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If you were to step back in time to China, about 122 million years ago,
you might have found yourself surrounded by a flock of small creatures that
looked a lot like crows, sporting glossy black feathers with an iridescent sheen.
These would not be crows, but the dinosaur Microraptor:
a little dinosaur with a bony tail, a mouth full of teeth, and two sets of wings,
with feathers on both their forelimbs and hindlimbs.
And we can describe this scene in all of this detail because
we know what this dinosaur would have looked like, down to the colors.
For a long time, we thought we’d only very rarely know the colors of dinosaurs,
or any other organism we only know from fossils.
But in the past decade or so, there has been an explosion of color.
Even though fossils are basically just rocks, with some clever paleontology
and chemistry, scientists can get them to reveal their vibrant secrets.
So first off, what do we mean when we talk about colors?
On a physical level, colors in animals are either made up of pigments,
or they are what’s known as structural colors.
Thirdly, there’s bioluminescence, but we’re not going to talk about that.
Pigments are molecules that absorb and reflect specific wavelengths of light.
Structural colors, on the other hand, are a bit weirder.
They don’t really have color themselves; instead, they have microscopic structures
that reflect or scatter light in specific ways.
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Sounds complicated, and it is complicated, but it’s fairly common in nature.
You can see it in the shiny blue sheen of certain butterfly wings or in bird feathers.
So why wouldn’t you expect to find these colors in fossils?
First off, because the outside bits of an animal, where the color usually is,
are soft and squishy and tend to not fossilize well.
And individual molecules like pigments don’t survive the process too well either.
Though the 21st century has seen rapid progress, it’s worth noting that there have
been hints that fossils could preserve at least some form of color for a while now.
We’ve known for centuries that ink from cephalopods, the group of animals that
includes squid and octopus, can actually be preserved in fossils, for instance.
Nineteenth-century fossil collectors on the rocky Dorset coast of England
would find fossil cephalopods with dark splotches
in the shape of ink sacs, for instance.
This fossil ink could even be re-wetted and used to write or paint.
Later analysis showed that it contained melanin, but figuring that out required
grinding up the fossils, which scientists were reluctant to do.
Scientists had also been occasionally able to note color patterns or blotches of light
and dark in things like fossil feathers, shells, and insects before.
But a major breakthrough came in 2008.
And even though we’re looking for a chemical,
so basically doing chemistry, the most important tool here was microscopes.
For a while, scientists had been scanning fossils with electron microscopes,
these are high-powered microscopes that can
use electrons to illuminate tiny objects.
Using these microscopes, they were able to see these strange,
microscopic, rod-like objects in fossils.
Many people assumed that these were the remains of bacteria,
since their shape resembled that of fossil microbes.
But in that 2008 paper, researchers looking at fossil feathers from the head of
an unnamed ancient Brazilian bird advanced a different theory:
these were not bacteria, but a structure called a melanosome.
Melanosomes are cellular structures that contain melanins, a type of pigment.
The same type that is in human skin, and eyes, and hair.
In fact, melanins are a whole group of molecules,
mostly black and red in color, that are made of rings of carbon and other atoms.
The scientists suggested these rod-like objects were melanosomes because,
when they compared these mysterious fossil structures to melanosomes
from the feathers of modern birds, the two structures looked similar
in terms of size, and shape, and how they were packed together.
This idea was really cool because it meant that scientists could make guesses
as to how these fossil feathers were colored, in this case, in a stripey pattern.
What’s more, the size and shape of melanosomes in living creatures
is linked to their particular color.
Black and grey melanosomes tend to be long and narrow,
while red and brown ones tend to be short and squat.
So by looking at the shape of the fossil melanosomes,
researchers could infer more about their color.
In 2010, for instance, scientists examining fossil melanosomes suggested that
the predatory dinosaur Sinosauropteryx may have
had reddish or chestnut-colored stripes on its tail.
That same year, scientists suggested the bird-like dinosaur Anchiornis
may have had a gray body, white limbs, and a red crest.
Now, this is not a total slam dunk.
People still argue about whether these are bacteria or not.
And it’s worth noting that melanosomes may have other uses in the body
besides coloration, so they might not tell us anything about color anyway.
But it’s opened up a whole field for studying fossil color.
What’s also really cool is, in living animals, these melanosomes can combo
with other structures to produce colors beyond black and brownish-red.
For instance, in the feathers of living birds, a layer of the protein keratin
over melanosomes can make them appear blue-ish thanks to structural color.
And scientists have found what they think is evidence of this in fossils too,
by comparing the exact shape of the fossil melanosomes
with extant ones used for structural color.
This is why scientists have suggested that Microraptor
was that black and iridescent color.
We’ve found other examples of structural color too using electron microscopes,
like in butterfly scales from the Jurassic period.
So we’ve been able to find what look like signs of color and structures
associated with color, but as scientists have looked more at these rocks,
they’ve also found specific pigments, thanks to some heavy-duty chemistry.
And the more colors we unlock,
the more we may be able to infer about the animals’ actual lives.
Because it turns out we can get chemicals from fossils.
Like I said, many chemicals are too unstable to last through time.
But, it turns out, with the right conditions many pigments are surprisingly robust.
Especially in places away from oxygen, like in lake or marine sediments,
or entombed in minerals or amber.
It turns out that melanins, for example, tend to not react with water that much,
which may help them stay relatively stable.
Time, pressure, and temperature can still wreck things a bit, but there’s apparently
enough left that we can identify what the original molecule was.
This may involve taking a small sample from the fossil
and doing a chemical analysis, though scientists are often hesitant
to do that because fossils are so rare.
But there are also more modern techniques that mean
you don’t need to touch the fossil at all.
Even if the chemical structure of melanin does break down, it leaves traces.
Scientists may even be able to tell different types of melanin,
and thus different colors, apart.
For example, in a 2019 paper in Nature Communications, scientists looked at what
seems to be traces of fur from a 3-million-year-old mouse called Apodemus atavus.
They used a technique called X-ray fluorescence imaging,
which involved bombarding the fossil with X-rays,
making it sort of light up based on the elements present.
Different types of melanin may contain slightly different elements.
In this case, they were looking for evidence of zinc and sulfur,
which in modern animals is associated with the reddish pigment eumelanin.
And they found them.
So not only did they find a ginger mouse;
they showed that we can tell melanins apart.
So melanins are generally black or brown, but we can find more colors!
For instance, let’s talk about carotenoids.
These are pigments, like melanin, but come in somewhat brighter colors,
like reds, yellows, and oranges.
Like melanins, carotenoids can be fairly stable in the right conditions,
even over the eons.
One way we’ve been able to extract them from fossils is through
high-performance liquid chromatography, or HPLC for short.
In it, a liquid solution made from the fossil is pumped past different solid materials.
And the compounds in the liquid stick to the solid material at slightly different rates,
making them move through the machine at different speeds.
This speed difference can let scientists separate
and identify the individual chemicals.
Another technique is Raman spectroscopy,
where a sample is lit up with a laser or X-rays.
The molecules in the sample scatter photons at particular wavelengths
depending on the chemicals present in it.
Researchers can then filter out the wavelength of the laser
and use the remaining sample as a kind of chemical fingerprint.
Using this technique, carotenoids have been found in the shells of
ancient brachiopods, which are hard-shelled molluscs
that look kind of like sideways clams.
We are still looking for examples of carotenoids in vertebrates, though.
Now those are the big players, but it’s worth noting that
there are other pigments and techniques.
There’s a class of pigment called porphyrins, for instance,
that can be a range of colors including greenish or bluish.
We’ve found those in eggs and plant remains.
And there are other ways to hunt for ancient colors,
like trying to detect trace elements or doing genetic analyses
of living creatures to see how ancient their coloration genes are.
And there are scientific reasons why we’d like to know what color
T. rex and friends were, in addition to just the awesomeness factor,
and having more accurate, great pictures and drawings that we can look at.
It could tell us things about how the animal lived,
like if it used its colors for camouflage or for display.
For instance, we’ve been able to find evidence of
what’s basically camouflage in an ankylosaur.
A dinosaur with big, heavy protective armor, but the researchers concluded it was
under so much evolutionary pressure from predators, it also needed to hide!
Maybe we’ll even be able to identify sexual dimorphism in dinosaurs someday,
like how some modern dinosaurs have
different colors between males and females.
I’m talking about birds.
But even though this new world of color is exciting,
there are still a few potential problems.
For instance, it’s always possible we’ve somehow gotten the chemistry wrong and
what we’re detecting in any given fossil isn’t actually pigments, but something else.
And even if they are pigments, it’s possible that they
weren’t actually used for coloration.
A lot of molecules play double duty in the body.
What adds color in a feather might play some other role
if it’s found in the liver or in muscle.
So it’s possible we are picking up real signals,
but not thinking about them the right way.
It’s also possible that, even if what we’re seeing is coloration,
we still might be wrong about the overall appearance of the animal.
Perhaps those colors were modified by some other thing,
like the keratin structures over melanin in structural color.
Or maybe they were combined with other pigments that haven’t been preserved.
Or hidden by feathers or other features.
It’s also possible that this “color” is actually contamination.
Like, if the animal died and sank into an algae-filled lake,
how do we know that the pigments come from the animal
and not from the plant matter in the sediment?
And, though we have made incredible discoveries in the past ten years that have
shown that finding color is possible, it’s still the case that there
may be some pigments, colors, or other structures
that are just too fragile to survive in a detectable way.
We also need to be cautious about how we interpret the color of individual fossils.
You might just have gotten a uniquely colored individual, like a black panther.
And some organisms change color during their life cycle.
But we are now done with all of the hedging!
That’s all the hedging we are going to do.
It is amazing that now we apparently know what color some dinosaurs were,
and some other ancient animals to boot.
Yes, there’s a lot of unknowns, but it is amazing
that we’re even having this discussion.
A brand new field with brand new techniques
has flowered in an amazingly short time.
Millions and billions of years of the history of life are a little bit lost to us.
We can only observe this whole world through the smallest of pictures,
thanks to the fossil record.
But now that scientists have developed amazing techniques like these,
we can start to get that picture in color.
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