A pregnant Tyrannosaurus rex has been found, shedding light on the evolution of egg-laying as well as on gender differences in the dinosaur.

Key points: Researchers discover medullary bone in leg bone of T rex fossil

Researchers discover medullary bone in leg bone of T rex fossil Medullary bone only present just before and during egg-laying

Medullary bone only present just before and during egg-laying Indicates dinosaur was pregnant female aged between 16 and 20

The remains could also contain the holy grail of all dinosaur fossils: DNA.

"Yes, it's possible," said Dr Lindsay Zanno, referring to genetic material that may be present in this as well as similar dinosaur finds.

"We have some evidence that fragments of DNA may be preserved in dinosaur fossils, but this remains to be tested further."

What has been confirmed so far is that the T rex, which was found in Montana and dates back 68 million years, retained medullary bone revealing the individual was pregnant.

Medullary bone is only present in female birds — which are related to dinosaurs — just before and during egg-laying. It is this type of bone that could retain preserved DNA.

This cross section of T rex bone shows medullary bone in the middle. ( Schweitzer M. H. et al/Scientific Reports )

Medullary bone lines the marrow cavity of the long bones of birds, Dr Zanno, who is an assistant research professor of biological sciences at North Carolina State University, explained.

"It's a special tissue that is built up as easily mobilised calcium storage just before egg-laying," Dr Zanno said.

"The outcome is that birds do not have to pull calcium from the main part of their bones in order to shell eggs, weakening their bones the way crocodiles do."

Crocodiles are related to the common ancestor of dinosaurs.

"Medullary bone is thus present just before and during egg-laying, but is entirely gone after the female has finished laying eggs," Dr Zanno said.

Analysis of leg bone helped determine fossil's gender

The project's lead researcher, palaeontologist Dr Mary Schweitzer, suspected that medullary bone was present in the dinosaur's remains, and was able to confirm her suspicions after she, Dr Zanno and their team conducted a chemical analysis of the T rex's femur.

The material, found to be consistent with known medullary tissues from ostriches and chickens, contained karatan sulfate, a substance not present in any other bone types.

"This analysis allows us to determine the gender of this fossil, and gives us a window into the evolution of egg-laying in modern birds," Dr Schweitzer, who is also at North Carolina State University, said.

Dr Zanno explained researchers now knew extinct dinosaurs inherited egg-laying from their ancestors, just as birds inherited this reproductive strategy from their dinosaur ancestors.

"The discovery of medullary bone is just one more piece of evidence that blurs the line between birds and other theropod [carnivorous two-legged] dinosaurs like T rex," she said.

The research is published in the journal Nature Scientific Reports.

Medullary bone found in other dinosaurs

A labelled diagram shows one pregnant T rex compared to another female. ( Mark Hallett )

In a prior study, Assistant Professor Sarah Werning of the University of California and Berkeley and her colleagues found medullary bone in the carnivorous dinosaur Allosaurus as well as in the plant-eating dinosaur Tenontosaurus.

The discoveries happened somewhat by chance, as she and the other researchers were studying dinosaur growth rates when they realised three of the dinosaurs were pregnant females.

"We were lucky to find these female fossils," Dr Werning said.

"Medullary bone is only around for three to four weeks in females who are reproductively mature, so you'd have to cut up a lot of dinosaur bones to have a good chance of finding this."

Dr Schweitzer agreed, and said that the femur her team studied was already broken when she received it.

She said most palaeontologists would not want to cut open, or demineralise, their fossils in order to search for the rare medullary bone.

Nevertheless, because much of the pregnant T rex's skeleton was found, including her skull, there is a very good chance palaeontologists will soon be able to provide a detailed description of her overall anatomy and general appearance.

They already know that the dinosaur mum-to-be was 16-20 years old when she died of as of yet unknown causes.

This story was originally published on DiscoveryNews.com