J. A. FAGERSTROM Dept. Geology, University of Nebraska, Lincoln, Nebr.

Fossil Communities in Paleoecology: Their Recognition and Significance

Abstract: Fossil assemblages consist of individuals mentary aggregate, lithofacies relationships, sur- that were members of one or more ancient com- face condition of fossils, and ratio of whole to frag- munities. Four types of fossil assemblages (fossil mented shells. Replacement, leaching, and crush- communities, residual or winnowed fossil com- ing of shells are the causes of most postbunal munities, transported fossil assemblages, and mixed alteration and may profoundly influence the cri- fossil assemblages) may be differentiated on the teria for preburial alteration. basis of amount, and in some cases, cause of pre- Analysis of a fossil assemblage from the Pennsyl- burial alteration. The most damaging agents to vanian of Nebraska indicates that the specimens marine communities are predators, scavengers, and have been subject to considerable postburial crush- especially bottom currents which selectively re- ing which has reduced the significance of some of move shells. To assess the preburial alteration of a the criteria for assessing preburial alteration. None- particular fossil assemblage one must evaluate as theless, right-skewed size-frequency distributions many of the following criteria as can be reasonably for two species of fossil invertebrates give strong applied: size-frequency distributions, ratio of indication that this assemblage is a fossil community. articulated to disarticulated valves, ratio of oppo- This conclusion is supported by the fact that the site valves, dispersion of fossils, population density, elongated shells show no tendency toward parallel community density, faunal composition and alignment and the absence of primary sedimentary diversity, orientation of clastic particles (including structures indicative of deposition from strong fossils), texture, sorting, and structure of sedi- directional currents.

CONTENTS Introduction and acknowledgments 1197 Discussion of criteria . . . 1208 Types of fossil assemblages 1198 Mode of formation . . . . 1212 Assessment of preburial alteration of fossil as- Environment of deposition . 1213 semblages 1200 Conclusions . 1214 Introductory statement 1200 References cited . 1214 Biotic criteria 1200 Size-frequency distributions 1200 Figure Shell disarticulation 1203 1. Tetrahedron showing the relationship between Dispersion of fossils 1203 types of fossil assemblages 1200 Population and community density .... 1204 2. Theoretical size-frequency distributions for Faunal composition and diversity 1204 various types of fossil assemblages .... 1201 Abiotic criteria 1204 3. Summary of the history of development of Sedimentary features 1204 fossil assemblages 1207 Lithofacies relationships 1205 4. Size-frequency distributions for a sample of Condition of fossils 1205 brachial valves of Juresania nebrascensis . . 1210 Assessment of postburial alteration of fossil as- 5. Size-frequency distributions for samples of semblages 1205 pygidia and cranidia of Ameura sangamon- Importance of proper collecting 1206 ensis 1211 Evaluation of criteria 1208 A fossil assemblage from the Pennsylvanian of Table Nebraska 1208 1. Biometrical data for samples of Juresania Introductory statement 1208 nebrascensis and Ameura sangamonensis . . 1212

whether or not the fossils he is studying were INTRODUCTION AND members of the same ecological community. ACKNOWLEDGMENTS Valid paleoecological conclusions concerning During the early stages of most paleoeco- such fundamental aspects of the community logical studies the investigator is generally faced concept as structure, function, composition, with the difficult problem of determining and distribution can be made only on the basis

Geological Society of America Bulletin, v. 75, p. 1197-1216, 5 figs., December 1964 1197

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of fossil communities. Communities such as This oversimplified, twofold division of the ancient reefs, which were dominated by firmly types of fossil assemblages has been expanded attached organisms, are the easiest to recognize by Hallam (I960, p. 30-31) and by Craig and and have been the subject of many significant Hallam (1963, p. 732). In the definitions pro- studies in paleoecology. Conversely, communi- posed in these papers stress was placed on the ties dominated by loosely attached or vagrant distance of transportation and the relationship organisms commonly undergo considerable between the fossils and the enclosing sediment alteration prior to burial and therefore are the rather than on the community concept and the most difficult to recognize. Despite the degree of preburial alteration. In addition, numerous criteria that have been suggested to their terms are awkward and the definitions are aid in their identification, very few fossil com- too short to be broadly applicable in paleo- munities dominated by loosely attached or- ecology. Therefore, in an attempt to bring ganisms have been described. some uniformity and simplicity in terminology The complex history of an organism from and to describe the natural conditions for the the time of its growth of preservable hard parts formation of various types of fossil assemblages and its ultimate collection as a fossil (see John- from living communities, the author proposes son, 1960, p. 1076-1077; Ager, 1963, p. 184- the following definitions for fossil assemblage 185) may be divided into two periods, pre- and for the specific types of fossil assemblages. burial and postburial. During both periods All definitions are applicable to assemblages of numerous factors operate to alter the original fossil plants, invertebrates, and vertebrates. community structure and composition. In A fossil assemblage is any group of fossils from marine communities the most damaging pre- a suitably restricted stratigraphic interval and geo- burial factor is selective removal of loose shells graphic locality. In most studies, a fossil as- by currents. Selective leaching, replacement, semblage contains only the specimens from one and crushing of fossils are the most damaging lithologic unit at a single outcrop and generally postburial factors. Certainly all fossil assem- consists of specimens belonging to more than blages have been subject to some alteration; one species. Assemblages of just one species are the primary problem, therefore, in recognizing monotyfie fossil assemblages. fossil communities is to assess the degree and The term "fossil assemblage" has inten- cause of alteration. tionally been made very general and carries no The manuscript for this paper was critically paleoecological connotations; the definition reviewed by A. J. Boucot, R. T. Paine, M. D. merely excludes isolated specimens and fossil Picard, and J. W. Valentine. The author is collections from more than one stratigraphic grateful for their many helpful suggestions. interval and/or geographic locality. Previous J. D. Boellstorff, R. R. Burchett, and R. K. workers have used two other terms which are Pabian helped in the field and in picking the closely related to the author's concept of fossil sample for immature brachiopods and trilobites. assemblage: The project was partially supported by a grant (1) Wood and others (1941, p. 7) introduced from the National Science Foundation. the term "local fauna" which has been used extensively by vertebrate paleontologists in TYPES OF FOSSIL ASSEMBLAGES several different ways; Wood and others never Most previous workers have recognized only precisely defined the term but suggested that two major types of fossil assemblages, i.e. "life some local faunas ". . . may later acquire assemblages" and "death assemblages." As definite stratigraphic significance as members noted by Craig (1953, p. 547), a life assemblage or formations. . . ." In such cases the term (biocoenose, biocoenosis) is simply an ecological "local fauna" would be a rock-stratigraphic community (see Carpenter, 1956, p. 42; Han- term and therefore is unacceptable for an as- son, 1962, p. 50). Paleoecologists have been semblage of fossils. more concerned than ecologists with death as- (2) Fenton and Fenton (1928, p. 2-20) re- semblages (thanatocoenose, thanatocoenosis). viewed the various usages of the term "fau- Numerous definitions have been proposed nule." They reject the use of the term for a which vary considerably in their scope (Boucot, collection of fossils from a stratigraphically re- 1953, p. 25; Olson, 1957, p. 312-313; Johnson, stricted body of rock at a single locality in 1960, p. 1076; Krumbein and Sloss, 1963, p. favor of using it for ecological and paleoeco- 277); none of these definitions has received logical communities. However, because of the general acceptance. various usages and redefinitions of the term

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"faunule" and the simplicity of "fossil as- semblage most closely fits the definition of semblage," the latter has been used in the "death assemblage" as used by previous present paper. authors. The author proposes the following defini- MIXED FOSSIL ASSEMBLAGE: A mixed fossil tions for various types of fossil assemblages. assemblage is a fossil assemblage containing FOSSIL COMMUNITY: A fossil community large numbers of specimens which belonged to (Craig, 1953, p. 547) is a fossil assemblage in the same ecological community; however, the which nearly all the specimens belonged to the assemblage also contains many specimens same ecological community and are present in transported from other contemporaneous com- about the same sizes and numbers as when they munities or derived from the erosion of pre- were alive. Fossil communities have undergone existing rocks. The untransported specimens a minimum of preburial alteration; most of the may represent a fossil community or a residual fossils are found in essentially their original fossil community, the structure and com- habitats and living positions. Fossil census position of which has been significantly altered communities are special types of fossil com- by the removal and/or addition of specimens munities in which the specimens are nearly from other communities. contemporaneous and are often the result of Mixed fossil assemblages, as here defined, mass mortality. Fossil communities consist of are partially equivalent to both "remanie" several fossilpopulations; each fossil population and "mixed assemblages" as described by includes only specimens of the same species. Hallam (1960, p. 31) and Craig and Hallam In fossil census populations the specimens are (1963, p. 732). They are the same as "fossil nearly contemporaneous. assemblage" as used by Fenton and Fenton The above definition of fossil community is (1928, p. 14). equivalent to Boucot's (1953, p. 25) original DISCUSSION: The four types of fossil as- usage of the term "life assemblage." semblages discussed heretofore are highly RESIDUAL FOSSIL COMMUNITY: A residual theoretical end members representing the prod- (or winnowed) fossil community is a fossil ucts of the complex operation of numerous assemblage in which nearly all the specimens preburial factors on ecologic communities. belonged to the same ecological community The end-member relationships between the but are not present in the same numbers and four types can be diagrammatically repre- sizes as when they were alive. A residual fossil sented by a tetrahedron (Fig. 1). Most fossil community has undergone moderate alteration assemblages will be within the tetrahedron and by preburial factors, the net effect of which will have some characteristics of each of the has been to remove selectively only part of end members; assemblages nearest the mixed- the original community. Most of the fossils are fossil-assemblage corner will predominate. found in essentially their original habitats The importance of determining to which of and living positons. Residual fossil census com- the four types a particular fossil assemblage munities, residual fossil populations, and residual belongs depends largely upon the purpose for fossil census populations are analogous to their which the fossils were collected. Fossil com- counterparts defined heretofore under "fossil munities are the most desirable type of as- community." semblage for paleoecological studies concerning Residual fossil communities, as here defined, all aspects of the community concept. Residual are the same as "life assemblage" as used by fossil communities may yield considerable in- Boucot and others (1958, p. 323). formation about community composition and TRANSPORTED FOSSIL ASSEMBLAGE: A trans- distribution, if the degree of alteration is not ported fossil assemblage is a fossil assemblage in too great. Both transported and mixed fossil which nearly all the specimens have been assemblages are of more limited value in subject to preburial transportation and there- paleoecology, but they may yield useful in- fore may have been derived from more than one formation regarding such abiotic aspects of contemporaneous community. Almost none of ancient environments as current directions, the fossils are found in their original habitats sedimentary source areas, and energy conditions or living positions. Transported fossil as- at the time of deposition. semblages are indicative of maximum pre- In much the same manner that ecologists burial alteration of one or more ecologic distinguish between major and minor com- communities. munities, paleoecologists may eventually be This definition of transported fossil as- able to distinguish between major and minor

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fossil communities. For this distinction, it is size-frequency histograms is one of the most apparent that a single fossil assemblage can useful criteria for determining the mode of lead only to the recognition of one minor fossil formation of fossil assemblages. The following community. Each minor fossil community is a discussion of the form and significance of consists of several fossil populations common to the characteristic size-frequency distribution other minor fossil communities. These more for each of the four types of fossil assemblage. ubiquitous fossil populations will provide the FOSSIL COMMUNITIES: Size-frequency distri- essential unifying aspect that characterizes butions based upon a direct census of the major communities and will therefore be most standing crop of many species of perennial useful in determining the vertical and lateral marine invertebrates are right-skewed (Per- limits of major fossil communities. cival, 1944, Fig. 1; Rowell, 1960, Figs. 1, 2) as All types of fossil assemblages are useful in biostratigraphy except for some mixed fossil Fossil Mixed assemblages containing specimens redeposited Community Assemblage from pre-existing rocks. Even assemblages of this type can be useful if the redeposited specimens can be identified and eliminated from biostratigraphical considerations.

ASSESSMENT OF PREBURIAL ALTERATION OF FOSSIL ASSEMBLAGES Introductory Statement Up to this point the discussion has con- cerned general principles for use in the study of fossil assemblages that may have lived under Residual a variety of conditions, such as marine, fresh Fossil Community water, terrestrial, etc. The remainder of this paper primarily concerns assemblages of shal- Transported low water, marine invertebrates, especially Assemblage brachiopods; with proper modification many Figure 1. Tetrahedron showing the relation- of the concepts may be used with other sorts of ship between types of fossil assemblages. assemblages. Fossil assemblages located at the corners of All fossil assemblages have been ultimately the tetrahedron are rare; most assemblages derived from living communities; the differ- will be nearest the mixed-assemblage corner. ences between the various types are simply the result of differences in the amount of a result of the interaction of high natality alteration they have undergone. Alteration of and infant mortality rates (Fig. 2, curve 1). communities may take place either prior to Such size-frequency distributions are similar in burial or after burial of the shells of the dead shape to the survivorship curves that are individuals. Both preburial and postburial known for a few species of marine inverte- alteration must be evaluated before the mode brates (Deevey, 1947, p. 312; Deevey, 1950, of formation and the paleoecologic significance p. 58-59; Paine, 1963, Fig. 17) and have been of a fossil assemblage can be determined. used as a criterion for the recognition of fossil Several criteria have been suggested as communities (Boucot, 1953, p. 26-31; Veevers, aids for distinguishing between life and death 1959, p. 891). assemblages of marine invertebrates (Boucot, Among marine invertebrates, highest values 1953, p. 26-35; Johnson, 1960, p. 1079-1082). for infant mortality rates characterize those With proper modification most of these aids species with large numbers of planktonic can be used to discriminate between the four larvae; those species which are viviparous, or, types of fossil assemblages described heretofore. in which the earliest development takes place The following criteria bear some relationship within egg cases, commonly have somewhat to the mode of formation of fossil assemblages. lower rates. The effect of this additional parental care on the form of size-frequency Biotic Criteria distributions is to lower the absolute frequency Size-frequency distributions. The shape of in the smallest size classes and to move the

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position of the modal class somewhat toward negative-sloping curve from the modal class the larger sizes (Fig. 2, curve 2). However, to a few individuals in the largest class. the over-all shape of the size-frequency distri- Fossil communities are also characterized bution will remain right-skewed, because by a very wide range in size of individuals, highest mortality rates occur at the termination and the modal class must contain relatively of parental care or emergence from egg cases. large numbers of small specimens. Right- Significant variation in the shape of size- skewed distributions, in which the modal class frequency distributions may occur among is not located among the very small specimens, populations of the same species. Rudwick are not indicative of fossil communities. In (1962, p. 332-334) discovered that the size of practice, taxonomic difficulties may arise in obtaining right-skewed distributions for some fossil communities containing small individuals of several closely related species. In such cases the necessary characters for valid taxonomic discrimination may not be present in very small individuals. Size-frequency distributions based on parts of more than one fossil popula- tion may be merely distorted and still retain their right-skewed shape, or, they may be altered to another shape, such as bimodal, and thus become the basis of false paleoecologi-

CT cal conclusions. O) Veevers (1959, p. 891) suggests that there may be an analogy between the right-skewed, polymodal size-frequency distributions for many recent perennial invertebrate species and between distributions of the same general o Size - shape in fossil populations. Craig and Hallam Figure 2. Theoretical size-frequency dis- (1963, p. 732-736) described an example of the tributions for various types of fossil as- use of polymodal distributions for Mytilus edulis semblages. Curves 1 and 2 indicate two dif- in studying change in population structure over ferent fossil populations in a fossil com- a 2-year period. Rudwick (1962, p. 332-334) munity. Curve 3 indicates either a residual also used size-frequency distributions to inter- fossil community or a transported fossil as- pret the age structure of a population of semblage. Curve 4 indicates a residual fossil Terebratella inconspicua. Theoretically, how- community dominated by very large in- ever, in a fossil population composed of numer- dividuals. ous generations, and one in which death has been reasonably uniform, it seems doubtful the area sampled and seasonal differences in that size-frequency distributions would be natality and mortality rates may produce expected to have secondary modes indicative marked differences in the shape of the dis- of well-defined age classes (cf. Hallam, 1961). tributions. For perennial species having rel- For example, if the four size-frequency distri- atively high growth rates and discrete spawn- butions illustrated by Craig and Hallam (1963, ing periods, distributions based on samples p. 734) are combined into one distribution, collected just prior to spawning will have the and the class interval changed to 3 mm, there modal class among larger sizes than will distri- is a primary mode at about 35 mm and a butions made just after an especially successful minor secondary mode at about 10 mm. The spatfall. over-all shape of this combined distribution Forrest (1963) recently reviewed the evi- most closely resembles a slightly right-skewed dence for constant mortality rates for Hydra normal curve. The secondary mode at 10 mm and supported the conclusion of previous is greatly reduced in appearance from Craig authors that this genus has a constant negative- and Hallam's Figures lb and Ic; with the sloping survivorship curve. If such a sur- inclusion of additional generations, this sec- vivorship curve also characterizes the living and ondary mode would presumably disappear. fossil Zoantharia, the expected size-frequency Despite the theoretical usefulness of size- distribution would contain large numbers of frequency distributions in recognizing fossil individuals in the smaller sizes and a constant communities, the shapes of the distributions are

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strongly influenced by the fact that natality, populations subject to current activity are of larval settling, mortality, and local occurrence this shape. of natural populations are neither random Residual communities dominated by very nor uniform. These aspects of marine ecology large individuals result from the activity of are among the most serious obstacles in the currents of high competence and sorting collection of precise data on age-specific efficiency. The shape of the size-frequency dis- mortality. Until more work of this type has tribution in such cases may depart from the been done, the usefulness of size-frequency usual bell-shaped curve and be right-skewed distributions in paleoecology is seriously instead (Fig. 2, curve 4; see also Craig and limited. Hallam, 1963, Fig. la). Such a curve is some- Another feature that may be of some use in what reminiscent of fossil communities. How- recognizing fossil communities is their char- ever, the location of the mode easily dis- acteristically large values for the standard tinguishes these two types of fossil assemblages. deviation and for the coefficient of variation TRANSPORTED FOSSIL ASSEMBLAGES: Pre- for various measures of size. Simpson and others burial transportation of loose shells by strong (1960, p. 91) noted that in "homogeneous" currents is a common phenomenon along the samples of living mammals, the coefficient of shores and in the shallow portions of the seas. variation is generally between 4 and 10. Values Shell pavements on storm beaches are examples greater than 10 may mean that the sample is of transported shell assemblages. In some cases heterogeneous with respect to ontogenetic age, the shells in transported assemblages have been an important characteristic of living popula- derived from more than one community. tions. The interaction of the numerous factors RESIDUAL FOSSIL COMMUNITIES: Large areas determining whether or not a loose shell will of the sea floor are swept by relatively weak be moved has not been studied in detail. currents of low competence. In these areas the Preliminary investigation, however, indicates small empty shells of the uncemented ben- that the hydrodynamic properties of the shells, thonic and pelagic species are removed by velocity of the currents, and nature of the sub- currents; the residue consists mostly of large stratum are important factors in this regard shells which, upon burial, become residual fossil (Menard and Boucot, 1951). During trans- communities. The effect of this selective re- portation, the currents tend to size-sort the moval of large numbers of small, empty shells shells to produce bell-shaped size-frequency is to change right-skewed size-frequency distri- distributions for most of the transported butions of unwinnowed populations to distri- species (Fig. 2, curve 3). Similar distributions butions that are bell-shaped or normal (Fig. result from the transportation and redep- 2, curve 3). The location of the mode depends osition of assemblages of living invertebrates largely on the competence of the current. (Paine, 1963, p. 200-204). Neither ecologists nor paleoecologists have The precise shape of the distribution and previously made a formal distinction between the location of the mode will depend upon the current-winnowed communities and com- interaction of the transportation factors listed munities that have not been winnowed. How- heretofore, the size-frequency distribution of ever, in view of the significant difference in the shells in the source area, the distance of trans- shape of the size-frequency distributions for port (Craig and Hallam, 1963, p. 736-738), these two types of communities, the distinction the competence and sorting efficiency of the seems justified. In addition, the literature con- currents, and the length of time during which tains examples of distributions of populations the currents operated. In the same transported from communities which, judging from the fossil assemblage, then, the shape of the size- collecting data, must be winnowed. Paine frequency distributions would generally differ (1962, p. 597-598) and Craig and Hallam for every species present. In most cases, the (1963, p. 732-736) presented size-frequency shape of the size-frequency distribution alone distributions of marine invertebrates collected cannot be used to distinguish between residual in the intertidal zone; these populations are fossil communities and transported fossil as- thus swept by currents at each change of the semblages; bell-shaped curves will commonly tide. The size-frequency distributions for the result for both types. species investigated by these authors are MIXED FOSSIL ASSEMBLAGES: Fossil assem- roughly bell-shaped and confirm the prediction blages of this type consist of an extremely of Boucot (1953, p. 30) that the distributions of heterogeneous mixture of specimens of highly

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diverse predepositional histories. Many of the that the entire assemblage consisted of immature specimens have not undergone any transporta- individuals. tion, others may have been carried to their In practice, the use of size-frequency dis- final site of deposition from numerous source tributions to distinguish between actual communities, and some specimens may even dwarfing and current-sorted diminutive as- have been eroded and redeposited from pre- semblages simply by inspection may prove existing rocks. Under such conditions of ac- difficult if not impossible. However, a few cumulation, the size-frequency distributions simple statistical measures may be of some for the various species are likely to be highly value. Size-frequency distributions from cur- irregular or even bimodal or polymodal, de- rent-sorted samples should show a high level pending upon variations in the competence of probability of having been drawn from a and sorting efficiency of the transporting normally distributed universe when tested by currents. the chi-square procedure (Simpson and others, DISCUSSION: Before size-frequency histo- 1960, p. 306-310), whereas right-skewed dis- grams and polygons can be properly interpreted tributions should have positive values for the for paleoecologic purposes, it is important that coefficient of skewness (Simpson and others, they be carefully prepared. Simpson and 1960, p. 143). others (1960, p. 49-52) illustrated the effect Shell disarticulation. The ratios of articu- of changing class intervals and class boundaries lated to disarticulated shells and the ratio of on the shape of size-frequency distributions. opposite values are so closely related that in Likewise, the shape may depend on the the paleoecologic investigation of most fossil variate measured. Assuming the sample is assemblages they would be treated together. sufficiently large to smooth out any gross The significance of both criteria has been irregularities, it may happen that, in those cases discussed by Boucot (1953, p. 33-31) and where more than one variate (i.e., length and Johnson (1960, p. 1081), and illustrative ex- width) can be measured, and where growth is amples presented by Boucot and others (1958, allometric, the shapes of the distributions may p. 323-329), Johnson (1962, p. 125-126), and differ. Amsden and Ventress (1963, p. 26-33). Although the occurrence of "dwarfed" or Since fossil communities have undergone a "stunted" fossil assemblages is not common, minimum of preburial alteration, the value of they nonetheless present some possible theoreti- the ratio of articulated to disarticulated cal uses of the variously shaped size-frequency shells for fossil communities should be higher distributions. Cloud (1948, p. 57) lists three than the ratio for any of the other types of types of diminutive brachiopod assemblages as fossil assemblages. The values of the ratio follows: for transported fossil assemblages should be lower than that for all other assemblages, ". . . (1) actual dwarfing due to physiological and the ratio for residual fossil communities retardation of growth, for whatever reason; (2) ac- and mixed fossil assemblages would be expected cumulations of immature specimens of normally to have intermediate values. larger species and adults of smaller species due to In fossil communities the populations of segregation by moving waters; and (3) accumula- bivalves should contain nearly equal numbers tion of immature specimens because of environ- mental factors tending to produce sub-normal life- of opposite valves among the disarticulated span or exceptionally high mortality of the younger shells; that is, the ratio of opposite valves shells." should be about 1. However, since the hy- drodynamic properties of the two valves are Case 1 is clearly a special type of fossil com- very different for most species of brachiopods munity, and the included fossil populations and for some pelecypod species (Martin-Kaye, should have right-skewed size-frequency dis- 1951, p. 434), the numbers of opposite valves tributions. Case 2 is a transported fossil as- in transported assemblages should be very un- semblage, and the size-frequency distributions equal. The ratio of opposite valves in residual for the included species should be bell-shaped. fossil communities and in mixed fossil assem- It is not readily apparent just what shape size- blages should have intermediate values. frequency distribution would result from case Dispersion of fossils. The dispersion of 3; a distribution similar to either of the above fossils refers to the horizontal and vertical examples seems possible. The main criterion for distribution of individuals belonging to the the recognition of case 3 would be evidence same fossil population. This criterion has some

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potential for the recognition of major fossil major groups of extinct organisms (e.g., communities, but, owing to the lack of eurypterids, early agnathans) is well-known to detailed information regarding the dispersion students of paleontology and attests to the of recent marine invertebrate populations, limitations of this criterion for the recognition the dispersion of fossils has not been used with of fossil assemblages from older rocks. much success in paleoecology (see Johnson, One important aspect of faunal composition 1960, p. 1082; 1962, p. 126-127). is faunal diversity or the number of species Population and community density. Popu- present in a community. Well-established lation density is the horizontal and vertical marine communities are characterized by a density of individuals of the same species; diversity of organisms proportional to the community density is the density of indi- number of niches available, and they include viduals of all species in the same community. individuals adapted to a variety of habitats. In fossil communities and residual communities, Fossil communities and residual fossil com- both population and community density munities are characterized by an assemblage of would be expected to approximate the density organisms known to have been members of the of individuals in comparable recent populations same community, i.e., they are ecologically and communities. compatible. Such communities of marine in- Although there is some data regarding vertebrates may contain individuals adapted density in a few populations of recent marine to a variety of niches and habitats (e.g., invertebrates (Percival, 1944, p. 2-5; Mattox, producers, consumers, bethonic, pelagic); fossil 1955; Paine, 1962, p. 598-599; 1963, p. 203), communities should show somewhat more the fact that most populations have a clumped diversity than residual fossil communities. distribution (Odum and Odum, 1959, p. Transported and mixed fossil assemblages 213-215; Rudwick, 1962, p. 330) makes the would be expected to contain ecologically in- use of these criteria in paleoecology extremely compatible individuals which may have oc- difficult. Furthermore, the use of population cupied an extremely wide variety of niches and community density in the interpretation and habitats (cf. Dunbar and Rodgers, 1957, of data from three-dimensional rock samples is p. 136-137). Conversely, however, sorting complicated by the fact that density may be during transport tends to segregate specimens primarily a function of the rate of sedimenta- according to their hydrodynamic properties tion (Bradley, 1957, p. 670-678); density in a rather than their ecologic incompatibility and, community may also change through time in extreme cases, may produce monotypic fossil (Lowenstam, 1957; Nicol, 1962). Despite these assemblages (van Straaten, 1956, PI. 1C; difficulties population and community density Ager, 1963; PI. 8B). have been used with notable success to de- scribe the distribution of fossil species in Abiotic Criteria several paleoecological studies (Eager, 1952, Sedimentary features. The criteria included p. 359-361; Imbrie, 1955, p. 655-669; Johnson, under this heading are orientation of clastic 1962, p. 125-127; Fox, 1962, p. 634-639). particles (including fossils) and texture, sorting, Fauna! composition and diversity. Faunal and structure of the sedimentary aggregate. composition refers to the well-known fact that These criteria primarily concern features of the modern communities are characterized by the rocks, rather than the fossils and thus reflect joint occurrence of several species as a result of abiotic factors in ancient sedimentary environ- similar environmental preferences and tolerance ments. Transported and mixed fossil assem- limits. These characteristic joint occurrences blages commonly occur in coarse-grained, well- provide a basis for recognizing and naming sorted clastic rocks having current ripples, communities (Thorson, 1957, p. 505-521). cross-bedding, and other primary sedimentary The criterion of faunal composition has been structures indicating deposition from moving used with great success in the recognition of water. In fossil assemblages that have been sub- fossil assemblages at the species level for the ject to currents, the elongate clastic particles Pleistocene (Johnson, 1962), with moderate (both shells and mineral fragments) often show success at the genus level for rocks as old as a preferred alignment parallel to the direction Miocene (Smith, 1959), and as the valid basis of the currents. There are numerous exceptions for only broad generalizations in Paleozoic to this condition. Elongate shells at the strand and Mesozoic rocks. Paleoecologic uncertainty line commonly occur with their long axes over the environmental preferences of some parallel to the shore, rather than parallel to the

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backwash currents. Similarly, oscillatory cur- shells of both predator and prey and drilled rents just prior to final burial may destroy the holes or pits in the shells of the prey or host. alignment produced by earlier directional cur- Attempts should be made to establish prey- rents. Van Straaten (1952, p. 503-504) dis- predator relationships within fossil communities covered that in recent shell assemblages that and to assign percentage mortality by size or have been subject to currents, more than half age classes resulting from predation for in- of the bivalve shells occur with the concave dividual species. Extreme selective predation or surfaces downward. scavenging could alter the form of size-fre- Fossil communities commonly occur in poor- quency distributions, could cause the dis- ly sorted, fine-grained clastic rocks and carbon- articulation of nearly every bivalve, or could ate rocks lacking sedimentary structures and cause the fragmentation of one valve of some alignment of elongate particles indicative of species. Some hosts or predators may even be deposition from moving water. Rocks contain- agents of transportation (Mattox, 1955, p. 75). ing residual fossil communities will be inter- Throughout this discussion the author has mediate with regard to each of these features. not mentioned those important fossil assem- Lithofacies relationships. Lithofacies studies blages that are dominated by pelagic organ- may yield useful information for the interpre- isms. Following death, the skeletons of pelagic tation of fossil assemblages. For example, lime- organisms begin their descent to the bottom; stone lenses in shale, indicative of locally re- during this time, they may be carried laterally duced turbulence, are likely sites for fossil by currents great distances from the habitat the communities; shoestring sandstones, deltaic organism occupied when it was alive. The a- deposits, and reef-flank debris mostly contain mount of lateral transportation prior to reach- transported and mixed assemblages. ing the sea floor is nearly impossible to deter- Condition of fossils. As a result of preburial mine and so can rarely be used in paleoecologic transportation of shells, it has generally been investigations. assumed that the surface will become scratched Once the skeleton of a pelagic organism and pitted; delicate structures such as spines, reaches the sea floor, it may be further trans- brachidia, and teeth will be broken, and many ported by bottom currents and subjected to all of the smaller and thin-shelled individuals will the processes described above for the formation become at least partially fragmented. of transported and mixed fossil assemblages. Data presented by Menard and Boucot Conversely, the skeletons may not be subject (1951) suggested to Elliott (1956) that perhaps to horizontal movement either before or after too much emphasis had previously been placed reaching the bottom, but they may, instead, on these criteria and that many transported and accumulate essentially as fossil communities mixed fossil assemblages showed no evidence of satisfying the same criteria as fossil communities abrasion or breakage. The fragmentation of of benthonic organisms. Because the skeletons shells during transportation has been studied of most pelagic organisms are light and fragile, quantitatively by van Straaten (1952, p. 513- transported fossil pelagic assemblages are prob- 515; 1956, Tables 1, 2). His data indicate that ably far more common than fossil pelagic fragmented shells of the pelecypod Cardium communities; residual fossil pelagic communi- edule are far more numerous in areas of rapidly ties must be very rare indeed. The joint oc- moving water (beaches, banks, and channels) currence of benthonic and pelagic organisms in than on the surface of tidal flats. The conflicting the same fossil assemblage may be evidence nature of the evidence cited by Elliott and van that the assemblage has not been subject to Straaten suggests that, on the basis of present important alteration by bottom currents. The knowledge, neither the surface condition of hydrodynamic properties of the skeletons of fossils nor the ratio of whole to fragmented pelagic and benthonic organisms are generally shells can be used with much certainty in de- so diverse that even very weak currents would termining the mode of formation of fossil as- be expected to segregate them. semblages. Another aspect of marine ecology that has ASSESSMENT OF received almost no attention in paleoecology is POSTBURIAL ALTERATION the fragmentation and similar alteration of OF FOSSIL ASSEMBLAGES shells by predators, scavengers, and encrusting Postburial diagenetic alteration (sensu latd) symbionts. These alterations may appear as may present great difficulty in the recognition chipped, cracked, or scratched edges of the of fossil communities; yet this aspect of paleoe-

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cology is commonly overlooked or inadequately p. 210-211) "crush-factor" so that it is doubt- assessed. The most important diagenetic proc- ful whether it can be used in paleoeceology. esses that alter or destroy fossil communities Postburial alteration of communities and are leaching and replacement of shells by in- their enclosing sediments as a result of the trastratal solutions and crushing of shells by activities of scavengers and microorganisms compaction of the enclosing sediments. living below the sedimentary surface is another Leaching and replacement are often highly important aspect of paleoecology. Several types selective so far as size, shell structure, and of marine organisms ingest sediments thereby composition are concerned. Thus small, thin altering such sedimentary features as the tex- calcareous shells are more likely to be leached ture, sorting, structure, and orientation of or replaced than large, heavy phosphatic clastic particles (see Dapples, 1938, p. 55-61; shells. Partial leaching may completely remove 1942, p. 120-121; Moore and Scruton, 1957, p. many of the small shells from what was 2741-2743; Ginsburg, 1957, p. 81-89). In addi- originally a population of articulate brachio- tion to these alterations of the sediments them- pods and leave unaltered a nearby population selves, certain marine "worms" may have pro- of inarticulate brachiopods. Similarly, partial found effects on the occurrence of shells of re- replacement of a fossil assemblage by silica cently dead animals contained within the sedi- may result in a disproportionately large ments. Van Straaten (1952, p. 507-508; 1956, number of complete small shells indicative of PI. 2) ascribed the concentration of shells in fossil communities, whereas the original as- Hydrobia beds to the ingesting activities of the semblage may have been a residual fossil com- lugworm Arenicola marina. Various types of munity or a transported or mixed assemblage. marine microorganisms, especially bacteria, The susceptibility of shells to crushing during inhabit the upper portion of the bottom sedi- diagenesis is also likely to be quite selective and ments and according to Zobell (1942, p. 132), depends, in part, upon the ontogenetic stage of "... are the principal Press dynamic agents development reached by the individual at which are responsible for changes in the hydro- the time of death. Thus small, thin, unribbed, gen ion concentration in sedimentary mate- convex shells will crush more easily than large, rials." By lowering the pH, bacteria may be re- thick heavily ribbed flat shells. This sort of sponsible for altering or even completely re- selective crushing could result in altered size- moving communities and populations of cal- frequency distributions indicative of residual careous-shelled animals. Other types of bacteria fossil communities or transported fossil as- may control the occurrence of the chitinous- semblages. shelled invertebrates (Zobell and Rittenberg, In addition to altering the shape of size- 1938; Hock, 1940). frequency distributions, selective leaching, re- It is apparent that living organisms of a placement, and crushing will influence several great many types can alter the structure and other criteria for the recognition of fossil com- composition of living communities both prior to munities. The observed size range will be and after burial. Assessing the degree and cause reduced either by elimination of the small or of such alteration in terms of the effect on the the large individuals; reduction in size range criteria for determining the mode of formation may also lower the values for the coefficient of of fossil assemblages is extremely difficult if not variation. Diagenesis may change the dispersion impossible in some cases. Nonetheless, failure to of fossils and reduce both population and com- estimate the alteration of communities by or- munity densities. Leaching and replacement ganisms, leaching, replacement, and crushing may produce pits and holes in the surface of may invalidate many otherwise sound paleoeco- shells. logic conclusions. Perhaps the most important limitation re- sulting from diagenesis involves the ratio of IMPORTANCE OF whole to fragmented shells as a criterion for the PROPER COLLECTING recognition of fossil communities. Leaching, Use of the above criteria depends upon careful replacement, and crushing will all reduce the observations and techniques in the field as well value of the ratio so that many fossil com- as in the laboratory. Criteria such as the disper- munities may be destroyed beyond recognition; sion of fossils, orientation of clastic particles, thus they may resemble transported and mixed texture, sorting, and structure of the sedimen- assemblages. Diagenetic crushing severely tary aggregate, and lithofacies relationships are limits the importance of van Straaten's (1956, primarily based upon field observations; the

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other criteria are based upon laboratory studies poorly indurated sandstones and shales can be of properly collected rock and fossil samples. boiled to free the fossils (Imbrie, 1955, p. 650- The collecting technique to be used in any 655). particular instance will depend primarily upon Fossil samples to be used for the recognition the nature and composition of the fossils and of fossil communities must be representative of their enclosing rock matrix. Studies based upon the assemblages from which they were collected well-indurated rocks from which fossils cannot with regard to the characteristics described be removed without excessive breaking are re- heretofore for determining thealteration of fossil duced to analyses of bedding-plane samples. In assemblages. As noted by Veevers (1959, p.

Transported 1 Pnccil / Assemblage I / Transported Assemblage Mixed Fossil Assemblage rr UJ

LU UJ CT O

1 Death Burial Diagenesis Weathering Collection TIME- Figure 3. Summary of the history of development of fossil assemblages. A, B, and C are contempo- raneous communities. D represents introduction of specimens from pre-existing rocks. Heavy lines indicate routes followed by most specimens. Horizontal and vertical scales are generalized.

some cases these may be essentially studies of 889-893), a sample picked at random from census-type assemblages. Techniques for bed- weathered, loose specimens on the surface of an ding-plane studies have been described by Fox outcrop is not adequate for paleoecological (1962, p. 634-635) and Ager (1963, p. 220-225); purposes. Such samples will generally not in- the "stretched-line" method (Johnson, 1960, p. clude the proper proportion of small individ- 1082-1083; 1962, p. 125) is an adaptation of uals and will result in left-skewed size-frequency these techniques for studying bedding planes in distributions. Regardless of the technique used a single dimension for use in cases where the to remove the fossils from the matrix, one must rock is either so hard or so loose that bedding be careful to collect a complete sample (Vee- planes cannot be exposed. Several authors have vers, 1959, p. 889); anything less than a com- described different methods for freeing fossils plete sample will commonly lead to false pale- from three-dimensional rock samples. Boucot oecological conclusions. (1953, p. 36—37) studied what was somewhat Figure 3 is a simplified summary of the vari- analogous to numerous, very closely spaced ous modes of formation of the four types of bedding planes. Silicified fossils are commonly fossil assemblages described heretofore. The removed from limestone by dissolving the chart is designed to show at which stages in limestone in acid (Veevers, 1959, p. 890); some the history of a fossil assemblage alteration

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may take place. Bias introduced during the interbedded, yellowish-brown, bedded shale later stages (diagenesis, weathering, and col- and light-gray crystalline limestone which lecting) is often uncertain. All fossil assem- occurs in 2-3-inch-thick, wavy lenticular beds; blages have followed a course of progressively and (3) the upper 1 foot is dark-yellowish- increasing alteration from the community of orange bedded shale with scattered light-gray •which they originally were a part up to the limestone nodules. The sample used in this time of collection. study was collected from the middle unit during the summers of 1962 and 1963. Many of EVALUATION OF CRITERIA the larger fossils had weathered free of the In view of the numerous factors that may enclosing matrix and were thus collected from alter a community from the time of death of the surface of the outcrop which extends for the individual members until their later about 500 feet along the quarry face. The small collection as fossils, it may seem doubtful that specimens were picked from a boiled 25-pound the mode of formation of any particular fossil sample of the shale. Several large slabs were assemblage can ever be determined with much collected for determining the density and certainty. Seldom will it be possible or even orientation of the fossils. feasible to collect enough data to evaluate all the criteria for every species in every as- Discussion of Criteria semblage. Neither will all the criteria in each Data pertaining to each of the criteria assemblage indicate the same mode of forma- discussed heretofore have been gathered and tion; thus each criterion must be assessed will be described and evaluated as evidence for separately. Nonetheless, the final interpreta- determining the mode of formation of the tion should depend upon the evidence from as fossil assemblage. many criteria as can be reasonably applied. FAUNAL COMPOSITION AND DIVERSITY: A In general, biotic criteria are more significant taxonomic study of the entire fauna is not than abiotic criteria for determining the mode necessary in determining the mode of forma- of formation of fossil assemblages. By them- tion of the assemblage, but the following selves the abiotic criteria can seldom give simplified faunal list will give some indication conclusive results. For younger rocks (Miocene of the composition and diversity of the as- to Recent), faunal composition and diversity semblage : are probably the most useful criteria; for pre- Bryozoa Miocene rocks, the evidence based on fossil Several species including ramose, fenestellid, and populations outweighs all others Of these encrusting forms latter criteria, the size-frequency distributions, Brachiopoda disarticulation ratio, and ratio of opposite Derby a sp. valves give the most easily interpreted results Juresania nebrascensis (Owen) for bivalve shells. For other organisms, size- Pukratia symmetrica (McChesney) Reticulatia huecoensis (King) frequency distributions will be most significant. Linoproductus sp. Composita subtilitia (Hall) A FOSSIL ASSEMBLAGE FROM THE Pelecypoda PENNSYLVANIAN OF NEBRASKA Aviculopecten sp. A nuculoid and an elongate form, both occurring Introductory Statement only as internal casts The remaining portion of this paper con- Gastropoda cerns the use of the above criteria for deter- Straparolus ? sp. mining the mode of formation of a fossil A high-spired form occurring only as internal casts assemblage from the Upper Pennsylvanian Cephalopoda Two nautiloid species—one straight-shelled, one Bonner Springs formation (see Condra and coiled Reed, 1959, p. 51) cropping out in an aban- Arthropoda doned quarry near the center of the SEJ^, Ameura sangamonensis (Meek and Worthen) SEJ4, Sec. 7, Eight Mile Grove Twp., Cass At least one species of ostracod County, Nebraska, about 1J-2 miles southwest Echinodermata of the village of Cedar Creek. At this locality Dorsal cups of at least two species of attached the Bonner Springs formation may be sub- crinoids. Crinoid columnals are only mod- divided into the following three units: (1) erately abundant, the lower 7% feet is a greenish-gray to olive, Fusulinids and coelenterates are the only blocky shale; (2) the middle 1 foot consists of major fossil groups commonly found in other

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Pennsylvanian rocks in Nebraska which have data for several other criteria rather difficult. not been identified in the sample. The absence In some cases such as determining population of these two groups is perhaps a result of the and community densities these criteria have fact that the Bonner Spings formation was been greatly reduced in their significance as a deposited in water shallower than these result of crushing. animals preferred (cf. Elias, 1937, p. 410). It Both the shale and the limestone are crowded is not possible to assess the ecological compati- with fossil debris. In view of the small size of bility of the various members of the assemblage; the fragments and their extremely large more work must be done on the faunal as- numbers, all attempts to estimate densities of sociations of the northern Mid-Continent individuals proved impossible. Nevertheless, Pennsylvanian rocks before significant con- some estimates of the relative abundance of the clusions can be made. However, in the experi- various species can be made: Juresania nebras- ence of the author, this assemblage is more censis dominates the assemblage; Aviculopecten diverse than most fossil assemblages in eastern sp. is probably next in abundance, followed Nebraska. successively by trilobites, internal casts of the CONDITION OF FOSSILS: Nearly all the fossils other two pelecypod species, and the other in the sample have been broken to some brachiopods and ostracods. Gastropods, cephal- extent; complete specimens of every species opods, and crinoids are rare. The bryozoan are rare. Samples of the shale that have been zoaria are entirely too fragmented to even boiled and decanted consist almost entirely of estimate their original importance in the as- small fossil fragments. The spines of the semblage, but they are very abundant in the productoids have nearly all been broken, but fossil debris. the surfaces of the shells are not scratched, At the locality near Cedar Creek, density of pitted, or bored. fossils is somewhat higher in the limestone than Because crushing has been such an important in the interbedded shale; this is probably the factor in the history of the assemblage, it is result of a slower rate of accumulation of lime essential that the cause and time of crushing be than mud. Many of the limestone beds are determined. If fragmentation of the shells was almost a coquina with the bedding planes the result of preburial transportation by essentially ancient shell pavements. relatively turbulent water, one would certainly ORIENTATION OF CLASTIC PARTICLES: Fossils expect a definite sorting of the fragments ac- are the only clastic particles in the Bonner cording to their hydrodynamic properties, Springs formation that are sufficiently large to i.e., size, shape, density, etc. Such is not the determine whether or not there is any ten- case, however; there is an extremely wide dency toward parallel alignment. Exam- variety of sizes and shapes among the fossil ination of bedding planes, especially the debris. Large and small specimens of the same limestone slabs, indicates that the elongate species lie side by side on the bedding planes, shells have no preferred orientation. all species are intimately mixed together, and Among the bivalves, however, there is a far relatively large fragments of delicate fenestellid greater number of productoid pedicle valves zoaria are associated with severely crushed oriented with the convex side upward than brachiopods. These aspects of the occurrence of with the convex side downward, whereas the fossils indicate that most of the fragmentation valves of Aviculopecten sp. occur with about as has come after burial and before exposure of many convex sides up as down. Since post- the rocks to weathering. There has been some burial crushing has been such an important selectivity in the crushing; large, flat, thickened factor influencing the preservation of fossils, brachial valves of the productoids are far more this observation is of very little importance common than thin, highly convex pedicle in determining the mode of formation of the valves; and complete trilobite cranidia far fossil assemblage. Instead, the occurrence of outnumber complete free cheeks bearing long, more productoid pedicle valves oriented with pointed genal spines. The original mud and the convex side upward means, simply, that lime of the Bonner Springs formation has isolated valves with this orientation are less undoubtedly undergone tremendous com- susceptible to crushing than are valves with paction to the present shale and limestone, the convex side downward. The orientation of and it was during this time that the included the valves of Aviculopecten sp. has very little fossils were crushed. bearing on their susceptibility to crushing. POPULATION AND COMMUNITY DENSITY: The SIZE-FREQUENCY DISTRIBUTIONS: Although importance of crushing has made collecting the fauna is abundant and diverse, only two

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species, Juresania nebrascensis and Ameura sangamonensis, were selected for special bi- 110 Juresania nebrascensis ometrical treatment. Size-frequency distri- 100. „ N = 298 butions were prepared (Figs. 4, 5), and some simple biometrical measures computed (Table 90. 1) for these species. Since relatively complete brachial valves of Juresania nebrascensis far 80. outnumber pedicle valves, only brachial 70. valves were measured. Strengthening of the hinge line by buttress plates, which results in o 60 c increased resistance to crushing, accounts for 0) the greater number of measurements for the S- 50j length of the hinge line than for the width of 0> the valve. it 40. In addition to Juresania nebrascensis, the 30_ sample also contains a few brachial valves of each of the other productoid species. In 20. general, specimens less than about 7 mm wide have not developed the critical morphological IQ features used to discriminate between species of productoids; thus a problem arose in being certain that only brachial valves of Juresania 03 9 15 21 27 33 nebrascensis were measured. Unfortunately, the Width of Brachial Valve author discovered no way to solve the problem (mm.) conclusively; all the very small spine-bearing A brachial valves were considered to be specimens of Juresania nebrascensis and have been included in Figure 4 and Table 1. However, because of 110 Juresania nebrascensis the very large numbers of positively identifi- able adult specimens of Juresania nebrascensis IOQ N=340 in comparison to the relatively few adult specimens of the other productoids, the bias 90- introduced by erroneously including a few 80_ small specimens of other species must be negligible; it is certainly not enough to change 70- the over-all right-skewed shape of the size- frequency distributions. 5" 60- Since the fauna contains only one species of § 50J trilobite, the problem of identifying the small CT a) specimens of Ameura sangamonensis never it 40J arose. Isolated cranidia, pygidia, and free cheeks are abundant in the sample. However, the genal 30_ spine has been broken on most free cheeks, and as a result these were not measured. 20. There is considerable discrepancy in the 10. shape of the size-frequency distributions for the pygidia and cranidia; the distribution for pygid- ia is moderately right-skewed (Fig. 5A; Table 0 2 10 14 18 22 1), whereas for cranidia, it is moderately left- skewed (Fig. 5B; Table 1). The reason for this Length of Hinge Line discrepancy is not certain, but it may result (mm.) from the fact that the sample of cranidia is B probably not representative of the universe Figure 4. Size-frequency distributions from which it was collected. Measurements on for a sample of brachial valves of large, complete specimens of Ameura sang- Juresania nebrascensis amonensis indicate that the pygidium is about

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25 per cent longer than the cranidium, whereas in the sample, there are many pygidia shorter 2CL than the shortest cranidium. The conclusion is that for some undetermined reason the sample Arneura SCwe, TIC>m 101 ?/?.SIS lacks the cranidia for those small trilobites of N = 15\\ which the pygidia are present. Thus, there is 15- very little significance to the left-skewed dis- o tribution for the cranidia and no paleoecologi- c. o> cal conclusions can be based on these measure- 3 CT 10. ments. V SHELL DIsARTICULATION: Crushing has made it impossible to determine precise values for the ratio of articulated to disarticulated shells and the ratio of opposite values, but it is pos- sible to estimate them for Juresania nebrascensis and the three pelecypod species. r n There is some bias in estimates for Juresania 0 2 4 6 8 10 12 nebrascensis because the small brachial valves and all the pedicle valves cannot be distinguished Total Pygidial Length from those of Pukratia symmetrica. Again it is (mm.) assumed that this bias is almost negligible as a result of the far greater number of adult brachial valves of Juresania nebrascensis. Of those speci- mens that are over half complete (to avoid using the same specimen twice), about 60 per cent are isolated brachial valves, about 25 per cent are isolated pedicle valves, and 25., Ameura sangamonensis about 15 per cent are articulated. These values require considerable modifica- N = IOO tion before they can be used in interpreting the 20 mode of formation of the fossil assemblage. First, the very low proportion of articulated shells is due primarily to the fact that pro- ductoid brachiopods lacked teeth and sockets to 15. hold the valves together after the animals died. u There is little reason to doubt that the activities c of scavengers and predators resulted in the dis- o> S- "0. articulation of large numbers of individuals or 0> that weak currents may have also been respon- sible for some disarticulation. Secondly, the far greater number of brachial valves than pedicle 5. valves is certainly the result of selective crush- ing. The pedicle valves of Juresania nebrascensis are very thin; have no strengthening structures such as ribs, buttresses, thickened muscle at- tachment areas, etc.; and are strongly convex, 0 2 4 6 8 10 12 making them much more susceptible to crush- Total Cranidial Length ing than the flat, thicker brachial valves. Exam- (mm.) ination of the small fossil fragments indicates that a very large proportion consists of crushed B productoid pedicle valves. Consequently, there is very little paleoecological significance to Figure 5. Size-frequency distributions for samples of pygidia and cranidia of Ameura estimates for the ratio of articulated and dis- sangamonensis articulated shells and the ratio of opposite valves for Juresania nebrascensis in the fossil assemblage.

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The limestone bedding planes are crowded of low turbulence at the time of deposition of with specimens of Aviculopecten sp. and a few the Bonner Springs. specimens of the nuculoid and elongate pelecy- TEXTURE, SORTING, AND STRUCTURE OF SEDI- pods. Most of the specimens of Aviculopecten MENTARY AGGREGATE: This criterion is far more sp. are disarticulated, the opposite valves oc- applicable to the study of fossil assemblages curring in about equal numbers, whereas the contained in coarse clastic rocks than it is to specimens of the nuculoid and the elongate shales and limestones like the Bonner Springs. species are mostly articulated. Apparently the Detailed sedimentological studies of the forma- valves of Aviculopecten sp. disarticulated very tion are beyond the scope of this report, but no

TABLE 1. BIOMETRICAL DATA FOR SAMPLES OF Juresania nebrascensis AND Ameura sangamonensis

Number of Observed size range mean Coefficient of Coefficient of Variate measurements (mm) (mm) variation skewness

Juresania nebrascensis Width, brachial valve 298 1.1-30.0 9.70 91.03 +1.42 Length, hinge line 340 0.8-22.8 7.66 83.90 + 1.35 Ameura sangamonensis Total pygidial length 121 1.7-12.0 6.31 46.59 +0.42 Total cranidial length 100 3.0-10.7 7.05 29.66 -0.77

easily and were about equally susceptible to megascopic evidence for strong currents at the crushing, whereas the valves of the other pele- time of deposition have been observed. cypod species were more firmly united by a tougher ligament. Mode of Formation LITHOFACIES RELATIONSHIPS; DISPERSION OF Determining the mode of formation of this FOSSILS: The Bonner Springs formation crops fossil assemblage requires careful consideration out in just a few localities in Nebraska; most of the paleoecological significance of each of of these outcrops are located in the vicinity the criteria described heretofore. Not all the of Cedar Creek, where the sample used in criteria are equally useful; indeed, some are of the present study was collected, and along the no use, and the significance of others is highly Platte River valley for about 4 miles west of speculative. The almost universal effects of Cedar Creek. Westward from Cedar Creek postburial crushing have been the most there is a progressive decrease in the limestone difficult obstacle for the determination of the content of the Bonner Springs formation; there degree of preburial alteration to which the is also a decrease in the abundance of fossils. At fossil assemblage has been subjected. the Stone Products Company quarry (NEJ4, On the basis of the present study the author Sec. 16, T. 12 N., R. 11 E., Sarpy County), concludes that the fossil assemblage has under- which is the westernmost outcrop of the Bonner gone a minimum of preburial alteration and is Springs in Nebraska, the limestone is reduced therefore best described as a fossil community. to a few scattered nodules, and fossils are rare. Right-skewed size-frequency distributions for In the vicinity of Omaha (SEJi, Sec. 28, T. 17 brachial valves of Juresania nebrascensis and N., R. 13 E., Washington County), about 27 pygidia of Ameura sangamonensis (Figs. 4, 5) miles north of Cedar Creek, the Bonner Springs are the best evidence to support this conclusion. is entirely shale, and no fossils have been found. Several histograms with different class bound- There are no outcrops of the Bonner Springs in aries and class intervals were prepared for Nebraska either east or south of Cedar Creek. both the width of the valve and length of the Thus, on the basis of this rather limited infor- hinge line for Juresania nebrascensis and the mation, there seems to be a close association length of the pygidium for Ameura sanga- between the occurrence of limestone and fossils monensis; all were right-skewed. Although in the Bonner Springs. The author has not many valves and pygidia were crushed and so attempted to determine the dispersion of fossils could not be used, there was no evidence of except in these very general terms. There is a size-selective crushing for either of these good possibility, however, that the occurrence species. The wide size range with large numbers of both limestone and fossils is restricted to areas of very small specimens and the extremely

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large values for the coefficient of variation Since no other suitably documented fossil (Table 1) suggest that the sample is very communities have been described from the heterogeneous with respect to the ontogenetic Pennsylvanian rocks of the Mid-Continent age of the specimens included in both species, region, the significance of faunal composition another important characteristic of fossil com- and diversity is difficult to assess. The as- munities. semblage is dominated by species of the sessile Olson (1957, p. 328-330) developed a tech- benthos (the brachiopods, pelecypods, bryozo- nique for determining theoretical size-fre- ans, and crinoids). Species of the vagrant quency distributions for various types of fossil benthos are second in abundance (trilobites, organisms. He discovered that, when he com- gastropods, and ostracods). The two relatively pared the shape of the distributions of actual rare species of nautiloids are the only pelagic samples with his theoretical curves, the shapes forms. The relative ecologic diversity of the of the theoretical curves resembled the shapes Cedar Creek assemblage, in comparison to for the samples in only 1 of the 14 cases he other fossil assemblages with which the author tested. From this, Olson concluded (p. 330) is familar in the Pennsylvanian of eastern that ". . . in the great majority of cases . . ." Nebraska, further supports the conclusion the shapes of size-frequency distributions of that this assemblage is a fossil community. fossil organisms were unrelated to the distri- The following criteria are of no value in butions of the original populations from which determining the mode of formation of the as- they were drawn. Assuming that Olson's semblage: ratio of whole to fragmented shells, techniques for estimating the shapes of his ratio of articulated to disarticulated shells, theoretical curves were correct and that his ratio of opposite valves, surface condition of samples were properly collected and sufficiently fossils, population density, community den- large for paleoecologic purposes, this conclusion sity, lithofacies relationships, and dispersion of can be restated in terms of the community con- fossils. The reasons for the lack of reliable data cept developed in this paper. Most fossil as- for use of these criteria have been described semblages have undergone preburial and/or in the section, "Discussion of criteria." postburial alteration, the effect of which has As noted heretofore much work remains to been to change the original structure of the be done on the Pennsylvanian faunal as- populations from which the fossil assemblages sociations in the Mid-Continent region, but, as have been derived. Thus, Olson's conclusion is a preliminary working hypothesis, the author not a reflection on either the validity of his suggests that this minor fossil community be theoretical curves or on the validity of size- named the "Juresania nebrascensis community" frequency distributions as criteria for the because of the dominance of this species. recognition of fossil communities; instead, it is a statement of the fact that most fossil as- Environment of Deposition semblages are not fossil communities. Study of the lithofacies relationships and Three other criteria lend support to the con- dispersion of fossils has considerable value for clusion that the fossil assemblage near Cedar determining some abiotic aspects of the envi- Creek is a fossil community: orientation of ronment in which the Juresania nebrascensis clastic particles; texture, sorting, and structure community lived. Recognition of similar en- of the sedimentary aggregate; and faunal com- vironments elsewhere may in turn lead to the position and diversity. Although elongate discovery of other minor Juresania nebrascensis shells are not abundant in the assemblage, the communities and in this manner establish the fact that there is no evidence of parallel horizontal and vertical limits of the major com- alignment indicates that directional bottom munity to which the minor communities be- currents were not moving the shells just before longed. burial. The absence of primary sedimentary The relatively restricted occurrence of fossils structures formed by moving water also sug- in what appears to be a lens-shaped unit of in- gests that currents were not active at the time terbedded limestone and shale centered in the of final burial. However, neither of these Cedar Creek area indicated that the members of criteria proves that currents were not active the Juresania nebrascensis community lived in prior to final burial; the best evidence against lime-rich water of lower turbulence and tur- preburial currents is the presence of numerous bidity rather than in surrounding areas where small shells of Juresania nebrascensis and small both fossils and limestone are not present in the pygidia of Ameura sangamonsis. Bonner Springs formation. The presence of

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maroon and black shale in the Bonner Springs (2) Several criteria may be used to determine just north of Omaha suggests that the shore the mode of formation of a particular fossil line was nearby and that the water in the Cedar assemblage, but each criterion for each assem- Creek area was probably rather shallow. blage must be evaluated separately. Deter- The regional significance of the present study termination of the mode of formation depends is uncertain since it is the first to stress the upon the assessed significance of as many crite- fossil-community approach to the interpreta- ria as can reasonably be applied. tion of depositional environments in the late (3) Size-frequency distributions may be one Paleozoic rocks of the northern Mid-Continent of the most important criteria for determining area. Most previous studies have been chiefly the mode of formation of fossil assemblages. concerned with such abiotic aspects of these Right-skewed size-frequency distributions with rocks as depth of water (Elias, 1937), lithofacies large numbers of specimens in the very small- distribution (Hattin, 1957; Lane, 1958), and size classes and large values for the observed size regional paleogeography (Laporte, 1962), with range and coefficient of variation for two species little or no attention to whether or not the of invertebrates in the Bonner Springs forma- fossils actually lived in the environments at- tion near Cedar Creek, Nebraska, are the most tributed to them. significant criteria for concluding that this is a fossil community. CONCLUSIONS (4) Fossil communities are probably not so (1) The terms "life assemblage" (biocoenose, rare as paleoecologists have assumed. With biocoenosis, indigenous assemblage) and "death proper collecting many more of them will be assemblage" (thanatocoenose, thantocoenosis) recognized and become the basis for conclu- represent an oversimplification of the funda- sions regarding such aspects of the community mental concepts involved in understanding the concept as structure, function, composition, mode of formation of fossil assemblages. The and horizontal and vertical distribution. The author proposes that the terms fossil com- Pennsylvanian rocks of the Mid-Continent munity, residual or winnowed fossil community region are an example of the need for more transported fossil assemblage, and mixed fossil paleoecological information of this sort before assemblage be used instead, since they more really significant conclusions can be reached. nearly describe the conditions under which fossil assemblages form.

REFERENCES CITED Ager, D. V., 1963, Principles of paleoecology: New York, McGraw-Hill Book Co., 371 p. Amsden, T. W., and Ventress, W. P. S., 1963, Early Devonian brachiopods of Oklahoma: Okla. Geol. Survey, Bull. 94, 238 p., 21 Pis. Boucot, A. J., 1953, Life and death assemblages among fossils: Am. Jour. Sci., v. 251, p. 25-40 Boucot, A. J., Brace, W., and DeMar, R., 1958, Distribution of brachiopod and pelecypod shells by cur- rents: Jour. Sed. Petrology, v. 28, p. 321-332 Bradley, W. H., 1957, Physical and ecologic features of the Sagadahoc Bay tidal flat, Georgetown, Maine, p. 641-682 in Ladd, H. S., Editor, Treatise on marine ecology and paleoecology, Volume 2, Paleo- ecology: Geol. Soc. America Mem. 67, 1077 p. Carpenter, J. R., 1956, An ecological glossary: New York, Hafner Publishing Co., 306 p. Cloud, P. E., 1948, Assemblages of diminutive brachiopods and their paleoecological significance: lour. Sed. Petrology, v. 18, p. 56-60 Condra, G. E., and Reed, E. C., 1959, The geological section of Nebraska: Neb. Geol. Survey, Bull. 14A, 82 p. Craig, G. Y., 1953, Fossil communities and assemblages: Am. Jour. Sci., v. 251, p. 547-548 Craig, G. Y., and Hallam, A., 1963, Size-frequency and growth-ring analyses of Mytilus edulis and Cardium edule, and their paleoecological significance: Palaeontology, v. 6, p. 731-750 Dapples, E. C., 1938, The sedimentational effects of the work of marine scavengers: Am. Jour. Sci., v. 236, p. 54-65 1942, The effect of macro-organisms upon near-shore marine sediments: Jour. Sed. Petrology, v. 12, p. 118-126

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/75/12/1197/3442421/i0016-7606-75-12-1197.pdf by guest on 25 September 2021 REFERENCES CITED 1215

Deevey, E. S., 1947, Life tables for natural populations of animals: Quart. Rev. Biology, v. 22, p. 283-314 1950, The probability of death: Scientific American, v. 182, p. 58-60 Dunbar, C. O., and Rodgers, J., 1957, Principles of stratigraphy: New York, John Wiley and Sons, 356 p. Eager, R. M. C., 1952, Growth and variation in the non-marine lamellibranch fauna above the Sand Rock Mine of the Lancashire Millstone grit: Geol. Soc. London Quart. Jour., v. 107, p. 339-373 Elias, M. K., 1937, Depth of deposition of the Big Blue (Late Paleozoic) sediments in Kansas: Geol. Soc. America Bull., v. 48, p. 403-432 Elliott, G. F., 1956, Post-Paleozoic brachiopod ecology—a reassessment: Geol. Mag., v. 93, p. 196-200 Fenton, C. L., and Fenton, M. A., 1928, Ecologic interpretations of some biostratigraphic terms: Am. Midland Naturalist, v. 11, p. 1-23 Forrest, H., 1963, Immortality and Pearl's survivorship curve for Hydra: Ecology, v. 44, p. 609-610 Fox, W. T., 1962, Stratigraphy and paleoecology of the Richmond group in southeastern Indiana: Geol. Soc. America Bull., v. 73, p. 621-642 Ginsburg, R. N., 1957, Early diagenesis and lithification of shallow-water carbonate sediments in south Florida: Soc. Econ. Paleontologists and Mineralogists, Special Publication 5, p. 80-100 Hallam, A., 1960, A sedimentary and faunal study of the Blue Lias of Dorset and Glamorgan: Royal Soc. London Philos. Trans., ser. B, v. 243, p. 1-44 1961, Brachiopod life assemblages from the Marlstone Rock-bed of Leicestershire: Palaeontology, v. 4, p. 653-659 Hanson, H. C., 1962, Dictionary of ecology: New York, Philos. Libr., 382 p. Hattin, D. E., 1957, Depositional environment of the Wreford megacyclothem (Lower Permian) of Kansas: Kans. Geol. Survey, Bull. 124, 150 p. Hock, C. W., 1940, Decomposition of chitin by marine bacteria: Biol. Bull., v. 79, p. 199-206 Imbrie, J., 1955, Quantitative lithofacies and biofacies study of Florena shale (Permian) of Kansas: Am. Assoc. Petroleum Geologists Bull., v. 39, p. 649-670 Johnson, R. G., 1960, Models and methods for analysis of the mode of formation of fossil assemblages: Geol. Soc. America Bull., v. 71, p. 1075-1086 1962, Mode of formation of marine fossil assemblages of the Pleistocene Millerton formation of California: Geol. Soc. America Bull., v. 73, p. 113-130 Krumbein, W. C., and Sloss, L. L., 1963, Stratigraphy and sedimentation, 2d edition: San Francisco, W. H. Freeman and Company, 660 p. Lane, N. G., 1958, Environment of deposition of the Grenola limestone (Lower Permian) in southern Kansas: Kans. Geol. Survey, Bull. 130, p. 117-164 Laporte, L. F., 1962, Paleoecology of the Cottonwood limestone (Permian), northern Mid-Continent: Geol. Soc. America Bull., v. 73, p. 521-544 Lowenstam, H. A., 1957, Niagran reefs in the Great Lakes area, p. 215-248 in Ladd, H. S., Editor, Treatise on marine ecology and paleoecology, Volume 2, Paleoecology: Geol. Soc. America Mem. 67, 1077 p. Martin-Kaye, P., 1951, Sorting of lamellibranch valves on beaches in Trinidad: Geol. Mag., v. 88, p. 432-434 Mattox, N. T., 1955, Observations on the brachiopod communities near Santa Catalina Island, p. 73-84 in Essays in the Natural Sciences in honor of Capt. Allan Hancock: Los Angeles, Univ. Southern Calif. Press, 345 p. Menard, H. W., and Boucot, A. J., 1951, Experiments on the movement of shells by water: Am. Jour. Sci., v. 249, p. 131-151 Moore, D. G., and Scruton, P. C., 1957, Minor internal structures of some recent unconsolidated sedi- ments: Am. Assoc. Petroleum Geologists Bull., v. 41, p. 2723-2751 Nicol, D., 1962, The biotic development of some Niagran reefs—an example of an ecological succession or sere: Jour. Paleontology, v. 36, p. 172-176 Odum, E. P., and Odum, H. T., 1959, Fundamentals of ecology, 2d. edition: Philadelphia, W. B. Saunders Company, 546 p. Olson, E. C., 1957, Size-frequency distribution in samples of extinct organisms: Jour. Geology, v. 65, p. 309-333 Paine, R. T., 1962, Filter-feeding pattern and local distribution of the brachiopod Discinisca strigata: Biol. Bull., v. 123, p. 597-604 1963, Ecology of the brachiopod Glottidia pyramidata: Ecol. Mon., v. 33, p. 187-213

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/75/12/1197/3442421/i0016-7606-75-12-1197.pdf by guest on 25 September 2021 1216 I. A. FAGERSTROM—FOSSIL COMMUNITIES IN PALEOECOLOGY

Percival, E., 1944, A contribution to the life-history of the brachiopod Terebratella inconspicua Sowerby: Royal Soc. New Zealand Trans., v. 74, p. 1-23 Rowell, A. J., 1960, Some early stages in the development of the brachiopod Crania anomala (Muller): Annals Mag. Nat. History, ser. 13, v. 3, p. 35-52 Rudwick, M. J. S., 1962, Notes on the ecology of brachiopods in New Zealand: Royal Soc. New Zealand Trans., v. 88, p. 327-335 Simpson, G. G., Roe, A., and Lewontin, R., 1960, Quantitative zoology, revised edition: New York, Harcourt Brace and Company, 440 p. Smith, A. B., 1959, Paleoecology of a molluscan fauna from the Trent formation: Jour. Paleontology, v. 33, p. 855-871 Straaten, L. M. J. U. van, 1952, Biogene textures and the formation of shell beds in the Dutch Wadden Sea: Koninklijke Nederlandsch Akad. Wetensch., ser. B, v. 55, p. 500-516 1956, Composition of shell beds formed in tidal flat environment in the Netherlands and the Bay of Arcachon (France): Geologic Mijnbouw, neu ser., Jaarg. 18, p. 209-226 Thorson, G., 1957, Bottom communities, p. 461-534 in Hedgpeth, J. W., Editor, Treatise on marine ecology and paleoecology, Volume 1, Ecology: Geol. Soc. America Mem. 67, 1296 p. Veevers, J. J., 1959, Size and shape variation in the brachiopod Schizophoria from the Devonian of western Australia: Jour. Paleontology, v. 33, p. 888-901 Wood, H. E., and others, 1941, Nomenclature and correlation of the North American continental Ter- tiary: Geol. Soc. America Bull., v. 52, p. 1-48 Zobell, C. E., 1942, Changes produced by micro-organisms in sediments after deposition: Jour. Sed. Petrology, v. 12, p. 127-136 Zobell, C. E., and Rittenberg, S. C., 1938, The occurrence and characteristics of chitinoclastic bacteria in the sea: Jour. Bacteriology, v. 35, p. 275-287

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