Sue the T. rex may be one of the most
famous dinosaur fossils ever discovered.
The skeleton was named after the person who found it,
American explorer Sue Hendrickson.
But while this fossil shares her name,
if you ask Sue –
or the person who runs the fossil’s social media accounts –
they’ll point out that Sue is actually gender-neutral.
Their media persona is neither male or female,
and Sue is referred to using they/them pronouns.
That’s because, well, we just don’t know
what sex Sue the actual Tyrannosaurus rex
was back when they were alive.
Which raises the question:
Can we ever be sure of the sex of any fossil creatures?
Well, there are clues to a fossil’s sex,
but they might not be what you think.
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How can you tell whether an ancient animal was male or female?
After all, reproductive organs are usually
soft tissue, and that doesn’t preserve well.
It’s incredibly rare, but sometimes we do get lucky
and find animals preserved with soft tissue, like with frozen mammoths.
In those cases you can just, well, look under the hood, so to speak.
In some other cases, an animals’ reproductive equipment does
leave clear traces behind even without their soft tissue.
Some male mammals have a baculum,
which is a bone that supports the penis.
So if you see one of those, it’s a pretty clear
indication that the animal was male.
At other times, we’ve been lucky enough to find fossils
that have eggs inside of them, or which we think might be pregnant.
In 2003, scientists announced that they had found a fossil of a marine
reptile called an ichthyosaur, that had two embryos inside its body.
Based on their skeletal features, orientation, and lack of trauma,
researchers think they really were embryos, not food.
And if there are babies inside a fossil animal, it’s probably a female.
Unless it’s, like, a seahorse, or something.
But what about eggs that have been laid?
Dinosaurs and other egg-layers have
been found alongside eggs that were already laid,
but male animals will also sometimes incubate or care for nests,
so that doesn’t end up telling us too much.
Though, put a pin in this, because we’ll come back
to something cool about egg layers later.
But maybe we don’t need to always find babies or eggs
to determine an animal’s sex. Maybe just their bones are enough.
And the reason for that is sexual dimorphism.
In many species of animal, males and females look different.
Think of how male peacocks are flashy
and colorful and females are more drab.
Or how male, silverback gorillas are much bigger
than the females, and have, well, silverbacks.
Sometimes these differences run so deep
that they’re visible in the skeleton.
If you’re looking at a whole bunch of animals of one species,
and there’s two really distinct size categories,
you may be looking at sexual dimorphism.
And many scientists have pored over fossil skeletons,
looking for any kind of evidence of sexual dimorphism.
In dinosaurs, scientists have looked at things
like tail and hip anatomy, which could be different
between males and females due to the need to lay eggs.
Others have looked at things like how big the animals were,
or looked for differences in the skull crests
that could suggest they had a fancy look to entice the other sex.
In 2021, scientists reported seeing sexual dimorphism
in the size of giant ground sloths’ canine teeth,
which indicates that one sex would have bigger fangs than the other.
Males of quite a few species of primates
will have larger canine teeth than the females do,
though our specific human lineage seems to have
lost that trait a while back.
We’re also less dimorphic in body size,
and researchers are still trying to figure out exactly
how long ago that happened.
Scientists have been debating whether species
in the hominin genus Australopithecus were dimorphic,
for instance, though recent evidence
points towards some size dimorphism in these guys.
And, while males and females in the genus Homo
have basically equal-sized canines,
we’re still not sure if the males and females
would have had roughly the same body mass,
or had body size dimorphism more like, say, chimpanzees.
Plus, modern humans have another odd distinction,
which is body fat distribution.
Human females have a higher percentage of body fat than males,
and we have very little way of knowing how long ago
that particular evolutionary quirk might have arisen.
But whether it’s our own ancestors or other animals,
even if we do see differences in the sizes or shapes of fossil skeletons,
figuring out whether that’s based on sex can be tricky.
For one thing, individual variation can be huge, even within a sex.
There’s always the possibility that there’d be overlap in the
size difference between the largest females and the smallest males,
or vice versa for species where females are larger.
And in many cases, we just don’t know a lot of the
nitty-gritty of what past species were really like.
We’re often working off of what we know about
existing relatives or general biological rules of thumb.
So it can be really hard to tease out which
skeletal differences are due to sex and which are due
to individual variation or from living in different areas or ecosystems.
This is made even harder by the fact that for many species,
there just aren’t that many fossils to examine.
T. rex might the most famous dinosaur ever,
but we have fewer than 100 fossil specimens of the species.
That means paleontologists need to work off of a small number
of individuals who may have been separated from each other
by thousands of miles and hundreds of thousands of years.
In 2017, a Canadian paleobiologist did statistical tests of fossils from
nine different dinosaur species, including T. rex, allosaurus, and
protoceratops, and his model found no evidence of sexual dimorphism.
There was too much overlap in the sizes,
and there weren’t two clear groups within the species
that we could argue were males and females.
A 2021 analysis found that some dinos, including allosaurus,
did appear to be dimorphic, but the results weren’t a slam dunk.
In both cases, the analyses were held back
by the limited number of fossils.
That said, there have been some finds
where we have managed to find enough fossils
from one species to make a judgment.
We’ve found hundreds of fossils of an early
Cretaceous bird called Confuciusornis.
It looks like males had two long,
pendant-like tail feathers, while females didn’t.
And that lines up with what we see in a lot of
modern-day dinosaurs, which you also know as birds.
In 2017, scientists working with tiny fossil crustaceans
were able to analyze over 6,000 individual fossils
and they did indeed find some dimorphism in the population.
But there’s one final trick up our sleeve for figuring out
what sex certain fossils, including dinosaurs, were.
Remember when I said to put a pin in the egg-laying question?
It turns out, one of the most promising pieces of evidence
for figuring out the sex of a fossil dinosaur may be medullary bone.
Medullary bone is a layer of calcium-rich tissue
on the inside of birds’ leg bones.
It’s found today in female birds as a way
to store calcium needed during egg-laying.
So we can look at their skeletons of any
egg-laying fossil species to see if we find that medullary bone.
And if we do, we can tell that individual was a female.
In 2005, researchers found medullary bone in one T. rex skeleton,
and since then it’s also been reported in other dinosaurs
like sauropods, as well as other critters like pterosaurs.
And the good news is we don’t need to
break open those leg bones to find it,
since we can use techniques like CT scans to peek inside.
Medullary bone only appears in mature,
reproductive females, which means that any fossil
that doesn’t have it can’t automatically be considered male.
But it’s still one of the most sure-fire ways we have
to know definitively what sex an individual fossil was.
That said, the sex of many fossils may continue to be a mystery.
We may never know for sure if Sue the T Rex was a male or a female.
But that doesn’t make their room at the Field Museum
any less crowded with visitors eager to take a look.
Thanks for watching this episode of SciShow.
If you liked it, you may want to go check out PBS Eons,
another show made by our parent company, Complexly.
PBS Eons tells the stories of the past and
how we learn about ancient life from the fossils they leave behind.
We really loved their recent episode
on past mass extinctions, and we think you will too.
So click the link wherever it is right now, and check ‘em out!
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