How Old Was Joe?

“Joe” certainly was a small dinosaur, compared to adults of its kind. It was only six feet long (compared to over 25 feet long for adult duck-billed dinosaurs), so “Joe” couldn’t have been fully grown. But, could scientists get any more precise?

A little bone biology turned out to be just the ticket to aging “Joe.” Even though bones are hard, they are actually made up of a dynamic, living tissue that undergoes constant remodeling, replacement, and growth. Many animals alive today lay down yearly layers in their bones—just like tree rings! Dinosaurs were no exception. So, if you slice up a dinosaur bone and count the rings under a microscope, you can figure out how old it was when it died.

This is a slice of bone tissue taken from the shin bone of <i>Tenontosaurus</i>, a cousin to <i>Parasaurolophus</i>. The five arrows point to five of the yearly rings. This  that tell us this animal was at least five years old when it died. Image modifed from a paper by Sarah Werning (link: http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0033539), specimen in the collections of the Sam Noble Oklahoma Museum of Natural History (link: http://www.snomnh.ou.edu/)

This is a slice of bone tissue taken from the shin bone of Tenontosaurus, a cousin to Parasaurolophus. The five arrows point to five of the yearly rings. This that tell us this animal was at least five years old when it died. Image modified from a paper by Sarah Werning, specimen in the collections of the Sam Noble Oklahoma Museum of Natural History.

Let’s take a closer look at these yearly rings! Under a microscope, every tiny detail of the bone was revealed, even the spaces once occupied by blood vessels and bone cells.

This is a close-up shot of the above <i>Tenontosaurus</i> shin bone showing what it looks like under a microscope. The white arrow points to a yearly ring. The red arrows point to spaces where blood vessels once coursed through the bone. All of the little pepper-like dots on the image are bone cells! Image modifed from a paper by Sarah Werning (link: http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0033539), specimen in the collections of the Sam Noble Oklahoma Museum of Natural History (link: http://www.snomnh.ou.edu/).

This is a close-up shot of the above Tenontosaurus shin bone showing what it looks like under a microscope. The white arrow points to a yearly ring. The red arrows point to spaces where blood vessels once coursed through the bone. All of the little pepper-like dots on the image are bone cells! Image modifed from a paper by Sarah Werning, specimen in the collections of the Sam Noble Oklahoma Museum of Natural History.

What do Joe’s bones look like? The researchers studying “Joe” took a small sample from the shin bone, and sliced it up very, very thin.

A close-up shot of the bone

A close-up shot of the bone in “Joe”. Just as in the pictures above, the red arrows point to some of the spaces where blood vessels once ran, and the tiny pepper-like dots are bone cells.

“Joe”’s bone showed signs that it was growing very fast—lots of big blood vessels, but no yearly rings. This meant “Joe” was a very young animal—probably under a year in age! In under 12 months, “Joe” had hatched at less than the size of a human infant and grown to over six feet in length. It takes a human 15-20 years to grow that much.