Dinosaurs and time: chronostratigraphic frameworks and their utility in analysis of dinosaur paleobiology
Fowler, Denver Warwick
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Stratigraphy is the study of the position of rock strata, in order to determine their age. Dinosaur fossils have been recovered from North America for over 150 years, yet the stratigraphy of the localities from which they were collected has rarely been analysed at high resolution, either due to lack of original locality data, or that precise correlation between depositional basins was not technologically possible. This dissertation analyses what effect the introduction of high-resolution chronostratigraphic data has on our understanding of dinosaur paleobiology. Terrestrial sequence stratigraphy was used in combination with previously published data in order to subdivide the uppermost Cretaceous Hell Creek Formation of Montana into lower, middle, and upper units, shown to be consistent across the formation type area. Similar stratigraphic data was gathered for each of the Late Cretaceous dinosaur-bearing formations of the North American Western Interior, which was combined with nearly 200 radiometric dates (newly recalibrated here) to plot a comprehensive high-resolution correlation chart. The stratigraphic occurrence of dinosaur taxa was consequently plotted upon this chart. Similarly, the new Hell Creek Formation stratigraphic framework was used to plot the stratigraphic occurrence of Triceratops fossils. From this it can be seen that many dinosaur groups form stacks of stratigraphically separated species, a pattern indicative of linear, non-branching evolution (anagenesis). A similar pattern is observed for two new taxa of chasmosaurine ceratopsid (horned) dinosaurs from New Mexico, which form morphologic and stratigraphic intermediates between the slightly older taxon, Pentaceratops, and the younger Anchiceratops. Phylogenetic and geometric morphometric analysis supports the hypothesis that the posterior embayment of the parietal deepens and closes in on itself over ~ 2 million years from Pentaceratops through the new taxa, to Anchiceratops, and suggests a deep split within Chasmosaurinae that occurs before the Middle Campanian. These findings imply that dinosaur evolution in the Late Cretaceous Western Interior was characterized mostly by anagenesis, punctuated by occasional speciation events, perhaps triggered by high sealevel creating a north / south geographic barrier. The evolutionary process of reinforcement is discussed as a possible mechanism for the development of cranial display organs, linked to speciation.