Discoveries made in the past decades help show how many species coped with cold temperatures near both poles
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| Dinosaurs found in Alaska's Prince Creek formation likely remained in the region when it snowed during the winter. (Julio Lacerda) |
The finds are coming fast and furious. A tiny jaw found in
Alaska’s ancient rock record, and written about in July, indicates that
dinosaurs nested in these places and stayed year-round. In 2018,
paleontologists published a study describing how microscopic details of polar dinosaur bones show that some dinosaurs slowed their growth during harsh
seasons to get by with less. The ongoing identification of new species, not
found anywhere else, highlighted how some dinosaurs adapted to the cold. Each
thread comes together to underscore how wonderfully flexible dinosaur species
were, adapting to some of the harshest habitats of their time.
Understanding when and where polar dinosaurs roamed takes a
little geological imagination. Earth’s continents are always shifting, so the
climates where fossils are found were once different. The environments recorded
in the strata of southern Australia, for example, were further south and within
the Antarctic Circle when dinosaurs thrived there in the Cretaceous. But in
reconstructing the tectonic jigsaw and tracking where fossils have been
uncovered, paleontologists have found dinosaurs that lived near both the
northern and southern poles at different times.
Some of the oldest polar dinosaurs are found among the rocks
of southern Australia’s aptly-named Dinosaur Cove. Over 110 million years ago,
says Monash University paleontologist Patricia Rich, this area was a temperate
rainforest carpeted with ferns and bushy-looking conifers called podocarps. And
while the Cretaceous world was a bit warmer, with no polar icecaps, winter
could still be harsh. “There would have been ice and snow in the
three-month-long, dark winters,” Rich says. Still, a variety of dinosaurs
thrived here, including small, feathery predators, parrot-like oviraptors and
Leaellynasaura, a small herbivore that walked on two legs and had one of the
longest tails for its body size of any dinosaur.
Some dinosaurs might have dug in to survive the harshest
months. Paleontologists working in southern Australia’s strata have found
burrow-like structures from the age of Leaellynasaura, and elsewhere these
structures actually contain small, herbivorous dinosaurs. “It’s possible that
dinosaurs might have burrowed as a way to escape the cold,” says paleontologist
Adele Pentland of the Australian Age of Dinosaurs Museum of Natural History.
“The clearest evidence we have of polar adaptations, or not,
is the composition of the fauna,” adds Monash University paleontologist Steve
Poropat. Which types of dinosaurs are found in cooler places, as opposed to
those that are missing, offers some insights into which dinosaurs were better
able to cope with or adapt to the long polar nights. “Theropods, ornithopods,
ankylosaurs? No problem. You find them at heaps of sites throughout Victoria,”
Poropat notes, referencing the state in southeastern Australia. These types of
dinosaurs could withstand the cold and dark months. But long-necked,
plant-eating dinosaurs called sauropods that lived at the same time are missing
from the same sites, which suggests that they were not able to survive or adapt
to the colder environments.
The Antarctic Circle wasn’t the only place to host
chill-adapted dinosaurs. The 70 million-year-old rock of Alaska’s Prince Creek
Formation contains the fossils of horned dinosaurs, tyrannosaurs, duckbilled
dinosaurs, raptors and more that lived within the Arctic Circle. And when these
dinosaurs began to catch researcher’s attention during the 1980s, they
presented some challenges to what paleontologists thought about dinosaur lives.
“When dinosaurs were first found in the Arctic, they
presented some serious problems to our understanding of dinosaurian
physiology,” Perot Museum of Nature and Science paleontologist Tony Fiorillo
says. Even as paleontologists considered that dinosaurs might keep warmer body
temperatures, the harshness of the Arctic cold was thought to be too much. Some
experts proposed that dinosaurs might migrate, drawing an analogy to modern-day
caribou, which don’t migrate long distances north and south, Fiorillo says.
Various lines of evidence indicate that the dinosaurs remained in their home
habitat through the winter. Just this past year, Fiorillo and colleagues were
the ones who published on a jaw from a very young raptor—evidence that
dinosaurs were nesting in the region and not just passing through.
The landscape would have looked a little familiar. At the
time the Prince Creek Formation was being laid down, Fiorillo says, the area
was similar to what it’s like today—a coastal plain dominated by stands of
conifers and flowering plants low to the ground. And while overall warmer than
the same spot today, it still got cold enough to snow during the winters.
Alaska’s dinosaurs had to contend with some of the same
stresses as their southern counterparts—such as harsher changes in seasons and
months of darkness—but evidence from their bones indicate that these dinosaurs
stayed year-round. Much like their relatives elsewhere, polar dinosaurs grew
fast when they were young but switched to more of a stop-and-start growth pattern as they got older. This means that polar dinosaurs were already
biologically predisposed to surviving on less during the cold months, with the
dinosaurs growing faster again during the lush summers. While certainly chilly
during the winter, the ground did not freeze in these places, providing enough
vegetation to support an ecosystem of resident dinosaurs.
There may have been no one way that dinosaurs adjusted to
the comparative harshness of life near the pole. The local tyrannosaur in the Prince
Creek Formation was not a familiar species seen elsewhere, but a unique and
smaller predator—roughly the size of a polar bear— that Fiorillo and colleagues
dubbed Nanuqsaurus. The comparatively small stature of this dinosaur, as well
as the downsized species of horned dinosaur called Pachyrhinosaurus in the
area, hints that types of dinosaurs that grew big elsewhere adapted to become
smaller and thereby get by on less food in the cool of ancient Alaska.
But some polar dinosaurs truly thrived. The raptor-relative
Troodon was a feathery, eight-foot-long dinosaur with large eyes. While rare
elsewhere, Fiorillo says, “it is the overwhelmingly abundant theropod
dinosaur.” The small-carnivore’s large eyes may have given it an advantage,
especially during the dark months.
Our visions of polar dinosaurs are still relatively new.
Determining which species lived in cooler areas is part of that task. Some,
like Nanuqsaurus which was named in 2014, are new. Others turn out to be
familiar—a duckbill dinosaur previously thought to be a new species has turned
out to be Edmontosaurus, a wide-ranging hadrosaur found elsewhere. “For me,”
Fiorillo says, “the story is even more fascinating knowing that some Arctic
dinosaurs became specialists within the ancient north while others were
generalists capable of surviving a wide array of environmental conditions.”
Many finds are left to be made, not just among the dinosaurs
but about the big picture of the habitats where they lived. “Discoveries are
being made every day,” Rich says, noting that fieldwork just this year in the
time of Leaellynasaura has uncovered dinosaur tracks, turtle shells, tree
trunks with termite damage and more, all parts of a lost polar world. Finds
like these will continue to highlight just how successful dinosaurs were, a
testament to their prehistoric versatility. In virtually any ancient landscape,
dinosaurs found a way.
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