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Birds are the living scion of the dinosaurs, and studying birds can offer clues to how dinosaurs worked under the hood. Paleontologists seldom find even the faintest trace of soft tissue clothing the bones of a fossilized dinosaur. Without prove of external cues like colors or chemical signatures that the dinosaurs could take used to distinguish between sexes, we have to rely on detective work across millions of years.

Even in terms of gross skeletal anatomy, as far equally nosotros tin can tell, dinosaurs were not sexually dimorphic. Lucky finds exercise happen: paleontologist Tamaki Sato and coauthors reported in 2005 on a birdlike oviraptorosaur, found preserved with her eggs still cradled between her hip basic. Plainly a female. But such rare and precious discoveries are the exception, non the dominion. So how do nosotros tell anything well-nigh the mode dinosaurs lived and died? One mode is to use birds every bit a living genetic Rosetta stone.

Medullary bone

All the stuff lit up in blue is medullary bone. Panels D and E are from the T. rex. For the total (huge) explanation, go read the paper at Nature Scientific Reports!

When birds lay their eggs, the eggs have a difficult, calcified vanquish. That calcification happens while the egg is still inside the mama bird, and the calcium has to come from somewhere. Somewhere, every bit it turns out, is the mama bird'due south ain bones. The spike in estrogen after a bird ovulates sends a betoken to its body to start stripping away os cells and scrapping them for their calcium content.

Just the mother bird'south torso won't just leave her without bones, or with Swiss cheese for a skeleton. In the place of the os stripped abroad to enclose the developing egg, the female parent bird lays downward what's called medullary os: loosely organized, messy ranks of soft placeholder bone cells full of daze-arresting structural sugars chosen glycosaminoglycans, that she'll remineralize subsequently when nutrient is more arable. We find medullary os only in gravid (pregnant egg-laying) animals, and we see it in their bigger bones, like the femur and humerus. Dinosaurs and birds share this trait. Molecular biologist Mary Schweitzer and team accept also found medullary bone, another office of the genetic Rosetta stone (full text!), deep inside the thigh bone of a Tyrannosaurus male monarch.

Other experts argued (too full text), based on spongy os they constitute in a male pterodactyl jaw, that what Schweitzer found in the T. rex femur merely looked like medullary bone, but was actually the consequence of some pathology. So Schweitzer spent the amend office of a decade applying sophisticated imaging and chemical testing to the T. rex thigh bone, to find out once and for all what was going on. In the work leading to Schweitzer'due south 2012 report on T. rex and gender, the squad practical an antigen-antibody examination that reacted only to bird osteocytes. They institute that the dinosaur bone had osteocytes similar enough to bird osteocytes to give a positive result. On this well-nigh recent sample, they used a chemical stain that adheres to glycosaminoglycans, which are institute in medullary bone but non in other kinds of os. Information technology lights up bluish under a microscope, clearly showing the presence of medullary bone. CT scans confirmed it: they had institute medullary os. Since medullary bone is just found in gravid females, that meant the T. king was gravid, in the process of developing an egg, when she died.

What this means is that now we can use fine skeletal anatomy to distinguish gravid (pregnant) dinosaurs from other fossils, even if the eggs they were developing weren't preserved along with the female parent's basic. Nosotros have a manner of telling male from female person reliably, fifty-fifty though soft tissue isn't preserved over the millennia. And using this kind of testing, we may exist able to tease out far more than data from the basic of our more than contempo relatives than we knew we could.