Embargo Till: 19:00 UTC December 10, 2009
19:00 UTC 12/10/2009
Dino Species: Early Meat-Eaters Crossed Continents
Thursday, December 10, 2009
Randall Irmis, University of Utah
New Dinosaur Species Tawa
hallae from New Mexico Had Evolutionary Roots in South America
reconstruction of the new Triassic carnivorous dinosaur Tawa
Nesbitt (left) and Michelle Stocker (right) excavate a new
dinosaur specimen at Hayden Quarry, Ghost Ranch, New Mexico.
Did the first
dinosaurs wander across continents or stay put where they first
evolved? The first dinosaurs evolved 230 million years ago when
the continents were assembled into one landmass called Pangea.
The question of early dinosaur movements remained unclear until
the discovery of some exciting new fossils.
In the Dec.
11, 2009, issue of Science, a team of paleontologists
presents the 213-million-year-old fossils of previously unknown
carnivorous dinosaur Tawa hallae, including several of the best
preserved dinosaur skeletons from the Triassic Period.
bones of Tawa, named after the Hopi word for the Puebloan sun
god, were recovered from a dig site in northern New Mexico known
as Hayden Quarry. The quarry is located on Ghost Ranch, where
late painter Georgia O’Keefe once lived. Fossil bones of
several individuals were recovered, but the type specimen is a
nearly complete skeleton of a juvenile that stood about 28 inches
(70 centimeters) tall at the hips and was approximately 6 feet
(about 2 meters) long, from snout to tail. Its body was about the
size of a large dog, but with a much longer tail.
an analysis of the relationships among Tawa and other early
dinosaurs, the researchers hypothesize that dinosaurs originated
in a part of Pangea that is now South America, diverging into
theropods (like Tyrannosaurus rex), sauropodomorphs (like
Apatosaurus) and ornithischians (like Triceratops); and then
dispersed more than 220 million years ago across parts of Pangea
that later became separate continents.
dinosaur Tawa hallae changes our understanding of the
relationships of early dinosaurs, and provides fantastic insight
into the evolution of the skeleton of the first carnivorous
dinosaurs” said Randall Irmis of the Utah Museum of Natural
History and University of Utah, a co-author of the study.
addition to Irmis, authors of the study included lead author
Sterling Nesbit of the University of Texas at Austin; Nathan
Smith of the University of Chicago and the Field Museum of
Natural History; Alan Turner of Stony Brook University; Alex
Downs of the Ruth Hall Museum of Paleontology in Abiquiu, N.M.;
and Mark Norell of the American Museum of Natural History.
you have continents splitting apart, you get isolation,”
said Nesbitt. “So when barriers develop, you would expect
that multiple carnivorous dinosaurs in a region should represent
a closely related endemic radiation. But that is what we don’t
see in early dinosaur evolution.”
research team found three distinct carnivorous dinosaurs –
including the newly discovered Tawa – in the fossil-rich,
Late Triassic beds they investigated at Ghost Ranch. “When
we analyzed the evolutionary relationships of these dinosaurs, we
discovered that they were only distantly related, and that each
species had close relatives in South America,” said Irmis.
“This implies that each carnivorous dinosaur species
descended from a separate lineage before arriving in [the part of
Pangea that is now] North America, instead of all evolving from a
At Ghost Ranch, the researchers
found fossils from a carnivorous dinosaur related to Coelophysis,
common to that region, and fossils from a carnivore closely
related to Herrerasaurus, which lived in South America. The 6.6-
to 13-foot-long (2- to 4-meter-long) skeletons of Tawa display
characteristics that exist in both species and features found in
neither, implying a separate lineage.
of multiple dinosaur species in one place that emigrated from
elsewhere got us wondering whether other Late Triassic reptiles
show similar patterns” said Irmis. “It turns out a
variety of other reptile groups made multiple trips from the
northern and southern continents [then parts of Pangea] and back
again during the Late Triassic, including other
Because so many different groups with
different life modes were able to move freely across Pangea, the
research team concluded that during the Late Triassic, there were
no major physical barriers, such as large mountain ranges, to the
movement of reptiles between parts of Pangea that later separated
into distinct continents.
But this presented a paradox to
the team: “We wondered: if reptiles, including dinosaurs,
were able to freely move around Pangea during the Late Triassic,
then why aren’t there any sauropodomorph and ornithischian
dinosaurs in North America during the Triassic?” said
Irmis. “Our conclusion is that climate, possibly related to
latitude, controlled the distributions of some reptile
“We think that all the major
dinosaur groups had the ability to get to North America [part of
Pangea] during the Late Triassic, and may have even passed
through, but for some reason, only the carnivorous dinosaurs
found the North American climate to be hospitable during this
time,” concluded Irmis.
The first Tawa fossils were
discovered in 2004 by volunteers taking a paleontology seminar at
the Ruth Hall Museum of Paleontology in New Mexico. Museum
scientists invited the team of paleontologists to come and take a
“The specimens are unusual because they are so
well preserved,” said Irmis. “Because dinosaur bones
are hollow, they are usually broken and crushed, but those of
Tawa are nearly pristine.”
Irmis and the rest of
the team began a full-scale excavation in 2006 and have continued
to unearth new material every summer since then. The fossil bone
bed extends for tens of yards along a hillside, promising many
years of potential significant finds.
The research was
supported by the National Science Foundation (NSF) through grants
to Mark Norell, Alan Turner, Nathan Smith and NSF Graduate
Research Fellowships to Sterling Nesbitt and Randall Irmis. The
research was also featured in the NSF-funded IMAX 3-D movie
Research was additionally
sponsored by the National Geographic Society with other
participating institutions including the University of Chicago,
the Field Museum of Natural History, the University of Utah,
Stony Brook University and the Ruth Hall Museum of Paleontology.
University of Utah
supported in part by “Readers Like You”