- Email: [email protected]
The evolution of grass-dominated ecosystems during the late Cenozoic
Palaeogeography, Palaeoclimatology, Palaeoecology 207 (2004) 199 – 201 www.elsevier.com/locate/palaeo
Preface
The evolution of grass-dominated ecosystems during the late Cenozoic Grass-dominated ecosystems, including steppes, temperate grasslands, and tropical –subtropical savannas, play a central role in the modern world, occupying about 1/4 of the Earth’s land surface (Shantz, 1954) and providing food and habitat for humans and many of the animals upon which humans have come to depend. Understanding the evolution of grass-dominated habitats and the creatures that inhabit them—including ourselves—is therefore crucial in providing future directions for conservation and management. However, this major ecological change, ‘‘The Great Transformation’’ (Simpson, 1951), that occurred on most continents during the latter part of the Cenozoic, is not just the key to the future. It is also a study system that is uniquely suited for terrestrial paleoecology and its application towards understanding evolutionary and ecological processes. As such, it has everything a paleobiologist could ask for: vast amounts of data, reasonable modern analogues, and distinct signals in more than one fossil record. The first two aspects of the Cenozoic record are familiar facts and partly explain the longstanding love affair between paleontologists and the topic of grassland evolution. The last point refers to the fact that the changes that lead to the formation of the grassland biome are visible not only in the floral and faunal record, but also in the isotopic record, paleosol record, climatic record, sedimentological record, and the records of fossil diet and behavior. This is a revelation that has started to be fully exploited only recently with the advent of several new techniques in paleontology, such as isotope geochemistry, tooth micro- and mesowear analysis, and climate inferences based on leaf physiognomy (Solounias and Dawson-Saunders, 0031-0182/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2004.01.017
1988; Cerling et al., 1993; Wolfe, 1995). Other advances include the construction of a robust stratigraphic framework, increased use of magnetostratigraphy and radiometric dating, and improved insight into taphonomic processes (e.g., Kidwell and Behrensmeyer, 1993; Woodburne and Swisher, 1995; Terry et al., 1998). Newly developed computer programs also allow more sophisticated analyses of the large amounts of data collected during the past 130 years by numerous workers (see Jacobs et al., 1999 for review). With these new techniques, it is possible to address the evolution of grass-dominated ecosystems from multiple angles, and as a result, test the timing of changes previously assumed to be linked. Research has already shown that the previously accepted scenarios for grassland evolution and associated biotic interactions are overly simplistic (e.g., Wang et al., 1994; Janis et al., 2000). Hence, there is great promise in that continued work with multiple proxies will eventually lead to a more synthetic picture of this undoubtedly complex transition in Earth’s history, and to a more refined understanding of evolutionary and ecological processes in general. It is certainly not a coincidence that the last 20 years has seen the emergence of several large-scale projects attempting to address the development of the grassland biome in a more panoptic manner, both in Europe (Neogene of the Old World: http://www.helsinki.fi/ science/now/) and the United States (MIOMAP: http://www.ucmp.berkeley.edu/miomap/; Paleobiology Database: http://paleodb.org/). The comprehensive review article on grassland evolution by Jacobs et al. (1999) is also emblematic of this trend. Moreover, several conferences during the last few years (Paleo-
200
Preface
Grassland Research 2000 and 2002) have focused on multi-proxy approaches to grassland evolution and ecology (see Wooller and Beuning, 2002 and associated papers). A theme session at the 2001 North American Paleontological Convention (NAPC) in Berkeley, California, USA, was a similar effort to bring together scientists using various proxies to promote a multilayered understanding of the development of grassdominated ecosystems, this time concentrating on results and new techniques pertaining to pre-Holocene grassland evolution. This thematic issue is the end product of that endeavor, in essence a ‘‘state of the art’’ of pre-Holocene grassland research. The invited papers accordingly represent a wide array of disciplines and approaches, including vertebrate paleontology, paleopedology, isotope geochemistry, and paleobotany. The scope of the papers also shows the range of unresolved questions concerning grassland evolution, from Eocene– Oligocene vegetation changes in North America to issues relating to the emergence of hominins in the Plio-Pleistocene of Africa. The papers are organized according to proxies used, starting with paleopedology, followed by paleobotany, isotope geochemistry, and, finally, vertebrate paleontology. The first paper, by Greg Retallack, shows the use of paleopedological data, in combination with other proxies, for the study of the evolution of grasslands in the Oligocene– Miocene of the northwestern US (Oregon). Retallack’s findings indicate a two-step development of grasslands with dry, bunch grasslands in the early Oligocene and sod grasslands in the early Miocene. Caroline Stro¨mberg adapts a traditionally archaeological tool, phytolith analysis, for the Tertiary fossil record, to provide an independent line of evidence for changes in vegetation structure during this time period. Using the new, synthetic approach to reexamine Eocene – Miocene samples from the central Great Plains (Nebraska), she suggests that grasslands evolved before or during the early Miocene in this region. Francesca Smith and James White highlight another aspect of phytoliths that can be used to shed light on shifts in C3 and C4 dominance in Neogene grasslands, namely the carbon isotope geochemistry of these microfossils. The authors examine modern iso-
tope data from phytoliths in grasses and grasslands in an attempt to calibrate this promising tool for use in the pre-Holocene fossil record. The carbon and oxygen isotopes of fossil soils have also shown to be helpful in documenting local and regional vegetation change. David Fox and Paul Koch analyze the carbon and oxygen isotope values of Neogene paleosol carbonates from the Great Plains of North America to assess when C4 grasslands become abundant. They dissect global versus local mechanisms involved in C4 dominance, and provide two testable hypotheses for C4 grassland expansion that are consistent with their data. Continuing with the isotopic approach to the evolution of grasslands, Paul Koch, Noah Diffenbaugh, and Kathryn Hoppe analyze the carbon and oxygen isotope values of large herbivore tooth enamel, and use these values to model the percentage of C4 grasses present during the late Pleistocene of Texas. The data predicted from the models are compared with modern and late Pleistocene climate variables in the hopes of determining which variable(s) primarily affect the C3/ C4 competitive balance. Robert Feranec further shows the versatility of isotopic analysis by testing this proxy for diet and paleoenvironment against other commonly used proxies. More specifically, he looks at tall-crowned herbivore tooth enamel from the late Pleistocene of Florida. His data show that diet can be very complex and a variety of techniques are necessary to appreciate the range in forage and habitats a taxon could use. Christine Janis, John Damuth, and Jessica Theodor develop their analysis (Janis et al., 2000) of a large faunal data set documenting the anomalously high species richness in short-crowned, hoofed mammals (browsers) during the Miocene of North America compared to the present day. In their study, they evaluate different explanations for the observed patterns, such as taphonomic influences, climatic change, and biotic interactions, and propose primary productivity as the most likely factor controlling species richness in browsers. Finally, Rene Bobe and Kay Behrensmeyer use detailed analysis of a comprehensive data set of mammal fossils from the Plio-Pleistocene of Kenya and Ethiopia to test different hypotheses relating human evolution and climate/vegetation change, against the background of expanding grasslands.
Preface
The faunal patterns that emerge from their highresolution study indicate that no single proposed mechanism is sufficient to explain the emergence of Homo. We thank the UCMP, who organized the NAPC 2001 meeting in Berkeley, the enthusiastic and hardworking participants in the theme session and in this issue, the many excellent referees that contributed insightful reviews, Hannan LaGarry (University of Nebraska) and David Lindberg (University of California at Berkeley) for their assistance and advice, and Femke Wallien at Elsevier Science for all her help. It is our hope that this effort will only be the first in a series of thematic issues, documenting the exciting work that will be undertaken on grassland evolution in the years to come.
References Cerling, T.E., Wang, Y., Quade, J., 1993. Expansion of C4 ecosystems as an indicator of global ecological change in the late Miocene. Nature 361, 344 – 345. Jacobs, B.F., Kingston, J.D., Jacobs, L.L., 1999. The origin of grass-dominated ecosystems. Annals of the Missouri Botanical Garden 86 (2), 590 – 643. Janis, C.M., Damuth, J., Theodor, J.M., 2000. Miocene ungulates and terrestrial primary productivity: where have all the browsers gone? Proceedings of the National Academy of Sciences of the Unites States of America 97 (14), 237 – 261. Kidwell, S.M., Behrensmeyer, A.K. (Eds.), 1993. Taphonomic Approaches to Time Resolution in Fossil Assemblages. Short Courses in Paleontology Number 6. The Paleontological Society, Knoxville,Tennessee. 309 pp. Shantz, H.L., 1954. The place of grasslands in the Earth’s cover. Ecology 35 (2), 143 – 145. Simpson, G.G., 1951. Horses: The Story of the Horse Family in the Modern World and through Sixty Million Years of History. Oxford Univ. Press, Oxford. Solounias, N., Dawson-Saunders, B., 1988. Dietary adaptations and paleoecology of the Late Miocene ruminants from Pikermi and
201 Samos in Greece. Palaeogeography, Palaeoclimatology, Palaeoecology 65 (3 – 4), 149 – 172. Terry, D.O., LaGarry, H.E., Hunt, R.M. (Eds.), 1998. Depositional Environments, Lithostratigraphy, and Biostratigraphy of the White River Group and Arikaree Groups (Late Eocene to Early Miocene, North America), vol. 325, pp. 1 – 216. Geological Society of America Special Paper. Wang, Y., Cerling, T.E., MacFadden, B.J., Bryant, J.D., 1994. Fossil horses and carbon isotopes: new evidence for Cenozoic dietary, habitat, and ecosystem changes in North America. Palaeogeography, Palaeoclimatology, Palaeoecology 107, 269 – 280. Wolfe, J.A., 1995. Paleoclimatic estimates from Tertiary leaf assemblages. Annual Review of Earth and Planetary Sciences 23, 119 – 142. Woodburne, M.O., Swisher III, C.C. 1995. Land mammal highresolution geochronology, intercontinental overland dispersals, sea level, climate, and vicariance. In: Berggren, W.A., Kent, D.V., Aubry, M.-P., Hardenbol, J. (Eds.), Geochronology, Time Scales and Stratigraphic Correlation; Framework for an Historical Geology. Special Publication - Society for Sedimentary Geology, vol. 54. Society of Sedimentary Geology, Tulsa, Oklahoma, pp. 335 – 364. Wooller, M.J., Beuning, K.R., 2002. Introduction to the reconstruction and modeling of grass-dominated ecosystems. Palaeogeography Palaeoclimatology Palaeoecology 177, 1 – 3.
Caroline A.E. Stro¨mberg * Department of Palaeobotany, Swedish Museum of Natural History, P.O. Box 50007, SE-104 05, Stockholm, Sweden Robert S. Feranec Department of Integrative Biology and Museum of Paleontology, 3060 Valley Life Sciences Building, University of California, Berkeley, CA, 94720, USA E-mail addresses: [email protected], [email protected] * Corresponding author. Tel.: +46-8-5195-4293; fax: +46-85195-4221.
Preface
The evolution of grass-dominated ecosystems during the late Cenozoic Grass-dominated ecosystems, including steppes, temperate grasslands, and tropical –subtropical savannas, play a central role in the modern world, occupying about 1/4 of the Earth’s land surface (Shantz, 1954) and providing food and habitat for humans and many of the animals upon which humans have come to depend. Understanding the evolution of grass-dominated habitats and the creatures that inhabit them—including ourselves—is therefore crucial in providing future directions for conservation and management. However, this major ecological change, ‘‘The Great Transformation’’ (Simpson, 1951), that occurred on most continents during the latter part of the Cenozoic, is not just the key to the future. It is also a study system that is uniquely suited for terrestrial paleoecology and its application towards understanding evolutionary and ecological processes. As such, it has everything a paleobiologist could ask for: vast amounts of data, reasonable modern analogues, and distinct signals in more than one fossil record. The first two aspects of the Cenozoic record are familiar facts and partly explain the longstanding love affair between paleontologists and the topic of grassland evolution. The last point refers to the fact that the changes that lead to the formation of the grassland biome are visible not only in the floral and faunal record, but also in the isotopic record, paleosol record, climatic record, sedimentological record, and the records of fossil diet and behavior. This is a revelation that has started to be fully exploited only recently with the advent of several new techniques in paleontology, such as isotope geochemistry, tooth micro- and mesowear analysis, and climate inferences based on leaf physiognomy (Solounias and Dawson-Saunders, 0031-0182/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2004.01.017
1988; Cerling et al., 1993; Wolfe, 1995). Other advances include the construction of a robust stratigraphic framework, increased use of magnetostratigraphy and radiometric dating, and improved insight into taphonomic processes (e.g., Kidwell and Behrensmeyer, 1993; Woodburne and Swisher, 1995; Terry et al., 1998). Newly developed computer programs also allow more sophisticated analyses of the large amounts of data collected during the past 130 years by numerous workers (see Jacobs et al., 1999 for review). With these new techniques, it is possible to address the evolution of grass-dominated ecosystems from multiple angles, and as a result, test the timing of changes previously assumed to be linked. Research has already shown that the previously accepted scenarios for grassland evolution and associated biotic interactions are overly simplistic (e.g., Wang et al., 1994; Janis et al., 2000). Hence, there is great promise in that continued work with multiple proxies will eventually lead to a more synthetic picture of this undoubtedly complex transition in Earth’s history, and to a more refined understanding of evolutionary and ecological processes in general. It is certainly not a coincidence that the last 20 years has seen the emergence of several large-scale projects attempting to address the development of the grassland biome in a more panoptic manner, both in Europe (Neogene of the Old World: http://www.helsinki.fi/ science/now/) and the United States (MIOMAP: http://www.ucmp.berkeley.edu/miomap/; Paleobiology Database: http://paleodb.org/). The comprehensive review article on grassland evolution by Jacobs et al. (1999) is also emblematic of this trend. Moreover, several conferences during the last few years (Paleo-
200
Preface
Grassland Research 2000 and 2002) have focused on multi-proxy approaches to grassland evolution and ecology (see Wooller and Beuning, 2002 and associated papers). A theme session at the 2001 North American Paleontological Convention (NAPC) in Berkeley, California, USA, was a similar effort to bring together scientists using various proxies to promote a multilayered understanding of the development of grassdominated ecosystems, this time concentrating on results and new techniques pertaining to pre-Holocene grassland evolution. This thematic issue is the end product of that endeavor, in essence a ‘‘state of the art’’ of pre-Holocene grassland research. The invited papers accordingly represent a wide array of disciplines and approaches, including vertebrate paleontology, paleopedology, isotope geochemistry, and paleobotany. The scope of the papers also shows the range of unresolved questions concerning grassland evolution, from Eocene– Oligocene vegetation changes in North America to issues relating to the emergence of hominins in the Plio-Pleistocene of Africa. The papers are organized according to proxies used, starting with paleopedology, followed by paleobotany, isotope geochemistry, and, finally, vertebrate paleontology. The first paper, by Greg Retallack, shows the use of paleopedological data, in combination with other proxies, for the study of the evolution of grasslands in the Oligocene– Miocene of the northwestern US (Oregon). Retallack’s findings indicate a two-step development of grasslands with dry, bunch grasslands in the early Oligocene and sod grasslands in the early Miocene. Caroline Stro¨mberg adapts a traditionally archaeological tool, phytolith analysis, for the Tertiary fossil record, to provide an independent line of evidence for changes in vegetation structure during this time period. Using the new, synthetic approach to reexamine Eocene – Miocene samples from the central Great Plains (Nebraska), she suggests that grasslands evolved before or during the early Miocene in this region. Francesca Smith and James White highlight another aspect of phytoliths that can be used to shed light on shifts in C3 and C4 dominance in Neogene grasslands, namely the carbon isotope geochemistry of these microfossils. The authors examine modern iso-
tope data from phytoliths in grasses and grasslands in an attempt to calibrate this promising tool for use in the pre-Holocene fossil record. The carbon and oxygen isotopes of fossil soils have also shown to be helpful in documenting local and regional vegetation change. David Fox and Paul Koch analyze the carbon and oxygen isotope values of Neogene paleosol carbonates from the Great Plains of North America to assess when C4 grasslands become abundant. They dissect global versus local mechanisms involved in C4 dominance, and provide two testable hypotheses for C4 grassland expansion that are consistent with their data. Continuing with the isotopic approach to the evolution of grasslands, Paul Koch, Noah Diffenbaugh, and Kathryn Hoppe analyze the carbon and oxygen isotope values of large herbivore tooth enamel, and use these values to model the percentage of C4 grasses present during the late Pleistocene of Texas. The data predicted from the models are compared with modern and late Pleistocene climate variables in the hopes of determining which variable(s) primarily affect the C3/ C4 competitive balance. Robert Feranec further shows the versatility of isotopic analysis by testing this proxy for diet and paleoenvironment against other commonly used proxies. More specifically, he looks at tall-crowned herbivore tooth enamel from the late Pleistocene of Florida. His data show that diet can be very complex and a variety of techniques are necessary to appreciate the range in forage and habitats a taxon could use. Christine Janis, John Damuth, and Jessica Theodor develop their analysis (Janis et al., 2000) of a large faunal data set documenting the anomalously high species richness in short-crowned, hoofed mammals (browsers) during the Miocene of North America compared to the present day. In their study, they evaluate different explanations for the observed patterns, such as taphonomic influences, climatic change, and biotic interactions, and propose primary productivity as the most likely factor controlling species richness in browsers. Finally, Rene Bobe and Kay Behrensmeyer use detailed analysis of a comprehensive data set of mammal fossils from the Plio-Pleistocene of Kenya and Ethiopia to test different hypotheses relating human evolution and climate/vegetation change, against the background of expanding grasslands.
Preface
The faunal patterns that emerge from their highresolution study indicate that no single proposed mechanism is sufficient to explain the emergence of Homo. We thank the UCMP, who organized the NAPC 2001 meeting in Berkeley, the enthusiastic and hardworking participants in the theme session and in this issue, the many excellent referees that contributed insightful reviews, Hannan LaGarry (University of Nebraska) and David Lindberg (University of California at Berkeley) for their assistance and advice, and Femke Wallien at Elsevier Science for all her help. It is our hope that this effort will only be the first in a series of thematic issues, documenting the exciting work that will be undertaken on grassland evolution in the years to come.
References Cerling, T.E., Wang, Y., Quade, J., 1993. Expansion of C4 ecosystems as an indicator of global ecological change in the late Miocene. Nature 361, 344 – 345. Jacobs, B.F., Kingston, J.D., Jacobs, L.L., 1999. The origin of grass-dominated ecosystems. Annals of the Missouri Botanical Garden 86 (2), 590 – 643. Janis, C.M., Damuth, J., Theodor, J.M., 2000. Miocene ungulates and terrestrial primary productivity: where have all the browsers gone? Proceedings of the National Academy of Sciences of the Unites States of America 97 (14), 237 – 261. Kidwell, S.M., Behrensmeyer, A.K. (Eds.), 1993. Taphonomic Approaches to Time Resolution in Fossil Assemblages. Short Courses in Paleontology Number 6. The Paleontological Society, Knoxville,Tennessee. 309 pp. Shantz, H.L., 1954. The place of grasslands in the Earth’s cover. Ecology 35 (2), 143 – 145. Simpson, G.G., 1951. Horses: The Story of the Horse Family in the Modern World and through Sixty Million Years of History. Oxford Univ. Press, Oxford. Solounias, N., Dawson-Saunders, B., 1988. Dietary adaptations and paleoecology of the Late Miocene ruminants from Pikermi and
201 Samos in Greece. Palaeogeography, Palaeoclimatology, Palaeoecology 65 (3 – 4), 149 – 172. Terry, D.O., LaGarry, H.E., Hunt, R.M. (Eds.), 1998. Depositional Environments, Lithostratigraphy, and Biostratigraphy of the White River Group and Arikaree Groups (Late Eocene to Early Miocene, North America), vol. 325, pp. 1 – 216. Geological Society of America Special Paper. Wang, Y., Cerling, T.E., MacFadden, B.J., Bryant, J.D., 1994. Fossil horses and carbon isotopes: new evidence for Cenozoic dietary, habitat, and ecosystem changes in North America. Palaeogeography, Palaeoclimatology, Palaeoecology 107, 269 – 280. Wolfe, J.A., 1995. Paleoclimatic estimates from Tertiary leaf assemblages. Annual Review of Earth and Planetary Sciences 23, 119 – 142. Woodburne, M.O., Swisher III, C.C. 1995. Land mammal highresolution geochronology, intercontinental overland dispersals, sea level, climate, and vicariance. In: Berggren, W.A., Kent, D.V., Aubry, M.-P., Hardenbol, J. (Eds.), Geochronology, Time Scales and Stratigraphic Correlation; Framework for an Historical Geology. Special Publication - Society for Sedimentary Geology, vol. 54. Society of Sedimentary Geology, Tulsa, Oklahoma, pp. 335 – 364. Wooller, M.J., Beuning, K.R., 2002. Introduction to the reconstruction and modeling of grass-dominated ecosystems. Palaeogeography Palaeoclimatology Palaeoecology 177, 1 – 3.
Caroline A.E. Stro¨mberg * Department of Palaeobotany, Swedish Museum of Natural History, P.O. Box 50007, SE-104 05, Stockholm, Sweden Robert S. Feranec Department of Integrative Biology and Museum of Paleontology, 3060 Valley Life Sciences Building, University of California, Berkeley, CA, 94720, USA E-mail addresses: [email protected], [email protected] * Corresponding author. Tel.: +46-8-5195-4293; fax: +46-85195-4221.