An artist's imagining of an ancient relative of today's rhinoceroses splashing through a stream next to turtles and fish in the Yukon. (Credit: Julius Csotonyi)
In 1973, a teacher named Joan Hodgins took her students on a hike near Whitehorse in Canada鈥檚 Yukon Territory. In the process, she made history for this chilly region.
A quarry where schoolteacher Joan Hodgins discovered 8-9 million year-old fossils from the Yukon. (Credit:听John Meikle)
While exploring the tailings left behind by a now-defunct copper mine, Hodgins and her students, part of a training program for juvenile delinquents,听stumbled across a few fragments of fossils鈥攂its and pieces of what seemed to be teeth alongside pieces of bone.
The ancient fragments of teeth were so small and in such bad shape that听鈥渕ost paleontologists may not have picked them up,鈥� said Jaelyn Eberle, a curator of fossil vertebrates at CU Boulder鈥檚 Museum of Natural History.听
But Hodgins did. Now, more than 40 years after the teacher鈥檚 fateful hike, an international team led by Eberle used modern technology to identify the origins of those enigmatic fossils.听
In a , Eberle and her colleagues report that the fossil tooth fragments likely came from the jaw of a long-extinct cousin of today鈥檚 rhinoceroses. This hefty animal may have tromped through the forests of Northwest Canada roughly 8 to 9 million years ago.
And it鈥檚 a first: Before the rhino discovery, paleontologists had not found a single fossil vertebrate dating back to this time period in the Yukon.听
鈥淚n the Yukon, we have truckloads of fossils from ice age mammals like woolly mammoths, ancient horses and ferocious lions,鈥� said Grant Zazula, a coauthor of the new study and Yukon Government paleontologist. 鈥淏ut this is the first time we have any evidence for ancient mammals, like rhinos, that pre-date the ice age.鈥�
Exploring Beringia
Top: Series of fossils recovered from the Yukon. They are pieces of shells from two different species of turtle (top), a fossil from a听relative of a modern pike fish (middle) and two fragments of ancient rhino teeth (bottom); bottom:听Enamel from a fragment of an ancient rhinoceros tooth听as seen under increasing levels of magnification. (Credits: Grant Zazula; Jaelyn Eberle)
It鈥檚 a gap in the fossil record that scientists have been keen to fill.
To understand why, imagine the Earth during the Tertiary Period, a span of time that began after the dinosaurs went extinct and ended about 2.6 million years ago. In that age, a narrow land bridge called Beringia connected what are today Russia and Alaska.
Paleontologists believe that animals of all sorts, including mammoths and rhinos, poured over that bridge.听
There鈥檚 just one problem: The geology and environment of the Yukon, which sat at the center of that mass migration route, isn鈥檛 conducive to preserving fossils from land animals.听
鈥淲e know that a land bridge must have been in operation throughout much of the last 66 million years,鈥� Eberle said. 鈥淭he catch is finding fossils in the right place at the right time.鈥�
In this case, the people at the right place and at the right time was a Yukon schoolteacher and her students.
When Eberle first saw Hodgins鈥� fossil听teeth, now housed in the Yukon Government's fossil collections in Whitehorse, she didn鈥檛 think she could do much with them. Then she and her colleagues landed on an idea: Eberle put one of the small pieces under a tool called a scanning electron microscope that can reveal the structure of tooth enamel in incredible detail.听
She explained that mammal teeth aren鈥檛 all built alike. The crystals that make up enamel can grow following different patterns in different types of animals, a bit like a dental fingerprint. The Yukon tooth enamel, the team found, carried the tell-tale signs of coming from a rhinoceros relative. 听
鈥淚 hadn鈥檛 thought that enamel could be so beautiful,鈥� Eberle said.
Museum treasures
The method isn鈥檛 detailed enough to determine the precise species of rhino. But, if this animal was anything like its contemporaries to the south, Eberle said, it may have been about the same size or smaller than today's black rhinos and browsed on leaves for sustenance. It also probably didn't have a horn on its snout.
The group also looked at a collection of fossils found alongside the rhino鈥檚 tooth chips. They belonged to two species of turtle, an ancient deer relative and a pike fish. The discovery of the turtles, in particular, indicated that the Yukon had a warmer and wetter听climate than it does today.听
Hodgins, now-retired, is excited to see what became of the fossils she and her students discovered more than four decades ago: It鈥檚 鈥渏ust so wonderful to learn what has developed with them from long ago,鈥� she said.听
Eberle added that the Yukon鈥檚 newly-discovered rhino residents are a testament to the importance of museums.听
鈥淭he fact that these specimens were discovered in the Yukon museum collection makes me really want to spend more time in other collections, including at CU Boulder, looking for these kinds of discoveries that are there but haven鈥檛 had the right eyes on them yet,鈥� Eberle said.
Coauthors on the study include Howard Hutchison of the University of California Museum of Paleontology; Kristen Kennedy of the Yukon Geological Survey; Wighart von Koenigswald of the Rheinische Friedrich-Wilhelms University in Germany; and Grant Zazula of the Yukon Paleontology Program in the Yukon Government.
