Happy Valentine’s Day: 50 Years Ago, Giant Sloth Remains Unearthed Nearby

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Our friends at the Highline Historical Society (we’re members – join here) remind us that one way to commemorate Valentine’s Day is to celebrate the 50-year anniversary of the unearthing of the remains of a giant Ice Age sloth from the north end of Sea-Tac Airport.

According to the HHS:

“…on February 13, 1961, workmen discovered the bones of an Ice Age sloth near the north end of Sea-Tac International Airport. The sloth was about 12 feet long. The animal was a relative of the present South American tree sloth. Radiocarbon dating of the peat in which the skeleton was found established the sloth to be about 12,500 years old.”

Currently, the remains of the now-extinct sloth, which is “about the size of a Mini Cooper,” are on display as a re-created, complete skeleton at the Burke Museum at the University of Washington.

“We’re planning to install a replica of his 17-foot long skeleton outside our new museum,” said Executive Directory Cyndi Upthegrove.

The 12,600-year old Giant Sloth was unearthed from a bog like this near the north end of Sea-Tac Airport.

Here’s more from the society’s newsletter:

Gordon Simmons found the first bone inside a steel caisson the Sellen Construction Co. was driving into the ground to provide an anchor for a tower on the airport’s runway-approach lighting system. Almost 2/3 of the skeleton was recovered, including some limbs from the right side, the left side, the majority of tail ribs and associated vertebrae. Lacking, and presumed either not present at burial or lost in excavation, was the head, most of the neck, and some limb bones. The missing bones and present skull were cast from another Megalonyx jeffersonii specimen. The original bones, as found, were preserved perfectly. The complete skeleton of this sloth is mounted in the University of Washington’s Burke Museum.

Ten to twelve thousand years ago, as a huge continental glacier retreated from the Puget Lowland, its melt waters carved out the depression in which first a lake and later a peat bog formed. This was the domain of the extinct ground sloth Megalonyx, a lumbering beast about the size of an ox. Its weight awkwardly rested on the outer margins of its huge hind feet, and on the knuckles of its smaller and more agile forefeet (equipped with powerful claws, from which the name “big claw” or “Megalonyx” comes). The animal was further supported by a massive tail when rearing up on its hind legs in search of low-hanging branches on which to browse. What caused its death we may never know, but the body of Megalonyx jeffersonii finally settled on the bottom of the bog to be covered, eventually, by thirteen feet of peat.

Some of the contemporaries of Megalonyx were the wooly mammoth, mastodon, long-horned bison, musk ox, horse, camel, caribou, elk, moose, various ground sloths, lynx, wolf, great short-faced bear, grizzly bear and sabre-tooth cat. The sabre-tooth cat probably was responsible for the death of many a ground sloth. It is believed that some of those species absent today were hunted to extinction by early man.

Of historic interest is the fact that Megalonyx was one of the first fossil mammals described in North America; the author was none other than Thomas Jefferson. Jefferson was interested in the natural history of Virginia, and in 1797 presented a paper to the American Philosophical Society on the discovery of the first specimen of Megalonyx found the previous year during saltpeter mining in a cave in what is now West Virginia. In the early 1800’s the specific name of jeffersonii was proposed by a French paleontologist to honor Jefferson’s contribution, and we now know the late Pleistocene species as Megalonyx jeffersonii. Since that time, fossils of this genus have been found throughout North America, from Florida to Washington and from Southern Mexico to Alaska.


  • GUTHRIE, RUSSELL D., “Recreating a Vanished World,” in National Geographic, Volume 141, Number 3, March 1972.
  • KURTEN, BJORN, The Ice Age, Putnam, New York, 1972.
  • MARTIN, PAUL S., “Sloth Droppings,” Natural History, Vol. 84, No. 7, August-September 1975.
  • ROMER, ALFRED S., Vertebrate Paleontology, Chapter 24: “Edentates,” University of Chicago, Revised Edition, 1962.
  • STOCK, CHESTER, Rancho La Brea, A Record of Pleistocene Life in California, Los Angeles County Museum, Science Series Number 20, Paleontology Number 11, 1965.

To learn more local history, be sure to check out the society’s Photo Exhibit currently on display at the Burien Community Center – read our previous coverage here.

(Photos courtesy the Highline Historical Society)

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One Response to “Happy Valentine’s Day: 50 Years Ago, Giant Sloth Remains Unearthed Nearby”
  1. Gordon Simmons Jr. says:

    We just sent this link to my dad, Gordon Simmons Sr. who has great stories of the events of that day and the days that followed. He has still never seen the exhibit.

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