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Monday, October 22, 2007

Pregnant Dinosaurs

Dinosaurs probably did not enjoy many carefree teenage years, since most were parents before they reached adulthood, according to recently announced research.

The find puts dinosaurs on the list of animals that had teenage pregnancies. Others on the list include crocodiles, lizards and humans.

The discovery, announced in Austin at the Society of Vertebrate Paleontology's annual meeting, also suggests why evolution favors early births in these groups.

"Dinosaurs did pretty much what we do and what most other vertebrates do," explained co-presenter Andrew Lee. "If these species had waited until full size to reproduce, they would have had very few years in which to produce offspring."

Lee, a microanatomy instructor and post-doctoral fellow at Ohio University, and colleague Sarah Werning extracted bone tissue from three types of dinosaurs: Tyrannosaurus rex, Allosaurus and Tenontosaurus. The latter was a medium sized, plant-eating cousin of duck-billed dinos, while the former two were big carnivores, so the sampling represented a broad spectrum.

Three of the examined dinos possessed a special type of bone tissue called medullary that was used as temporary calcium storage before eggshells were made.

Lee explained to Discovery News that living birds possess this same tissue, which they form a few weeks before they are ready to create eggs. This means the analyzed dinosaurs were females that were ready to lay eggs just before they died. Their cause of death remains unknown.

Based on the growth stages of the dinosaurs, the scientists determined that T. rex was laying eggs by the age of 18, Allosaurus by age 10 and Tenontosaurus by the very young age of 8. Individuals within all three species would have reached full adult size between their 17th and 21st years. Their lifespan was from 25-30 years.
Since smaller animals tend to reach sexual maturity earlier than larger animals, the smallest of the three dinosaurs studied, Tenontosaurus, was on a slightly faster track.

Florida State University paleontologist Gregory Erickson and his team recently came to similar conclusions using a different technique.

Instead of extracting bone tissue, Erickson and his colleagues counted growth rings in the bones of T. rex and other related dinosaurs, all of which had been found buried with brooding eggs.

Erickson and his group also concluded that T. rex gave birth before it reached full maturity.

"We now know that T. rex lived fast and died young," he said.

While birds are the modern descendants of dinosaurs, birds never give birth as teens. They finish growing and then may wait a year, or even longer, before reproducing.

Lee thinks several factors caused birds to change from their dino birthing ways.

"Flight is definitely part of the answer," he said. "Most birds need to be able to fly when they fledge, so they have to reach adult size."

Birds also now require a steep learning curve for survival, and the learning process takes time. Additionally, he said, "If birds matured early, their offspring might be born during the late summer or fall when food for growth is more scarce."

As for future humans losing their ability to produce as teens, Lee believes that's unlikely.

He said, "There is little benefit to delay sexual maturity in humans, so I can't imagine that selection might operate to delay sexual maturity before reaching adult size."

Posted by Ajay :: 5:45 PM :: 0 comments

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