A COMPARISON OF FISH COMMUNITY STRUCTURE AMONG SEVERAL PALEOENVIRONMENTS OF EOCENE FOSSIL LAKE, WYOMING: A PRELIMINARY REPORT

Terry R. Finger
9882 Rt. N, Columbia, Missouri 65203


Abstract—Genus, standard length, and stratigraphic position were recorded for 3341 fossil fishes at three sites within the basin of Eocene Fossil Lake near Fossil Butte National Monument, southwestern Wyoming. Two sites were at different stratigraphic positions near the lake center (Lewis Ranch sites) and one site, nearly contemporaneous with the stratigraphically lower mid-lake site, was near the lake shore (Thompson Ranch site). Preliminary analysis has documented distinct differences in fish community structure between the near-shore and stratigraphically lower mid-lake sites. The near-shore site is dominated by Knightia and small Diplomystus, while the lower mid-lake site is characterized by proportionately fewer Knightia, larger Diplomystus, and Priscacara. Overall fish fossil density at the near-shore site was approximately five times that of the lower mid-lake site. The fish community structure of the stratigraphically upper mid-lake site was similar to the near-shore site, suggesting that the mid-lake habitat may have become shallow during later lake stages. Further data analysis is continuing, including detailed documentation of the size distribution of the fishes at each site and an examination of stratigraphic changes in fish community structure within each site. Sampling is also underway at a second near-shore site.

Introduction

Ecologists have long been interested in determining factors that affect biological community structure, i.e., the species present in a given area and their relative abundances. The effects of species interactions and environmental perturbations on fish community structure is a topic of considerable interest and debate (Grossman et al. 1982, 1985, 1990, Rahel et al. 1984, Yant et al. 1984, Herbold 1984, Moyle and Vondracek 1985, Ross et al. 1985, Matthews 1982, 1986, Finger and Stewart 1987). Ecological studies of extant communities are, however, limited in their time perspective. At best, such investigations can track changes in communities over several decades, while significant alterations in community structure may only occur over much longer time periods.

Ecological analysis of the fossil record provides an opportunity to assess long-term trends in community structure (Boucot 1978). Sites where the fossil record is sufficiently complete to consider such analyses are, however, rare, especially for vertebrates. The fossil fish localities of Eocene Fossil Lake (Fossil Butte Member, Green River Formation), in and near Fossil Butte National Monument, southwestern Wyoming, provide a unique opportunity for the study of long-term trends in fish community structure. Fishes from Fossil Lake are among the most numerous and best preserved vertebrate fossils in the world, and commercial quarries in the area provide access to large quantities of fossil material in several stratigraphic sections. The purpose of this study is to document fish community structure at localities within the basin of Eocene Fossil Lake and to relate those findings to paleoenvironmental conditions. In this paper I present preliminary findings from three distinct paleoenvironments.

Study Area

Fossil Lake persisted for several million years during the late early Eocene, and at its largest size covered several hundred km2. It was the smallest and most short-lived of a system of three lakes that existed for approximately 15 million years from the late Paleocene to the middle or late Eocene. The environment was subtropical and the lake supported a freshwater fauna, although there may have been periods when at least part of the water column was saline (McGrew and Casilliano 1975, Grande 1994, Grande and Buchheim 1994). The taxonomy of the fish fauna of Fossil Lake has been thoroughly summarized by Grande (1984, 1994) and Grande and Buchheim (1994). Sedimentological studies assessing paleoenvironmental conditions have been conducted by Buchheim (1993, 1994) and others.

This paper includes preliminary findings from three fossil fish sites within the deposits of the middle and upper units of the Fossil Butte Member.

Sample Site A-1: Lewis Ranch quarry site of James E. Tynsky; SE1/4, SE1/4, Sec. 19, T.21N., R.117W., and NE1/4, NE1/4, Sec. 30, T.21N., R117W., Kemmerer 15-minute quadrangle (U.S.G.S.). This quarry is locality A of Grande and Buchheim (1994). Stratigraphically, this sample site encompassed the "18-inch layer" deposits; i.e., the F-1 deposits of Grande (1984) and Grande and Buchheim (1994). Paleontological and sedimentological evidence indicates that these are mid-lake deposits.

Sample Site A-2: The same quarry location as sample site A-1. Stratigraphically, however, this sample site encompassed deposits approximately 10 m above the strata sampled at site A-1, and approximately 3 m above the "K-spar" tuff layer (see Grande and Buchheim 1994, Figure 2). The paleontological characteristics of this later lake stage are not well known, but sedimentological data suggest that there may have been periods of fluctuating salinity and depth.


Table 1—Sample characteristics for sites A-1, A-2, and H-1. Sampling occurred during 1993 and 1995 field seasons.


Sample                              Sample Sites

Characteristic              A-1         A-2         H-1



volume sampled (dm3)        14197.3     1577.3      2126.0     



stratigraphic column 

sampled (cm)                36.5        94.3        291.1


Table 2—Number of fish of each genus collected at each sample site. See text for descriptions of sites. Data from site A-1 are compiled separately with and without 312 very young juvenile Diplomystus (<25 mm SL). For each site, no. = number of fish collected; % = percentage of the total number of specimens collected at that site.

Genus                      Sample Sites     

              A-1 w/juv.      A-1 w/o juv.     A-2             H-1

              no.     %       no.      %       no.     %       no.     %     

Knightia      484     32.1    484      40.5    498     59.9    566     56.4

Diplomystus   620     41.2    308      25.8    268     32.2    329     32.8

Priscacara    298     19.8    298      25.0    11      1.3     3       0.3

Mioplosus     39      2.6     39       3.3     25      3.0     21      2.1

Phareodus     16      1.1     16       1.3     0       0       6       0.6

Notogoneus    12      0.8     12       1.0     0       0       0       0

Amphiplaga    0       0       0        0       0       0       3       0.3

Eohiodon      0       0       0        0       0       0       1       0.1

Heliobatis    0       0       0        0       0       0       1       0.1

Unknown       37      2.5     37       3.1     29      3.5     74      7.4



Total No.     1506            1194             831             1004     

No./m3        106.3           84.1             526.8           472.2


Sample Site H-1: Thompson Ranch quarry site of Richard Hebdon, NW1/4, SW1/4, Sec. 22, T.22N., R117W., Kemmerer 15-minute quadrangle (U.S.G.S.). This is locality H of Grande and Buchheim (1994), located approximately 11 km northeast of sample sites A-1 and A-2. Stratigraphically, this sample site encompassed the F-2 deposits of Grande (1984) and Grande and Buchheim (1994). Paleontological and sedimentological evidence indicates that these are near-shore deposits.

The deposits sampled at site H-1 are the sandwich bed deposits that can be traced throughout the lake basin because of their distinct pattern of tuff layers (see Grande and Buchheim 1994, Figure 2). At site A-1, the top of these deposits is less than 2 m below the sampled 18-inch layer deposits, and the two deposits were considered nearly contemporaneous by Grande and Buchheim (1994). Thus, comparison of sites A-1 and H-1 provides a view of mid-lake and near-shore fish communities at approximately the same stage in the lake's history, while comparison of sites A-1 and A-2 provides a perspective on temporal changes in the fish community within the same geographical location in the lake.

Methods

Fish were collected in 1993 and 1995 (Table 1). Each locality was sampled by digging through the entire targeted stratigraphic column with techniques appropriate for the site. Similar techniques were described and illustrated by Grande (1984). At site A-1, where large, thin sheets of the deposits can be excavated, sampling was conducted in conjunction with the work of a quarry crew. Areas of 15 to 30 m2 were excavated by defining layers averaging approximately 2 cm in thickness. Within each layer, sheets were removed and split as finely as possible, typically in sublayers 1.0 to 1.5 cm thick. At sites A-2 and H-1, areas of approximately 1 m2 were excavated by defining layers approximately 4 cm in thickness. Within each layer, blocks were removed and split as finely as possible, typically in sublayers approximately 0.5 cm thick. At all sites, fish were identified to genus, measured for standard length (SL; the distance parallel to the vertebral column from the tip of the snout to the posterior end of the hypurals), and recorded separately for each defined stratigraphic layer. Stratigraphic positions were determined by measuring distances from tuff layer markers.

Results and Discussion

At total of 3341 fish were collected from the three localities, including 312 very young juvenile Diplomystus less than 25 mm SL from site A-1 (Table 2).

The A-1 site, geographically located near the center of the basin, is dominated by Knightia, Diplomystus, and Priscacara. Mioplosus, Phareodus, and Notogoneus are also relatively common. Many of the Diplomystus are large, exceeding 300 mm SL, and the only juveniles in abundance are very young Diplomystus. These findings parallel those of Grande (1984) and Grande and Buchheim (1994) for mid-lake deposits. Sedimentological analyses of these deposits also indicate a mid-lake site (Buchheim 1993, 1994). The composition of the fish community at site A-1 therefore appears to be characteristic of a relatively deep-water, pelagic habitat. The presence of very young Diplomystus may be an indication of open-water spawning by this genus.

The H-1 site, geographically located near the northeast shore of the basin, is dominated by a greater proportion of Knightia than site A-1. Diplomystus is also abundant, but unlike site A-1, few individuals exceed 150 mm SL and very young juveniles are rare. Mioplosus and Phareodus are quite common, as at site A-1, but Priscacara is considerably less common and Notogoneus was not collected. These findings parallel those of Grande (1984) and Grande and Buchheim (1994) for near-shore deposits. Sedimentological analyses of these deposits also indicate a near-shore site (Buchheim 1993, 1994). The composition of the fish community at site H-1 therefore appears to be characteristic of a relatively shallow-water, near-shore habitat.

Overall fish fossil density at site H-1 is approximately five times that of site A-1. Although this difference may be an artifact of differential preservation at the two sites, the overall excellent quality of specimens from both localities suggests that the difference in fossil density may be the result of an actual difference in fish density between the communities. This finding is consistent with observations from most extant lakes, where fish density in near-shore areas is considerably higher than off-shore, pelagic habitats.

The fish community structure of site A-2, although from a site geographically located near the center of the lake basin, is very similar to that of near-shore site H-1. The sites contained similar percentages of Knightia, Diplomystus and Mioplosus, and nearly all Diplomystus were less than 150 mm SL. At site A-2 Priscacara was slightly more abundant than at site H-1 and Phareodus and several uncommon genera were not collected, but the overall structure of the fish communities at sites A-2 and H-1 were remarkably similar. Overall fish fossil density was also similar at the two sites. These findings strongly suggest that site A-2 contained a typical near-shore fish fauna, although perhaps slightly less diverse than site H-1.

Sedimentological studies of strata contemporaneous with site A-2 suggest that in the later lake stages, there may have been instability in both lake depth and salinity (Buchheim 1993, Grande and Buchheim 1994). The preliminary results of this study indicate that, in response to this instability, the fish community near the historic lake center shifted from a deep-water community to one typical of near-shore areas, but did not decline dramatically. Fish may have been able to survive in freshwater areas around the margins of the lake or the salinity may have been confined to the lower portions of the water column.

Ongoing work includes more refined data analyses to fully document the size distribution of each genus at each site, compare the fish faunal composition among sites with multivariate ordination techniques (e.g., see Gauch 1982), and investigate stratigraphic changes in fish community structure within each site. The objective of the latter analysis is to examine changes during the time period over which the targeted stratigraphic section at each site was deposited. The time involved in deposition is unknown, but is currently thought to be from a few hundred to a few thousand years (Grande and Buchheim 1994). Sampling is also underway at another locality approximately 8 km east-northeast of sample sites A-1 and A-2. This site encompasses the same sandwich bed strata as site H-1, and also appears to be a near-shore site.

Acknowledgments

This study would not have been possible without the cooperation of James E. Tynsky and Richard Hebdon, who graciously granted full access to their quarry sites. I am also indebted to Arvid Aase, who provided considerable expertise and guidance during field work. Rachel Benton, Peter Ambrose, Vincent Santucci, David McGinnis, Liz Parker, Jim Parker, and the staff of Fossil Butte National Monument introduced me to Fossil Lake and provided much encouragement and logistic support. Funding for this study was provided in part by Dinosaur Nature Association.

References

Boucot, A. J. 1978. Community evolution and rates of cladogenesis. Evolutionary Biology 11:545-655.

Buchheim, H. P. 1993. Paleo-historical fluctuations in paleogeography, depositional environment, and chemistry of Eocene Fossil
Lake. Final Report. University of Wyoming National Park Service Research Station. Contract CA-1463-5-0001.

Buchheim, H. P. 1994. Paleoenvironments, lithofacies and varves of the Fossil Butte Member of the Eocene Green River Formation, southwestern Wyoming. Contributions to Geology, University of Wyoming 30:3-14.

Finger, T. R., and E. M. Stewart. 1987. Response of fishes to flooding regime in lowland hardwood wetlands. Pages 86 - 92 in Matthews, W. J., and D. C. Heins, eds. Evolutionary and community ecology of North American stream fishes. University of Oklahoma Press, Norman.

Gauch, H. G., Jr. 1982. Multivariate analysis in community ecology. Cambridge University Press, Cambridge.

Grande, L. 1984. Paleontology of the Green River Formation, with a review of the fish fauna. Bulletin 63, Geological Survey of Wyoming.

———. 1994. Studies of paleoenvironments and historical biogeography in the Fossil Butte and Laney members of the Green River Formation. Contributions to Geology, University of Wyoming 30:15-32.

———, and H. P. Buchheim. 1994. Paleontological and sedimentological variation in Eocene Fossil Lake. Contributions to Geology, University of Wyoming 30:33-56.

Grossman, G. D., P. B. Moyle, and J. O. Whittaker, Jr. 1982. Stochasticity in structural and functional characteristics of an Indiana stream fish assemblage: a test of community theory. American Naturalist 120:423-454.

———, M. C. Freeman, P. B. Moyle, and J. O. Whittaker, Jr. 1985. Stochasticity and assemblage organization in an Indiana stream fish assemblage. American Naturalist 126:275-285.

———, J. F. Dowd, and M. Crawford. 1990. Assemblage stability in stream fishes: a review. Environmental Management 14:661-671.

Herbold, B. 1984. Structure of an Indiana stream fish association: choosing and appropriate model. American Naturalist 124:561-572.

Matthews, W. J. 1982. Small fish community structure in Ozark streams: structured assembly patterns or random abundance of species? American Midland Naturalist 107:42-54.

———. 1986. Fish community structure in a temperate stream: stability, persistence, and a catastrophic flood. Copeia 1986:388-397.

McGrew, P. O., and M. Casilliano. 1975. The geological history of Fossil Butte National Monument and Fossil Basin. National Park Service Occasional Paper No.3.

Moyle, P. B., and B. Vondracek. 1985. Persistence and structure of the fish assemblage in a small California stream. Ecology 66:1-13.

Rahel, F. J., J. D. Lyons, and P. A. Cochran. 1894. Stochastic or deterministic regulation of assemblage structure? It may depend on how the assemblage is defined. American Naturalist 124:583-589.

Ross, S. T., W. J. Matthews, and A. A. Echelle. 1985. Persistence of stream fish assemblages: effects of environmental change. American Naturalist 126:24-40.

Yant, P. R., J. R. Karr, and P. L. Angermeier. 1984. Stochasticity in stream fish communities: an alternative interpretation. American Naturalist 124:573-582.