Oreodont adaptation, evolution, and extinction in Oligocene-Miocene North America
Open Access
- Author:
- Cleveland, Claire
- Graduate Program:
- Geosciences
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- August 13, 2021
- Committee Members:
- Timothy Bralower, Major Field Member
Mark Patzkowsky, Chair & Dissertation Advisor
Sarah Ivory, Major Field Member
Erica Smithwick, Outside Unit & Field Member
Russell Graham, Major Field Member & Dissertation Advisor
Mark Patzkowsky, Program Head/Chair - Keywords:
- oreodonts
grassland expansion
North America
Cenozoic
Oligocene
Miocene
morphology
ecometrics
browsers
grazers
hypsodonty
cursoriality
Principal Component Analysis
Procrustes Analysis
evolutionary paleoecology
Artiodactyls
Merycoidodonts
Merycoidodontidae
Merycoidodontinae
evolution
ecology
paleobiology
trilateration
3D models
adaptation
extinction
extirpation
flora
grass
mammals
Central High Plains
High Plains
climate
seasonality
deciduous
evergreen
frost - Abstract:
- This dissertation is an examination of oreodont adaptation and evolution in North America. Over millions of years from the late Eocene to the early Pliocene, closed-canopy forests under warm and wet conditions transitioned to open grasslands under cool and dry conditions with low-frost winters. This was likely the most significant ecological transition during the Cenozoic and caused profound changes to all types of mammals. Oreodonts, herbivorous ungulates most closely related to camelids, were abundant, diverse, and geographically widespread across North America before going extinct in the late Miocene. The extinction of such an abundant, diverse, and geographically widespread clade by chance was unlikely. Grasses began to dominate open habitat in central western North America for the first time in the latest Oligocene, but Miocene oreodonts never adapted hypsodonty, high crowned teeth, or cursoriality, long, unguligrade-form limbs, that were inferred adaptations to grazing. For over a century, oreodont extinction was explained by their lack of adaptation to grazing and openness. To test the hypothesis that oreodonts did not adapt to grazing and openness, morphologic variation in oreodonts was quantified in Chapter Two using traditional measurement methods across a broad suite of grazing traits, not just hypsodonty and limb length proportion. Then, in Chapter Three, a subset of skull measurements from Chapter Two were used to develop three-dimensional models of oreodont skulls using trilateration, a method applied here for the first time. Trilateration is a mathematical method that uses only the measurements of distances between points to identify point coordinates in three-dimensional space. This method allowed for the removal of body size from interpretations of morphological variation and provided deeper insights into coordinated changes between measured traits. Trilateration also provided an alternative to existing methods that were too costly and time consuming for the large number of specimens and large size of specimens required in this research. In both Chapter Two and Chapter Three, Principal Component Analysis was used to construct an ecological morphospace, identify which traits were driving the greatest variation in body form, and assess if trends toward more grazing-type body forms were observed. Chapter Four investigated additional lines of evidence to explain oreodont extinction through a review of the literature. Oreodonts went extinct through a series of extirpations that occurred from north to south on the Columbia Plateau ca. 15 Ma, the Central High Plains ca. 9 Ma., and southern California ca. 7 Ma. Based on the north to south pattern of extirpation then extinction in oreodonts, five questions were asked: (1) Were regional histories in western central North America similar or distinct? (2) Did grasses appear, expand, and dominate landscapes earlier in the north where the first oreodont extirpation occurred on the Columbia Plateau then in the Central High Plains, and finally in southern California where oreodonts went extinct? (3) Did seasonality with the introduction of frost arrive earlier where oreodont extirpations occurred first and last where their final extinction occurred? (4) Did true-ruminants arrive earlier in the north where oreodont extirpation occurred first and last in the south where their extinction occurred? (5) Might tectonism have caused the isolation of oreodont populations into smaller groups that put them at higher risk for extinction? Results from investigations of morphological variation and comparisons of the timing of oreodont extirpations with the paleontological record do not support the hypothesis that oreodonts went extinct due to grassland expansion. Oreodont extinction was a complex process, as are most extinctions, and is likely best explained by a combination of an overall reduction in forage, increased seasonality associated with reduction of winter resources, and competition with true-ruminants. These results have broad implications that challenge existing explanations for the near mass extinction of terrestrial mammals during the Miocene that depend on the appearance, expansion, and dominance of grassland habitats.