Tyrannosaurus rex meet Tyrannosaurus regina. For the first time, paleontologists have identified a T. rex as female by discovering a type of bone tissue that appears to be related to egg-laying. The trademark may help scientists distinguish the sex of other bird-like dinosaurs, but its delicate and transient nature could preclude wide use.
Most direct indicators of sex—reproductive organs, for example—don't fossilize. In a few cases, paleontologists have discovered eggs inside a skeleton (Science, 15 April, p. 375) allowing them to know for sure they found a female.
Now a team led by Mary Schweitzer of North Carolina State University in Raleigh has found in a dinosaur fossil the remains of tissue associated with egg-laying. Schweitzer and her colleagues were studying the 70 million-year-old bones of a remarkably well-preserved Tyrannosaurus (Science, 25 March, p. 1852) when Schweitzer noticed a type of bone tissue she had never seen before—at least not in a dinosaur. Located inside a leg bone, the brownish tissue was very fibrous and disorganized. "It's completely distinctive," Schweitzer says.
This texture resembles that of medullary bone, a thin layer of collagen and calcareous mineral that birds rapidly deposit inside their limb bones after they ovulate. The highly vascularized tissue provides birds with an easily-tapped supply of calcium for making eggshell and prevents any calcium loss that might weaken the so-called cortical tissue that strengthens bones.
To compare the Tyrannosaurus fossil with living birds that were at least somewhat comparable in size to the dinosaur, the team cut up the legs of a pregnant ostrich and an emu that had died before laying all its eggs. Like the Tyrannosaurus tissue, the medullary bone of these so-called ratites was relatively thinner than in chickens; that makes sense because birds with larger bones are inherently stronger, so calcium loss is less of a threat.
The potential payoff of identifying sexes in dinosaurs is great, says Anusuya Chinsamy-Turan of the University of Cape Town South Africa: one could better study population dynamics, for example. The problem with medullary bone, she cautions, is that it's delicate and only present ephemerally during a female's life. Schweitzer hopes to study egg-bearing dinosaurs to confirm that the tissue is medullary bone.