Pregnant T. Rex sheds light on sex-linked traits in extinct dinosaurs

Paleontologists have found the presence of a gender-specific reproductive tissue in a Tyrannosaurus Rex (T.Rex) that roamed Montana in the US 68 million years ago, thereby allowing them to determine the gender of this fossil.

The T.Rex femur may be the key to discerning gender differences among theropod, or meat-eating dinosaur species, the researchers said, adding that the findings shed light on the evolution of egg laying in modern birds.

"Just being able to identify a dinosaur definitively as a female opens up a whole new world of possibilities," said one of the study authors Lindsay Zanno, paleontologist at North Carolina Museum of Natural Sciences in the US.

The investigation confirmed that the tissue found in the T.Rex femur was a medullary bone -- found only in female birds and just during the period before or during egg laying because it has to be laid down and mobilised quickly in order for birds to shell their eggs.

Also, the presence of keratan sulfate -- a substance not present in other bone types, further confirmed that the tissue from the T. Rex was a medullary bone.

"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," said Mary Schweitzer, paleontologist at North Carolina State University in the US.

To confirm that the tissue from the T. Rex was medullary bone, a number of different tests were performed on the sample, including the testing for keratan sulfate using monoclonal antibodies, and comparing their results to the same tests performed on known medullary tissue from ostrich and chicken bone.

Before this analysis, the researchers pointed out, they did not have a reliable way to tell males from females in the dinosaurs from their fossils.

The findings were detailed in the journal Scientific Reports.

Medullary bone is chemically distinct from other bone types, like the dense cortical bone that makes up the outer portion of our bones, or the spongy cancellous bone found inside them, the researchers explained.

Theropod dinosaurs, the broader dinosaurian group that includes modern birds and other toothy relatives such as T. Rex, also laid eggs in order to reproduce, and paleontologists have hypothesised that they may have had medullary bone as well.