Date of Award

Spring 1-1-2012

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geological Sciences

First Advisor

Karen Chin

Second Advisor

Alex Cruz

Third Advisor

Jaelyn Eberle

Fourth Advisor

Hans Larsson

Fifth Advisor

Tom Marchitto

Abstract

The Late Cretaceous was a time of greenhouse climate and high sea levels that led to environmental conditions distinctly different from those today. A group of flightless, pursuit-diving seabirds called hesperornithiforms thrived during this time in Northern Hemisphere marine environments, and have a particularly abundant fossil record from Campanian Western Interior Seaway deposits of North America. Their North American distribution from Arkansas to Ellesmere Island provides an exceptional opportunity to compare the evolution, biology, and ecology of a group of ancient marine birds along a latitudinal gradient. In this research project, I describe two new high latitude hesperornithiform specimens, analyze the body size evolution of North American hesperornithiforms, evaluate the trophic ecology of Arctic hesperornithiforms, and compare the effects of climate and behavior on the bone microstructure of these birds by drawing on results from analyses of extant penguin osteohistology.

All analyses are placed within a paleoecological context, and the findings demonstrate that environmental conditions appear to have affected multiple aspects of hesperornithiform paleobiology. The discovery of large-bodied Arctic hesperornithiforms suggests that seasonal temperature and photoperiod fluctuations likely influenced body size evolution in high latitude populations. In lower latitude populations, however, competition-driven character displacement appears to have affected body size. Seasonality also likely influenced the trophic ecology of Arctic hesperornithiforms. Dental microwear analysis suggests that these birds had a more diverse diet (that probably varied seasonally) than previously assumed. Despite differences in body size evolution between populations, osteohistologic analyses of high and mid latitude hesperornithiforms reveal that neither Late Cretaceous climatic conditions nor long-distance migration appear to have affected bone growth dynamics. This conclusion is supported by a complementary study comparing modern pygoscelid penguin bone microstructure, which found that long-distance migration does not affect histologic patterns in these penguins, but that climatic factors and chick growth dynamics do. Overall, the results of this project provide a better understanding of this unique group of ancient seabirds, and shed light on the evolution of a group of marine vertebrates that thrived in a greenhouse world.

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