The most diverse Blancan vertebrate assemblage in North America has been collected from the Glenns Ferry Formation within Hagerman Fossil Beds National Monument. The Glenns Ferry Formation has also produced the single largest sample of the earliest species of Equus, E. simplicidens (MacFadden, 1992). Although horse is the dominant large species from the quarry (over 200 individuals of all ages); camel, mastodon, peccary and antelope have been recovered. A diverse microvertebrate fauna including fish, frog, turtle, snake, bird, rabbit, gopher, vole and shrew has also been found. Invertebrates recovered from the quarry include both gastropods and pelecypods.

There has previously only been a cursory examination of the causes for this large accumulation of horse remains and previous explanations for the large number of individuals has been based on minimal data. Hagerman Fossil Beds National Monument received a grant from Canon U.S.A. to conduct an excavation at the quarry to specifically collect data on the depositional environment. The study permitted a more complete understanding of this assemblage and its origin.

Discussion

The Horse Quarry fossil assemblage was deposited within a fine-grained, poorly sorted sandstone bed that is lithostratigraphically in the lower portion of the Glenns Ferry Formation (Pliocene, Blancan). The quarry is located 9.5 meters below the unconformable contact with the overlying Pleistocene Tuana Gravel. Chronostratigraphically the Glenns Ferry Formation at Hagerman Fossil Beds ranges from about 3.7 to 3.0 mya. Magnetostratigraphy suggests that the quarry is younger than 3.4 mya and its position above the Shoestring Basalt places it younger than 3.2 mya. During excavation of the quarry a siliceous ash directly overlying the quarry sandstone bed was uncovered and is currently being dated using Ar-Ar techniques.

The fluvial system in which the fossil accumulation was deposited had a high width/depth ratio (F= 56.6) and low sinuosity (P= 1.17) indicating that the channel was a relatively straight bedload system. Lateral accretion sets resulting from point bar migration of a meandering fluvial system are not evident in the quarry. A longitudinal bar present in the
western portion of the 1997 excavation consists of a large accumulation of poorly sorted (1.95 phi) medium-grained (2.0 phi) sandstone, intraformational mudstone rip-up clasts and bones. The mean grain size for the quarry sandstone is fine (.25 - .125 mm). The sandstone is poorly sorted (mean of 1.47 phi) lithic arenite containing subangular grains larger than 2 mm in diameter with a few (>5 mm) pebble-sized grains.

Considering the .25 mm grain size and dune bedforms, the paleoflow velocity can be bracketed between 60 and 100 cm/sec. Flow depth was less than half a meter. Measurement of thirty trough cross-sets of the fluvial channel exposed during excavation of the quarry indicates a south-southwest (mean 226o azimuth) transport direction (Figure 1). The majority of bones are aligned transverse to fluid flow, supporting the sedimentological evidence for a low flow velocity and a shallow water depth. The channel sediments fine upward and represent a single waning flood event.

The historical Hagerman Horse Quarry consisted of three informal quarries: the original main quarry of the Smithsonian also known as the red sandstone quarry, the white sandstone quarry, and the green sandstone quarry. The Smithsonian red sandstone quarry, excavated in 1929, 1930 and 1931 yielded the highest percentage of excavated Smithsonian fossil material. The white and green sandstone quarries were excavated in 1934 by the Smithsonian and were enlarged by later excavations by the Los Angeles County Museum and Idaho Museum of Natural History. The location of the 1997 excavation was positioned to connect these separate quarries and determine their spatial relationships to each other. The red sandstone quarry is topographically lower than the other quarries and differences in elevation reflect the topography of the bottom of the paleochannel. The red sandstone quarry is interpreted to be a paleochannel that was infilled by sediment and bones during deposition of the overlying braided channel. At present, due to subsequent erosion, only a small portion of the red sand quarry sediments are exposed. The white sandstone quarry is well-cemented, intraformational rip-up conglomerate at the base of the braided channel. The green sandstone quarry consists of the overlying friable trough cross-bedded sandstone portion of the braided channel. The entire package of sediments represents a fining upward sequence deposited during a waning flood event. Bones were recovered from all three sedimentary packages during the 1997 field season.

Figure 1—Rose diagrams showing azimuth of trough cross sets and of long bones of Equus simplicidens from the Hagerman Horse Quarry.

Conclusions

Historical explanations for the large accumulation of horse bones have ranged from long term accumulation around a water hole (Gazin, 1936) to a single catastrophic accumulation during a flood event in a deep river (McDonald, 1996). As a result of our recent study of the quarry we interpret the accumulation, consisting of thousands of bones, to have resulted from a Phase II drought followed by a seasonal flash flood. The moderate drought resulted in the mass mortality of Equus simplicidens and other members of the fauna, including the microvertebrates in the area of what is now the Hagerman Horse Quarry. The horses and other animals were attracted to remnant water holes in low areas of the shallow river bed where they died of starvation and dehydration. This mass mortality produced a considerable accumulation of disarticulated horse remains of individuals of all ages ( 2-3 weeks old to > 20 years) scattered on the dry river bed. The lack of weathering on most bones indicates only a short interval of time of exposure prior to a short-lived flash flood that refilled the river channel and transported, entrained, deposited and buried the bones. No obvious modification of bones by scavengers has been observed suggesting that the surplus of carcasses minimized the need for scavengers to extract nourishment from the bones. Geologic and taphonomic characteristics suggest the bones traveled a very short distance prior to burial. A general inventory of bones indicates all three Voorhies Groups are present, indicating an autochtonous fossil concentration.

References

Gazin, C.L. 1936. A study of the fossil horse remains from the Upper Pliocene of Idaho. Proceedings of the United States National Museum 83(2985):281-320.

MacFadden, B.J. 1992. Fossil Horses, Systematics, Paleobiology, and Evolution of the Family Equidae. Cambridge University Press, Cambridge.

McDonald, H.G. 1996. Population structure of the late Pliocene (Blancan) zebra Equus simplicidens (Perissodactyla: Equidae) from the Hagerman Horse Quarry, Idaho. Pp. 134-155 in Palaeoecology and Palaeoenvironments of Late Cenozoic Mammals. K.M. Stewart and K.L. Seymour eds. University of Toronto Press, Toronto