Up to 1.7 billion T. rex dinosaurs lived on Earth, a new study found. But scientists aren't sure where all the bones went

Link to Business Insider article

Dinosaurs are popular topics for people of all ages. We have a fascination for them but especially the big ones. Tyrannosaurus rex (T. rex, "tyrant lizard king") is probably the most well known. But dinosaurs ranged in size from small dogs or horses to the behemoths like T. rex. So, how many dinosaurs lived? Charles Marshall, who is a paleontologist at the University of California, Berkeley, has recently done some calculating to answer that question for T. rex.

T. rex illustration from the article

Marshall and his team looked at reptiles alive today and came up with a ratio of their body size and the population density. In other words, how big they are has some bearing on how many can live in a certain number of square miles. That number was used to estimate the density, distribution, total biomass, and persistence of T. rex using fossil evidence we have now. They claim the numbers showed over 2 billion T. rexes lived overall (about 20,000 at any one time). Most T. rex fossils have been found in North America but some also in Asia and Australia. Only 32 complete skeletons have been found so far.

Range of T. rex in western North America

But the Business Insider article explains that Eva Griebeler from the Johannes Gutenberg University of Mainz, Germany figured slightly fewer of the T. rex species existed, about 1.7 billion. She said Marshall's number came from calculating one figure incorrectly. It's their rate of survival. We have no living T. rex today, so we have to estimate how long each one lived. But Griebeler used other animals still living for her calculations, as follows.

For small, birds, mammals, and lizards alive today, their survival looks like a straight line on a graph, with high percent of survival for young animals and lower as they get older. But the survival of large, long-lived reptiles living today are a different shape on the graph, and that data was what she used instead. Her paper carefully explained with very complex math just what the differences were compared to Marshall's work, and Marshall later complimented her on a better estimation of T. rex populations. Take this link for a simpler explanation.

Examples of 3 types of survivorship curves (Wikipedia)

Studying dinosaurs is difficult because they are no longer around. We have to rely on fossils and other evidence that can be found around them. But what is a fossil anyway? It's Latin name means "unearthed", which is a clue.

According to the British Geological Survey:

Fossils are the preserved remains of plants and animals whose bodies were buried in sediments, such as sand and mud, under ancient seas, lakes and rivers. Fossils also include any preserved trace of life that is typically more than 10,000 years old.

The first dinosaur fossil that we recognize was found in 1676 by Reverend Robert Plot, a curator of an English museum and a professor of chemistry. He found a 20-pound lower part of a thigh bone and thought it belonged to an ancient species of giant humans from the Bible. Although the bone was lost over time, drawings he made of it suggested many years later that it actually belonged to a dinosaur Megalosaurus. 

Plot's 1676 dinosaur bone and publication (Wikipedia)

Many other dinosaur bones were discovered after Plot. In 1827, geologist William Buckland was the first to accurately describe some as belonging to a giant lizard, the Megalosaurus ("large lizard"). But it wasn't until 1841 when English anatomist Sir Richard Owen came up with the name "dinosaur", meaning "terrible lizard". 

Megalosaurus fossils in the Oxford University Museum of Natural History

Fossils are usually found in sedimentary rock, the type that is formed when soil and other organic material settled (sediments) to the bottom of water. But they don't stay around as bone material. Skin and organs decay rapidly, but bones last longer. Since dinosaurs were around 65 to 240 million years ago, something has to preserve even hard material such as bone for that long a time.

• If the buried bone is exposed to mineral-rich fluids with a lot of minerals in them, the minerals move into the bone and replace the organic (carbon-containing) material to make stone fossils.
• Soft material like leaves can be compressed onto soft mud by top layers of sediment.

Photo from the British Geological Survey

• Another type of fossilization process is when a shell or bone decays completely, but the empty space that it leaves behind is filled in with sediment. This leaves a mold of the original shape.
• The rarest type of fossil preserves the original skeletons and soft body parts. Insects trapped in amber (tree sap that hardens) are a good example.

If you want to read more about Tyrannosaurus rex, check out fossilguy.com

Here's a link to fossils, how they are formed, and why we study them.

Here's a link to the history of dinosaurs.