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Current projects

Ecology and Evolution of the Notoungulata

Once South America became geographically (and biotically) separated from other continents the mammals on South America evolved in isolation, mixing little with more familiar mammal groups on other continents. These endemic South American mammals have traditionally been divided into six or seven major groups, only two of which (marsupials and the group that includes sloths, armadillos, and anteaters) have living representatives. The most diverse extinct group is the Notoungulata, a collection of rat-sized to rhino-sized hoofed, plant-eating mammals. I am particularly interested in notoungulates because they represent a completely separate radiation of mammals into the herbivore niche. As such, they demonstrate adaptations for herbivory that are sometimes unique and sometimes convergent compared to those exhibited by herbivorous mammals living today. My current research on the typothere notoungulates (the generally smaller-bodied half of the notoungulate family tree) involves documenting new species, refining taxonomy and phylogenetic relationships, and investigating the paleobiology (e.g., diet, locomotion) of these animals.      Archaeohyracids:  Archaeohyracids are a group of small (typothere) notoungulates known from the Casamayoran through Deseadan South American Land Mammal "Ages" (SALMAs). They are generally rare animals in the faunas in which they are found, although they are unusually common in the Deseadan fauna of Salla, Bolivia, as well as the Tinguiririca Fauna of central Chile. My collaborators and I have published two papers in Fieldiana that describe new archaeohyracids from the Tinguiririca Fauna, revise a significant portion of archaeohyracid taxonomy, and include a phylogenetic tree. These new taxa were included in the most recent summary of the Tinguiririca Fauna, in which we formally named the Tinguirirican SALMA. More recently, I  published a paper with Argentine colleagues that describes a new, very small species from northwest Argentina.      Hegetotheriids: Hegetotheriids are another group of small notoungulates that are closely related to archaeohyracids. Unlike archaeohyracids, they did not go extinct at the end of the Oligocene, but rather persisted all the way through to the Pleistocene. The specific phylogenetic relationships between archaeohyracids and hegetotheriids are a key focus of my research, as are the relationships among hegetotheriids themselves. And since they are relatively common in many Oligo-Miocene faunas, I often run across them while doing fieldwork and therefore usually have lots of specimens awaiting description. I recently published a paper (along with Federico Anaya) describing a new species of hegetotheriid (pictured at left, illustrated by Velizar Simeonovski) from the middle Miocene (Laventan SALMA) of Quebrada Honda, Bolivia; this paper also included a phylogenetic analysis of the family. This species is the first hegetotheriid known from this particular period of time. I didn't happen to find these specimens in the field, however, but rather in the drawers of a museum in La Paz, Bolivia. We have collected some beautiful hegetotheriid specimens from Chile, but most have yet to be described.      Mesotheriids:  Mesotheriids are typothere notoungulates that are related to archaeohyracids and hegetotheriids; they span a size range greater than that of archaeohyracids, with larger members generally described as being sheep-sized.  They first appear in the Divisaderan (?late Eocene) and persist into the Pleistocene.  The last revision of the group (actually, of one of the two subfamilies) was a study by Francis in 1965.  Since then, new specimens have been collected from Argentina, Bolivia, and Chile, and several new taxa have been described.  In our three field seasons at Chucal, we have collected a large number of mesotheriid specimens. We presented a preliminary review of these new mesotheriines in a short JVP article, and named three new species from Chucal in a subsequent Fieldiana paper. We recently described an additional new species from younger strata to the south.  Based on these investigations, it appears as though mesotheriines underwent a significant radiation in the middle latitudes of South America in the early Miocene. I continue to be interested in the evolution and biogeography of this group and am currently studying specimens from Chile and Bolivia. I am also studying the functional morphology of their appendicular skeleton in collaboration with Bruce Shockey and Federico Anaya.      Basicranial Morphology:  The base of the skull and the ear region are anatomically complex areas of the mammalian skull that have been valuable in phylogenetic studies of many groups of mammals. Although several early studies of notoungulate skulls have highlighted important distinctions in basicranial anatomy among various groups, these findings have been underutilized in investigations of notoungulate systematics. I will soon begin a comprehensive examination of notoungulate ear and basicranial anatomy using high resolution computed tomography (CT), a technology that will allow me to look not only at the external structure of the skull, but also at the internal anatomy. The data from this study should significantly help clarify the evolutionary history of the Notoungulata.      Dietary Inference:  Because notoungulate have no living representatives, it is difficult to figure out what they were eating.  Pretty much everyone agrees notoungulates were herbivores (i.e., eating leaves, fruits, twigs, etc.) but it is less certain how important various types of plants were in the diets of different species. Diet is a very important aspect of a mammal's biology and dietary data can sometimes be used to infer habitat, and so I am quite interested in figuring out what sorts of plants these animals were eating. One way Beth Townsend and I have been investigating diet in notoungulates is by studying "enamel microwear." Microwear is the term applied to very small marks - such as scratches, pits, and gouges - that are left on a mammal's teeth by the food it eats. It is called microwear because the marks left on the tooth can only be seen well using a microscope. For several decades, scientists have been studying the correlation between microwear and diet in living (extant) mammals and have found that certain types of food usually leave characteristic marks.  Based on analogy, if you can "read" the microwear on the teeth of a fossil specimen, you can then get an idea of what that animal was probably eating. (There are lots of other factors involved, but that's the short version of the process.) Beth and I were the first to apply this method to notoungulates and were also the first to examine microwear in living caviomorph rodents. Our notoungulate studies have been particularly interesting because they haven't supported the traditional dietary interpretation that these animals were eating abrasive vegetation like grasses, at least in the early Miocene of Argentina. Future studies using mesowear (a different type of tooth wear) should help test the accuracy of our results. A student of mine and I recently worked together on a project that applied the mesowear technique to Oligocene notoungulates from Bolivia, demonstrating its efficacy for endemic South American ungulates (another first such study of its kind). Beth and I have recently received funding from the National Science Foundation to further explore these methods of dietary inference for endemic ungulates so that we can better characterize the ways that ancient plant-eaters were partitioning the available resources.

Mammal communities on large islands

Due to their isolated nature, islands have long been known as places for evolutionary innovation; more than 150 years ago, careful study of the faunas of differently-sized islands in various locations contributed significantly to the ideas of both Charles Darwin and Alfred Wallace, the co-founders of the idea of evolution via natural selection. Although continents are seldom thought of as islands, Australia (today) and South America (until a few million years ago) both demonstrate unique faunas, typical of isolated land masses. In contrast to other continents – where immigration and dispersal have had significant roles in shaping and re-shaping the mammal fauna over the past 65 million years – in South America and Australia, autochthonous origination has been the norm. These island continents are therefore particularly well-suited for testing macroecological models based exclusively (or almost exclusively) on data from other continents. I am interested in investigating these models by incorporating data from the fossil record and trying to figure out what the paleocommunities can tell us about communities and species diversity today.      Paleoenvironment:  One type of macroecological model is that of a "cenogram" analysis. Using data from modern faunas, a cenogram analysis correlates characteristics of faunal body size distributions with habitat and environmental attributes such as rainfall and vegetational structure. Cenogram analyses have been  widely used for interpreting habitats of northern hemisphere paleofaunas, but a study I published a few years ago was the first to apply this method to South American paleofaunas. One of the most interesting results of this study was that the habitat interpretations based on cenogram analyses were in conflict with those based on other lines of evidence (e.g., craniodental morphology of the herbivores) for several South American localities. This suggests that these types of macroecological models may not apply to endemic faunas in places such as South America, and that previous studies using these models in other areas may or may not have reached accurate ecological conclusions. Of course, I'd now like to figure our why they do or do not work under certain circumstances.  In collaboration with Beth Townsend, I am expanding our repertoire of techniques for inferring ancient habitats by using ecological diversity analysis (EDA). In 2005 I presented preliminary results of our study of the Santa Cruz Fauna of southern Argentina using EDA and at some point we plan to put together the manuscript.      Predator-Prey Diversity:  A common attribute of Australia and South America throughout most of the Cenozoic is low mammalian predator diversity, consisting solely of marsupial predators. Although this relative lack of diversity is well documented, little research has investigated what effects this might have had on prey diversity and community dynamics. At a Society of Vertebrate Paleontology meeting, I presented preliminary results from a new study examining the relationship between mammal predator and prey species diversity in modern habitats. The final version of this study was recently published in Evolutionary Ecology Research. Relative diversity data from both South America and Australia strongly support the qualitative observation that marsupial predators - as a group - are not as "successful" as placental mammals in terms of species diversity. Abundance data suggest that marsupial predators were also rarer in their respective faunas than typical placental predators. These trends do not appear to result from differences in continental area or habitat diversity, nor do they appear to be attributable to effects of fossilization. In future investigations, I'd like to examine how such low levels of predator diversity and abundance might have affected the structure of fossil ecosystems. I'd also like to examine morphological diversity and disparity in these extinct predatory mammals.       Island Dwarfism: One of the best examples of an evolutionary phenomenon that has occurred repeatedly through geologic time is that of island dwarfing: when a large mammal becomes smaller through evolutionary time after becoming isolated on an island. Examples of dwarf mammoths, hippos, deer, and other animals abound, but this phenomenon had never previously been reported in the cattle group (Bovinae in technical terms). Thanks to my connections at The Field Museum, I became involved in a collaboration with the unique opportunity to describe some bones of an extinct species of dwarf water buffalo from the Philippines (see reconstruction at right by Velizar Simeonovski). These bones had been discovered almost 50 years ago by Michael Armas, a mining engineer, in a phosphate mine on Cebu Island, Philippines. He collected them and kept them safe for nearly four decades until they came to the attention of Dr. Hamilcar Intengan, a physician.  Dr. Intengan recognized the importance of these fossils and in 1995 brought them to the Field Museum for scientific study. After comparing the bones with those of the modern water buffalo, the tamaraw (an endangered Philippine water buffalo), and an anoa (a more distantly related buffalo from Sulawesi), we verified the distinctiveness of the bones and estimated that this tiny buffalo would have stood only 2.5' high at the shoulder and probably would have weighed a mere 350 lbs. The full report was published in the Journal of Mammalogy and featured as an open access article by BioOne. Because the Philippines has a relatively poor fossil record, we are very interested in finding additional mammal fossils there.

New fossil mammal localities

South America has an extremely rich fossil mammal record, but most of the classic fossil-producing localities are located in the far southern end of the continent, in Argentine Patagonia. Our research group has been working to increase geographic and temporal sampling of South American faunas by finding and describing new localities throughout Chile, a country spanning half the length of western South America. Such increased sampling has allowed us to document provinciality within the continent, to constrain and calibrate parts of the sequence of South American Land Mammal Ages (SALMAs), and to elucidate the geologic and tectonic histories of different rock units. I am also studying new or recently described faunas from Bolivia and northern South America.       Tinguiririca:  Perhaps the most important of these new Chilean localities is the Tinguiririca Fauna of central Chile. In the transition between the warm, equitable climate of the Eocene Epoch (54.8 to 33.7 million years ago) and the cooler climate of the Oligocene Epoch (33.7 to 23.8 mya), numerous "archaic" mammal groups went extinct while newer lineages of mammals, most with extant representatives, became more diverse. Before the discovery of the Tinguiririca Fauna, a significant gap spanning this period of time was present in the South American fossil mammal record. Understanding this critical time in the evolution of mammals worldwide (known as the Eocene-Oligocene transition) is essential to understanding the historic factors that influence the distribution of animals today. Our publication on the Tinguiririca Fauna is the first to describe the evolutionary effects the Eocene-Oligocene Transition on South American mammals and habitats, and it demonstrates that "open" (i.e., non-forested) habitats appeared earlier in South America than anywhere else in the world. A more recent publication, incorporating an ecological diversity analysis of Tinguiririca and taxonomic comparisons with other faunas of similar age, was published in 2008.      Chucal:  One of our more recent published localities is the Chucal Fauna from the Altiplano (high plateau) of northern Chile. At an elevation of some 4500 m, it is the highest vertebrate fossil locality in the western hemisphere (and the second highest in the world). The area was at a much lower elevation when the fossils were deposited there, and at that time the region supported a diverse and abundant mammal fauna (at least 18 different species have been collected). Comparisons with well-known faunas of potentially the same age from southern South America, however, illustrate that quite different mammal groups are represented at Chucal, thus documenting marked provinciality within the continent at the time. For example, the mesotheriid notoungulates are common and diverse at Chucal but are unknown in comparable faunas from southern Chile and Argentina.  Similarly, interatheriids (another family of small notoungulates) and ground sloths are common in Patagonia but are completely absent at Chucal. By combining the information from Chucal with that from other Chilean faunas of different ages, we will better understand when and how such provinciality originated and what effects it has had on modern faunas. The ungulates of Chucal were described a few years ago and we recently tackled the cingulates (armadillos and glyptodonts).       Other central Chilean localities:  A variety of other central Chilean faunas have been discovered and collected by our group, and although certain specimens from these faunas have been published, most of them have not yet been described. A current major focus of our group is compiling faunal lists for these faunas in order to make preliminary age and biostratigraphic interpretations. These, in turn, will be useful for elucidating the tectonic and uplift histories of these areas, for expanding the temporal spread of fossil mammal localities in Chile, and widening the geographic spread of fossil mammal localities in South America. As one example, the Miocene strata at Laguna del Laja preserve a remarkable variety of endemic rodents, illustrating the uniqueness of that component of the region's fauna during this interval.     Quebrada Honda, Bolivia:  The middle Miocene locality of Quebrada Honda was first discovered in the late 1970s and was collected more extensively in the early 1980s. Although a pair of papers were published in 1987 and 1988 describing some of the mammals, little detailed research had occurred until a few years ago, when interest resurfaced in the marsupials of Quebrada Honda. Federico Anaya and I recently published a paper describing a new hegetotheriid notoungulate from this locality, and I reviewed the other notoungulates in another paper published in Palaeontology. In 2007 we revisited the site, as well as another, slightly older, Bolivian locality.