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 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 1198 J. A. FAGERSTROM—FOSSIL COMMUNITIES IN PALEOECOLOGY 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 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 TYPES OF FOSSIL ASSEMBLAGES 1199 "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