It’s a Girl! Paleontologists Examine Pregnant T. rex

Sexing a dinosaur isn’t easy.

As far as gross skeletal anatomy is concerned, male and female dinosaurs are practically identical. And the shape of saurian bones provides no help. So far as anyone has been able to tell, the skeletons of dinosaurs were not sexually dimorphic (or, in other words, different between males and females). Even in highly-ornamented species of horned dinosaurs, armored dinosaurs, and others, all the gnarly spikes and plates and crests don’t show a definitive split in form that can be taken as a marker of different sexes.

But the evolutionary architecture of bones isn’t everything. A pair of surprises provided opportunities for paleontologists to identify some female dinosaurs, at least. In 2005 paleontologist Tamaki Sato and coauthors reported on a fossil of a parrot-like oviraptorosaur that had been preserved with a pair of eggs nestled between her hip bones. This dinosaur was definitely a female.

Better yet, just a few months later molecular biologist Mary Schweitzer and her colleagues reported on another way female dinosaurs could be identified through their pregnancies. A thigh bone of a Tyrannosaurus rex—MOR 1125 or “B. rex” to fossil fans—had a special tissue inside the main cavity called medullary bone. The same type of bone is seen in living birds, and is laid down when estrogen skyrockets following ovulation. In short, if you see medullary bone, you’ve found a pregnant female.

Art by Mark Hallett.
Art by Mark Hallett.

Not everyone agreed with this interpretation. Other experts suggested that the messy, rapidly-deposited bone tissue inside the T. rex was from a disease and that similar types of bone found in the jaws of male and juvenile pterosaurs—the flying relatives of dinosaurs—meant that medullary bone can’t be taken as a reliable indicator of a dinosaur’s sex.

But Schweitzer and a multidisciplinary team of experts have now answered these criticisms with a new study of the original T. rex clue. What did they find? That MOR 1125 truly was pregnant when she died.

Schweitzer and her colleagues approached the bone tissue from multiple avenues, re-examining the structure of the tissue with CT scans but also looking at its chemical composition. This is the key in teasing out tricky tissues, the researchers write, because medullary bone is chemically different from other bone types.

That’s because true medullary bone contains a higher proportion of mineral content and biomolecules called glycoaminoglycans than the surrounding tissue. So when the researchers used a stain to reveal the presence of biomolecules known to be abundant in medullary bone, the reaction fit with what they had suspected: the stain literally highlighted the fact that MOR 1125 had a femur infilled with the mineral-rich tissue.

Watch: Dinosaurs May Have Danced Like Birds. According to famed paleontologist Jack Horner, dinosaurs may have had courting behaviors similar to today’s birds.

The bone in MOR 1125 was not a pathology, and the superficially similar tissues in the pterosaurs must be attributable to some other condition or process. (Medullary bone is estrogen-dependent, Schweitzer and coauthors point out, so similar tissues in male and immature animals have to be something different.)

Pregnant dinosaurs really did lay down true medullary bone inside themselves, and this discovery holds fantastic possibilities for investigating how dinosaurs actually lived. But there’s a more subtle point that’s just as important to the way we think about these animals.

In fossiliferous shorthand, it’s easy to say that dinosaurs turned to rock during their long tenure in the earth. And yes, their bones and other tissues come down to us as permineralized versions of the originals. But it’s not as if everything of the real creatures was obliterated.

“Original organic components are assumed to be completely destroyed during burial and fossilization processes over millions of years,” Schweitzer and colleagues write. “However, we have shown that tissues, cells, and fragments of original molecules can persist across geological time.” Dinosaurs didn’t turn to stone like mythological trolls caught in sunlight. After all this time, tatters of the living creatures remain.

Look at dinosaurs as once-living animals, not piles of bone-shaped rocks, and you can start to see them.


Schweitzer, M., Zheng, W., Zanno, L., Werning, S., Sugiyama, T. 2016. Chemistry supports the identification of gender-specific reproductive tissue in Tyrannosaurus rex. Scientific Reports. doi: 10.1038/srep23099

Schweitzer, M., Wittmeyer, J., Horner, J. 2005. Gender-specific reproductive tissue in ratites and Tyrannosaurus rex. Science. doi: 10.1126/science.1112158

3 thoughts on “It’s a Girl! Paleontologists Examine Pregnant T. rex

  1. There have been two suggestions of morphological signs of the sex of T-rex. Two “morphs,” robust and gracile, have been identified by at least some researchers, and a suggestion made that they might be sexual dimorphs. (Since, in the best known extant carnivorous dinosaurs, the hawks and owls, the female tends to be bigger than the male, I’d a priori guess that the robust morph is the girl.) Second (I think this may have been observed in extant crocodilians) there is a difference in the hemal arch of a vertebra near the base of the tail (I assume one form would get in the way of egg-laying and the other not).
    Now that we have an independently sexed specimen, is any light thrown on the validity of these suggestions?

    1. @Allen Hazen

      In reference to “the robust morph is the girl”, you’re probably right (See the Larson quote; Also, see Figure 8.18 for how they plot on a graph: ).

      In reference to “extant crocodilians”, chevron anatomy has been known to be useless for sexing dinos since 2005:

      Quoting Larson ( ): “By use of morphometric analysis, gracile and robust morphs are confirmed to be present within the clade Tyrannosaurus rex. Extant phyloge- netic bracketing (comparison with living crocodiles and birds) leads us to conclude that the existence of these 2 morphs most parsimoniously repre- sents sexual dimorphism. The discovery of medullary bone within the medullary cavity of a robust specimen of T. rex established MOR 1125 as female (Schweitzer et al. 2005), and therefore all other robust T. rex specimens are, in all probability, also female.”

  2. Actually, the MOR 1125 specimen – ‘B-rex’ – is quite small for a T. rex, and only consists of a partial skull, a femur, and few other isolated bones. The ‘robust’ description is based on the femur – which is given different measurements, in Horner, Larson, and other researchers. Femur dimensions is one of the least effective way to differentiate between the two morphs – if they exist at all. It could be nothing but age difference, individual variation, or even different species or sub-species (this is an animal that flourished over a period of 4 million years).
    Christopher Brochu, who did most of the initial analysis of ‘Sue’, the most robust specimen of all points out that in birds – even where females are larger – the female’s bones are gracile, not robust – which is reasonable in that it allows more body cavity space for carrying eggs. Also, Sue – again the MOST robust specimen – clusters near the middle of catalogued specimens in the thickness of its femur – the criteria upon which B-rex was declared ‘robust.’ Most likely, it was simply a smaller animal with shorter legs.

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