Scientists Discover Pregnant T. Rex Fossil

442 Scientists Discover Pregnant T. Rex Fossil
Mark Hallett

Researchers examining a giant thighbone unearthed in Montana reveal that it belonged to a pregnant Tyrannosaurus rex that lived 68 million years ago. The femur contained a gender-specific tissue, and it could help with sex determination of fossils in the future, according to new findings published in Scientific Reports this week. This is the first time researchers have been able to study a dinosaur fossil that definitely belonged to a female. 

Birds are the living descendants of carnivorous dinosaurs called theropods, and many of today’s bird species have a gender-specific reproductive tissue called the medullary bone. Only mature female birds have it and only right before or during egg laying. While this short-lived bony tissue is typically found in long bones (like femurs), it doesn’t provide much structural support. Rather, it’s an easily mobilized source of calcium for eggshells. 


Since extinct theropods also laid eggs, researchers hypothesized that medullary bone would be present in those toothy predators as well. In fact, four dinosaurs have been previously proposed to have this estrogen-dependent tissue – including T. rex, based on a femur fragment known as MOR 1125. However, some pathologies are known to mimic the appearance of medullary bone even under the microscope, so a chemical diagnosis is needed to know for sure. 

Medullary bone is chemically distinct from other types of bone such as dense cortical bone, which makes up the outer layer of our bones. In birds, the bone-forming cells of medullary bone have receptors on their nuclei for estrogen, so it can be rapidly deposited when estrogen levels get high after ovulation. Also, medullary bone doesn’t have as much collagen. Instead, it incorporates a substance called keratan sulfate, which isn’t found in the matrix of any other bone type. 

A team led by Mary Schweitzer of North Carolina State University compared dinosaur tissue with that of an ostrich and a chicken, and then CT scans and chemical tests were conducted to study the differences between their medullary bone and cortical bone.

Sure enough, their analyses confirmed the presence of medullary bone in the MOR 1125 T. rex femur. It seems the unique chemical composition of medullary bone in birds was retained from their theropod ancestors, and it may be used to identify sex and reproductive status in fossils. 


"It’s a dirty secret, but we know next to nothing about sex-linked traits in extinct dinosaurs," study coauthor Lindsay Zanno also of NC State said in a statement. "Dinosaurs weren’t shy about sexual signaling, all those bells and whistles, horns, crests, and frills, and yet we just haven’t had a reliable way to tell males from females. Just being able to identify a dinosaur definitively as a female opens up a whole new world of possibilities."


  • tag
  • sex,

  • T. rex,

  • bone,

  • pregnancy,

  • estrogen,

  • sex determination