Late Cretaceous and Cenozoic Mammals of North America: Biostratigraphy and Geochronology

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Late Cretaceous and Cenozoic Mammals of North America

Late Cretaceous and Cenozoic Mammals of North America

Biostratigraphy and Geochronology

Edited by Michael O. Woodburne

Columbia University Press

New York

COLUMBIA UNIVERSITY PRESS

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Copyright © 2004 Columbia University Press

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E-ISBN 978-0-231-50378-5

Library of Congress Cataloging-in-Publication Data

Late cretaceous and cenozoic mammals of North America : biostratigraphy and geochronology / edited by Michael O. Woodburne.

      p. cm.

      Includes bibliographical references.

      ISBN 0-231-13040-6 (cloth)

      1. Mammals, Fossil—North America. 2. Paleontology—Cenozoic. 3. Paleontology—Cretaceous. 4. Paleontology—North America. I. Woodburne, Michael O.

QE881.L26 2004

569.′097—dc21

2003046251

A Columbia University Press E-book.

CUP would be pleased to hear about your reading experience with this e-book at [e-mail].

Contents

Preface M. O. Woodburne

List of Contributors

Definitions

Introduction  M. O. Woodburne

1.   Principles and Procedures

M. O. Woodburne

2.   Mammalian Biochronology of the Latest Cretaceous

R. L. Cifelli,* J. J. Eberle, D. L. Lofgren, J. A. Lillegraven, and W. A. Clemens

3.   Paleocene Biochronology: The Puercan Through Clarkforkian Land Mammal Ages

D. L. Lofgren, * J. A. Lillegraven, W. A. Clemens, P. D. Gingerich, and T. E. Williamson

4.   Wasatchian Through Duchesnean Biochronology

P. Robinson, * G. F. Gunnell, S. L. Walsh, W. C. Clyde, J. E. Storer, R. K. Stucky, D. J. Froehlich, I. Ferrusquia-Villafranca, and M. C. McKenna

5.   The Chadronian, Orellan, and Whitneyan North American Land Mammal Ages

D. R. Prothero and R. J. Emry

6.   Mammalian Biochronology of the Arikareean Through Hemphillian Interval (Late Oligocene Through Early Pliocene Epochs)

R. H. Tedford,* L. B. Albright III, A. D. Barnosky, I. Ferrusquia-Villafranca, R. M. Hunt Jr., J. E. Storer, C. C. Swisher III, M. R. Voorhies, S. D. Webb, and D. P. Whistler

7.   The Blancan, Irvingtonian, and Rancholabrean Mammal Ages

E. L. Lundelius Jr. and C. J. Bell,* A. D. Barnosky, R. W. Graham, E. H. Lindsay, D. R. Ruez Jr., H. A. Semken Jr., S. D. Webb, and R. J. Zakrzewski

8.   Global Events and the North American Mammalian Biochronology

M. O. Woodburne

Systematic Index

Subject Index

---

*Chairman of the committee of contributors

Preface

THIS BOOK UPDATES the information contained in its 1987 progenitor, Cenozoic Mammals of North America: Geochronology and Biostratigraphy, to further refine the tempo and mode of mammalian faunal succession in North America, with the major steps being recognized as discrete intervals known as North American land mammal ages. In the present work, the coverage is extended temporally to include the Lancian part of the Late Cretaceous, as precursor to the Cenozoic, and geographically to include information from Mexico, an integral part of the North American fauna, past and present.

This work incorporates new information on the systematic biology of the fossil record inspected herein but also uses the many advances in geochronologic methods and their results obtained since 1987. It is hoped that what follows here can lead to an increasingly high-resolution stratigraphy in which all available temporally significant data and applications are integrated. Fundamental to achieving this goal are using procedures to enable chronologic units to be recognized and their boundaries defined (no gaps or overlaps), establishing the units in actual field settings so that they are both replicable and realistically complete, and using radioisotopic, cyclostratigraphic, and magnetostratigraphic means to assist in developing as highly refined a correlation network as possible. The goal is a robust high-resolution chronology and, potentially, a chronostratigraphy.

As discussed more fully in the Introduction, highresolution chronostratigraphy involves a detailed integration of lithostratigraphic, faunal or (better) biostratigraphic, magnetostratigraphic, cyclostratigraphic, and radioisotopic data to arrive at the best possible interpretation of the age of a given fossiliferous level.

Whereas radioisotopic data used in 1987 had the advantage of the results of the K—Ar method pioneered by Evernden et al. (1964) unavailable to the original promulgation of the mammal age framework developed by Wood et al. (1941), the present effort benefits from the newly developed ⁴⁰Ar/³⁹Ar laser fusion techniques, unavailable before 1987. Similarly, the 1987 work saw the beginning of the now almost ubiquitous application of paleomagnetic stratigraphy to nonmarine mammal-bearing deposits, and a much richer array of this data set is available for the present book. Isotopic geochemistry provides information on changes in isotopes of oxygen and carbon that are proxies for changes in sea level and climate with implications for the nonmarine record, both as an impetus for faunal change and as tools for correlation. Advances in cyclostratigraphy improve the calibration of the magnetic polarity chronology paradigm, with feedback to the nonmarine correlation framework used here.

Thus the present work differs from the earlier volume in representing improvements in all aspects of the data set designed to promote correlation between fossil mammal–bearing successions in North America and thereby to improve our understanding of the times of faunal change represented by the mammal ages and their chronologic relationship to other important geologic, biological, or climatic events that transpired in the past 80 million years or so and may have shaped the tempo and mode of land mammal faunal succession during that time.

The goal of this book, then, is to place in modern context the information by which North American mammalian paleontologists recognize, divide, calibrate, and discuss intervals of mammalian evolution known as North American land mammal ages.

I dedicate this book to the memory of Donald Elvin Savage and Remmert Daams, two persistent advocates from North America and Europe, respectively, of the efforts and approaches documented herein.

Michael O. Woodburne

Running Springs, California

REFERENCES

Evernden, J. F., D. E. Savage, G. H. Curtis, and G. T. James. 1964. Potassium–argon dates and the Cenozoic mammal chronology of North America. American Journal of Science 262:145–198.

Wood, H. E. II, R. W. Chaney, J. Clark, E. H. Colbert, G. L. Jepsen, J. B. Reeside Jr., and C. Stock. 1941. Nomenclature and correlation of the North American continental Tertiary. Bulletin of the Geological Society of America 52:1–48.

Woodburne, M. O. (ed.). 1987. Cenozoic mammals of North America: Geochronology and biostratigraphy. Berkeley: University of California Press.

List of Contributors

L. Barry Albright III

Museum of Northern Arizona, Flagstaff, Arizona 86001

Anthony D. Barnosky

Museum of Paleontology, University of California, Berkeley, California

Christopher J. Bell

Department of Geological Sciences, University of Texas, Austin, Texas

Richard L. Cifelli

Oklahoma Museum of Natural History, University of Oklahoma, Norman, Oklahoma

William A. Clemens

Museum of Paleontology, University of California, Berkeley, California

William C. Clyde

Department of Geology, University of New Hampshire, Durham, New Hampshire

Jaelyn J. Eberle

University of Colorado Museum, Boulder, Colorado

Robert J. Emry

Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, Washington, D.C.

Ismael Ferrusquia-Villafranca

Instituto de Geologia, UNAM, University of Coyoacan, Mexico

David J. Froehlich

Vertebrate Paleontology Laboratory, University of Texas, Austin, Texas

Philip D. Gingerich

Museum of Paleontology, The University of Michigan, Ann Arbor, Michigan

Russell W. Graham

Denver Museum of Natural History, Denver, Colorado

Gregg F. Gunnell

Museum of Paleontology, The University of Michigan, Ann Arbor, Michigan

Robert M. Hunt Jr.

University of Nebraska State Museum, Lincoln, Nebraska

Jason A. Lillegraven

Department of Geology, The University of Wyoming, Laramie, Wyoming

Everett H. Lindsay

Department of Geosciences, University of Arizona, Tucson, Arizona

Donald L. Lofgren

Raymond M. Alf Museum of Paleontology, Claremont, California

Ernest L. Lundelius Jr.

Department of Geological Sciences, University of Texas, Austin, Texas

Malcolm C. McKenna

Department of Vertebrate Paleontology, American Museum of Natural History, New York, New York

Donald R. Prothero

Department of Geology, Occidental College, Los Angeles, California

Peter Robinson

University of Colorado Museum, Boulder, Colorado

Dennis R. Ruez Jr.

Department of Geological Sciences, University of Texas, Austin, Texas

Holmes A. Semken Jr.

Department of Geology, University of Iowa, Iowa City, Iowa

John E. Storer

Yukon Government Heritage Branch, Whitehorse, Yukon Territory, Canada

Carl C. Swisher III

Department of Geological Sciences, Rutgers, The State University of New Jersey, New Brunswick, New Jersey

Richard K. Stucky

Denver Museum of Natural History, Denver, Colorado

Richard H. Tedford

Department of Vertebrate Paleontology, American Museum of Natural History, New York, New York

Michael R. Voorhies

University of Nebraska State Museum, Lincoln, Nebraska

Stephen L. Walsh

San Diego Natural History Museum, San Diego, California

S. David Webb

Florida Museum of Natural History, University of Florida, Gainesville, Florida

David P. Whistler

Division of Earth Sciences, Natural History Museum of Los Angeles County, Los Angeles, California

Thomas E. Williamson

New Mexico Museum of Natural History, Albuquerque, New Mexico

Michael O. Woodburne

Department of Earth Sciences, University of California, Riverside, California

Richard J. Zakrzewski

Department of Earth Sciences and Sternberg Memorial Museum, Fort Hays State University, Hays, Kansas

Definitions

APTS. Astronomical polarity time scale. Time scale based on cyclical variations in the stratigraphic record interpreted to reflect astronomical cyclical variations in Earth’s orbital progression (Hilgen et al. 1997).

ASSEMBLAGE CHRON. This is a new biochronologic unit based on the antecedent assemblage zone and is an interval of time characterized by a distinctive assemblage or association of three or more fossil taxa that, taken together, distinguishes it in biochronologic character from adjacent intervals of time. To the degree that the replication of boundaries is hindered by the number of taxa involved (derived from the antecedent assemblage zone), the utility in precise correlation for the assemblage chron is diminished thereby. Many mammal ages originally were assemblage chrons with little attention given to biostratigraphic data and therefore are not comparable to the assemblage biochron of Walsh (1998) for which the antecedent assemblage zone (Walsh, 1998:160L and figure 5) is based on a biostratigraphic range zone.

ASSEMBLAGE ZONE. According to Salvador (1994:62–63), this is a stratum or body of strata characterized by a distinctive assemblage or association of three or more fossil taxa that, taken together, distinguishes it in biostratigraphic character from adjacent strata. This is generally similar to Hedberg’s (1976:50–52) definition, except for his explicit biofacies connotation. Because of stratigraphic vagaries in ranges of the associated taxa when considered regionally, stratigraphic limits of assemblage zones may be equally variable (Salvador 1994:63). The North American Commission on Stratigraphic Nomenclature (NACSN 1983:863) considers taxon ranges irrelevant and doesn’t define boundaries for assemblage zones, apparently because of their ambiguity, whereas boundaries are defined for interval zones and range zones. This treatment differs from the assemblage zone (assemblage fossizone or fossilzone) of Walsh (1998, 2000, respectively) in that the latter are based on specified taxon ranges, an approach that effectively synonymizes assemblage and range zones and differs from the language and concept of Hedberg (1976), Salvador (1994), and NACSN (1983).

BIOCHRON. According to Salvador (1994), this is The total time represented by a biozone. Williams (1901:579) originally defined this term as an interval of geologic time based on the duration of organic characters.

BIOCHRONOLOGY. "Geochronology based on the relative dating of geologic events by biostratigraphic or paleontologic methods or evidence" (Bates and Jackson 1987:69). To the extent that a biochron is based on a biozone, biochronology has a connection to biostratigraphy because the duration of organic characters cannot be demonstrated usefully without recourse to a stratigraphic framework that includes an ordinal paleontologic scale, with or without the addition of numerical data.

BIOSTRATIGRAPHIC UNIT. A body of rock strata that [is] defined or characterized on the basis of [its] contained fossils (Salvador 1994:53). Kinds of biostratigraphic units include range zone, taxon-range zone, concurrent-range zone, interval zone, lineage zone, assemblage zone, and abundance zone (= acme zone). Fossizone of Walsh (1998) or fossilzone (Walsh 2000) is not used here because it is equivalent in concept to a biozone.

BIOZONE. This is a general term for a biostratigraphic zone (Salvador 1994:55).

CHRON. Chron is the corresponding geochronologic term for a chronozone, the formal lowest-ranking member of the chronostratigraphic hierarchy (Hedberg 1976:69). This means that the chronostratigraphic unit (chronozone) must be established first in order for the chron (geochronologic unit) to be proposed. On this basis, a biozone (biostratigraphic unit) must be developed before an equivalent biologically based chronozone can be identified. In that the time span of a chronozone is usually defined in terms of the time span of a previously designated stratigraphic unit, such as … a biozone (Hedberg 1976:69), that interval of time is a biochron.

CHRONOFAUNA. Following Olson (1952:185), this is a geographically restricted, natural assemblage of interacting animal populations that has maintained its basic structure over a geologically significant interval of time. See also Tedford (1970), who stresses that chronofaunas are ecologically interpretive units.

CHRONOSTRATIGRAPHY. Chronostratigraphy is "the element of stratigraphy that deals with the age of strata and their time relations" (Hedberg 1976:66). Salvador (1994:77) replaces strata with rock bodies, which is not appropriate. According to Aubry et al. (1999:99), chronostratigraphy is the temporal ordering of geologic strata. For the purposes of this book, chronostratigraphy deals with strata. Contrary to Walsh (2001), chronostratigraphy is neither solely a method of age determination nor a means of age classification of strata, nor is it a subset of geochronology. In Hedberg (1976) and Salvador (1994), the purpose of a chronostratigraphic classification is to organize systematically the Earth’s sequence of rock strata into named units (chronostratigraphic units), corresponding to intervals of geologic time (geochronologic units), to serve as a basis for time-correlation and a reference system for recording events of geologic history (Hedberg 1976:66). Included objectives are to determine local time relations (because this is where the gathering of evidence must begin) and to establish a Standard Global Chronostratigraphic Scale (for global correlation and communication). The determination of the rock record precedes its interpretation (by whatever means) as to the age of that record. The basic chronostratigraphic unit, the stage, therefore precedes the establishment of its geochronologic counterpart, the age, contrary to Walsh (1998, 2001, and references therein).

CLASSICAL TIME SCALE (CTS; AUBRY 1995). Time scale based on radioisotopic dating of the stratigraphic record chosen to characterize certain temporal intervals, such as the system, series, and stage.

CONCURRENT-RANGE CHRON. Following from the antecedent concurrent-range zone, this is a new biochronologic term based on the time of the concurrent, coincident, or overlapping parts of the range chrons of two specified taxa selected from among the total forms contained in a temporal array. This is comparable to the strict overlap biochron of Walsh (1998:161, 2000:771) when two taxa are specified.

CONCURRENT-RANGE ZONE. According to Salvador (1994:58) this is the body of strata including the concurrent, coincident, or overlapping parts of the range zones of two specified taxa selected from among the total forms contained in a sequence of strata. This is preferred over the definition of Hedberg (1976:55–57) (parts of the range-zones of two or more … taxons) because it simplifies boundary definition and recognition. Still, these zones are not as useful in leading to biochronologic correlations as are others. The present definition is comparable to the strict overlap assemblage fossizone (or fossilzone) of Walsh (1998:161, 2000:770) when two taxa are considered.

CORRELATION. Stratigraphic correlation shows correspondence in character or stratigraphic position (Salvador 1994:15), but as modified from Aubry (1998:43) as stratigraphic correlation, it must mean temporal correlation as based on temporal analysis. Neither diachrony nor synchrony may be accepted on the basis of stratigraphic correlations alone but must be demonstrated on the basis of temporal analysis (Aubry 1995), and a dual terminology for stratigraphic and temporal terms must obtain.

CYCLOSTRATIGRAPHY. A discipline of stratigraphy wherein successive repetitions of sedimentary features are considered to be cyclical in nature. Some sedimentary cycles (i.e., varves) are interpreted as being annual features of climatic origin. Others are thought to reflect perturbations in orbital precession and obliquity caused by Earth’s behavior as it orbits the Sun, commonly known as Milankovitch cycles (Hilgen et al. 1997).

FAD. First appearance datum. This is a change in the fossil record with extraordinary geographical limits (Berggren and Van Couvering 1974:IX). As a chronostratigraphic concept, a FAD expresses an interpretation that the first stratigraphic appearance of a taxon is likely to have been synchronous over a specified geographic region (Woodburne 1996). The origin for a FAD (= appearance) was not constrained by Berggren and Van Couvering (1974, 1978), except that the dispersing taxon would have been newly evolved. For the paleobiotic event to be of extraordinary geographical limits, dispersal of an organism at a major scale clearly is the primary consideration, presumably from an indigenous source at some location. Aubry (1995:215) paraphrased this as the FAD being the first (temporal; evolutionary) appearance datum. Also, LO corresponds to FAD if the LO is of global significance (Aubry 1997:18, 22).

FAUNA. For paleontology, this is an assemblage of vertebrate fossils of similar taxonomic composition obtained from a small number sites considered to have a limited temporal range. A fauna is commonly composed of a number of local faunas. See Tedford (1970). Depending on historical context and author intent, stratigraphic limits of a fauna may be supplied.

FAUNULE. Association of taxa interpreted directly or intentionally for its ecological significance. See Tedford (1970).

FOD. First occurrence datum. Aubry (1997:18–19) distinguishes FOD from LO and FAD as a diachronous LO and therefore not an isochronous FAD. The word datum in the name signifies the temporal connotation rather than the biostratigraphically descriptive LO. If a given LO can be demonstrated as temporally later than the time of the FAD of that taxon, then it can segregated from the list of LOs that contribute to the FAD and be designated as a FOD. The FAD is of global significance; the FOD may be regionally important. The FOD is comparable to the dispersal lag of Woodburne and Swisher (1995) if its age can be demonstrated.

GEOCHRONOLOGY. According to Hedberg (1976) and Salvador (1994), this is defined as the science of dating and determining the time sequence of events in the history of the Earth (Hedberg 1976:15). As expressed by Berggren and Van Couvering (1978:40), geochronology is geologic time as perceived by the progress in one or another ordinal series of events, with those events being parts of irreversible systems, such as organic evolution or radioisotopic decay. It is critically important that these ordinal systems provide a theoretical basis outside of the preserved geologic record by which the nature and relation of the events in the progression can be recognized or predicted, and according to which missing parts of the record can be identified (Berggren and Van Couvering 1978:40). Other methods useful to geochronology include paleomagnetic stratigraphy, isotope stratigraphy, and Milankovitch cyclostratigraphy. Geochronology is not merely geochronometry, by which numerical ages are applied to rocks or events.

GEOMAGNETIC POLARITY TIME SCALE (GPTS). A chronology based on counting reversals of Earth’s magnetic field (Bates and Jackson 1987:272).

HO. Highest stratigraphic occurrence (Aubry 1997:18–19). This is effectively similar to HSD. An HO may correspond to a LAD (Aubry 1997:22) if it is of effectively global significance. A series of diachronous HOs can become LODs if of regional significance. An HO also may have no temporal significance because of poor representation, scarcity, and truncation by an unconformity (Aubry 1997:22). See also Walsh (2000).

HSD. Highest stratigraphic occurrence of a taxon in a local section (Opdyke et al. 1977). A biostratigraphic term (Lindsay et al. 1987; Woodburne 1996); see LSD. Aubry (1997:18–22) prefers to use HO for (mostly) the same intent but to reserve the term datum for chronologic inference.

INTEGRATED MAGNETOBIOCHRONOLOGIC SCALE (IMBS; Berggren et al. 1985a, 1985b, 1985c, 1995a). A time scale consisting of a magnetochronology, a numerical scale, and a magnetobiochronologic framework.

INTERNATIONAL COMMISSION ON STRATIGRAPHY (ICS), accepted as such by the International Union of Geological Sciences in 1986. The mandate of the ICS is to develop a standard global stratigraphic scale (Cowie et al. 1986).

INTERNATIONAL UNION OF GEOLOGICAL SCIENCES. The IUGS promotes and supports the study of geological problems of worldwide significance and facilitates international and interdisciplinary cooperation in the Earth sciences.

INTERVAL CHRON. Following from the terminology of the interval zone (Salvador 1994), this is the interval of time defined on the earliest age of two successive biohorizons and is comparable to that of Walsh (1998) in representing the span of time between the first or last occurrence of one taxon and the first or last occurrence of another taxon. This is interpreted herein to mean that the boundaries of such a unit would be based on the ages of the LO and HO, respectively, of the taxa in question.

INTERVAL ZONE. According to Hedberg (1976:60) this is a biostratigraphic unit defined as the body of fossiliferous strata between two distinctive biostratigraphic horizons. Salvador (1994:123) defines this as a biozone consisting of the body of fossiliferous strata between two specified biostratigraphic horizons (biohorizons). This is interpreted herein to mean that the boundaries of such a unit would be based on the LOs, respectively, of the taxa in question. Although defining a boundary on an HO is theoretically possible, it generally has a greater potential for stratigraphic inconsistency than a LO (but see Cooper et al. 2001).

LAD. Last appearance datum; counterpart to a FAD. A LAD may be identical to the HO if the latter is of global significance (Aubry 1997:22).

LINEAGE CHRON. This is a new biochronologic unit. It is based on the corresponding biostratigraphic unit, the lineage zone (Salvador 1994). Thus a lineage chron is the interval of time defined on the earliest age of a taxon or part thereof in a specific evolutionary lineage and on the earliest age of its evolutionary successor. There is no counterpart in Walsh (1998).

LINEAGE ZONE. According to Hedberg (1976:58), a lineage zone comprises the body of strata containing specimens representing a segment of an evolutionary … line or trend, defined above and below by changes in features of the line or trend. In Salvador (1994:125) this is a body of strata containing specimens representing a specific segment of an evolutionary lineage. These criteria are interpreted herein to mean that the boundaries of such a unit would be based on the LOs, respectively, of the evolutionary first stratigraphic appearance of the taxon in question and the subsequent evolutionary first stratigraphic appearance of the derivative taxon of the lineage in question (see also NACSN 1983:862). Lineage zones offers one of the best assurances of reliable time-correlation on a biostratigraphic basis (Hedberg 1976:59).

LOCAL FAUNA. An aggregate of fossil vertebrate species that have a limited distribution in time from a number of closely grouped localities in a limited geographic area. See Tedford (1970). A local fauna could be based on taxa from a single locality.

LOD. Last occurrence datum. A series of regionally diachronous highest stratigraphic occurrences can form a number of LODs if they can be documented. See FOD.

LO. Lowest stratigraphic occurrence (Aubry 1995:17). This may be an LSD. It also may equate to an FAD (Aubry 1997:22) if it is of regional significance. Aubry (1995, 1997) differentiates LO as a stratigraphic (descriptive) first occurrence and, although LSD is equivalent in concept, reserves the term datum to signify a temporal connotation. Aubry (1977:18–19) distinguishes a LO from a FOD as well as an FAD. See also Walsh (2000).

LSD. Lowest stratigraphic datum (Opdyke et al. 1977:324). This is a biostratigraphic concept of the lowest known occurrence of a taxon in a local stratigraphic sequence (see also Lindsay et al. 1987; Lindsay and Tedford 1990:609; Woodburne 1996). The LO (Aubry 1997:18–22) is in part identical to the LSD.

MAGNETOSTRATIGRAPHIC POLARITY UNITS. Throughout the history of its development, workers applied a variety of names to parts of the Geomagnetic Polarity Time Scale, such as epoch, event, or interval. Recent codes or guides have stabilized the nomenclature of magnetic polarity units (e.g., Hedberg 1976; Salvador 1994). The following terminology implies that magnetostratigraphic and chronostratigraphic polarity units are analogous to those based on lithostratigraphy (tables 1.1 and 1.2). In practice, however, the original magnetostratigraphic chrons have no lithostratigraphic or chronostratigraphic base because the magnetic interval is inferred to be present in unseen sea floor lavas as sensed from magnetometers towed through the seas by ships.

RECOMMENDED TERMINOLOGY FOR MAGNETOSTRATIGRAPHIC POLARITY UNITS (AFTER SALVADOR 1994:TABLE 2)

MAMMAL AGES. Mammal ages make up the basic chronologic system used to describe the age and succession of events in mammalian evolution in North America. Mammal ages (commonly known as North American land mammal ages [NALMAs]), are biochronologic units. The interval of time corresponding to each of these is recognized on the basis of mammalian evolution loosely (at least originally) tied to their stratal succession in sedimentary rocks (Wood et al. 1941; Woodburne 1987). In terms of the definitions presented here, mammal ages typically are assemblage chrons, although some have been interval chrons or lineage chrons (Archibald et al. 1987) with varying degrees of biostratigraphic documentation. To the extent that many mammal ages have been defined on the basis of immigrant taxa (Repenning 1967; Woodburne and Swisher 1995), they are effectively interval chrons whose the boundaries are based on first appearance datums. The biostratigraphic counterpart of most mammal ages is the assemblage zone, an assemblage zone based on a fossil fauna (Salvador 1994:63).

MEGANNUM (MA). One million years in the radioisotopic time scale. For example, 10 Ma refers to the 10-miIlion-year level of the radioisotopic scale.

M.Y. (OR m.y.). A segment of geologic time 1 million years in duration, or the age of an event (e.g., 10 m.y. ago) without reference to a given point or set of points on the radioisotopic time scale.

NEOGENE. This follows Berggren et al. (1995b) to embrace the Miocene through Pleistocene series/epochs.

NORTH AMERICAN LAND MAMMAL AGE (NALMA); see Mammal ages.

PALEOGENE. This follows Berggren et al. (1995b) to embrace the Paleocene through Oligocene series/epochs.

RANGE CHRON. This is a biochronologic unit. Following from the language of the antecedent range zone (Salvador 1994), it represents the span of time defined on the age of selected element or elements of a biochronologic sequence. This is interpreted herein to mean that the boundaries of such a unit would be based on the ages of the LO and HO, respectively, of the taxon or taxa in question. The range chron of Walsh (1998) is a subset of the range chron as defined here.

RANGE ZONE. According to Salvador (1994:135) this is a biostratigraphic unit comprising the body of strata representing the known stratigraphic and geographic range of occurrences of any selected element or elements of the assemblage of fossils present in a stratigraphic sequence. That is interpreted herein to mean that the boundaries of such a unit would be based on the LO and HO, respectively, of the selected element or elements in question.

TAXON-RANGE CHRON. A taxon-range chron is a new biochronologic unit. Following from the language of the antecedent taxon-range zone (Salvador 1994), a taxon-range chron is defined on the known age range of a specified taxon.

TAXON-RANGE ZONE. According to Salvador (1994:140), this is a biostratigraphic unit comprising the body of strata representing the known range of occurrence (stratigraphic and geographic) of specimens of a certain taxon (species, genus, family, etc.). That is interpreted herein to mean that the boundaries of such a unit would be based on the LO and HO, respectively, of the taxon in question (see also NACSN 1983:862).

REFERENCES

Archibald, J. D., P. D. Gingerich, E. H. Lindsay, W. A. Clemens, D. W. Krause, and K. D. Rose. 1987. First North American land mammal ages of the Cenozoic Era. In Cenozoic mammals of North America: Geochronology and biostratigraphy, ed. M. O. Woodburne. Berkeley: University of California Press, pp. 24–76.

Aubry, M.-P. 1995. From chronology to stratigraphy: Interpreting the stratigraphic record. In Geochronology, time scales and global stratigraphic correlation, ed. W. A. Berggren, D. V. Kent, M.-P. Aubry, and J. Hardenbol. Tulsa: SEPM Special Publication 54, pp. 213–274.

———. 1997. Interpreting the (marine) stratigraphic record. In Actes du Congrès BiochroM’97, eds. J.-P. Aguilar, S. Legendre, and J. Michaux. Mémoires et Travaux E.P.H.E., Institut de Montpellier 21:15–32.

———. 1998. Stratigraphic (dis)continuity and temporal resolution of geological events in the Upper Paleocene–Lower Eocene deep sea record. In Late Paleocene–Early Eocene climatic and biotic events in the marine and terrestrial records, ed. M.-P. Aubry, S. G. Lucas, and W. A. Berggren. New York: Columbia University Press, pp. 37–66.

Aubry, M.-P., W. A. Berggren, J. A. Van Couvering, and F. Steininger. 1999. Problems in chronostratigraphy: Stages, series, unit and boundary stratotypes, Global Stratotype Section and Point and tarnished golden spikes. Earth Science Reviews 46:99–148.

Bates, R. L. and J. A. Jackson (eds.). 1987. Glossary of geology. Alexandria, VA: American Geological Institute.

Berggren, W. A., F. J. Hilgen, C. G. Langereis, D. V. Kent, J. D. Obradovich, I. Raffi, M. Raymo, and N. J. Shackleton. 1995a. Late Neogene (Pliocene–Pleistocene) chronology: New perspectives in high resolution stratigraphy. Geological Society of America Bulletin 107:1272–1287.

Berggren, W. A., D. V. Kent, J. J. Flynn, and J. A. Van Couvering. 1985a. Cenozoic geochronology. Geological Society of America Bulletin 96:1407–1418.

———. 1985b. Paleogene geochronology and chronostratigraphy. In The chronology of the geologic record, ed. N. J. Snelling. Geological Society of London Memoirs 10:141–195.

———. 1985c. Neogene geochronology and chronostratigraphy. In The chronology of the geologic record, ed. N. J. Snelling. Geological Society of London Memoirs 10:211–260.

Berggren, W. A., Kent, D. V., Swisher, C. C. III, and Aubry, M.-P. 1995b. A revised Cenozoic geochronology and chronostratigraphy. In Geochronology, time-scales and global stratigraphic correlations: A unified framework for an historical geology, ed. W. A. Berggren, D. V. Kent, M.-P. Aubry, and J. Hardenbol. Tulsa: SEPM Special Publication 54, pp. 129–213.

Berggren, W. A. and J. A. Van Couvering. 1974. The late Neogene: Biostratigraphy, geochronology and paleoclimatology of the last 15 million years in marine and continental sequences. Palaeogeography, Palaeoecology, Palaeoclimatology 16:1–216.

———. 1978. Biochronology. In Contributions to the geologic time scale, ed. G. V. Cohee, M. F. Glaessner, and H. D. Hedberg. Tulsa: American Association of Petroleum Geologists, Studies in Geology 6:39–55.

Cooper, R. A., J. S. Crampton, J. I. Raine, F. M. Gradstein, H. E. G. Morgans, P. M. Sadler, C. P. Strong, D. Waghorn, and G. J. Wilson. 2001. Quantitative biostratigraphy of the Taranaki Basin, New Zealand: A deterministic and probabilistic approach. American Association of Petroleum Geologists Bulletin 85(5):1469–1498.

Cowie, J. W., W. Ziegler, A. J. Boucot, M. G. Bassett, and J. Remane. 1986. Guidelines and Statutes of the International Commission on Stratigraphy (ICS). Courier Forschungsinstitut Senckenberg 83:1–14.

Hedberg, H. C. (ed.). 1976. International stratigraphic guide. New York: Wiley.

Hilgen, F. J., W. Krijgsman, C. G. Langereis, and L. Lourens. 1997. Breakthrough made in dating of the geologic record. EOS, Transactions, American Geophysical Union 78(28):285, 288–289.

Lindsay, E. H., N. M. Johnson, N. D. Opdyke, and R. F. Butler. 1987. Mammalian chronology and the magnetic polarity time scale. In Cenozoic mammals of North America; Geochronology and biostratigraphy, ed. M. O. Woodburne. Berkeley: University of California Press, pp. 269–290.

Lindsay, E. H. and R. H. Tedford. 1990. Development and application of land mammal ages in North America and Europe, a comparison. In European Neogene mammal chronology, ed. E. H. Lindsay, V. Fahlbusch, and P. Mein. New York: Plenum. NATO Advanced Science Institute Series 180:601–624.

North American Commission on Stratigraphic Nomenclature. 1983. North American stratigraphic code. American Association of Petroleum Geologists Bulletin 67(5):841–875.

Olson, E. C. 1952. The evolution of a Permian vertebrate chronofauna. Evolution 6:181–196.

Opdyke, N. D., E. H. Lindsay, N. M. Johnson, and T. Downs. 1977. The paleomagnetism and magnetic polarity stratigraphy of the mammal-bearing sections of Anza Borrego State Park, California. Quaternary Research 7:316–329.

Repenning, C. A. 1967. Palearctic–Nearctic mammalian dispersal in the late Cenozoic. In The Bering land bridge, ed. D. M. Hopkins. Stanford, CA: Stanford University Press, pp. 288–311.

Salvador, A. (ed.) 1994. International stratigraphic guide. Boulder, CO: Geological Society of America.

Tedford, R. H. 1970. Principles and practices of mammalian geochronology in North America. Proceedings, North American Paleontological Convention Pt. F, pp. 666–703.

Walsh, S. L. 1998. Fossil datum and paleobiological event terms, paleontostratigraphy, chronostratigraphy, and the definition of land mammal age boundaries. Journal of Vertebrate Paleontology 18(1):150–179.

———. 2000. Eubiostratigraphic units, quasibiostratigraphic units, and assemblage zones. Journal of Vertebrate Paleontology 20(4):761–775.

———. 2001. Notes on geochronologic and chronostratigraphic units. Bulletin of the Geological Society of America 113(6):704–713.

Williams, H. S. 1901. The discrimination of time-values in geology. Journal of Geology 9:570–585.

Wood, H. E. II, R. W. Chaney, J. Clark, E. H. Colbert, G. L. Jepsen, J. B. Reeside Jr., and C. Stock. 1941. Nomenclature and correlation of the North American continental Tertiary. Bulletin of the Geological Society of America 52:1–48.

Woodburne, M. O. (ed.). 1987. Cenozoic mammals of North America: Geochronology and biostratigraphy. Berkeley: University of California Press.

———. 1996. Precision and resolution in mammalian chronostratigraphy: Principles, practices, examples. Journal of Vertebrate Paleontology 16(3):531–555.

Woodburne, M. O. and C. C. Swisher III. 1995. Land mammal high resolution geochronology, intercontinental overland dispersals, sea-level, climate, and vicariance. In Geochronology, time-scales and global stratigraphic correlations: A unified framework for an historical geology, ed. W. A. Berggren, D. V. Kent, M.-P. Aubry, and J. Hardenbol. Tulsa: SEPM, Special Publication 54:335–364.

Introduction

Michael O. Woodburne

PERSPECTIVE

The chronologic framework of the present book remains the North American mammal age concept articulated by Wood et al. (1941) and Savage (1951) and displayed in a great variety of sources, including Woodburne (1987), hereafter identified as the 1987 volume. It is taken as given that practitioners of stratigraphic paleontology or stratigraphic paleobiology recognize and embrace the principle of paleontological correlation (Smith 1815, 1817) and of Steno’s (1669) principles of superposition, original horizontality, and original continuity of strata so that the rock record can be used to order the succession of mammalian (and other) taxa and serve as an empirical basis for recording that succession irrespective of theories of evolution or philosophies of systematic analysis. Even though mammal ages are nominally biochrons (Williams 1901:579; intervals of geologic time based on the duration of organic characters), their succession (Wood et al. 1941) was framed by the stratigraphic sequences in which they were found (Tedford 1970; Emry 1973 and references cited therein.). Thus the succession of mammal ages depended on the lithostratigraphic framework for their documentation. Similarly, it follows that refinements in the chronology of mammal ages also depend on increasingly refined documentation of the stratigraphic and chronologic framework in which they occur.

Chapters 1 and 2 of the 1987 volume summarized the variety of biostratigraphic and chronostratigraphic proposals developed to describe mammalian faunal succession and correlation up to that time. Chapter 3 of the 1987 volume nominated a succession of new biostratigraphic zones for faunas of nominal Paleocene age, and other 1987 chapters evaluated the mammal succession in early and late (or finer-scale) subdivisions of the traditional mammal ages. These frameworks are essentially followed herein. Woodburne and Swisher (1995) gave an update of the mammal age chronology in North America, with emphasis on evidence for the age of the immigrations that define a majority of the mammal ages and the extent to which these corresponded to major episodes of global sea level lowering. Alroy (1992, 1994, 1998a, 1998b) presented a subdivision of the mammalian faunal record in North America based on quantitative analysis and indicated that whereas immigration is a rapid process, the observed sampling-influenced diachroneity is far too great to allow favoring immigrant first occurrences as time indicators. In Alroy’s view, only quantitative analyses of entire faunas have any chance of recovering robust biochronological patterns. However, the only way in which quantitative or any other analyses can be improved is by developing new chronologically significant information with which to assess the age of taxa having a taxonomic precision that is underwritten by the experts directly familiar with the fossils they represent.

HIGH-RESOLUTION STRATIGRAPHY AND BIOCHRONOLOGY

The present work continues with the integration of stratigraphic and other temporally significant data with the mammal record in its primary physical context so as to provide an empirical basis on which the tempo and mode of mammalian evolution can be measured. An underlying concern is the degree to which the patterns of mammalian succession are replicable geographically and found to be chronologically consistent, whether these patterns are described as various kinds of biochrons (mammal ages), subdivisions of them, or biostratigraphic or chronostratigraphic zones.

A goal is the development of a high-resolution chronologic network that, to paraphrase Woodburne (1996), involves the development of a detailed stratigraphic framework for the fossil data, whether they are portrayed in a biostratigraphic array or not, determining an approximate age for the fossiliferous levels with respect to radioisotopic calibration or with respect to a magnetozone whose age limits are confidently known. The independent relative chronologic framework of magnetostratigraphy (and assignment of numerical ages to polarity reversal boundaries by various means; Cande and Kent 1992, 1995; Berggren et al. 1995a, 1995b) allows calibration of the fossil level and temporal correlation with any other similarly placed fossil level in another stratigraphic section. See chapter 1 for further consideration of this topic.

This is not the end of the operation, however. In recent decades, increasing emphasis has been placed on addressing the fidelity of the stratigraphic record through both statistical aspects (Strauss and Sadler 1989; Marshall 1990) and graphic methods (Aubry 1995, 1998; Mann and Lane 1995). Such operations may become increasingly meaningful in recognition of the fact that the ± factor as applied for the ⁴⁰Ar/³⁹Ar radioisotopic dating method can produce ancient ages with very small ± dimensions (e.g., 249.9 ± 0.1 Ma; Siberian Traps flood basalts; Renne et al. 1998:130). This can lead to the notion that ⁴⁰Ar/³⁹Ar ages are usually better than those derived from, say, the K–Ar method. But as discussed further in chapter 1, this notion can be somewhat misleading. In any case, there are numerous examples wherein mammalian stratigraphers attempt to use accumulation rate reconstructions (based on extrapolations from or interpolations between radioisotopic or magnetic polarity ages) to estimate the age of biostratigraphic or biochronologic units (Woodburne et al. 1990:474), but almost none use the kinds of procedures outlined in Aubry (1995, 1998) to test rigorously for hidden unconformities or other condensations of stratigraphic section, even though it is a given that any sharp geologic boundary (including a magnetic polarity reversal) may reflect an unconformity in the record (Sadler 1999). In fact, the frequent mismatches in the continental magnetostratigraphic record relative to the Geomagnetic Polarity Time Scale must result at least as much as from the effect of apparently unappreciated unconformities in the rock record as from imperfections, overprints, or technical errors in the magnetostratigraphic analysis. Before asserting diachrony in the lowest stratigraphic datum of fossil mammals when considered regionally (Alroy 1998), it is necessary to rule out the effect of imperfections in the stratigraphic record. Finally, in order to be precise, boundaries must be defined and the proposed interval characterized (Woodburne, 1977, 1987, 1996), with single-taxon definitions being preferred over those based on multiple taxa because they are less ambiguous.

REFERENCES

Alroy, J. 1992. Conjunction among taxonomic distributions and the Miocene mammalian biochronology of the Great Plains. Paleobiology 18(3):326–343.

———. 1994. Appearance event ordination: A new biochronologic method. Paleobiology 20(2):191–207.

———. 1998a. Diachrony of mammalian appearance events: Implications for biochronology. Geology 26(1):23–26.

———. 1998b. Diachrony of mammalian appearance events: Implications for biochronology—Reply. Geology 26:956–958.

Aubry, M.-P. 1995. From chronology to stratigraphy: Interpreting the stratigraphic record. In Geochronology, time scales and global stratigraphic correlation, ed. W. A. Berggren, D. V. Kent, M.-P. Aubry, and J. Hardenbol. Tulsa: SEPM Special Publication 54, pp. 213–274.

———. 1998. Stratigraphic (dis)continuity and temporal resolution of geological events in the Upper Paleocene–Lower Eocene deep sea record. In Late Paleocene–Early Eocene climatic and biotic events in the marine and terrestrial records, ed. M.-P. Aubry, S. G. Lucas, and W. A. Berggren. New York: Columbia University Press, pp. 37–66.

Berggren, W. A., F. J. Hilgen, C. G. Langereis, D. V. Kent, J. D. Obradovich, I. Raffi, M. Raymo, and N. J. Shackleton. 1995a. Late Neogene (Pliocene–Pleistocene) chronology: New perspectives in high resolution stratigraphy. Geological Society of America Bulletin 107:1272–1287.

Berggren, W. A., D. V. Kent, C. C. Swisher III, and M.-P. Aubry. 1995b. A revised Cenozoic geochronology and chronostratigraphy. In Geochronology, time-scales and global stratigraphic correlations: A unified framework for an historical geology, ed. W. A. Berggren, D. V. Kent, M.-P. Aubry, and J. Hardenbol. Tulsa: SEPM Special Publication 54, pp. 129–213.

Cande, S. C. and D. V. Kent. 1992. A new geomagnetic polarity time-scale for the Late Cretaceous and Cenozoic. Journal of Geophysical Research 97:13,917–13,951.

———. 1995. Revised calibration of the geomagnetic polarity time scale for the Late Cretaceous and Cenozoic. Journal of Geophysical Research 100:6093–6095.

Emry, R. J. 1973. Stratigraphy and preliminary biostratigraphy of the Flagstaff Rim area, Natrona County, Wyoming. Smithsonian Contributions to Paleobiology 25:1–20.

Mann, K. O. and H. R. Lane (eds.). 1995. Graphic correlation. Tulsa: SEPM Special Publication 53.

Marshall, C. R. 1990. Confidence intervals on stratigraphic ranges. Paleobiology 16:1–10.

Renne, P. R., C. C. Swisher III, A. L. Deino, D. B. Karner, T. L. Owens, and D. J. K. DePaolo. 1998. Intercalibration of standards, absolute ages and uncertainties in ⁴⁰Ar/³⁹Ar dating. Chemical Geology 145:117–152.

Sadler, P. M. 1999. The influence of hiatuses on sediment accumulation rates. GeoResearch Forum 5:15–40.

Savage, D. E. 1951. Late Cenozoic vertebrates of the San Francisco Bay region. University of California Publications in Geological Sciences 28:215–314.

Smith, W. 1815. Memoir to the map and delineation of the strata of England and Wales with a plat of Scotland. London: Cary.

Smith, W. 1817. Stratigraphic system of organized fossils with reference to the specimens of the original collection in the British Museum explaining their state of preservation and their use in identifying the British strata. London: E. Williams.

Steno, N. 1669. De solido intra solidum naturaliter contento dissertationis prodromus. Florence.

Strauss, D. and P. M. Sadler. 1989. Classical confidence intervals and Bayesian probability estimates for ends of local taxon ranges. Mathematical Geology 21:411–427.

Tedford, R. H. 1970. Principles and practices of mammalian geochronology in North America. Proceedings of the North American Paleontological Convention Pt. F, pp. 666–703.

Williams, H. S. 1901. The discrimination of time-values in geology. Journal of Geology 9:570–585.

Wood, H. E. II, R. W. Chaney, J. Clark, E. H. Colbert, G. L. Jepsen, J. B. Reeside Jr., and C. Stock. 1941. Nomenclature and correlation of the North American continental Tertiary. Bulletin of the Geological Society of America 52:1–48.

Woodburne, M. O. 1977. Definition and characterization in mammalian chronostratigraphy. Journal of Paleontology 51(2):220–234.

———(ed.). 1987. Cenozoic mammals of North America: Geochronology and biostratigraphy. Berkeley: University of California Press.

———. 1996. Precision and resolution in mammalian chronostratigraphy: Principles, practices, examples. Journal of Vertebrate Paleontology 16(3):531–555.

Woodburne, M. O. and C. C. Swisher III. 1995. Land mammal high resolution geochronology, intercontinental overland dispersals, sea-level, climate, and vicariance. In Geochronology, time-scales and global stratigraphic correlations: A unified framework for an historical geology, ed. W. A. Berggren, D. V. Kent, M.-P. Aubry, and J. Hardenbol. Tulsa: SEPM, Special Publication 54:335–364.

Woodburne, M. O., R. H. Tedford, and C. C. Swisher III. 1990. Lithostratigraphy, biostratigraphy, and geochronology of the Barstow Formation, Mojave Desert, southern California. Geological Society of America Bulletin 102:459–477.

1

Principles and Procedures

Michael O. Woodburne

ADISCUSSION OF THE PRINCIPLES and procedures in methodology and the goal of producing a time scale based on the evolution of fossil mammals that contains neither gaps nor overlaps is as pertinent now as it was in 1987 or, indeed, in 1941 (Wood et al. 1941). Whether or not it is formally identified as biostratigraphy, students of mammalian chronology in North America have continually worked to improve the stratigraphic framework associated with fossil mammals and to integrate it with other chronologic information. Although still biochrons, mammal ages and subdivisions have become stratigraphically assisted (stratigraphically characterized but not defined) to varying degrees since 1941, and this trend continues here. There still are only a few instances in which sufficient stratigraphic information has been added to the fossil mammal biochronologic concept to support the development of a chronostratigraphic stage, and thus a true geochronologic age, hence the common convention of categorizing the biochrons as mammal ages. The main purpose of this chapter is to review the traditional stratigraphic procedures and some innovations designed to improve the development of a correlation network for fossil mammals that is empirically based and noncircular in reasoning. A thesis developed here is that not only is there a distinct progression from biostratigraphy, through biochronology, to chronostratigraphy, and then to geochronology, but that it is appropriate to use a formalized set of biochronologic units as part of that process. In at least one example cited in this chapter, almost the entire process has been achieved for early Paleocene strata of the Hanna Basin, Wyoming, even though the final procedural documentation of a chronostratigraphic unit (statement of intent, selection of stratotypes, reference sections) has not been completed. In a much larger set of examples (described elsewhere in this book), the mammal age data set is becoming increasingly documented in detail with respect to stratigraphy and with respect to radioisotopic, paleomagnetic, and stable isotope chronology. It is therefore appropriate to review the fundamentals of stratigraphic classification and correlation here.

THE GEOCHRONOLOGIC FRAMEWORK

This is the framework within which the geologist or paleobiologist understands not only the passage of geologic time but also the age and interrelation of past events important for study. In the present case, interest is focused on mammalian evolution and the means by which the results therefrom can be used to develop a framework of data that lead to a chronologic system by which that evolution can be perceived and documented: an increasingly high-resolution stratigraphy in which all available temporally significant data and applications are integrated. The progression toward this goal begins with a consideration of a chronology derived from a physical stratigraphic framework and turns to its calibration, estimation of completeness or fidelity, operations in correlation, the role of biochronology, and the relationship of these factors to mammal ages.

STRATIGRAPHIC CLASSIFICATION

A major task of a mammalian stratigrapher is to devise and work within a framework of data that leads to the establishment of a succession of temporal intervals that account for all of geologic time, with no overlaps or hiatuses. The following describes the classification of units and concepts in the conventional stratigraphic hierarchy. Table 1.1 shows the formal chronostratigraphic and geochronologic terms used in modern stratigraphic guides and codes, and table 1.2 summarizes the classification of units and concepts important to biostratigraphy, chronostratigraphy, and geochronology. The operation of working within this hierarchy to develop a temporal correlation is taken up later in this chapter. With respect to table 1.1, the System–Period pair was the focus of geologists contemporaneous with William Smith, but in subsequent years, increasing attention has been paid to smaller-scale increments of the hierarchy as attempts were made to more finely subdivide (and recognize) intervals of geologic time.

TABLE 1.1  Conventional Hierarchy of Formal Chronostratigraphic and Geochronologic Terms

aIf additional ranks are needed, the prefixes sub- and super- may be used with these terms.

bSeveral adjacent stages may be grouped into a superstage.

Hedberg (1976:69–70) considers chronozone and chron to be members of the formal hierarchy. Salvador (1994:83–84) treats them as formal but nonhierarchical units.

Stratigraphers are increasingly concerned with identifying and using stratigraphically and temporally shorter intervals and use an increasingly sophisticated and refined set of analytical and procedural tools to further those goals. But the first step is to clearly separate physical and tangible units from purely inferential and intangible ones. At one extreme is the lithostratigraphic base on which all other stratigraphic endeavors must be founded. At the other extreme are geochronologic (geologic time) units that are explicitly intangible and inferential. To varying degrees, biostratigraphic units (based on the physical disposition of fossils in the rock) and chronostratigraphic units (sections of rock that document intervals of geologic time) are both physical (tangible) and inferential (intangible), as summarized in table 1.2 and as discussed more fully later in this chapter. In brief, lithostratigraphic units provide the objective physical framework for geologic data; biostratigraphic units provide the objective physical framework for paleontologic data; chronostratigraphic units give a physical, stratigraphic record of the passage of time, drawn in large part from biostratigraphic information; and geochronologic units are intangible representations of the intervals of time contained in chronostratigraphic units.

TABLE 1.2  Independence of Lithostratigraphic Units From and Potential Relationships Between Biostratigraphic, Chronostratigraphic, and Geochronologic Units

aPhysical unit; descriptive, based on lithological characteristics without regard to age of deposition. A formation may be lithologically heterogeneous or homogeneous. A member usually is lithologically more homogeneous and may be interpreted as to lithogenesis. A bed commonly is of limited thickness and is at least as homogeneous lithologically as a member. A horizon is of very limited (conceptually zero) thickness but is a traceable marker.

bFundamentally a physical unit, descriptive of the occurrence of fossils in their stratigraphic context. Procedurally independent of other stratigraphic units, biostratigraphic units can be developed for their biochronological significance and ultimately transformed into the paleontological basis for chronostratigraphic units (see text). The biozones most useful in chronostratigraphy are the taxon-range zones and lineage zones that describe the stratigraphic range of a single taxon without regard to sampling factors (e.g., abundance). The interpretive aspect is the subjective identification that the specimens on which the zone is based pertain to a given paleospecies. Thus if Lineage zone α is followed stratigraphically by Lineage zone θ, the lower boundary of θ may depend on an arbitrary decision on the part of the stratigrapher. Thus to some extent the stratigraphic range of each biozone has an interpretive aspect.

cThis is a physical unit in that it is the rock deposited during an interval of geologic time. It is conceptual in that the means by which the unit is recognized (most commonly fossils) are presumed to have a temporal component that is unique. Once a chronostratigraphic (time–rock) unit is created, the corresponding geochronologic (geologic time) unit of equal rank is thereby defined (Table 1.1). The chronostratigraphic unit (e.g., Ypresian Stage) is defined in a given type section or reference sections, and stratigraphic sequences in other areas are referred to this stage based on having sufficient defining or characteristic criteria (usually fossils) to warrant such a correlation. Chronostratigraphic units are the fundamental means for building a time–rock record that accounts for all of geologic time that has neither overlaps nor hiatuses. In contrast to geochronologic units, chronostratigraphic units are limited by the rock record. The chronozone is the basal element of the hierarchy (Table 1.1).

dThis is a conceptual and intangible unit that stands for an interval of geologic time. It is not a stratigraphic unit, even though it may correspond to the time span of a stratigraphic unit. Thus one may speak of events that transpired during the Ypresian age without reference to a specific section of strata. The chron is the basal element of the hierarchy (Table 1.1).

Lithostratigraphic Units   With regard to lithostratigraphic units, present North American and international codes and guides are consistent in separating the concepts and operations of lithostratigraphy as distinct from those dealing directly, or potentially, with biostratigraphy, chronostratigraphy, or geochronology. This stems from the conviction that the basic physical and descriptive framework for historical geology should be separate from any interpretive concepts or operations. Schenk and Muller (1941) clearly articulated this principle.

Lithostratigraphic units are bodies of rock, bedded or unbedded, that are defined and characterized on the basis of their observable lithologic properties (Salvador 1994:31). The objectively observed lithologic criteria are paramount in establishing a lithostratigraphic unit, regardless of age. Thus whereas fossils can be recognized as an identifying component (e.g., a coquina), they are treated as lithologic properties similar to kinds of rocks, minerals, and the like.

A primary purpose of lithostratigraphic units is to demonstrate a physical framework at a level pertinent to the study at hand, not always necessitating the construction of a geologic map. The typically mappable unit is the formation (table 1.2), but other and generally thinner but not necessarily areally less extensive units may be used. Examples of the latter include air-fall or ash-flow tuffs, debris flows, or other (usually thin, measured in meters or less) beds of distinctive lithology relative to those above or below. Whereas formations or other units may be homogenous lithologically, others may be differentiated by being lithologically heterogeneous in contrast to those above and below. Also, it is convenient if the boundaries of the lithologic unit are sharp and unambiguously detected, but in other cases boundaries may be gradational. As Schenk and Muller (1941:1424) point out, boundaries of lithologic units commonly are chosen at unconformities, across which trenchant changes in lithology may be observed. These authors further assert that this is in distinct contrast to the goals of time stratigraphy, in which it is desirable to have a setting in which deposition was effectively continuous, especially at the boundaries between the units. For lithostratigraphic units, the basic issue is developing a physical stratigraphic framework that is empirically constructed and reliably replicable in the district under study. Here, and for other units, base defines boundary. Hedberg (1976) and Salvador (1994) summarize the need to specify stratotypes or type localities of lithostratigraphic units.

Biostratigraphic Units   As given in Salvador (1994:53), biostratigraphic units (biozones) are bodies of rock strata that are defined or characterized on the basis of their contained fossils. As summarized in tables 1.2 and 1.3, biostratigraphic units are material, physical units (also Walsh 1998:163). Determining the base of the unit in places other than the stratotype is only as valid as its definition. As discussed later in this chapter (see Definition and Characterization), the best definition is based on the lowest stratigraphic occurrence of a single taxon. Whereas some biozones illustrate variations in abundance of paleospecies, others clearly are intended for use in correlation, including the development of chronostratigraphic units. Some biozones are based on the record of single taxa, others on the occurrence of several.

The reason for establishing biostratigraphic units is to develop an empirical record of taxonomic occurrence in the rock record. Whether codified as a given category (table 1.3) or not, the pattern of biostratigraphic information that may be constructed forms an empirical framework parallel in concept to the development of lithostratigraphic information. Both frameworks are conceptually independent of other kinds of considerations, such as time or ecology, and on this basis can form the legitimate foundation from which those other considerations may be developed. Regarding biostratigraphy, the species from which biozones are described have a distinct, limited sojourn in geologic time. Once described and found to be replicated geographically and consistently with respect to geologically isochronous markers, biostratigraphic data can be interpreted for their temporal significance and form the basis for defining and characterizing chronostratigraphic units (table 1.2). Thus biozones are basically descriptive units but also have the potential for temporal interpretation. This aspect of biostratigraphy is taken up later in this chapter (The Role of Biochronology).

Salvador (1994:57–64) summarizes the various kinds of biozones. They are categorized as range zone, interval zone, lineage zone, assemblage zone, and abundance zone (table 1.3). Table 1.3 also compares biostratigraphic categories of Salvador (1994) with those of Walsh (1998) and concepts used herein. Either taxon-range zones and concurrent-range zones represent the total stratigraphic and geographic range of taxa. The taxon-range zone (range zone of Walsh 1998) is based on the range of a single taxon, whether of specific rank or greater. Its boundaries therefore are defined on the presence of the taxon in question. Thus the zone begins and ends stratigraphically with the known range of the specified taxon. The concurrent-range zone (strict overlap assemblage fossi-zone of Walsh 1998) is similar, except that its extent is defined on the shared ranges of two taxa, although other taxa can help characterize the zone. As Salvador (1994:58) notes, a succession of concurrent-range zones can have gaps or overlaps between them.

TABLE 1.3  Biostratigraphic and Biochronologic Categories

FHA; HSD, highest stratigraphic datum; LHA; LSD, lowest stratigraphic datum.

aWalsh (1998) uses the term fossizone for an equivalent concept, modified to fossilzone in Walsh (2000).

bRange zones and interval zones are each divided into a range epizone, range entozone, lineage epizone, and lineage entozone. Epizones are based on LSDk and HSDk (empirical zones based on the LSD and HSD as originally defined by Opdyke at al. (1977). Entozones are based on LSDa and HSDa, the actual occurrence in the rock record, but this is a theoretical concept in that the fossils involved have not yet been found. Even though they are used in a stratal context, LSDa and HSDa are comparable to interpretive, biochronologic units and not recognized here as a separate biostratigraphic category. Range zone of Walsh (1998) is most similar to taxon-range zone of Salvador (1994).

cRange chrons are based on FHA and LHA (first and last historical occurrence as a temporal interpretation). If a taxon is thought to have been present in a given area (FHA), it presumably would leave a potential rock record (LSDa) that, when demonstrated stratigraphically, would become an LSDk. Regardless of being oriented toward a potential rock record rather than a potential temporal record, an LSDk is interpretive to the same extent as is an FHA. Both are interpretive units. Thus entozones are not proper biostratigraphic units and are not used here. Range chron of Walsh (1998) appears most comparable to taxon-range chron as used here.

dWalsh (1998) also nominates assemblage interval fossizones and biochrons (renamed in 2000 as multiple-taxon interval fossizones and biochrons). These are not discussed further here.

eA lineage zone has strong temporal significance and approaches a chronozone (basic unit of chronostratigraphy). Once defined, however, a chronozone contains all strata that can be shown to correspond to the specified interval of time, regardless of fossil content.

fThis apparently reflects Walsh’s (1988) view that procedures leading to the development of chronostratigraphic units via the usual methods of performing a biochronologic correlation based on biostratigraphic data are irrelevant to finding geochronologic events that can be formalized as a Global Boundary Stratotype Section and Point and then used to define the boundaries of chronostratigraphic units.

An interval zone is a unit of fossiliferous rock having boundaries specified by two bounding biohorizons, although the fossil content of the zone within the interval itself is not specified. The interval zone of Walsh (1998) is comparable. This kind of zone appears to be most useful in analyzing cores of subsurface drilling but also has been used in mammalian biochronology (Archibald et al. 1987). This exemplifies the trend reflected in this book whereby mammal paleontologists strive to increase the role of biostratigraphy as applied to rock sequences bearing fossil mammals.

Lineage zones may be chronostratigraphic in character. These comprise a body of strata having specimens of a specific part of an evolutionary lineage, which could include the entire range of a given species. In that evolutionary innovations are temporally unique, this type of zone approaches a chronozone, the basic hierarchical unit of chronostratigraphy (table 1.1). To the extent that limits between species or other parts of the lineage must be interpreted by a paleontologist, there may be an intangible aspect to these zones that could have a temporal ramification (Woodburne 1987b, 1996a). A lineage zone still is empirical in that its presence is recognized solely on the stratigraphic occurrence of the taxon in question and has the additional provision that the base of the lineage defines the base of the zone, and its top is defined by the base of the descendant taxon. A chronozone based on a lineage zone theoretically would have boundaries with neither gaps nor overlaps. Walsh (1998) has no counterpart to the lineage zone.

An assemblage zone is characterized by the co-occurrence of three or more taxa that together distinguish the stratigraphic interval in which they occur from those above or below. In addition to usually being limited to specific areas or regions, the boundaries of assemblage zones are imprecise. In that the zone is based on the ranges of three or more taxa, it is possible that