WHAT IS claimed to be the world’s oldest dinosaur has been uncovered
by Argentine researchers in the foothills of the Andes. The fossil, which
is nearly 230 million years old, is the nearly complete skeleton of a bipedal
carnivore that was between 2 and 2.5 metres tall and weighed more than 100
kilograms. ‘It gives us the clearest idea of the primitive early stages
of dinosaur evolution that we ever had,’ said Paul Sereno of the University
of Chicago last week. He went on the Argentine expedition, which was led
by researchers from the University of San Juan.
Dinosaurs evolved during the Triassic Period between 250 and 200 million
years ago. Researchers have found evidence of ‘protodinosaurs’, which had
a dog-like stance and resembled long-legged crocodiles, originating from
the early Triassic. However, until the Argentine discovery, the earliest
complete dinosaur fossils to be found probably originated later in the Triassic
(This Week, 23 May 1985). By that time, dinosaurs had split into two major
families, the ‘bird-hipped’ ornithischians such as the duck-billed dinosaurs
and the stegosaurs, and the ‘lizard-hipped’ saurischians, such as the giant
sauropod dinosaurs and Tyrannosaurus.
The expedition, led by Alfredo Monetta of the University of San Juan,
found two nearly complete fossils of the same species, called Herrerasaurus,
in sandstone deposits by a river. They also found many fragmentary fossils
of similar animals. Lightly built, and with a long, bird-like skull, Herrerasaurus
was an active predator, which ran on its two hind legs, chasing live prey.
Its front limbs were short and had very long claws and semi-opposable thumbs,
evidently for grasping prey. It was named after Victorino Herrera, an amateur
fossil hunter who discovered the first fragmentary bones many years ago.
Palaeontologists draw the line between protodinosaur and dinosaur on
the basis of changes in the hips and hind limbs that determined posture.
Herrerasaurus and other dinosaurs like it had a more efficient, upright
stance than the more dog-like protodinosaurs. Sereno believes that Herrerasaurus
was not a direct ancestor of later dinosaurs, but was closely related to
more primitive dinosaurs: it had many primitive dinosaurian features. However,
it also had some surprisingly advanced features – notably a dual-hinged
jaw, which did not appear in other dinosaurs for some 50 million years.
This finding indicates that it was on an evolutionary side branch. That
extra hinge behind the teeth allowed Herrerasaurus to hold its prey more
tightly than with a single-hinged jaw.
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The Argentines had earlier found fragmentary Herrerasaurus fossils in
the same deposits. The best-preserved fossil was a fortuitous discovery;
its skull and neck were just starting to weather out from the rock. ‘You
might have walked right by it 10 years ago,’ said Sereno. Although the animal
was comparatively small, its skeleton was so well preserved that tiny earbones
and plates in the iris of the eyes showed in the rock. The Argentines are
still extracting the fossil from the rock; when they have finished, they
will display the find at the San Juan Museum, part of the university.