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| The Columbia mammoth was a giant of Pleistocene North America |
Understanding the life of the
past can sometimes be quite challenging. For some groups of organisms there are
no living descendents or even any close living relatives. In such cases understanding their biology can
be difficult and in a few cases almost impossible. Right now I and two
colleagues are studying several well preserved skeletons of a new and very
enigmatic fossil reptile. Even though we have excellent
fossils of this new animal and the details of the skeletal anatomy are clear,
we are confounded as to the ecology of these creatures and how they made a
living in the ancient desert ecosystem they inhabited. There are no living animals with
such bizarre skeletons. Some suggestions
we have discussed amongst ourselves we would not dare utter in public. It is an intriguing and yet remarkably
frustrating problem.
However, in other cases,
fossil organisms do have living descendents or close relatives and those living
species can provide remarkable insight into the biology of their long gone
antecedents. A case in point is the
spectacular Mammoth Site in Hot Springs, South Dakota. This is the largest deposit of fossil
mammoths in North America and a famous site studied by paleontologists around the world. It is also a place where the public can go
and see a most remarkable fossil deposit, where mammoths are exposed but left in place.
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| To see a mass of dead mammoths, there's no better place to visit than the Mammoth Site Hot Springs, South Dakota |
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In 1974 excavation work for
a new subdivision uncovered bones, one of which was from a mammoth. Excavation in the same year soon revealed a
skull and tusks. Additional work
uncovered more fossils and local residents worked together to eventually
create the Mammoth Site. There is wonderful structure built over this unique
fossil deposit in order to protect and exhibit the bones. Discoveries continue to be made as excavations are still on-going. For an on-line 360 degree
tour of the mammoth quarry, go to http://blackhills360.com/bhgalleries/mammogallery.html
By far the most common
fossils in the site are Columbia mammoths (Mammuthus columbi), proboscidean brutes measuring 14
feet high at the shoulder and weighing up to 10 tons. These behemoths ranged across North America
and into Central America during the Pleistocene and could live up to 80 years.
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| The skull and tusks of a male Columbia mammoth. |
As of 2012 some 57 Columbia
Mammoths have been uncovered at the site and excavations are far from complete.
Like at my own institution, Dinosaur
National Monument, the fossils at the Mammoth Site are exposed but left in
place, just as they were buried tens of thousands of years ago. However, the mammoth remains do not occur as
one mass accumulation at one level.
Rather, they occur as numerous isolated, articulated skeletons, at many
different levels. Even odder is the
fact that they are all males and mostly young males at that. How did such odd deposit come to be?
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The geological history of the
site is well understood, thanks to the work of Robert Laury.
Phase 1: Some 26,000 years
ago the collapse of a cave system created a sinkhole 120 feet by 150 feet and
sixty feet deep. The walls were steep to overhanging. Spring water bubbled up into the sinkhole
creating a 15 foot deep pond that filled the sinkhole. This was a thermal spring, so the water
in the sinkhole was warm. The invertebrate fossils in the sinkhole indicate
that the pond water was very warm --- about 95-100oF. This prevented the pond
from freezing and eventually allowed for year-round plant life around the margin of the
sinkhole.
Phase 2: The year-round water supply
would attract mammoths, especially during the cold Pleistocene winters that
occurred during the time of the sinkhole. Attracted to the sinkhole, some mammoths would
slip and slide or fall into the sinkhole only to find that they could not climb
out. They would eventually become
exhausted, drown in the pond, sink to the bottom, and become
buried in sediments.This occurred repeatedly for hundreds of years.
Phase 3: Sediments continue to be
washed into the sinkhole from the surrounding land as well as the sides of the
sinkhole itself. After some 700 years the sinkhole filled up with sediment and
the spring ceased flowing, leaving a mass of mud and mammoths that remained
buried until a bulldozer blade uncovered them in 1974.
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So the accumulation of
mammoths at the Mammoth Site is not the result of the death of a herd as a
single event but rather the result of many mammoths falling into the sinkhole,
one-at-a-time, over hundreds of years. Dr.
Larry Agenbroad has been leading excavations at the site since 1975. He is an
expert on the paleobiology of mammoths and his research has revealed much about
the lives of the mammoths entombed in the sinkhole.
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| A herd of African elephants, led by a dominant female. |
Studies of living elephants
shows that males and females differ in a number of skeletal features. This is fortunate, because it allows us to
determine the sex of fossil elephant skeletons.
Applying these techniques to the skeletons at the Mammoth Site shows
that all the skeletons are of males.
Other skeletal features, especially involving the teeth, allow for pretty
accurate estimates of an individual’s age at death. That data shows that about
87% of the mammoths were 12-29 years old at time of death. So not only are most
of the mammoths at the site males, they are adolescent males. Why?
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| A herd of mammoths keep a wary eye out for the American lion as they cross a Pleistocene plain. |
In living elephants the herds
are centered around families, i.e. females and their calves. If there are several groups of females and
calves in a herd, the oldest female, the matriarch, leads the group. Upon
reaching puberty (10-12 years old) male elephants leave the herd of their birth and wander,
living often alone or with a few other adolescent males. When an adult male elephant reaches sexual maturity (35 years), he will seeks out the herds to find a mate during breeding season.
So a bull’s interaction with herds is temporary and restricted to
reproductive purposes. After breeding
season the bulls leave the herds. This knowledge provides a critical piece of evidence in
unraveling the mystery of the Mammoth Site.
Mammoths had the same social
organization as living elephants, with wandering adolescent males leaving the
herd of their birth, and seeking out other herds when they were ready to mate. As Dr. Larry Agenbroad wryly observed about the mammoths in the sinkhole “There is 20 to 25 years of no supervision and no protection [for the
young mammoths]. So they wander as ‘loners’ or in small bachelor groups
and get into really dumb places!”
Attracted
by the year round water in the sinkhole, some males would get too close, slip
into the sinkhole, and drown. While this was not a common event, it happened
frequently enough over the 700 years of the sinkhole’s existence to compile the
spectacular deposit of nearly 60 Columbia mammoths that we can see today.
So, as is often the case,
the tragic story closes with the observation “A number of
adolescent males were involved.”
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| After being entombed for 26,000 years the skeleton of an adolescent male Columbian mammoth is again visible at the Mammoth Site. |
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SOURCES
Agenbroad, L.D. 1984. Hot Springs, South
Dakota: Entrapment and taphonomy of Columbian mammoths.
in: Quaternary Extinctions: A Prehistoric Revolution:
113-127.
Agenbroad quote: http://www.cosmosmagazine.com/weird-animals/columbian-mammoth/
Dutrow, B. L.
"Preliminary postcranial metric analysis of mammoths from Hot Springs
Mammoth site, South Dakota."
Transactions of the Nebraska
Academy of Sciences and
Affiliated Societies 4 (1977): 223-227.
Hayes, G. 1991. Mammoths, Mastodons, and Elephants: Biology, Behavior, and the Fossil Record. Cambridge University Press: 413 pages.
Hayes, G. 1991. Mammoths, Mastodons, and Elephants: Biology, Behavior, and the Fossil Record. Cambridge University Press: 413 pages.
Laury, R.L. 1980.
Paleoenvironment of a late Quaternary mammoth-bearing sinkhole deposit, Hot Springs, South
Dakota. Geological Society of America
Bulletin 91: 465-475.
The Mammoth Site, Hot Springs South
Dakota: http://www.mammothsite.com/
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IMAGES
Battling Columbia mammoths:
Artwork of the geological events at the Mammoth Site:
African elephant herd:
Mammoth herd: http://www.hanskrause.de/HKHPF/hkhpf_53_03.htm
All other images: Dr George Englemann, University of Nebraska - Omaha