Dinosaurs became sexually active as half-grown adolescents and were
able to get pregnant as early as age 8, according to a new study.
Allosaurus, a carnivorous relative of Tyrannosaurus rex
from the Jurassic Period, and Tenontosaurus, a herbivorous relative of
the duckbilled dinosaurs, became pregnant well before they were full
grown.
Scientists estimate Allosaurus took about 20 to 30 years to reach full maturity and Tenontosaurus about 15 to 20 years.
The study team cut open the arm and leg bones from 10 to 20 specimens
of these two species of dinosaurs, ranging from juvenile to almost
fully-grown. Similar to tree rings, growth lines in bones can be used
to determine the age and growth rate of dinosaurs
and other reptiles. The team found that it took these dinosaurs several
years to three decades to grow up, and they most likely didn't live
much longer afterwards.
“If you only live to be 30 or so and it takes about 25 years to finish
growing, it really limits the amount of time you have to reproduce if
you wait until you're done growing,” said study co-author Sarah
Werning, a graduate student at University of California, Berkeley.
“There's a definite advantage in doing it young rather than waiting.”
Dinosaurs,
which laid eggs, deposited calcium in the marrow cavity of their bones
just before laying eggs as a resource for making eggshells. Werning and
her colleagues found that the leg bones of Allosaurus and Tenontosaurus
contained a layer of calcium-rich bone tissue called medullary bone,
indicating these adolescents died shortly before laying eggs. They also
found this structure in T. rex based on data from other studies.
“Life was tough for dinosaurs,” said study co-author Andrew H. Lee, a
postdoctoral fellow at Ohio University's College of Osteopathic
Medicine in Athens. “Maturity while still growing is common in animals
that have fairly precocial offspring and don't live long after reaching
full-size.”
The finding shows that dinosaurs grew like birds but had a reproductive strategy similar to mammals and crocodiles.
“In the family tree of life, birds descended from dinosaurs, but dinosaurs are also cousins to crocs and alligators,” Werning told LiveScience.
Birds grow very fast, finish growing within a year, and don’t reproduce
until after they’re finished growing. Crocs grow much slower but start
reproducing before they’re done growing.
“We know dinos had a lot in common with birds, but we weren't sure when
they reproduced. The evidence we had before this wasn't conclusive,”
she added.
The study is detailed in the Jan. 15 issue of the journal Proceedings of the National Academy of Sciences.
