When you go to the natural history museum and you see all the fossils, most of us are like, “Wow!

Look at all those fossils!” And yeah, the museum probably has  a bunch of neat stuff on display. But those rows of bones aren’t the whole story.

See, most of the fossils in the world are missing. Or, to be precise, most of the fossils  never existed in the first place. And that’s a problem for the people who want to use them to discover  the secrets of the ancient past.

So here’s how researchers are solving  the world’s biggest jigsaw puzzle, when like, 90% of the pieces are missing. [intro music] To start, let’s talk about how to make a fossil. Step one, die. That’s kind of the easy part, since everybody’s gotta go some time.

The trickier bit is the switch from a  dead thing to a fossil of a dead thing. And the best way to do that is  for your dead thing to get buried. Now our writer pointed out that technically, you can be buried first and die second, but that’s not nice to think about, so we’ll just ignore that for now.

Okay, so the body of an organism becomes buried in some kind of sediment. It could be the sand on a beach or  the mud at the bottom of a lake, anything that will cover up the body and seal it away from the  wind and rain and scavengers. Once they’re underground, the organic remains gradually break down and react with the surrounding minerals.

Eventually, the original materials become filled in or replaced  with mineral components. This is how we get most of our famous fossils, like dinosaur bones, shark  teeth, and ammonite shells. Those fossils are essentially rocks, made up of some combination of  original material and minerals from the surrounding sediment.

And they were kind of halfway to  being rocks even when they were alive. Bones are mostly calcium, so when they get buried, the porous sections can  fill in with other minerals. And a shell is just a hunk of calcium carbonate, so it’s most of the way  towards being a rock already.

There are other things that  can be fossils too, of course. An insect can be preserved in amber, for example, or a leaf can get squished between rock layers and preserved as a carbon film. But all of these modes of fossilization  follow a pretty similar pattern: get buried and get preserved in rock.

The study of fossilization is called taphonomy. It’s basically the study  of everything that happens between when an organism dies and when somebody finds it as a fossil, if it was one of the lucky ones. See, researchers know that not all organisms have equal odds of fossilizing.

This is called preservation bias, and it can make a big difference in  how we see these ancient ecosystems. For example, trilobites are super famous fossils. These ancient ocean bugs had hard exoskeletons and spent a lot of their lives  crawling around on the muddy seafloor.

They’re practically custom-made to  be turned into gorgeous fossils. But most of their fellow sea creatures were small, squishy swimmers that  would almost certainly be destroyed before ever made it to burial and mineralization. So while the fossil record is full of trilobites, researchers think that between 80 to  90 percent of ancient marine species that have ever lived never fossilized at all.

Now this sounds like a bummer, but preservation bias isn’t  quite as bad as it sounds. The fact that we know about preservation bias means that paleontologists have a  pretty good idea of what factors are tipping the scales. Like we said before, hard  parts like bones, shells, and wood are super durable, so they’re much more likely to survive  the ravages of weather and decomposition long enough to be buried at least somewhat intact.

On the other hand, softer body parts like organs, feathers, and  flowers tend to break down before they ever had a chance to turn rock-solid. Size matters, too. The gigantic bones of a T. rex will  tend to survive the full process much better than the delicate  little bones of a mouse.

But it isn’t just the organism’s bodies that affect their odds of mineralizing. The environment itself can have an impact. To make a really good fossil, the best kind of environment  is one that has consistent, gentle buildup of sediment over time.

It also helps if it’s cold,  dark, and has low oxygen levels, to keep decay from happening. So if you were trying to pick the  ideal environment for fossil-making, something like the bottom of a  deep lake couldn’t get much better. On the other hand, your odds of finding fossils from times and places that have harsh weather, temperature fluctuations, or just don’t build up sediment layers, are basically nil.

So if you’re a paleontologist  searching for ancient squishy worms that lived on mountain slopes … you might want to consider a career change. Preservation bias is a problem, but it’s not a new problem. And yet, solving it has been  way harder than we’d hoped.

The more we study fossils and fossilization, the more we realize just how complicated it can be to figure out exactly who is  missing from the fossil record. Like, you might expect that similar animals buried in the same environment would  probably fossilize equally well. But it turns out that’s not necessarily true.

A 2025 study conducted an experiment where they closely monitored  the decomposition process of a variety of ocean animals. They found that the rate of decomposition not only depended on the  animals’ size and squishiness, but also on the different types of  molecules that made up their tissues. And as each animal’s body decomposed  in slightly different ways, the chemical reactions of  decomposition affected how much oxygen was sucked out of the surrounding water.

And when the oxygen levels change, it could affect the ability  of the decomposing microbes to break down the body. All of this means that a small  squishy shrimp and small squishy worm that died side-by-side might have  different fossilization potential, just because of what molecules  make up their bodies. And it gets weirder.

Some other studies have found that certain factors we thought were universally bad for fossilization can sometimes actually make it more  likely for something to fossilize. Take scavengers, for instance. It makes sense that if scavengers find a carcass, they’ll get in the way of fossilization because they’ll chow down and destroy the body and not leave much to preserve.

But in some cases, it looks like scavengers might actually help the preservation process. A 2022 study examined an  extraordinary fossil of a dinosaur called Edmontosaurus. This was so well preserved that the  researchers even found patches of skin on its legs and tail, making it a dinosaur mummy.

It was in such good shape that paleontologists had assumed that this lucky dino must  have been buried incredibly quickly, and scavengers didn’t have  a chance to snack on it. But when they looked closely  at the preserved skin, they found signs of damage from teeth and claws. This prompted them to examine  scavenging behavior in modern animals, to see if they were missing something.

They found that when  scavengers dig into a carcass, they usually start with the soft  flesh and organs, which is… gross. But to get to those tasty bits, the  scavengers end up opening holes in the body that can release the various fluids and gases and microbes of decomposition  into the outside world. Scavenger activity also removes a  lot of the decomposers from the body, giving the remaining tissues  such as skin a better chance of surviving, drying out, and  becoming buried and fossilized.

So in the case of that Edmontosaurus mummy, getting picked over by scavengers  might have been the key to fossilizing so beautifully. And speaking of scavengers, we also know that sometimes, the things that an animal did during its  life affect its odds of fossilization, down to its favorite meal. Take for example, the famous  La Brea Tar Pits of California.

There, paleontologists have  uncovered hundreds of fossils of mammoths, ground sloths, and  more from the end of the Ice Age. These animals all stepped  into patches of sticky asphalt and became stuck like flies on fly paper, eventually becoming buried and fossilized. But while there’s plenty of  fossils of those other animals, sabertooth cats and dire wolves  outnumber them almost ten to one, by some estimates.

Literally thousands have been uncovered, which is way more than any of  the herbivores at the site. That’s an unusual ratio. In living  ecosystems and at fossil sites, herbivores tend to be much  more abundant than carnivores - think of the size of African lion  prides compared to African zebra herds.

And that makes sense, because if you had more  carnivores than herbivores, there just wouldn’t be enough food to go around, unless they started eating other carnivores. But La Brea is different because those  tar pits acted as a predator trap. A mammoth or sloth would get caught in the asphalt and become easy pickings for hungry carnivores.

Then some cats or wolves would go to snack on it and, oops, oh no, now they’re stuck, too. So, tar pits are a great place  for finding fossil predators, but that also means that the  fossils we find in the pits aren’t as good of a snapshot of  the whole ecosystem as we’d hoped. And on top of all that, even when we’ve accounted for the variations in ancient environments and body types, we can still end up with a skewed record because of one of the most classic  dilemmas of science - human error!

Two different studies from  2019 found there’s a sex bias in museum collections of mammals and birds. In collections of both modern and fossil species, males were significantly more common than females. Now that might make sense for the living things, since males of bird and mammal species also tend to be bigger and flashier than females, making the living ones more likely  to catch the attention of collectors.

But why would this be true for the fossils, too? Part of this might be our own human impulses. When museums build their collections, they might want to acquire the  really exceptional fossils, and that often means the biggest.

And since for a lot of vertebrate species, males are bigger than females… well, you get it. And what’s really interesting is that  this bias might not be all our fault. Some of it could be due to the animals’ behavior.

In many species, males tend to be more solitary, more wide-ranging, and more prone to risky behavior. Which means that males might be predisposed  to ending up trapped in a tar pit. So all in all, there are a  thousand reasons that the fossils we’ve collected provide an incomplete  vision of the distant past.

Some species are overrepresented and others are hardly represented at all. It’s a pretty big problem, but there are plenty of science-based solutions. For one thing, there are lots  of different types of fossils, and they can complement each other.

Where one type of evidence is missing, others can help fill in the gaps. A study from early in 2025 identified  fossil footprints of club-tailed dinosaurs called ankylosaurs from fossil sites dating to the mid Cretaceous  Period of western Canada. Which is cooler than it sounds, because we don’t actually have any of  their bones from that place and time.

So while we don’t know what caused  this particular preservation bias, the footprints alone help us fill in  the gaps left by unequal fossilization. We can go back to the very  earliest life forms on Earth, too. Those tiny microbes had a very  low chance of becoming fossilized, because they were, you know, tiny.

But while their bodies are gone, they left traces of their  existence in the form of lipids, molecules of fat that tell us there were  living things there once upon a time. Scientists can also compare ancient  environments to modern ones, and try to use what’s around  today to estimate what’s missing. Basically, you count up the numbers of species  of all your different categories in a modern ecosystem, and you can use the relative ratios to estimate what might be missing in the fossil ecosystem.

That’s obviously never going to be perfect, and you need to be really careful  with the example ecosystem, but it’s a start. Or you can go further, and try  to figure out population sizes for extinct animals based on  what we know about present ones. A 2021 study identified a relationship between body size and population  size in living animals, and used that correlation to estimate  population sizes of Tyrannosaurus rex.

Their calculations predicted that around 20,000 adult T. rex  would have existed at any given time, and during the species’  entire tenure on our planet, there would have been around  two-and-a-half billion individuals stomping around. Which is, of course, way more than the  number of T. rex fossils we’ve ever found. As of this video, there’s like, thirty.

Naturally, there’s probably a big  margin of error on those estimates given how many variables are involved. But it’s still a cool concept for  how we could take modern populations and use them to decipher things about the past. And beyond all those tricks of the trade, the best way to get around the  issues of an incomplete fossil record is to keep studying the  fossilization process itself.

Paleontologists are constantly  discovering new quirks and details about the ways that organisms  preserve in the geologic record. Each new discovery brings  us closer to understanding exactly how biased our fossil record  is, and how to fill in those gaps. Sadly, there are loads of ancient species  that we’ll simply never know about.

And we’ll never find a perfectly  preserved fossil ecosystem. But researchers are able to piece  together the past in quite a bit of detail by understanding the fossils that aren’t  there as much as the ones that are. [ OUTRO ]