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Bones in Rivers: Observations and Experiments in Fluvial Taphonomy
At the invitation of Dr. Luna B. Leopold of the University of California, Berkeley, C. Bruce Hanson and I initiated experiments in bone transport in natural rivers in 1974. This followed a series of flume experiments (with Joe Gregory) that indicated it was important to observe bone transport in real river systems in order to understand processes that could have affected fossil bones and teeth preserved in fluvial channel deposits. The East Fork River in west central Wyoming, where Leopold and his graduate students were studying fluvial sedimentology and hydraulogy, was used in the first series of experiments. This river is a tributary of the Green River and is sourced in the Wind River Mountains. It is perennial, meandering and ~30m wide and 1.7-2.5 m deep at flood stage in the study reach. In the 1970’s, discharge varies from 75 cubic feet per second (cfs) in low to moderate flow stages to 1100 cfs during flood maxima. An unusual flood in 1974 (a 35 year high) had a discharge of 1578 cfs, with highest mean current velocities of 154 cm/sec.
Bones were obtained from naturally exposed, clean skeletons of domestic animals, mainly cow. Marked bones were laid out on sand and gravel bars during low water stage in 1974, 1975, and 1983, prior to the spring flood season, and then relocated periodically, usually during low water in August through October. In 1974, we used a coding system of drill holes to mark the bones but in 1975 and 1983 we inscribed numbers on the bone surfaces. In the 1983 set, 2” aluminum screws were inserted into the bones so that they could be relocated if buried using a metal detector. The bone surveys were done by 3-5 people positioned across the channel, wading upstream (to avoid muddying the water where we were looking for the bones). Most of the relocated bones were left to continue their journeys, but some were collected to document abrasion and other types of damage as well as distance traveled.
From 20-50% (depending on the input year) of the experimental bones were relocated downstream from their starting points, some multiple times over the years of the study. Bones show both abrasion and corrosion damage from being in the river, and one robust tibia was broken due to large animal (probably cow) trampling on an exposed gravel bar. Significant observations (still being analyzed and written up) are that: 1) bones were not highly abraded even by 5 km and up to 10 years of transport in this sand+gravel bed river, 2) breakage did not occur during transport, 3) vertebrae and other light elements tended to go farther downstream than limb bones and mandibles, following the predictions of Voorhies Group sorting experiments in a flume.
In 1983, the study was expanded to include 3 other rivers with different flow regimes, the South Platte in northeastern Colorado (a seasonally fluctuating braided system), Lost Creek in central Wyoming (ephemeral, meandering), and the Calamus in the Sand Hills of Nebraska (perennial flow, meandering). Identical sets of bones were placed in each system and then recorded in 1984 and 1985. The results of this comparative research are still pending.
“Wild” bones in the East Fork and other rivers were also documented during surveying for the experimental bones. These are naturally occurring bones in the channel, on the exposed bars, and on the floodplain. They are fairly abundant and provide an excellent way of censusing what animals live (and die) in the drainage basin. We collected many of these bones, which provide a reference assemblage for the East Fork and several other rivers. A number of bones were being eroded out of cut-banks formed on previous floodplain deposits and dropped into the channel, including bison remains that were at least 100 years old. This indicates significant time-averaging – bones from recent deaths along the river are being mixed with relatively fresh-looking bones that have been buried in the floodplain for ecologically long periods of time.
Fun Science Note: Documenting bones in river channels is a great way to learn about an ecosystem, and a fine outdoor activity for children (with appropriate adult supervision).
References Relating to Bones in Fluvial Deposits:
Behrensmeyer, A. K. 1982. Time resolution in fluvial vertebrate assemblages. Paleobiology 8: 2ll-227.
Behrensmeyer, A. K. 1987. Miocene fluvial facies and vertebrate taphonomy in northern Pakistan. In: F. G. Ethridge, R. M. Flores, M. D. Harvey (Editors), Recent Developments in Fluvial Sedimentology. SEPM Special Publication No. 39: 169-176.
Behrensmeyer, A. K. 1988. Vertebrate Preservation in Fluvial Channels. In: Behrensmeyer, A. K. and S. M. Kidwell, eds., Ecological and Evolutionary Implications of Taphonomic Processes, Palaeogeography, Palaeoclimatology, Palaeoecology 63 (1-3): 183-199.
Behrensmeyer, A. K. 1990. Transport/Hydrodynamics of Bones. In: Palaeobiology: A Synthesis, D. E. G. Briggs and P. R. Crowther, Eds., Blackwell Scientific Publications, Oxford, pp. 232-235.
Behrensmeyer, A. K. 1991. Terrestrial Vertebrate Accumulations. Pp. 291-335 in Allison, P. and D. E. G. Briggs, Taphonomy: Releasing the Data Locked in the Fossil Record. New York: Plenum.
Behrensmeyer, A. K. and R. W. Hook. 1992. Paleoenvironmental contexts and taphonomic modes in the terrestrial fossil record. In: Behrensmeyer, A. K., J. D. Damuth, W. A. DiMichele, R. Potts, H.-D. Sues, and S. L. Wing, Terrestrial Ecosystems Through Time, pp. 15-136. (Chicago: University of Chicago Press).
Kidwell, S. M. and Behrensmeyer, A. K. 1993. Taphonomic approaches to time resolution in fossil assemblages: introduction. In: Taphonomic Approaches to Time Resolution in Fossil Assemblages: 1-8. Edited by S. Kidwell and A. K. Behrensmeyer, Short Courses in Paleontology No. 6. Knoxville, Tennessee: Paleontological Society.
Kidwell, S. M. and Behrensmeyer, A. K. 1993. Summary: estimates of time- averaging.
In: Taphonomic Approaches to Time Resolution in Fossil Assemblages: 301-302. Edited by S. Kidwell and A. K. Behrensmeyer, Short Courses in Paleontology No. 6. Knoxville, Tennesee: Paleontological Society.
Willis, B. J., and Behrensmeyer, A. K. 1994. Architecture of Miocene overbank deposits in northern Pakistan. Journal of Sedimentary Research B64(1):60-67.
Behrensmeyer, A. K., B. J. Willis and J. Quade. 1995. Floodplains and paleosols in the Siwalik Neogene and Wyoming Paleogene: implications for the taphonomy and paleoecology of faunas. Paleo-3 115:37-60.
Badgley, C. E., and A. K. Behrensmeyer. 1995. Preservational, paleoecological and
evolutionary patterns in the Paleogene of Wyoming-Montana and the Neogene of Pakistan. Paleo-3 115: 319-340.
Badgley, C., W. S. Bartels, M. E. Morgan, A. K. Behrensmeyer, and S. Mahmood
Raza. 1995. Taphonomy of vertebrate assemblages from the Paleogene of northwestern Wyoming and the Neogene of northern Pakistan. Paleo-3 115: 157-181.
Aslan, A. and A. K. Behrensmeyer. 1996. Taphonomy and time resolution of bone
assemblages in a contemporary fluvial system: The East Fork River, Wyoming. Palaios:
Behrensmeyer, A. K. 2002. Bones through Time: The Importance of Biotic versus Abiotic Taphonomic Processes in the Vertebrate Fossil Record. In: De Renzi, M., M. V. P. Alonso, M. Belinchón, E. Peñalver, P. Montoya, and A. Marquez-Aliaga, Eds. Current topics on Taphonomy and Fossilization. Pp. 297-304. Valencia, Spain: Col Leccio Encontres, Adjuntament de Valencia. (Proceedings of the International Conference Taphos 2002)
Behrensmeyer, A. K., C. E. Badgley, J. C. Barry, M. Morgan and S. M. Raza. 2005. The Paleoenvironmental Context of Siwalik Miocene Vertebrate Localities. Pp. 48-62. In: Daniel E. Lieberman, Richard J. Smith, and Jay Kelley (Editors), Interpreting the Past: Essays on Human, Primate, and Mammal Evolution in Honor of David Pilbeam. Brill Academic Publishers, Inc., Boston.
Behrensmeyer, A. K. Bonebeds through geologic time. 2007. In: Rogers, R., D. Eberth, and T. Fiorillo, Eds., Bonebeds: Genesis, Analysis, and Paleobiological Significance, pp. 65-102. Chicago: University of Chicago Press.
Behrensmeyer, A. K. 2008. Paleoenvironmental context of the Pliocene A.L. 333 “First Family” Hominin Locality, Denen Dora Member, Hadar Formation, Ethiopia. GSA Special Paper 446: The Geology of Early Humans in the Horn of Africa, edited by Jay Quade and Jonathan Wynn, pp. 203-214.
Hanson, C. B. 1980. Fluvial taphonomic processes: models and experiments. In A. K. Behrensmeyer and A. Hill (eds.), Fossils in the Making (Chicago: Univ. Chicago Press), pp. 156-181.
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