A new dinosaur called Ingentia prima appeared to have
a air sacs in its neck, shown in this artist's impression in green, and walked
on flexed feet. (Supplied: Jorge A. González)
ABC Science By science reporter Belinda Smith
Think of a plant-eating dinosaur, and chances are you
picture something along the lines of a brachiosaurus.
These iconic giants belonged to a group of dinosaurs called
sauropods — massive creatures with thick, column-like legs and a long neck and
tail.
Now, the discovery of a new dinosaur species suggests
gigantism in sauropods evolved about 30 million years earlier than previously
thought.
The newly uncovered bones, belonging to a plant-eater dubbed
Ingentia prima, also suggest that there were a couple of different ways these
giant dinosaurs evolved.
The fossil find was unveiled in Nature Ecology and Evolution
today by an Argentinian crew led by National University of San Juan
palaeontologist Cecilia Apaldetti.
"Now we are rethinking the evolution of giant size in
dinosaurs," Dr Apaldetti said.
Most palaeontologists think sauropod dinosaurs first
appeared on the scene in the early Jurassic period, starting with the 11-metre
Vulcanodon about 180 million years ago.
The discovery of the Ingentia prima — a dinosaur that
existed about 205 million years ago and was about 8-10 metres long — has
challenged this idea.
But whether the new dinosaur walked on two legs or four, and
was a sauropod at all, is up for debate.
The story of Ingentia prima starts with a cattle farmer in
northern Argentina.
"He [told the local museum], 'I have seen bones inside
rocks but they don't seem to be from cows'," Dr Apaldetti said.
When she and her colleagues went to check them out, they
found a rich trove of fossils dating back to the Triassic and Jurassic periods,
or 190 million to 210 million years ago.
Today, the region is dry and scrubby. But back when the
fossilised specimens were living, breathing animals, it was part of the
supercontinent Pangaea.
Being near the equator, the climate was warm with periodic
rains, Dr Apaldetti said: "Something similar to a current savannah,
probably with a large amount of shrubs to satisfy the great herbivores of that
time."
Among the fossilised remains, in 2015 they found a new
dinosaur species — Ingentia prima — alongside other, previously known relatives
called Lessemsaurus sauropoides.
Weighing in at an estimated 7–10 tonnes, both dinosaurs
sported a long neck and tail, as well as air sacs in their body, like later
sauropods did.
Ingentia prima, a new dinosaur species discovered in
Argentina, was about 8 to 10 metres long.(Supplied: Jorge A González)
Air sacs are thought to help keep the massive beasts cool.
But unlike their more recent four-legged counterparts, which
stood on straight tree-trunk-like legs, Ingentia prima seemed to stand on
flexed feet.
Their bones also showed signs of seasonal growth, also
different to how palaeontologists think later sauropods grew.
Dinosaurs like brachiosaurus, Dr Apaldetti said, probably
grew at a fairly rapid but consistent rate.
Ingentia prima's bones show it grew even faster at times,
but slowed at others.
"With this discovery we can see that the first steps
toward gigantism occurred 30 million years before the giants dominated
practically the entire planet," Dr Apaldetti said.
"It [also] shows that there were other ways to be
giant, and not necessarily implied the same anatomical changes that all other
giants such as the titanosaurs required."
Four legs or two?
But Steve Salisbury, a palaeontologist at the University of
Queensland who was not involved with the study, isn't convinced that Ingentia
prima was a sauropod.
Rather than walking on four legs, he suspects they probably
walked on two.
Scientists are hopeful the discovery of a fossilised
dinosaur skeleton in outback Queensland will shed light on what was on the menu
around 95 million years ago.
"Dinosaurs similar to Ingentia prima are typically
referred to as 'basal sauropodomorphs', meaning they are on the side of the
dinosaurian family tree that includes sauropods, but they are not there quite
yet," he said.
"Most basal sauropodomorphs are thought to have walked
on their hind legs like their meat-eating cousins, the theropods, but had a
long neck and ate plants, like their descendants, the sauropods."
The new fossil haul was missing a few vital bones, such as
Ingentia prima's hip, thigh bone and most of the lower leg.
Without them, Dr Salisbury said, it's hard to gauge the
animal's posture.
And based on the few bones reported in the paper,
"Ingentia prima's hand doesn't look ideal for putting on the ground,"
he added.
"They don't have a lot of the skeleton and, overall,
the bits that are there don't look all that different from other basal
sauropodomorphs."
So Ingentia might have been able to get on all fours, but
probably spent most of its time on two legs — a little like another basal sauropodomorph
that lived about 210 million years ago called Plateosaurus engelhardti, which
is known from much more complete material.
"Although Ingentia prima looks like it was large for a
basal sauropodomorph, unfortunately, they just don't have enough in this paper
to make it appear too different to something like Plateosaurus engelhardti, and
it's pretty much agreed that it was bipedal," he said.
"If that's the case, the transition to four-legged
locomotion must have happened later."
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