New predator “dawn runner”
discovered in early dinosaur graveyard
A team of paleontologists and geologists from Argentina and
the United States on Jan. 13 announced the discovery of a lanky dinosaur that
roamed South America in search of prey as the age of dinosaurs began,
approximately 230 million years ago.
Sporting a long neck and tail and weighing only 10 to 15
pounds, the new dinosaur has been named Eodromaeus, the “dawn runner.”
“It really is the earliest look we have at the long line of
meat eaters that would ultimately culminate in Tyrannosaurus rex near
the end of the dinosaur era,” said Paul Sereno, University of
Chicago paleontologist and National Geographic Explorer-in-Residence.
“Who could foretell what evolution had in store for the descendants of this
pint-sized, fleet-footed predator?”
Sereno and his colleagues describe a near-complete skeleton
of the new species, based on the rare discovery of two individuals found
side-by-side, in the Jan. 14, 2011 issue of the journal Science. The
paper presents a new snapshot of the dawn of the dinosaur era—a key period that
has garnered less attention than the dinosaurs’ demise. “It’s more complex than
some had supposed,” Sereno said.
Set in picturesque foothills of the Andes, the site of discovery
is known as the “Valley of the Moon,” said the report’s lead author, Ricardo
Martinez of Argentina’s National University of San Juan. For dinosaur
paleontologists, it is like no other.
“Two generations of field work have generated the single
best view we have of the birth of the dinosaurs,” Martinez said. “With a hike
across the valley, you literally walk over the graveyard of the earliest
dinosaurs to a time when they ultimately dominate.”
The area was once a rift valley in the southwest corner of
the supercontinent Pangaea. Sediments covered skeletons over a period of five
million years, eventually accumulating a thickness of more than 2,000 feet (700
meters).
Volcanoes associated with the nascent Andes Mountains
occasionally spewed volcanic ash into the valley, allowing the team to use
radioactive elements in the ash layers to determine the age of the sediments.
“Radioisotopes—our clocks in the rocks—not only placed the
new species in time, about 230 million years ago, but also gave us perspective
on the development of this key valley,” said Paul Renne, director of the Berkeley
Geochronology Center in California. “About five million years of time are
represented in these layers, from one end to the other.”
In the oldest rocks Eodromaeus lived alongside Eoraptor,
a similar-sized, plant-eating dinosaur that Sereno and colleagues discovered in
the valley in 1991. Eoraptor’s descendants would eventually include the
giant, long-necked sauropods. Eodromaeus, with stabbing canine teeth and
sharp-clawed grasping hands, is the pint-sized precursor to later meat-eaters
called theropods, and eventually to birds. “We’re looking at a snapshot of early dinosaur life. Their
storied evolutionary careers are just unfolding, but at this point they’re
actually quite similar,” Sereno said.
Eodromaeus at the root of the dinosaur family tree
Vexing scientific questions at the dawn of the dinosaur era
include what gave them an edge over competitors, and how quickly did they rise
to dominance? In Eodromaeus’ day, other kinds of reptiles outnumbered
dinosaurs, such as squat lizard-like rhynchosaurs and mammal-like reptiles. The
authors logged thousands of fossils unearthed in the valley to find, as
Martinez remarked, that “dinosaurs took their sweet time to dominate the
scene.” Their competitors dropped out sequentially over several million years,
not at a single horizon in the valley.
In the red cliffs on the far side of the valley, larger
plant- and meat-eating dinosaurs had evolved many times the size of Eoraptor
and Eodromaeus, but it would be even later when they dominated all land
habitats in the succeeding Jurassic and Cretaceous periods.
“The story from this valley suggests that there was no
single advantage or lucky break for dinosaurs but rather a long period of
evolutionary experimentation in the shadow of other groups,” Sereno said. Other
researchers on the paper tracked climate change and other conditions across the
layers of the valley. “The dawn of the age of dinosaurs,” Martinez remarked,
“is coming into focus.”
Citation
Martinez, R. N., P. C. Sereno, O. A. Alcober, C. E. Colombi, P. R. Renne, I.
P. Montañez, B. S. Currie. A Basal Dinosaur from the Dawn of the Dinosaur Era in Southwestern
Pangaea. Science,
2011; 331 (6014): 206-210 DOI: 10.1126/science.1198467