Thisland/South Dakota: mammoth site

"Please come to Hot Spring. We think we have mammoths!" In the summer of 1974, this message drew me to a construction site in the Black Hills of South Dakota. In June, George Hanson, a heavy-equipment operator who was in the process of clearing land on the southern edge of the town of Hot Spring, had uncovered some intriguing bones. When his son Dan visited the site, he literally stubbed his toe on a object that he recognized, thanks to his knowledge of geology, as a mammoth tooth. I arrived in early July, fresh from a mammoth excavation in Arizona. Judging by the bones and tusks exposed by the construction machinery, I estimated that the site contained the remains of at least four to six Columbian mammoths.

The next summer, with the permission of Phil Anderson, the landowner, I gathered a crew of volunteers to analyze the site. On the final day of our work stint, a crew member discovered a mammoth skull complete with tusks and lower jaw. We extended our schedule for a few days to cast and remove the huge skull. The value of the site was confirmed, and we planned to return.

Twenty-three years later, the work continues. We estimate that we have now excavated about 40 percent of the site. As of the close of the 1997 summer field season, the site has yielded (according to tusk count) remains of forty-nine Columbian mammoths and two woolly mammoths, along with a host of other creatures that inhabited the area some 26,000 years ago, during the Pleistocene. it is the world's largest known primary accumulation of mammoth fossils; that is, the remains were not rafted in by water and cached at the site but are the bones of animals that died on the spot.

Early on, we found help from donors, from excavators both professional and volunteer, and from local citizens who formed a corporation to oversee and protect the site. An exhibition center was built directly over the fossils, which, rather than being extracted and removed, now remain in situ. Known as the Hot Springs Mammoth Site, this Ice Age graveyard presented us with a murder mystery, and we searched not only for fossils but also for clues as to what lured the animals to the scene and caused their death.

Thousands of years ago, the site was a sinkhole that contained a thermal pond fed by artesian springs. Fossil pollen analyses suggest that the water was warm year round (as are the present-day springs that give the town its name) and that the climate may have been similar to that of steppe-tundra areas in Alaska today.

Encircling the 70-million-year-old Black Hills is an elliptical geological formation known as the Spearfish Shale. As water seeped through this red shale, it dissolved an underlying layer of porous limestone, forming a number of subterranean caverns. Such caverns, and the shale above them, occasionally collapsed, creating depressions called sinkholes. Over time, sinkholes flu up with sediment. At the Mammoth Site, these sediments were much more resistant to erosion than were the relatively soft surrounding shales. What had been the bottom of a pit slowly became the top of a hill -- the site that was being smoothed over by George Hanson and his bulldozer. The Mammoth Site sinkhole is an oval, some 120 by 150 feet. Based on the rate at which present-day sinkholes fill in, we believe that this ancient sinkhole filled with sediment and entombed the bones and tusks over a period of three hundred to seven hundred years.

The Columbian mammoths that predominate at the site were temperate-zone contemporaries of the long-haired woolly mammoths usually found in more northerly zones. Some twelve feet tad at the shoulder and weighing up to eight tons, Columbian mammoths roamed steppe grasslands of North America, from about 130,000 to 11,000 years ago. Their remains have been found from Canada to Mexico and from California to Florida.

Of particular interest were the tusks we found (tusks are enlarged upper incisors). Where they emerged from their sockets, they were sometimes thicker than my thigh -- a strong indication that most of the mammoths were males, the larger of the sexes in mammoths. Later on, a British colleague, Adrian Lister, developed a method for determining sex from a ratio of two pelvic measurements. When tested, all our fossil pelvis bones indeed proved to be those of males. And about 84 percent of the animals were between the ages of eleven and twenty-nine, young for mammoths. (Elephants have a life span of more than sixty years; presumably this would hold for mammoths too.)

What could explain the phenomenon of a natural death trap for young male mammoths? in the Pleistocene, as now, the area was apparently well watered, with numerous artesian springs and the ancestral Fall River nearby, so the presence of water in the sinkhole seemed unlikely to be the main lure. Could the animals' behavior have had something to do with the deaths? In her book Elephant Memories, zoologist Cynthia Moss describes her studies of the life cycle of today's African elephants. When we applied this information to our fossils, we were able to put flesh on the bones and to hypothesize about the behavior of our mammoths. Elephants are born into matriarchal societies. For their first decade or so of life, the young are carefully guarded and protected by their mother, aunts, and sisters. As males become sexually mature, they are driven from the female-dominated herd. Young males tend to wander, eventually gathering in small groups of similarly displaced young males or taking up life on their own as solitary bachelors.