How to Tell if a Fossil T-Rex Is Pregnant

Credit: Mark Hallett
Credit: Mark Hallett

Researchers have confirmed that a 68-million-year-old fossil found in Montana came from a pregnant Tyrannosaurus Rex. This not only will make it easier for other scientists to sex different fossils, it gives us an insight into the evolution of birds.


Scientists from North Carolina State University and the North Carolina Museum of Natural Sciences have discovered a way to determine whether certain fossilized dinosaurs were pregnant at the time they died. The findings were published today in Scientific Reports.

Illustration for article titled How to Tell if a Fossil T-Rex Is Pregnant

The clue came from the changes in physiology displayed in modern birds. When a female bird is pregnant, it supplements its skeleton with something called medullary bone (MB). Scientists believe that the bone is there to provide material for the egg shells. It’s visually and chemically different from dense cortical bone (CB). Bone found in a cracked T. Rex femur looked a great deal like medullary bone.

But the scientists involved weren’t going to declare the T. Rex pregnant just by eyeballing a femur. Paleontologist Mary Higby Schweitzer knew that bone diseases could cause regular bone to look like medullary bone. But finding traces of a chemical component of medullary bone called keratin sulfate would be the equivalent of finding a pregnancy testing stick with a blue line on it.

There was some doubt that any of the chemical would be left in the fossil remains after millions of years. Schweitzer and the team searched for it by using monoclonal antibodies—antibodies made from one parent white blood cell, meant to react to only one substance.


Their results showed no reaction with the T. Rex cortical bone (I, J above) and a faint but definite reaction with the T. Rex medullary bone (K, L) — strong evidence the T. Rex was pregnant. For contrast, there was no reaction with a chicken’s cortical bone (A, B) and medullary bone (C, D), or with the medullary and cortical bones of an ostrich (second row).

The fact that we can find solid evidence of medullary bone means that researchers could be sure about sexing their dinosaur fossils. Although it’s not guaranteed that female fossils will have medullary bone, since it’s only present while the female is gestating eggs, we might get a lot of gender surprises in the next few years. Fossils that researchers believed were male could turn out to be female, and vice versa.


This could be a major advance for biology and paleontology. The presence of medullary bone shows a common trait among birds and non-avian dinosaurs. This gives us a look at the evolution of egg laying in birds: the way modern birds shell their eggs could have been in place tens of millions of years ago.

[Scientific Reports]




It is a strange and beautiful time in your life right now, your body is developing medullary bone tissue and dedicating precious resources to your new family. Nutrition is doubly important right, so remember to supplement your diet with a healthy serving of lawyers.