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Biting down on the origin of a tooth


Tyrannosaurus rex skeleton in the Dinosaurs hall of the Smithsonian's National Museum of Natural History in Washington, D.C.

Tyrannosaurus rex skeleton in the Dinosaurs hall of the Smithsonian's National Museum of
Natural History in Washington, D.C. (John Gibbons/Smithsonian photo)

Posted by L.F. Chambers, Office of Communication, and Michael Fracasso, Minerals and Geology Management, U.S. Forest Service

The big female sniffed at the dry Late Cretaceous air as she trotted – delicately, considering her 7-ton frame – along a game trail through a stand of towering conifers, whose needled lower branches trembled slightly at her passing.

Tiny mammals scurried in the underbrush of magnificent ferns and little blue flowers. A small flock of red birds rose from the scant cover of a magnolia as the dinosaur passed, their calls carried on the wind, sending other distant flocks to flight in this corner of what is today the Custer National Forest in South Dakota.

The Tyrannosaurus rex in her prime was the undisputed queen of her forest. Pausing to sniff the breeze, her nostrils flared as she determined that the rotting carcass she’d been tracking all morning was close now. She continued on toward the river – in just a few more miles she would find the delicious remains of an Edmontosaurus.

As she ate, her jaws rending tendon and bone with more than 5 tons of pressure on the dead duck-billed dinosaur’s thigh, several of her teeth – built like a cross between a steak knife and a railroad spike – broke loose and came to rest along the debris-strewn shore of the river.

Soon after that driving rains flooded the riverbed and washed the teeth and other bones downstream where they separated and settled out onto sandy point bars forming at bends in the river. There, they were covered more and more deeply through succeeding years in sediments laid down by floods, volcanism and the dust of ages.

The business end of an isolated Tyrannosaurus rex tooth

The business end of an isolated Tyrannosaurus rex tooth found by Barb Beasley during a
2012 Passport in Time excavation in the Late Cretaceous Hell Creek Formation on the
Custer National Forest in South Dakota. Unlike mammals that only possess two sets of teeth
during their lifetime, dinosaurs replaced worn teeth by shedding them continuously
throughout their lives. A single Tyrannosaurus may have shed hundreds or even thousands
of teeth during its lifetime. (Rhonda Fore/U.S. Forest Service photo)

As millennia passed, the strata above piled higher and the organic material in the teeth and bones was slowly replaced by minerals. The sand that surrounded them became cemented to form a rocky matrix known today as the Hell Creek Formation. The cycads and conifers of the Late Cretaceous gave way to new trees, birds, flowers and mammals. Species evolved and disappeared, leaving imprints of their own in the strata above the dinosaur remains.

Millions more years of wind and rain then began to erode the stone away, finally exposing a time-blackened tooth to sunlight for the first time in 66 million years. That’s how one of those new mammals, a Homo sapiens, who happened to work for the U.S. Forest Service, came to find it.

Barbara Beasley, regional paleontologist, was leading a crew of 20 volunteers when she found the tooth just a few inches away from where she had been swinging an awl moments before.

“These teeth are so rare, at first you aren’t totally sure it’s what you’ve found,” said Beasley. “Then once you pick it up, you’re just in awe. It’s a real adrenaline-pumping moment.”


A local 4-H group was visiting the dig site, and the kids clambered to touch the serrations, a visceral connection to an ancient world and the some of the wonder that can be found on the national forests and grasslands.

Fossils of various types have been found on nearly two-thirds of Forest Service lands, most notably in the western United States. Beasley and her colleague Bruce Schumacher have been coordinating volunteer-staffed Passport in Time paleontological excavations to conserve fossil resources in various locations in Wyoming, North and South Dakota, Nebraska and Colorado since 1998.

“For a lot of volunteers, working on a dig is a dream-come-true,” said Beasley. “One of the best parts of working for the Forest Service is getting to provide these sorts of opportunities – our volunteers have been 10-year-old kids with their parents, all the way up to people in their 80s.”


US Forest Service
Last modified April 05, 2013

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