Evolution of the earliest horses driven by climate change in the Paleocene-Eocene Thermal Maximum (original) (raw)
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Paleobiology
Interpreting the impact of climate change on vertebrates in the fossil record can be complicated by the effects of potential biotic drivers on morphological patterns observed in taxa. One promising area where this impact can be assessed is a high-resolution terrestrial record from the Bighorn Basin, Wyoming, that corresponds to the Paleocene–Eocene thermal maximum (PETM), a geologically rapid (~170 kyr) interval of sustained temperature and aridity shifts about 56 Ma. The PETM has been extensively studied, but different lines of research have not yet been brought together to compare the timing of shifts in abiotic drivers that include temperature and aridity proxies and those of biotic drivers, measured through changes in floral and faunal assemblages, to the timing of morphological change within mammalian species lineages. We used a suite of morphometric tools to document morphological changes in molar crown morphology of three lineages of stem erinaceid eulipotyphlans. We then com...
The impact of regional climate on the evolution of mammals: A case study using fossil horses
Evolution, 2010
horses with low-crowned molar teeth to faunas with hipparionine horses characterized by high-crowned teeth. The spread of hipparionine horses is associated with increased seasonality and the expansion of open habitats. It is generally accepted that anchitheriine horses did not display an evolutionary increase in tooth crown height prior to their extinction. Nevertheless, to test whether anchitheriines showed any changes interpretable as adaptation to local conditions, we analyzed molar teeth from multiple populations of Anchitherium in three dimensions. Our results show differences in tooth morphology that suggest incipient hypsodonty in Spain, the first region experiencing increasingly arid conditions in the early Miocene of Europe. Furthermore, analyses of tooth wear show that Spanish specimens cluster with present ungulates that eat foliage together with grasses and shrubs, whereas German specimens cluster with present-day ungulates that eat mostly foliage. Taken together, even a taxon such as Anchitherium, with a long and successful history of forest adaptation, did respond to regional environmental changes in an adaptive manner.
Special Paper 369: Causes and consequences of globally warm climates in the early Paleogene, 2003
The Paleocene-Eocene transition in North American land-mammal faunas is well documented on the south side of Polecat Bench and in surrounding badlands of northwestern Wyoming. Here a rich fossil record is known from a stratigraphic section with an established geomagnetic polarity time scale and medium-to high-resolution carbon and oxygen isotope records. The Paleocene-Eocene carbon isotope excursion (CIE) and temperature maximum (PETM) associated with greenhouse warming are both represented, starting at ca. 55 Ma and lasting ∼ ∼86 k.y., during the transition from Paleocene to Eocene epochs and from Clarkforkian to Wasatchian land-mammal ages.
To better understand how past climatic change influenced mammalian communities, we used fossils from the Pit Locality of Porcupine Cave, to evaluate how two taxa responded to climatic events spanning two glacial–interglacial transitions of the middle Pleistocene in Colorado. We analyzed the isotopes of carbon, oxygen and strontium in 84 specimens of rabbits and marmots to infer (1) if feeding and habitat preferences differed across glacial–interglacial transitions, and (2) whether these taxa responded similarly and synchronously to climatic events. Our results showed no significant differences in any of the isotopic values within taxa across levels. Stable carbon isotope values revealed a C3-dominated environment around Porcupine Cave during the middle Pleistocene, similar to what is present around the cave today. Oxygen isotopes did not change significantly across levels suggesting consistent water sources over time and preventing any correlation to the Marine Isotope Stages. Marmots did show significantly more positive oxygen isotope values than rabbits over most of the Pit levels likely indicative of hibernation. Lack of significant change in Sr isotopes indicates similarity in habitat range through time, or homogenization of landscape Sr values due to atmospheric inputs. These results suggest that middle Pleistocene climatic change had a negligible effect on the ecology of the sampled individuals around Porcupine Cave. The effects of climate on mammals are complex and these results cannot be extrapolated globally; research is needed to differentiate how global climate change affects mammals in different regions and of different life history to provide insight into how current global warming will affect extant species.
Mammal extinctions, body size, and paleotemperature
Proceedings of the National Academy of Sciences, 1994
There is a general inverse relationship be. tween the natural logarithm of tooth area (a body size indicator) of some fossil mammals and paleotemperature during approximately 2.9 million years of the early Eocene In the Bighorn Basin of northwest Wyoming. When mean temperahures became warmer, tooth areas tended to become smaller. During colder times, larger species predominated; these generally became larger or remain the same size. Paleotemperature trends also markedly affece patterns of local (and, perhaps, regional) e con d immigration. New species appeared as immigrants during or near the hottest(smaller forms) and coldest (larger forms) intervals. Paleotemperature trend reversals commonly resulted In the ultimate e n of both small forms (during cooling intervals) and larger forms (during warming intervals). These immgtions and extinctions mark faunal turnovers that were also modulated by sharp increases in sediment accumulation rate.
The role of climatic change in the evolution of mammals
Bioscience, 2007
The paleontological record of mammals offers many examples of evolutionary change, which are well documented at many levels of the biological hierarchy-at the level of species (and above), populations, morphology, and, in ideal cases, even genes. The evolutionary changes developed against, backdrop of climatic change that took place on different scales, from rapid shifts in climate state that took only a few decades, to those that occurred over a millennial scale, to regular glacialinterglacial transitions with cycles of roughly a hundred thousand years, to long-term warming or cooling trends over hundreds of thousands to millions of years. Are there certain scales of climatic change that accelerate evolution? And what will the current global warming event do to evolutionary rates? Here we use paleontology-the study of fossils-to illustrate the scientific method behind answering such complex questions, and to suggest that current rates of global warming are far too fast to influence evolution much and instead are likely to accelerate extinctions.
Global climate change and North American mammalian evolution
2000
We compare refined data sets for Atlantic benthic foraminiferal oxygen isotope ratios and for North American mammalian diversity, faunal turnover, and body mass distributions. Each data set spans the late Paleocene through Pleistocene and has temporal resolution of 1.0 m.y.; the mammal data are restricted to western North America. We use the isotope data to compute five separate time series: oxygen isotope ratios at the midpoint of each 1.0-m.y. bin; changes in these ratios across bins; absolute values of these changes (= isotopic volatility); standard deviations of multiple isotope measurements within each bin; and standard deviations that have been detrended and corrected for serial correlation. For the mammals, we compute 12 different variables: standing diversity at the start of each bin; per-lineage origination and extinction rates; total turnover; net diversification; the absolute value of net diversification (= diversification volatility); change in proportional representation of major orders, as measured by a simple index and by a G-statistic; and the mean, standard deviation, skewness, and kurtosis of body mass. Simple and liberal statistical analyses fail to show any consistent relationship between any two isotope and mammalian time series, other than some unavoidable correlations between a few untransformed, highly autocorrelated time series like the raw isotope and mean body mass curves. Standard methods of detrending and differencing remove these correlations. Some of the major climate shifts indicated by oxygen isotope records do correspond to major ecological and evolutionary transitions in the mammalian biota, but the nature of these correspondences is unpredictable, and several other such transitions occur at times of relatively little global climate change. We conclude that given currently available climate records, we cannot show that the impact of climate change on the broad patterns of mammalian evolution involves linear forcings; instead, we see only the relatively unpredictable effects of a few major events. Over the scale of the whole Cenozoic, intrinsic, biotic factors like logistic diversity dynamics and within-lineage evolutionary trends seem to be far more important.
Rapid body size decline in Alaskan Pleistocene horses before extinction
Nature, 2003
About 70% of North American large mammal species were lost at the end of the Pleistocene epoch. The causes of this extinction-the role of humans versus that of climate-have been the focus of much controversy. Horses have figured centrally in that debate, because equid species dominated North American late Pleistocene faunas in terms of abundance, geographical distribution, and species variety, yet none survived into the Holocene epoch. The timing of these equid regional extinctions and accompanying evolutionary changes are poorly known. In an attempt to document better the decline and demise of two Alaskan Pleistocene equids, I selected a large number of fossils from the latest Pleistocene for radiocarbon dating. Here I show that horses underwent a rapid decline in body size before extinction, and I propose that the size decline and subsequent regional extinction at 12,500 radiocarbon years before present are best attributed to a coincident climatic/vegetational shift. The present data do not support human overkill and several other proposed extinction causes, and also show that large mammal species responded somewhat individualistically to climate changes at the end of the Pleistocene.
Proceedings of the National Academy of Sciences, 2004
Mid-Pleistocene vertebrates in North America are scarce but important for recognizing the ecological effects of climatic change in the absence of humans. We report on a uniquely rich mid-Pleistocene vertebrate sequence from Porcupine Cave, Colorado, which records at least 127 species and the earliest appearances of 30 mammals and birds. By analyzing >20,000 mammal fossils in relation to modern species and independent climatic proxies, we determined how mammal communities reacted to presumed glacial–interglacial transitions between 1,000,000 and 600,000 years ago. We conclude that climatic warming primarily affected mammals of lower trophic and size categories, in contrast to documented human impacts on higher trophic and size categories historically. Despite changes in species composition and minor changes in small-mammal species richness evident at times of climatic change, overall structural stability of mammal communities persisted >600,000 years before human impacts.
Climate directly influences Eocene mammal faunal dynamics in North America
Proceedings of the National Academy of Sciences, 2009
The modern effect of climate on plants and animals is well documented. Some have cautioned against assigning climate a direct role in Cenozoic land mammal faunal changes. We illustrate 3 episodes of significant mammalian reorganization in the Eocene of North America that are considered direct responses to dramatic climatic events. The first episode occurred during the Paleocene–Eocene Thermal Maximum (PETM), beginning the Eocene (55.8 Ma), and earliest Wasatchian North American Land Mammal Age (NALMA). The PETM documents a short (<170 k.y.) global temperature increase of ≈5 °C and a substantial increase in first appearances of mammals traced to climate-induced immigration. A 4-m.y. period of climatic and evolutionary stasis then ensued. The second climate episode, the late early Eocene Climatic Optimum (EECO, 53–50 Ma), is marked by a temperature increase to the highest prolonged Cenozoic ocean temperature and a similarly distinctive continental interior mean annual temperature (...