Theses Doctoral

Multidisciplinary investigations on the origins and evolution of the extinct ungulate order Notoungulata (Mammalia: Placentalia) and the extinct muskox genus Bootherium (Mammalia: Artiodactyla: Bovidae)

West, Abagael Rosemary

This dissertation is an exploration of phenomena on varying scales, built on the backbone of Cenozoic mammalian biochronologic units (Land Mammal ‘Ages’): the integration of fossil and geological data to constrain spatiotemporal patterns in evolution. I develop and test hypotheses about the origins and ordinal-level relationships of the extinct South American endemic placental order Notoungulata, as well as about some more specific macroevolutionary patterns at a familial level within notoungulates. Major novel outcomes include a new biochronologic timescale for the terrestrial Cenozoic of South America, numerically calibrated through synthesis of new and existing high-precision geochronological data (particularly U-Pb and 40Ar/39Ar dating), along with an explicit logical framework for Land Mammal “Age” calibration; description of two new interatheriid notoungulate taxa from the central Chilean Andes; a new phylogenetic hypothesis for the position of Notoungulata within Placentalia; and the first ever DNA sequences obtained from the extinct musk ox relative Bootherium bombifrons.
The first study, chapter two, is a review of geochronologic (radiometric and magnetostratigraphic) constraints for the South American Land Mammal “Age” timescale. I present a revised, updated timescale, and a descriptive, logical framework for the synthesis of geochronologic and biochronologic data from a variety of sources and analytical methods. Significant changes to the calibration of individual SALMAs in this update are concentrated in the Paleogene, while Neogene calibrations have remained fairly stable, with small refinements to the core age ranges of the Huayquerian, Chasicoan, Colloncuran, and Friasian. This section also investigates the influence of latitudinal biotic provinciality on correlations and chronologic calibrations, particularly as far as provinciality reflects the climatic evolution of the continent. Marked provinciality is evident at least as early as the early Eocene, with faunas like Itaboraí differing from potential correlative faunas at higher latitudes, potentially representing unique periods in mammalian evolution, both faunally and chronologically. Instead of trying to recognize and correlate the classical high latitude SALMAs to highly distinctive tropical assemblages, the SALMA timescale should allow for the development of separate mammalian biochronologic zonations for low and high latitudes.
Chapter three presents and describes two new notoungulate taxa, representing the first species formally described from the Los Queñes Fauna, a late Eocene mammal assemblage from the Andean Main Range of central Chile. These two taxa, Anabalcarcel ignimbritae and Jackconrad carreterensis, represent the earliest hypsodont interatheres known. Based on ancestral state reconstructions using parsimony, hypsodonty appeared no later than the latest Eocene (34.6 ± 0.8 Ma; likely correlative with the Mustersan South American Land Mammal Age) in interatheres, a time when this dental specialization was not yet pervasive among other mammalian herbivores. Tree-based comparative analyses revealed two significant taxonomic radiations of interatheres, the early radiation of basal interatheriids and a later radiation of hypselodont taxa after the interval of dramatic global climatic change associated with the Eocene/Oligocene boundary.
In chapter four, I attempted to apply some of the geochronologic methods incorporated in the first two chapters, to date a newly discovered site in Abanico Formation, central Chilean Andes. This formation yields fossil mammals in numerous areas, including at several localities in the Río Las Leñas and Río Cachapoal drainages. In the Cachapoal Valley, steeply-dipping beds have yielded fossils of Tinguirirican age, including a polydolopine marsupial and an interatheriid notoungulate. The results of this study, the first 40Ar/39Ar analysis from the Cachapoal Valley, are a date of 11.1 ± 1.8 Ma, from stratigraphically higher levels loosely constraining the minimum age for fossiliferous deposits.
Chapters five and six change tack a little, and investigate analytical methods in ancient-DNA phylogenomics, time-calibrated with radiocarbon dates, using as a study system Bootherium bombifrons, the endemic North American helmeted musk ox that went extinct, along with most of its megafaunal colleagues, at the end of the Pleistocene. This switch to a North American, rather than South American, endemic mammal was driven by availability of molecular comparative material and the presence of hundreds of helmeted musk ox specimens in the American Museum paleontology collections, which I was able to sample destructively. I present the first complete mitochondrial genome of the extinct musk ox Bootherium bombifrons, and new mitochondrial sequence data from seven individuals referred to Bootherium bombifrons Additionally, I obtained new radiocarbon dates from these same specimens, of 38580 ± 720, 30240 ± 260, 44570 ± 190, 41040 ± 910, 44240 ± 1500, 39080 ± 770, 28370 ± 210, and 47190 ± 2100 calibrated 14C years BP. Phylogenetic analysis supports placement of Bootherium as sister to the living musk ox, Ovibos moschatus, in agreement with morphological taxonomy. I also tested hypotheses regarding the impact that locus choice has on divergence date estimates using tip calibrations in this taxon. Estimates of the root age of three different gene trees for Bootherium specimens ranged from 19 ka, for cytochrome B, to over 50 ka for the mitochondrial control region.
The final study in this dissertation (chapter seven) is a superordinal scale approach to fitting Notoungulata into the picture of all placental mammals through synthesis and expansion of existing total-evidence matrices. Results presented here corroborate previous findings that notoungulates are most closely related to Afrotheria, but do not support afrothere and notoungulate reciprocal monophyly, suggesting a complex biogeographic relationship between South America and Africa persisting from Late Cretaceous through mid-Paleocene.


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More About This Work

Academic Units
Earth and Environmental Sciences
Thesis Advisors
Flynn, John J
Ph.D., Columbia University
Published Here
July 22, 2017