Bonebeds: Genesis, Analysis, and Paleobiological Significanceby Raymond R. Rogers
The vertebrate fossil record extends back more than 500 million years, and bonebeds—localized concentrations of the skeletal remains of vertebrate animals—help unlock the secrets of this long history. Often spectacularly preserved, bonebeds—both modern and ancient—can reveal more about life histories, ecological associations, and
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The vertebrate fossil record extends back more than 500 million years, and bonebeds—localized concentrations of the skeletal remains of vertebrate animals—help unlock the secrets of this long history. Often spectacularly preserved, bonebeds—both modern and ancient—can reveal more about life histories, ecological associations, and preservation patterns than any single skeleton or bone. For this reason, bonebeds are frequently studied by paleobiologists, geologists, and archeologists seeking to piece together the vertebrate record.
Thirteen respected researchers combine their experiences in Bonebeds, providing readers with workable definitions, theoretical frameworks, and a compendium of modern techniques in bonebed data collection and analysis. By addressing the historical, theoretical, and practical aspects of bonebed research, this edited volume—the first of its kind—provides the background and methods that students and professionals need to explore and understand these fantastic records of ancient life and death.
"This is a thorough analysis of the taphonomic phenomenon known as the 'bonebed' in the stratigraphic record.. . . The emphasis of this volume is practical, making it most useful for vertebrate paleontologists, stratigraphers, and sedimentologists."—Choice
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BONEBEDS Genesis, Analysis, and Paleobiological Significance
The University of Chicago Press Copyright © 2007 The University of Chicago
All right reserved.
Chapter One A Conceptual Framework for the Genesis and Analysis of Vertebrate Skeletal Concentrations
Raymond R. Rogers and Susan M. Kidwell
The record of vertebrate skeletal concentration begins with accumulations of agnathan dermal armor preserved in lower Paleozoic rocks (Behre and Johnson, 1933; Denison, 1967; Allulee and Holland, 2005). From these relatively modest beginnings, the record soon expands to include fossil deposits that yield the more diversified skeletal remains (calcified cartilage, endochondral elements, teeth) of marine and freshwater gnathostomes (Elles and Slater, 1906; Wells, 1944; Conkin et al., 1976; Antia, 1979; Adrain and Wilson, 1994). By the Late Devonian, tetrapods had ventured into marginal terrestrial ecosystems (Campbell and Bell, 1978; DiMichelle et al., 1992), and this major ecological foray marked a dramatic increase in the potential diversity of vertebrate taphonomic modes. Relative concentrations of vertebrate skeletal hardparts are found throughout the remainder of the Phanerozoic in a wide spectrum of marine and terrestrial depositional settings (Behrensmeyer et al., 1992; Behrensmeyer, Chapter 2 in this volume; Eberth et al., Chapter 3 in this volume).
Like macroinvertebrate shell beds, which are commonly studied for paleontological, paleoecological, paleoenvironmental, and stratigraphic information (Kidwell 1991a, 1993; Brett and Baird, 1993; Brett, 1995; Abbott, 1998; Kondo et al., 1998; del Rio et al., 2001; Mandic and Piller, 2001), vertebrate skeletal concentrations, or "bonebeds," provide a unique opportunity to explore an array of paleobiological and geological questions. These questions revolve around an array of fundamental "quality of data" issues, such as the degree of time averaging recorded by skeletal material and its fidelity to the spatial distribution and species or age-class composition of the source community. How do vertebrate paleoecology and behavior translate into bone-rich deposits? What are the genetic links between local sedimentary dynamics (event sedimentation, sediment starvation, erosion) and bonebed formation? To what extent are bonebeds associated with important stratigraphic intervals and discontinuity surfaces, such as well-developed paleosols, marine flooding surfaces, and sequence boundaries, and how faithfully does the nature of the skeletal material record the "time significance" of such features?
Clearly, understanding the diverse mechanisms of vertebrate hardpart concentration-and, needless to say, having the ability to recognize these in the fossil record-is vital to accurate paleoecological and paleoenvironmental reconstructions and is also essential for the development of productive collection strategies. Moreover, the stratigraphic distribution and taphonomic signatures of vertebrate skeletal concentrations have geological significance because of their potential to provide critical insights into sedimentary dynamics, local geochemical conditions, and basin-fill history in both marine and terrestrial settings (e.g., Badgley, 1986; Kidwell, 1986, 1993; Behrensmeyer, 1987, 1988; Bartels et al., 1992; Rogers and Kidwell, 2000; Straight and Eberth, 2002; Rogers, 2005; Eberth et al., 2006; Walsh and Martill, 2006).
Our aims here are first to clarify terminology by providing operational definitions, and then, by focusing on the dynamics of vertebrate hardpart accumulation, to distill a relatively simple intuitive scheme for categorizing bonebeds genetically. Like the benthic marine macroinvertebrate record, some concentrations of vertebrate skeletal elements reflect primarily biological agents or activities, such as gregarious nesting habits (yielding bone-strewn rookeries), predation (yielding bone-rich feces), and bone collecting (e.g., hyena dens and packrat middens), whereas others are the result of predominantly physical phenomena such as erosional exhumation (yielding bone lags) and sediment starvation (yielding time-averaged attritional accumulations). In addition, some concentrations reflect single, ecologically and geologically brief events, such as mass-kill deposits, whereas others have complex formative histories recording the interplay of multiple ecological and/or geological agents and events, generally over longer periods. Genetic scenarios are considered here both from a conceptual standpoint and from empirical observations (both actualistic and stratigraphic record-based studies), and the characteristic taphonomic signatures of different genetic themes are explored.
Here we essentially follow Behrensmeyer's (1991, Chapter 2 in this volume) and Eberth et al.'s (Chapter 3 in this volume) definitions of a "bonebed," which at its most basic is a "relative concentration" of vertebrate hardparts preserved in a localized area or stratigraphically limited sedimentary unit (e.g., a bed, horizon, stratum) and derived from more than one individual. Within this broad characterization, two distinct types of bone concentrations are commonly recognized by vertebrate paleontologists: (1) "macrofossil" bonebeds and (2) "microfossil" bonebeds (see Appendix 2.1 in Chapter 2 of this volume for a comparison of terminology [Behrensmeyer, this volume]).
Macrofossil bonebeds (sensu Eberth et al., Chapter 3 in this volume) are herein considered concentrated deposits of skeletal elements from two or more animals in which most bioclasts (>75%, be they isolated elements or entire skeletons) are >5 cm in maximum dimension. Macrofossil bonebeds are known from many different facies and depositional contexts, and they occur throughout the Phanerozoic history of the Vertebrata. Classic examples include the many Jurassic and Cretaceous dinosaur quarries of the western interior of North America (e.g., Hatcher, 1901; Brown, 1935; Sternberg, 1970; Lawton, 1977; Hunt, 1986; Rogers, 1990; Varricchio, 1995; Ryan et al., 2001; Gates, 2005). Mammalian counterparts also abound and include the spectacularly bone-rich Agate Spring locality and the Poison Ivy Quarry in the Miocene of Nebraska (Peterson, 1906; Matthew, 1923; Voorhies, 1981, 1985, 1992), among many others (e.g., Borsuk-Bialynicka, 1969; Voorhies, 1969; Barnosky, 1985; Voorhies et al., 1987; Fiorillo, 1988; Turnbull and Martill, 1988; Coombs and Coombs, 1997; Smith and Haarhoff, 2004).
Macrofossil bonebeds are also known to preserve aquatic and semiaquatic animals. Examples include abundant amphibian-dominated assemblages from the Late Paleozoic of Texas and Oklahoma (Case, 1935; Dalquest and Mamay, 1963; Sander, 1987), and extensive fish-dominated bonebeds from Cretaceous deposits in Lebanon (Hückel, 1970), the Eocene Green River Formation of Utah and Wyoming (McGrew, 1975; Grande, 1980; Ferber and Wells, 1995), and elsewhere (e.g., Anderson, 1933; Pedley, 1978; Martill, 1988; Adrain and Wilson, 1994; Johanson, 1998; Davis and Martill, 1999; Fara et al., 2005). Interestingly, macrofossil bonebeds are seemingly less common in the marine realm-or at least less commonly analyzed and reported by vertebrate paleontologists-than might be expected given the great abundance and diversity of marine vertebrates and the potential for the addition of terrestrially derived skeletal material via bedload delivery and "bloat and float" (see Brongersma-Sanders, 1957; Schäfer, 1962, 1972). Well-documented examples of ancient marine macrofossil bonebeds include ichthyosaur lagerstätten from the Triassic of Nevada and the Jurassic of Europe (Ulrichs et al., 1979; Camp, 1980; Hogler, 1992). Less well documented but more common are shark-tooth beds and ecologically mixed assemblages of marine, estuarine, and terrestrial vertebrates associated with marine unconformities and surfaces of maximum transgression in Cretaceous to Neogene records (e.g., Barnes, 1977; Myrick, 1979; Norris, 1986; Kidwell, 1989; Schröder-Adams et al., 2001).
Microfossil bonebeds, which are commonly termed "vertebrate microsites" or "vertebrate microfossil assemblages" in the literature (McKenna, 1962; Estes, 1964; Sahni, 1972; Korth, 1979; Dodson, 1987; Brinkman, 1990; Eberth, 1990; Peng et al., 2001, among others), are sometimes construed as preserving the abundant remains of animals that have body masses on average 5 kg or less (e.g., Behrensmeyer, 1991). Instead of this overall body size criterion, Eberth et al. (Chapter 3 in this volume; see also Wood et al., 1988) propose that microfossil bonebeds be defined as relative concentrations of fossils where most component elements (>75%) are [less than or equal to] 5 cm in maximum dimension. This would include a variety of skeletal material (including entire carcasses) from small animals (such as frogs, salamanders, small snakes, fish, mammals, etc.) and small skeletal components or skeletal fragments from large animals (such as the teeth of crocodiles, dinosaurs, and sharks). In keeping with the operational definition of a bonebed, microfossil bonebeds should occur in a stratigraphically limited sedimentary unit, should demonstrably include the remains of at least two animals, and should preserve bones and teeth in considerably greater abundance than in surrounding strata (i.e., they should be "relatively enriched" with vertebrate bioclasts). Microfossil bonebeds have been described from many different facies (e.g., Nevo, 1968; Estes et al., 1978; Maas, 1985; Breithaupt and Duvall, 1986; Bell et al., 1989; Eberth, 1990; Henrici and Fiorillo, 1993; Khajuria and Prasad, 1998; Gau and Shubin, 2000; Rogers and Kidwell, 2000; Perea et al., 2001; Ralrick, 2004), and like macrofossil bonebeds, they occur throughout much of the Phanerozoic record in both terrestrial and marine depositional settings. Thoughts pertaining to the genesis of multitaxic microfossil bonebeds are presented later in this chapter.
Finally, "bone sands" are occasionally encountered in the stratigraphic literature and, as the name implies, consist mostly consist of sand-sized grains (0.0625-2 mm) to granules (2-4 mm) of bone. Particles are generally rounded fragments and are usually not identifiable beyond assignment to Vertebrata, but intact skeletal elements (e.g., teeth, vertebrae, phalanges, scales) and large bone pebbles are occasionally dispersed in the bone-sand matrix. Bone sands can vary in geometry from localized lenses to a really widespread sheets associated with unconformities (e.g., classic Rhaetic bone sand of Reif, 1982; SM-0 of Kidwell, 1989) and are most commonly found intercalated in marine strata.
GENETIC FRAMEWORK OF SKELETAL CONCENTRATION
Vertebrates today inhabit virtually every depositional environment, from the deepest ocean basins to mountain lakes, and they exhibit tremendous variation in life strategies, ecological interactions, and body sizes (Pough et al., 2005). Given their current distribution (global) and diversity (~50,000 extant species), the array of factors with potential to cause mass mortality or otherwise play a role in the concentration and preservation of vertebrate skeletal debris is staggering, and the possibilities only multiply when vertebrates are considered in an evolutionary context that spans more than 500 million years.
Nevertheless, a few general themes can be exploited in order to construct a genetic framework that is applicable across a broad spectrum of vertebrate occurrences, and we believe that the major formative scenarios explored here will provide general guidelines for the analysis of taphonomic history. This review is not exhaustive, however, and the reader is especially referred to the classic works of Weigelt (1927, 1989), Brongersma-Sanders (1957), and Schäfer (1962, 1972) for additional insights into an array of mortality and preservation scenarios in both terrestrial and marine settings. The reader is also referred to works by Behrensmeyer and Hill (1980), Shipman (1981), Behrensmeyer (1991), Martill (1991), Behrensmeyer and others (1992, 2000), and Lyman (1994) for in-depth considerations of vertebrate taphonomic modes and methodological approaches to the reconstruction of taphonomic history.
In keeping with the goal of developing a conceptual framework comparable to those already in existence for the macroinvertebrate fossil record ( Johnson, 1960; Kidwell, 1986, 1991a; Kidwell et al., 1986), vertebrate skeletal concentrations are grouped here according to their inferred relations to biological and physical agents (Fig. 1.1). Most vertebrate skeletal concentrations can be readily categorized as either biogenic (intrinsic versus extrinsic) or physical (hydraulic versus sedimentologic) in origin, although the potential for mixed formative histories abounds. Mass mortality assemblages that have been reworked, sorted, and reconcentrated by fluvial processes are good examples of such mixed-origin concentrations (e.g., Voorhies, 1969; Wood et al., 1988; Eberth and Ryan, 1992). Contrary to the macroinvertebrate record, diagenetic processes such as compaction and pressure solution are deemed largely inconsequential with regard to the primary concentration of vertebrate hardparts, although diagenesis is certainly a factor in long-term preservation potential, and early diagenetic prefossilization may play a role in the reworking of bones into some erosional lags (see below).
Biogenic concentrations of vertebrate hardparts are by definition produced by biological agents or events, and two general types are recognized here (Fig. 1.1). Intrinsic biogenic concentrations (Fig. 1.2) result from the "normal" activity or behavior of the vertebrates preserved in the death assemblage. Examples include concentrations that reflect gregarious behavior in life, such as colonial nesting habits, or in death, such as fatal spawning events. In some instances, the key behavior is forced by environmental conditions, specifically unusual hazards or perturbations that elicit behavior that results in a concentrated death assemblage. A prime example is drought, during which gregarious and nongregarious animals alike may modify their behavior so that their carcasses become concentrated in the vicinity of persistent food and water resources (Shipman, 1975; Haynes, 1988, 1991; Rogers, 1990; Dudley et al., 2001; among others). The key factor is that the formation of a death assemblage results ultimately from the behavior or activity of the hardpart producers and is not the result of direct action by other organisms or physical processes such as aqueous flows.
Biogenic concentrations of vertebrate hardparts also can be produced by the actions of other animals, acting on the hardpart producers (Fig. 1.3). Good examples of such extrinsic biogenic concentrations are predator-generated bone accumulations, such as bone-rich fecal masses and regurgitates (Mayhew, 1977; Dodson and Wexlar, 1979; Hoffman, 1988; Kusmer, 1990; Schmitt and Juell, 1994; Laudet and Selva, 2005; among others). Nonpredatory animals such as porcupines and packrats also concentrate vertebrate hardparts in appreciable quantities due to habitual collecting (e.g., Brain, 1980, 1981; Shipman, 1981; Betancourt et al., 1990). More rarely, intimate predator-prey associations are preserved, such as instances of fatal ingestion (or partial ingestion, see Grande, 1980; Davis and Martill, 1999) and dead carnivores with osseous gut contents (e.g., Eastman, 1911; Romer and Price, 1940; Eaton, 1964; Ostrom, 1978; Colbert, 1989; Charig and Milner, 1997; Chen et al., 1998; Varricchio, 2001; Hu et al., 2005; Nesbitt et al., 2006). This final category is somewhat of a hybrid mode of biogenic concentration (intrinsic plus extrinsic), in that it preserves the remains of the animal that generated the concentration via its typical behavior (the predator) and its ingested prey.
Animals are unlikely to engage in activities as part of their regular life strategies that result in mass mortality. However, some vertebrates, by virtue of their reproductive physiology or behavior, are predisposed to localized, multi-individual (n [greater than or equal to] 2) mortality. At least three routine life-history events linked to reproduction predictably generate localized concentrations of bones and teeth.
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Meet the Author
Raymond R. Rogers is a professor and chair of the geology department at Macalester College, St. Paul, Minnesota. David A. Eberth is a senior research scientist at the Royal Tyrrell Museum in Drumheller, Alberta, Canada. Tony R. Fiorillo is a faculty member in the Department of Geological Sciences at Southern Methodist University and Curator of Paleontology at the Dallas Museum of Natural History, Dallas, Texas.
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