Browsing by Author "Zelenitsky, Darla K."

Now showing 1 - 6 of 6
  • Thumbnail Image
    ItemOpen Access
    An Analysis of North American Taeniolabidoid Multituberculate (Mammalia, Allotheria) Dentitions Using Mammalian Dietary Proxies
    (2018-12-20) Robson, Selina Viktoria; Theodor, Jessica M.; Scott, Craig S.; Cote, Susanne; Syme, Douglas A.; Zelenitsky, Darla K.
    In this thesis, a set of dietary proxies – dental microwear analysis, cusp row ratios (CRR) (similar to shearing ratios), relief index (RFI), orientation patch count rotated (OPCR), and Dirichlet normal energy (DNE) – was used to infer diets of North American taeniolabidoid multituberculates. Based on the signals recovered by these proxies, taeniolabidoid diets did not vary consistently with body size: small-bodied and large-bodied taeniolabidoids had similar dietary signals for almost all proxies, the only difference being in microwear feature dimensions. Dental microwear signals suggest that taeniolabidoids and non-taeniolabidoid cimolodontans may have had different diets, but all other proxies have recovered equivalent signals between the two groups. Dietary classifications are inconsistent among CRR, RFI, OPCR, and DNE. This suggests that these proxies are not equally good predictors and that their generalizability to non-therian mammals may need to be re-evaluated.
  • Thumbnail Image
    ItemOpen Access
    Description and phylogenetic implications of extant and fossil oologic remains
    (2004) Zelenitsky, Darla K.; Currie, Philip J.
    Rigorous analytical techniques are combined with phylogeneticsystematic methodology to provide a new perspective on the scientific informativeness of the eggs and eggshells of both extant and extinct dinosaurs. Re-description of the eggs of paleognathous birds reveals that numerous eggshell characteristics were overlooked by previous investigators. A cladistic analysis of discrete egg characters of paleognaths reveals that although trees based on egg data provide less resolution among taxa than those based on skeletal data, egg data do provide phylogenetic information and can help resolve disagreements in relationships derived from skeletal data. In an analysis based on the egg data alone, tinamous are basal among paleognaths, Apteryx is basal among ratites and the sister taxon of an unresolved clade consisting of large ratites, within which Aepyornis and dinornithids form a subclade and Casuarius and Dromaius form a subclade. When the egg data are combined with the skeletal data of Bledsoe (1988), the clade of Apteryx, Casuarius, and Dromaius is dissolved, and Apteryx moves to a basal position within Ratitae. Analysis of 13 absolute measurements and ratios from the eggs reveals that only 4 of these continuous characters are suitable for cladistic analysis. A cladistic analysis of the continuous and discrete characters provides further resolution among taxa within the tree based on egg data in that Struthio and Rhea form a clade. Fossil eggs of several dinosaurs (including birds) are described and included in a cladistic analysis of 24 ootaxa, the results of which, in general, are comparable to those drawn from skeletal-based studies. Sauropod eggs are basal in Saurischia, allosauroid eggs are basal in Theropoda, the eggs of oviraptorids and dromaeosaurids form a derived clade, and the eggs of troodontids and birds form a clade. Unexpected results include the basal position of therizinosaurid eggs, a clade consisting of Protoceratopsidovum (Protoceratops eggs) and troodontid eggs, and a clade consisting of Deinonychus and oviraptorid eggs. These anomalies may call the alleged parentage of these eggs into question. The evolutionary-systematic method that was used to construct the traditional parataxonomic system for fossil eggs is eschewed, and a new nomenclatural system employing a phylogenetic-systematic methodology is proposed. Phylogenetic definitions are erected for seven ooclades of Saurischia, following the tenets of phylogenetic nomenclature.
  • Thumbnail Image
    ItemOpen Access
    Description of juvenile specimens of Prosaurolophus maximus (Hadrosauridae: Saurolophinae) from the Upper Cretaceous Bearpaw Formation of southern Alberta, Canada, reveals ontogenetic changes in crest morphology
    (Journal of Vertebrate Paleontology (Taylor and Francis), 2019-03-19) Drysdale, Eamon T.; Therrien, Francois; Zelenitsky, Darla K.; Weishampel, David B.; Evans, David C.
    Three juvenile specimens of Prosaurolophus maximus, represented by articulated to disarticulated skeletons, are the smallest known individuals for the taxon. Cranial anatomy of the juvenile specimens indicates that diagnostic characters of P. maximus are ontogenetically variable. In the smallest individual, the crest and deeply excavated fossa at the caudal margin of the circumnarial depression are poorly developed or absent. The crest approaches adult-like morphology in large juveniles, whereas crest robusticity and the deep excavation of the circumnarial depression occur only in subadult and adult individuals. The shape of the caudal margin of the circumnarial depression is consistent between juvenile and adult individuals, potentially making this feature a reliable character for taxonomic identification at younger ontogenetic stages. The crest of P. maximus grows isometrically during ontogeny, unlike the positive allometric growth of lambeosaurine hadrosaur crests, suggesting that this taxon may have had soft tissue structures associated with the narial-crest region, rather than the bony crest itself, selected for sexual display. Recovered from sediments of the Bearpaw Formation deposited during the Baculites compressus ammonite zone and magnetochrons 33n.3n to 33n.2n, the juvenile specimens are stratigraphically younger than P. maximus specimens from the Dinosaur Park Formation (Alberta) and contemporaneous with most specimens from the Two Medicine Formation (Montana), extending the temporal range of the taxon to 75.7–74 Ma. The occurrence of P. maximus in well-drained terrestrial deposits of the Dinosaur Park and Two Medicine formations and marine sediments of the Bearpaw Formation indicates that this taxon inhabited various paleoenvironments in western North America.
  • Thumbnail Image
    ItemOpen Access
  • Thumbnail Image
    ItemOpen Access
    Juvenile specimens of Prosaurolophus maximus (Hadrosauridae, Saurolophinae) from the marine Bearpaw Formation (upper Campanian, Upper Cretaceous) of southern Alberta, Canada reveal ontogenetic change in crest morphology
    (2019-01-25) Drysdale, Eamon Thomas; Zelenitsky, Darla K.; Therrien, François; Theodor, Jessica M.; Dutchak, Alexander
    Three juvenile specimens of Prosaurolophus maximus, represented by both articulated and disarticulated skeletal material, are the smallest known individuals for the taxon. Their cranial anatomy indicates that the diagnostic characters of P. maximus are ontogenetically variable. In the smallest specimen, the crest and deeply-excavated fossa at the caudal margin of the circumnarial depression are poorly-developed or absent. In the larger specimens, the crest approaches the morphology observed in subadult and adult specimens but lacks the robusticity and deep excavation of the circumnarial depression typical of P. maximus. In contrast, the shape of the caudal end of the circumnarial depression is ontogenetically consistent, potentially making it a reliable character for taxonomic identification. Histological sections from the three juvenile specimens and one large individual indicate that the minimum biological age of the known specimens of P. maximus ranges from two to five years. Given that the large P. maximus individual has yet to reach skeletal maturity, the species may reach a maximum body size larger than represented by currently known specimens, perhaps with a skull length approaching the size of Saurolophus angustirostris. The lack of positive allometric growth associated with the P. maximus crest, which undergoes isometric growth from age three onward, differs from that observed in lambeosaurine hadrosaurs suggesting that the crest may not have been the result of sexual selection. However, positive allometric growth of the circumnarial region and the rugosity of the bony crest suggests that soft tissue structures associated with the narial-crest region, rather than the bony crest itself, may have been selected for sexual display in P. maximus. The juvenile specimens were recovered from sediments of the Bearpaw Formation deposited during the Baculites compressus ammonite zone and magnetochrons 33n.3n to 33n.2n. They are stratigraphically younger than P. maximus specimens from the Dinosaur Park Formation, and contemporaneous to 100,000 years younger than specimens from the Two Medicine Formation, extending the taxon’s temporal range. The occurrence of P. maximus in the marine deposits of the Bearpaw Formation and well-drained upland terrestrial deposits of both the Dinosaur Park and Two Medicine formations suggests that this taxon inhabited various palaeoenvironments.
  • Thumbnail Image
    ItemOpen Access
    Two exceptionally preserved juvenile specimens of Gorgosaurus libratus (Tyrannosauridae, Albertosaurinae) provide new insight into the timing of ontogenetic changes in tyrannosaurids
    (Journal of Vertebrate Paleontology (Taylor and Francis), 2022-04-13) Voris, Jared. T.; Zelenitsky, Darla K.; Therrien, Francois; Ridgely, Ryan C.; Currie, Philip J.; Witmer, Lawrence, M.
    Known from dozens of specimens discovered since the early 20th century, Gorgosaurus libratus has arguably contributed more than any other taxon to our understanding of the life history of tyrannosaurids. However, juvenile material for this taxon is rare. Here, we describe two small, articulated Gorgosaurus specimens (skull lengths of ca. 500 mm) that help advance our knowledge of the anatomy and ontogeny of this taxon and of tyrannosaurids in general. The new specimens exhibit hallmark juvenile tyrannosaurid features, including long, low, and narrow skulls, large circular orbits, absent or incipient cranial ornamentation, ziphodont dentition, and an overall gracile skull frame. Comparison with other Gorgosaurus specimens of various ontogenetic stages allows for an examination of the timing of morphological changes that occurred through ontogeny in this taxon relative to other tyrannosaurids. Of particular note, Gorgosaurus and the larger Tyrannosaurus rex are found to have experienced similar ontogenetic transformations at similar percent skull length relative to the large known individuals for each respective taxon but at different absolute body sizes and biological ages, occurring at a larger size and older age in Tyrannosaurus than in Gorgosaurus. These results suggest a dissociation between the timing of cranial development and body size in tyrannosaurids. Finally, the recognition of ontogenetically invariant characters in Gorgosaurus makes it possible to determine the taxonomic identity of previously misidentified specimens.