nest site suggest that the fossil clutch belongs to a large, freshwater turtle. Numerous embryonic bones described from virtual reconstructions of computed tomographic data reveal that the embryonic bones are well-ossified. Comparisons with modem turtles indicate the fossil embryos were at a late stage of development, near batching, at the time of death. Morphology of the maxillae, dentary, and plastron suggests affinities with Adocidae, an extinct freshwater turtle clade. Additional lines of evidence, including egg and eggshell morphology, the predicted size of the female, and geographic and stratigraphic location, also suggest that the fossil clutch belongs to adocids.
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    A new tyrannosaurine (Theropoda:Tyrannosauridae) from the Campanian Foremost Formation of Alberta, Canada, provides insight into the evolution and biogeography of tyrannosaurids
    (Cretaceous Research (Elsevier), 2020-01-23) Voris, Jared Thomas; Therrien, Francois; Zelenitsky, Darla Karen; Brown, Caleb Marshall
    Upper Cretaceous tyrannosauroid material from North America was primarily known from upper Campanian through Maastrichtian formations until the recent discovery of derived tyrannosaurid taxa from lower-to-mid Campanian deposits in the southwestern United States. However, diagnostic material from contemporaneous deposits further north in Alberta (Canada) and Montana (USA) has yet to be documented. Here we report the discovery of a new tyrannosaurine tyrannosaurid from the mid- Campanian Foremost Formation of Alberta, Thanatotheristes degrootorum gen. et. sp. nov, which helps fill this gap. The new tyrannosaurine, diagnosed by five autapomorphies, is found to be the sister taxon to the late Campanian genus Daspletosaurus. Thanatotheristes is distinct from Daspletosaurus based on several features, and lacks at least two apomorphies of the latter taxon. Together, these taxa form the newly established Daspletosaurini, a clade of long-, deep-snouted tyrannosaurines endemic to northern Laramidia during the Campanian. Our study demonstrates that Tyrannosauridae is composed of several geographically-segregated clades rather than a series of monogeneric successive sister taxa as recovered by previous studies. The geographic segregation of tyrannosaurid clades within North America provides renewed evidence for provinciality among large theropods during the Late Cretaceous.
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    Dinosaur eggshells from the Santonian Milk River Formation of Alberta, Canada
    (Cretaceous Research (Elsevier), 2017-02-16) Zelenitsky, Darla Karen; Therrien, Francois; Tanaka, Kohei; Kobayashi, Yoshitsugu; DeBuhr, Christopher Leonard
    The North American fossil record of dinosaur eggshells for the Cretaceous is primarily restricted to formations of the middle (Albian–Cenomanian) and uppermost (Campanian–Maastrichtian) stages, with a large gap in the record for intermediate stages. Here we describe a dinosaur eggshell assemblage from a formation that represents an intermediate and poorly fossiliferous stage of the Upper Cretaceous, the Santonian Milk River Formation of southern Alberta, Canada. The Milk River eggshell assemblage contains five eggshell taxa: Continuoolithus, Porituberoolithus, Prismatoolithus, Spheroolithus, and Triprismatoolithus. These ootaxa are most similar to those reported from younger Campanian–Maastrichtian formations of the northern Western Interior than they are to ootaxa reported from older middle Cretaceous formations (i.e., predominantly Macroelongatoolithus). Characteristics of the Milk River ootaxa indicate that they are ascribable to at least one ornithopod and four small theropod species. The taxonomic affinity of the eggshell assemblage is consistent with the dinosaur fauna known based on isolated teeth and fragmentary skeletal remains from the formation, although most ornithischians and large theropods are not represented by eggshell. Relative to the Milk River Formation eggshell, similar oospecies occurring in younger Cretaceous deposits tend to be somewhat thicker, which may reflect an increase in body size of various dinosaur lineages during the Late Cretaceous.
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    Mandibular force profiles and tooth morphology in growth series of Albertosaurus sarcophagus and Gorgosaurus libratus (Tyrannosauridae: Albertosaurinae) provide evidence for an ontogenetic dietary shift in tyrannosaurids
    (Canadian Journal of Earth Sciences (Canadian Science Publishing), 2021-01-26) Therrien, Francois; Zelenitsky, Darla Karen; Voris, Jared Thomas; Tanaka, Kohei
    The albertosaurines Albertosaurus sarcophagus and Gorgosaurus libratus are among the best represented tyrannosaurids, known from nearly complete growth series. These specimens provide an opportunity to study mandibular biomechanical properties and tooth morphology to infer changes in feeding behaviour and bite force through ontogeny in tyrannosaurids. Mandibular force profiles reveal that the symphyseal region of albertosaurines is consistently stronger in bending than the mid-dentary region, indicating that the anterior extremity of the jaws played an important role in prey capture and handling through ontogeny. The symphyseal region was better adapted to withstand torsional stresses than in most non-avian theropods, but not to the extent seen in Tyrannosaurus rex, suggesting that albertosaurine feeding behaviour may have involved less bone crushing or perhaps relatively smaller prey than in Tyrannosaurus rex. The constancy of these biomechanical properties at all known growth stages indicates that although albertosaurines maintained a similar feeding strategy through ontogeny, prey size/type had to change between juvenile and mature individuals. This ontogenetic dietary shift likely happened when individuals reached a mandibular length of ∼58 cm, a size at which teeth shift from ziphodont to incrassate in shape and bite force begins to increase exponentially. The fact that large albertosaurines were capable of generating bite forces equivalent to similar-sized tyrannosaurines suggests that no significant differences in jaw-closing musculature existed between the two clades and that the powerful bite of Tyrannosaurus rex is the result of its large body size rather than of unique adaptations related to a specialized ecology.
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    Nest humidity and egg water vapor conductance of archosaurs: Implications for nesting modes
    (2012-08-10) Tanaka, Kohei; Zelenitsky, Darla Karen
    Absolute nest humidity (Pnest) and egg water vapor conductance (GH2O) of 295 taxa of living archosaurs are analyzed to test if Pnest and GH2O are correlated to nest types. Pnest is shown to be significantly higher in covered nest types than in non-covered nest types, likely because enclosed environments retain more humidity. Also, estimated Pnest of some waterbirds (e.g., grebes) with non-covered cup nests is high and comparable to that of covered nests, probably because the nest material is wet. Species that incubate eggs in humid nests are shown to have significantly higher GH2O than those incubating in regular nests (less humid), and these differences in GH2O between nester types prevent excess humidification or desiccation of the eggs. Therefore, Pnest and GH2O of the eggs appear to be closely related to nesting types in archosaurs, the latter of which can potentially be used to infer nest types of extinct archosaurs.
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    Nesting and Egg Incubation in Dinosaurs: Morphological and Statistical Investigations into the Study of Eggs, Eggshells, and Nests
    (2016) Tanaka, Kohei; Zelenitsky, Darla Karen; Brinkman, Donald; Anderson, Jason; Therrien, François; Dutchak, Alexander; Theodor, Jessica
    Archosaurs (e.g., crocodylians, dinosaurs, and birds) are the most diverse and successful terrestrial vertebrates. An understanding of the nesting strategies in both extinct (e.g., non-avian dinosaurs) and extant archosaurs (i.e., crocodylians and birds) is crucial for advancement of our knowledge on the evolution and diversification of this group. However, nesting methods and behaviors of non-avian dinosaurs are still poorly understood due to the limitations of the fossil record. In this dissertation, certain features of eggs and nests in dinosaurs (e.g., clutch size, egg mass, substrates of nests, water vapor conductance of eggs, and eggshell porosity) are compared with those of their closest living relatives (i.e., birds and crocodylians) and aspects of dinosaur nesting (i.e., nest type, incubation behavior, incubation heat source, and incubation period) are inferred and reconstructed. Findings in this dissertation suggest that nests and nesting styles among non-avian dinosaurs were diverse, and that bird-like traits were acquired throughout their evolution. Analyses of eggs and eggshell porosity indicate that more basal dinosaurs (i.e., ornithischians, sauropodomorphs, Lourinhanosaurus) completely covered their eggs with nest materials during incubation, although more derived forms (e.g., oviraptorosaurs, troodontids) used open nests, like modern birds, in which the eggs were not fully buried. The lithologies of the clutches of basal dinosaurs reveal their nests were probably incubated with external heat sources (e.g., microbial respiration, solar radiation), like those of modern crocodylians and megapode birds. Distribution and lithologies of some ornithischian and some sauropodomorph clutches show that heat from microbial respiration, in particular, was used for incubation, whereas other sauropodomorphs may have used inorganic heat sources, such as solar radiation. More derived dinosaurs (i.e., maniraptorans) had eggshell porosities and clutch lithologies that indicate their nests were partially open, indicating that these taxa brooded their eggs. Regardless of the type of nest, heat source, or incubation behavior, incubation period of most non-avian dinosaurs examined was relatively short, more comparable to that of birds than crocodylians. Major dinosaur (and archosaur) clades show diversity in their nesting and incubation, and also reveal a transition to more bird-like nesting features through evolution.
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    The upper cretaceous Horseshoe Canyon formation: using palesols to reconstruct ancient environments, climates, and record of sea level change in a dinosaur-dominated terrestrial ecosystem
    (2012) Quinney, Ann; Zelenitsky, Darla Karen
    Paleosols are investigated to reconstruct Late Cretaceous paleoenvironments and paleoclimates of the Horseshoe Canyon Formation. Based on hydrology, degree of development, and pedogenic features, paleosols record the transition from wetland environments to well-drained environments and back to poorly-drained environments through the formation. Paleoclimatic reconstructions indicate that the lower half of the formation was warm and humid, but a period of cooling and drying occurred in the middle of the formation. A complex relationship exists between paleoenvironmental and paleoclimatic changes and a terrestrial vertebrate faunal turnover within the Horseshoe Canyon Formation. The distribution of paleosols was also compared to sequence stratigraphic interpretations of the HCFm to determine if paleosols recorded evidence of base level changes. Although HCFm paleosols do record some marine transgressive-regressive cycles, the ability to resolve these cycles depends on the distance to the shoreline and the magnitude of the sea­level change.