Researchers now confirm that a Tyrannosaurus rex that is now in Montana was apparently pregnant when it died some 68 million years ago. In this new study, chemical tests confirmed that this T. rex apparently possessed a special bone specifically its thigh bone's medullary bone, indicating that this belongs to a dinosaur that is laying eggs or about to lay eggs. 

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According to paleontologist Mary Schweitzer from the North Carolina State University, this medullary bone was first identified when it was viewed under a microscope, revealing the structure's characteristics. Prior to this discovery, scientists were still not sure if dinosaurs even possessed a medullary bone.

However, scientists know that this special bone can be seen in long bones such as thigh bones from female birds when they are about to lay some eggs. This medullary bone can act as a calcium deposit that can be immediately accessed to aid in the formation of eggshells.

Schweitzer who is also from the North Carolina Museum of Natural Sciences, thought that since birds and carnivorous dinosaurs like the T. rex are close relatives, then it would be possible that some dinosaurs have the medullary bone as well.

However, other scientists were skeptical about these conclusions, suggesting that this strange structure that seems out of place could have resulted from an injury or effects from a disease.

Schweitzer says that in order to be completely sure, a chemical analysis of the bone tissue is needed. In order to do this, she and her team utilized antibodies and stains to detect a special carbohydrate known as keratan sulfate, which is only found in the medullary bone and not in any other type of bones.

The researchers compared this to other birds with medullary bones and regular bones as well, from ostriches and chickens. The tests confirmed that this special bone found in this T. rex specimen was in fact a medullary bone.

According to co-author of the study, Lindsay Zanno from the North Carolina State University and the North Carolina Museum of Natural Sciences, up until now, there is no sure way to identify dinosaurs if they are male or female.

She adds that this new study has proven a definitive way to identify a female dinosaur, that opens up to a new world of possibilities. Scientists can now show that pregnant dinosaurs can reveal a chemical fingerprint and that more efforts are crucial in order to determine more.

Since the medullary bone was confirmed in this specimen, the team proceeded to report several other dinosaurs from a past 2005 study of Schweitzer that includes a meat eater known as the Allosaurus and two Tenontosauruses which are herbivores but are not closely related to birds.

This new study is published in the journal Scientific Reports.