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

Even in terms of gross skeletal anatomy, every bit far as nosotros can tell, dinosaurs were not sexually dimorphic. Lucky finds do happen: paleontologist Tamaki Sato and coauthors reported in 2005 on a birdlike oviraptorosaur, plant preserved with her eggs nevertheless cradled between her hip bones. Apparently a female. But such rare and precious discoveries are the exception, non the rule. So how do we tell annihilation well-nigh the way dinosaurs lived and died? I way is to use birds as a living genetic Rosetta rock.

Medullary bone

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

When birds lay their eggs, the eggs have a hard, calcified crush. That calcification happens while the egg is still inside the mama bird, and the calcium has to come from somewhere. Somewhere, every bit information technology turns out, is the mama bird's own bones. The spike in estrogen after a bird ovulates sends a signal to its torso to start stripping away bone cells and scrapping them for their calcium content.

But the female parent bird's trunk won't but leave her without basic, or with Swiss cheese for a skeleton. In the place of the bone stripped away to enclose the developing egg, the female parent bird lays down what's called medullary bone: loosely organized, messy ranks of soft placeholder bone cells full of shock-absorbing structural sugars chosen glycosaminoglycans, that she'll remineralize subsequently when food is more abundant. We find medullary bone only in gravid (pregnant egg-laying) animals, and we run across it in their bigger bones, like the femur and humerus. Dinosaurs and birds share this trait. Molecular biologist Mary Schweitzer and team have also institute medullary bone, another part of the genetic Rosetta stone (full text!), deep inside the thigh os of a Tyrannosaurus male monarch.

Other experts argued (also full text), based on spongy bone they found in a male pterodactyl jaw, that what Schweitzer found in the T. king femur only looked like medullary bone, merely was actually the consequence of some pathology. So Schweitzer spent the better function of a decade applying sophisticated imaging and chemical testing to the T. rex thigh bone, to find out one time and for all what was going on. In the piece of work leading to Schweitzer's 2022 report on T. male monarch and gender, the squad applied an antigen-antibody test that reacted only to bird osteocytes. They found that the dinosaur bone had osteocytes like enough to bird osteocytes to give a positive effect. On this near recent sample, they used a chemical stain that adheres to glycosaminoglycans, which are found in medullary bone but not in other kinds of os. It lights upward blue under a microscope, clearly showing the presence of medullary bone. CT scans confirmed it: they had found medullary bone. Since medullary bone is simply constitute in gravid females, that meant the T. male monarch was gravid, in the procedure 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 mother's bones. We have a style of telling male from female reliably, even though soft tissue isn't preserved over the millennia. And using this kind of testing, nosotros may be able to tease out far more than information from the basic of our more recent relatives than we knew we could.