Paleontologists have discovered a group of more than 20
polar dinosaur tracks on the coast of Victoria,
Australia,
offering a rare glimpse into animal behavior during the last period of
pronounced global warming, about 105 million years ago.
The discovery, reported in the journal Alcheringa, is the largest and best collection of polar dinosaur
tracks ever found in the Southern Hemisphere.
“These tracks provide us with a direct indicator of how
these dinosaurs were interacting with the polar ecosystems, during an important
time in geological history,” says Emory University
paleontologist Anthony Martin, who led the research.
The three-toed tracks are preserved on two sandstone blocks
from the Early Cretaceous Period. They appear to belong to three different
sizes of small theropods—a group of bipedal, mostly carnivorous dinosaurs whose
descendants include modern birds.
The research team also included Thomas Rich, from the Museum of Victoria;
Michael Hall and Patricia Vickers-Rich, both from the School
of Geosciences at Monash University
in Victoria; and Gonzalo Vazquez-Prokopec, an ecologist and expert in spatial
analysis from Emory’s Department of Environmental Studies.
The tracks were found on the rocky shoreline of remote Milanesia Beach,
in Otways National Park. This area, west of Melbourne, is known for
energetic surf and rugged coastal cliffs, consisting of layers of sediment
accumulated over millions of years. Riddled with fractures and pounded by waves
and wind, the cliffs occasionally shed large chunks of rock, such as those
containing the dinosaur tracks.
One sandstone block has about 15 tracks, including three
consecutive footprints made by the smallest of the theropods, estimated to be
the size of a chicken. Martin spotted this first known dinosaur trackway of Victoria June 14, 2010,
around noon. He was on the lookout, since he had earlier noticed ripple marks
and trace fossils of what looked like insect burrows in piles of fallen rock.
“The ripples and burrows indicate a floodplain, which
is the most likely area to find polar dinosaur tracks,” Martin explains.
The second block containing tracks was spotted about three
hours later by Greg Denney, a local volunteer who accompanied Martin and Rich
on that day’s expedition. That block had similar characteristics to the first
one, and included eight tracks. The tracks show what appear to be theropods
ranging in size from a chicken to a large crane.
“We believe that the two blocks were from the same rock
layer, and the same surface, that the dinosaurs were walking on,” Martin
says.
The small, medium, and large tracks may have been made by
three different species, Martin says. “They could also belong to two
genders and a juvenile of one species—a little dinosaur family—but that’s
purely speculative,” he adds.
The Victoria Coast marks the seam where Australia
was once joined to Antarctica. During that
era, about 115 to 105 million years ago, the dinosaurs roamed in prolonged polar
darkness. The Earth’s average temperature was 68 F—just 10 degrees warmer than
today—and the spring thaws would cause torrential flooding in the river
valleys.
The dinosaur tracks were probably made during the summer,
Martin says. “The ground would have been frozen in the winter, and in
order for the waters to subside so that animals could walk across the
floodplain, it would have to be later in the season,” he explains.
Lower Cretaceous strata of Victoria have yielded the best-documented
assemblage of polar dinosaur bones in the world. Few dinosaur tracks, however,
have been found.
In the February 2006, Martin found the first known
carnivorous dinosaur track in Victoria,
at a coastal site known as Dinosaur Dreaming.
In May 2006, during a hike to another remote site near Milanesia Beach,
he discovered the first trace fossil of a dinosaur burrow in Australia. That
find came on the heels of Martin’s co-discovery of the first known dinosaur
burrow and burrowing dinosaur, in Montana.
The two discoveries suggest that burrowing behaviors were shared by dinosaurs
of different species, in different hemispheres, and spanned millions of years
during the Cretaceous Period.
