Browsing by Author "Carroll, Nathan"

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Avian tail ontogeny, pygostyle formation, and interpretation of juvenile Mesozoic specimens
(2018-06) Rashid, Dana J.; Surya, Kevin; Chiappe, Luis M.; Carroll, Nathan; Garrett, Kimball L.; Varghese, Bino; Bailleul, Alida M.; O'Connor, Jingmai K.; Chapman, Susan C.; Horner, John R.
The avian tail played a critical role in the evolutionary transition from long- to short-tailed birds, yet its ontogeny in extant birds has largely been ignored. This deficit has hampered efforts to effectively identify intermediate species during the Mesozoic transition to short tails. Here we show that fusion of distal vertebrae into the pygostyle structure does not occur in extant birds until near skeletal maturity, and mineralization of vertebral processes also occurs long after hatching. Evidence for post-hatching pygostyle formation is also demonstrated in two Cretaceous specimens, a juvenile enantiornithine and a subadult basal ornithuromorph. These findings call for reinterpretations of Zhongornis haoae, a Cretaceous bird hypothesized to be an intermediate in the long- to short-tailed bird transition, and of the recently discovered coelurosaur tail embedded in amber. Zhongornis, as a juvenile, may not yet have formed a pygostyle, and the amber-embedded tail specimen is reinterpreted as possibly avian. Analyses of relative pygostyle lengths in extant and Cretaceous birds suggests the number of vertebrae incorporated into the pygostyle has varied considerably, further complicating the interpretation of potential transitional species. In addition, this analysis of avian tail development reveals the generation and loss of intervertebral discs in the pygostyle, vertebral bodies derived from different kinds of cartilage, and alternative modes of caudal vertebral process morphogenesis in birds. These findings demonstrate that avian tail ontogeny is a crucial parameter specifically for the interpretation of Mesozoic specimens, and generally for insights into vertebrae formation.
Pterosaurs as a Teaching Tool
(2013-03) Carroll, Nathan; Lee, Ilse-Mari; Varricchio, David
The description of a new azhdarchid pterosaur from the Two Medicine Formation of Montana provides new information on this distinctive group of animals. The body plan of this pterosaur also provides for a unique public outreach opportunity. Both the size and shape of this pterosaur makes it ideal for constructing a 1:1 scale puppet that can be used to introduce audiences to pterosaur biology and ecology. The puppet is accompanied with an engaging, interactive presentation about the diversity and evolution of life in the Cretaceous. This time period is often portrayed to the public as the “Age of Dinosaurs,” but the diversity of the Mesozoic included much more than just dinosaurs, as evidenced by the assemblage of animals found at Jack's Birthday Site, a fossil bone bed. This site, found in the Two Medicine Formation of Montana, is one of the best faunal assemblage samples from this area. Although this site is rich in dinosaur fauna, it also documents the presence of birds, pterosaurs, lizards, and mammals. The life-sized puppet is an experiment in engaging and teaching broad audiences about pterosaurs, the site from which this specimen was found, and methods of paleontology.