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 A new study provides a step-by-step account of the evolution of
this distinctive feature possessed by the heavily armored dinosaur
Ankylosaurus and its cousins, a bludgeon that may have given even
the ferocious Tyrannosaurus rex reason to worry.
The researchers studied fossils of the group called ankylosaurs
including early, primitive species with no tail club and later ones
with a fully developed one.
Ankylosaurs began to evolve tail clubs much earlier than previously
thought, the researchers found, and the clubs evolved in two steps
over tens of millions of years.
 First, vertebrae in the back part of the tail changed so that the
tail became stiff. Next, bones that form in the skin to provide body
armor, called osteoderms, became very large at the tip of the tail
and completely enveloped the tail's end to form a club that could be
swung at an enemy.
Ankylosaurs lived at a time when the largest land predators in
Earth's history including T. rex roamed the landscape, dismembering
other dinosaurs with powerful jaws and serrated teeth. In an arms
race, some plant-eaters developed defensive weaponry.
"A tail club was definitely an effective weapon and could have
broken the ankle of a predator," said paleontologist Victoria Arbour
of North Carolina State University and the North Carolina Museum of
Natural Sciences, who led the study published this week in the
Journal of Anatomy.
"But in living animals today, weapons are also often used for
battling members of your own species – consider the horns of bighorn
sheep or the antlers of deer – so perhaps ankylosaurs did something
similar."
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Ankylosaurs were wide-bodied, four-legged dinosaurs covered in bony
plates and spikes. The oldest known ankylosaur dated from around 160
million years ago during the Jurassic Period, Arbour said. The first
fully formed ankylosaur tail club appeared around 75 million years
ago during the Cretaceous.
Ankylosaurus, measuring around 20 feet (6 meters), was the largest
and last of the ankylosaurs, living at the end of the age of
dinosaurs about 65 million years ago.
Ankylosaurs from China were crucial for understanding the tail
club's origins, including Gobisaurus, from about 92 million years
ago, and Liaoningosaurus, from about 122 million years ago, Arbour
said.
(Editing by Eric Walsh)
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