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Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Geology

Supervisor

Longstaffe, Fred J.

2nd Supervisor

Webb, Elizabeth

Co-Supervisor

Abstract

This thesis uses stable isotopes to investigate the adaptive advantages of Ovibos moschatus, a muskox, relative to now-extinct Pleistocene megafauna in eastern Beringia (Yukon Territory and Alaska), including another muskox Bootherium bombifrons. Ecological niches were defined for Ovibos and Bootherium using stable carbon and nitrogen isotope compositions of bone collagen. Plant remains trapped in modern Ovibos molars were used to determine the carbon and nitrogen isotope collagen-diet discrimination factors, which were then applied to ancient specimens. Ovibos possessed larger isotopic – and hence ecological – niches than Bootherium across eastern Beringian sites. The larger niche suggests that Ovibos consumed a wider range of forage than Bootherium – an advantage during the major vegetation change in eastern Beringia (and the Mammoth Steppe) during the terminal Pleistocene. Ecological niche size comparisons between Ovibos and other megafauna for the North Slope, Alaska also showed that species facing extinction tended to have small isotopic niches. During the post-Last Glacial Maximum, Ovibos also had a small isotopic niche size, but it increased in size by the Holocene, likely because of Ovibos’ advantageous digestive and feeding strategies relative to other genera that went extinct.

Bioapatite oxygen isotope compositions for Ovibos and Bootherium from eastern Beringia were examined as proxies for drinking water isotopic composition – and hence warmer versus cooler conditions. Modern herbivores (Ovibos, Equus, Capra) were used to establish a baseline for assessing bioapatite oxygen isotope alteration in ancient specimens. The results confirmed that the oxygen isotope composition of bioapatite phosphate provides a more robust record of drinking water isotopic composition than structural carbonate. Bone collagen oxygen isotope compositions were also compared to those of bioapatite. In terms of oxygen-isotope preservation quality, collagen is poorer than phosphate but similar to bioapatite structural carbonate. Collagen oxygen isotope composition also varied with drinking water isotopic composition, the signal was attenuated relative to that provided by bioapatite oxygen. This difference likely reflects a greater contribution of food water oxygen to the collagen isotope signal relative to bioapatite.

This study demonstrates the power of the multi-isotope, multi-tissue, isotopic niche concept for evaluating the survival potential of megaherbivores faced with changing ecosystems.

Summary for Lay Audience

About 10,000 years ago, at the end of the last Ice Age, ~70% of large mammal types in North America went extinct. What allowed some other large mammal species to avoid extinction is of interest, because – like now – this was a period of significant warming. To better understand the responses to warming in Arctic regions, we studied the muskox (Ovibos moschatus). It has persisted in the face of climate change for more than 50,000 years. We used samples of ancient and modern bone and a chemical tracer technique known as stable isotopes to learn why Ovibos was more adaptable than the other, now extinct, large herbivores in eastern Beringia (modern day Yukon Territory and Alaska). One of latter was another type of muskox called Bootherium bombifrons, and its comparison with Ovibos was a particular focus of this thesis.

Stable isotopes are varieties of an element that differ in their numbers of neutrons, which causes differences in their relative incorporation into animals. Those differences can be traced back to diet, drinking water, climate and environment. Differences in diet and habitat were detected between Ovibos and Bootherium using the stable isotopes of nitrogen and carbon from organic matter in bone (collagen). Ovibos was able to survive on a wider range of plant types than Bootherium. This was a big advantage during the vegetation changes that accompanied warming. Ovibos also had advantageous digestive and feeding strategies relative to species that went extinct.

We also explored the stable isotopes of oxygen in bone, which should reflect drinking water isotopic composition. That composition can be related to geographical location and in Arctic regions, to warmer versus cooler conditions. Oxygen isotopes in bone from modern muskox, horse and goat were used to refine the relationship with drinking water composition. The oxygen isotope composition of phosphate in bone mineral provided a better record of drinking water than bone mineral carbonate. Oxygen in bone collagen also varied in isotopic composition with drinking water, but additional contributions from food make this signal more difficult to interpret in terms of climatic variation alone.

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