'Terror crocs' grew
more slowly than their dinosaur cousins
BY DAVID F. SALISBURY
Imagine a crocodile that
is more than 30 feet long and weighs about 10,000 pounds.
Such a creature actually
existed 80 million years ago, when dinosaurs still ruled
the Earth. Paleontologists Gregory Erickson, a
postdoctoral fellow in mechanical engineering at Stanford
University, and Christopher A. Brochu at the Field Museum
of Natural History in Chicago, have figured out how they
grew so large.
The giant crocs, dubbed Deinosuchus,
or terror crocodile, needed their large stature to
compete with the dinosaurs that also tended to be
extremely large by today's standards. The prehistoric
crocs achieved their large size by growing at about the
same annual rate as modern crocodilians but continuing to
grow for decades longer, the researchers report in the
March 18 issue of the journal Nature.
The researchers studied
the annual growth layers in the fossilized bony armor
plates called scutes from two terror crocs that were
unearthed in Texas and Montana. By comparing them with
the scutes of existing crocodilians, the researchers have
found that the Deinosuchus grew a foot a year,
about the same as modern crocodiles, but they continued
to grow for a much longer period, taking about 35 to 40
years to reach adult size. By comparison, today's
crocodiles seldom live beyond the age of 30 in the wild.
"This is a much
different growth pattern from that found in
dinosaurs," says Erickson. "For example,
duckbilled dinosaurs that were about the same size as Deinosuchus
grew to adult size in only seven to eight years. It
illustrates one of the key differences between dinosaurs
and other reptiles."
Scutes are bony plates
within the skin of crocodiles that serve as body armor
located on their back and sides. The plates contain a
series of layers that are laid down annually. The
thickness of a given layer is proportional to how much
the animal grew that year. The scientists counted these
layers to determine the age of the terror crocs that they
examined. Then, by taking the relative thickness of the
annual layers into account, the researchers reconstructed
how much each of the crocs grew each year.
They also examined jaws,
ribs, vertebrae and other long bones on the Deinosuchus
skeletons. They found that the giant crocs of yesteryear
had the same type of slow-growing bones, called
lamellar-zonal bones, as do today's crocodilians: a
finding consistent with the slow growth indicated by
their analysis of the scutes.
This slow but steady
growth pattern is characteristic of cold-blooded
creatures like reptiles. At one time, scientists thought
dinosaurs grew in a similar fashion and reached such
large sizes because they continued growing for a long
time, like the terror croc. In the last 20 years,
however, they have found considerable evidence that
dinosaurs grew much more quickly, at rates comparable to
those of large mammals today. This has led some
paleontologists to argue that dinosaurs must have had a
high metabolic rate, one comparable to warm-blooded
animals. But exactly how dinosaurs managed to grow so
large and so fast remains a scientific mystery. SR
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