Everything we’ve learned about dinosaurs essentially comes from fossils. But million-year-old rocks and bones have left a few hulking gaps in our understanding of the prehistoric world. Dinosaur Mysteries digs into the more secretive side of the “terrible lizards,” and all the questions that keep paleontologists up at night.

WE ARE IN THE MIDDLE of history’s greatest fossil rush. Forget about the 19th-century Bone Wars or the early 20th-century rise of US museums—paleontologists today are finding more dinosaurs faster than before. On average, they name a new nonavian dinosaur species every two weeks. Some of this year’s fresh arrivals include the long-necked herbivore Chucarosaurus, the duckbill Malefica, and the dome-headed Platytholus.

Despite this incredible rate of discovery, however, plenty of dinosaurs are missing from the paleontological history we’re trying to piece together.

If dinosaur seekers had their druthers, Earth’s geology would look something like an onion. Experts would work through perfectly stacked layers of sedimentary rock that contain comprehensive records of all the species that lived in ancient habitats through time. But such good fortune has eluded scientists. Since the 1800s, geologists and paleontologists have recognized that the fossil record is uneven and sporadic, made up of sediment that accumulated in environments such as streams, oceans, and dune-covered deserts. Most living things were eaten or decayed long before they could become fossilized.

Circumstances have to be just right for a fossil to form. The most ideal settings include relatively wet lowlands where rivers, streams, and other flowing waters could carry the requisite sand and silt to cover bodies. The blanket of sediment helped keep fossils-to-be from being nibbled on by scavengers or destroyed by the elements. As sediment turned to stone, mineral-laden water trickled through the encased body and replaced bone and sometimes soft tissues in a process called permineralization. The nature of the reaction varied from case to case, affected by everything from the size of the dinosaur to the local environment. This explains why we find some prehistoric creatures as partial, jumbled skeletons and others as delicately preserved fossils surrounded by feathers with not a bone out of place. 

In the end, paleontologists need to work with a fraction of a fraction of life’s story. Even some of the best fossil-hunting spots in the world are far from perfect. Consider the gorgeous banded rock layers of Dinosaur Provincial Park in Alberta, Canada, a hotspot for the discovery of stunning Late Cretaceous species such as the crested duckbill Lambeosaurus and the toothy tyrannosaur Gorgosaurus. In a 2013 review of fossils discovered in the park, paleontologists found that dinosaurs that weighed more than 130 pounds are often found at about 78 percent completeness while those below 130 pounds are usually found at about 7.6 percent completeness. (Paleontologists can often differentiate species even from such limited remains based on subtle anatomical traits that experts catalog over time.) Evidently, ancient ecosystems were much harsher on small specimens, masking how numerous they were in thriving times.

The fossil record runs rampant with sampling biases as well. Paleontologists come into the field with their own ideas of what to look for, and many are motivated to study megafauna, which hold more public allure and pose less of a challenge to excavate. A little more than a century ago, when paleontologists were beginning to search Alberta’s 75-million-year-old rocks, the big dinosaurs were much easier to find. Museums—both in the province and in faraway cities like New York—were hungry for near-complete, showstopping reptiles to lure in visitors. No surprise then that the same 2013 assessment from Dinosaur Provincial Park found it took paleontologists an average of 33.6 years to discover and name species above the 130-pound threshold and 65.9 years for those below.

The pattern holds for other dinosaur-bearing rocks, like the famous multistate Hell Creek Formation that preserves the last days of the dinosaurs in western North America. Even though paleontologists have named small dinosaurs when they’ve happened across their fossils, experts have been actively considering the more diminutive reptiles only in the past decade or so.

According to one estimate, more than 70 percent of discoverable dinosaur species are still hidden beyond detection.

Of course, it’s a wonder that we know about any dinosaurs at all. Every single fossilized skeleton or footprint has beaten long odds to tell us about ecosystems that we’ll never get to experience directly. Details of how these ancient habitats changed are critical to debates on whether dinosaurs were flourishing or struggling as the great Age of Dinosaurs approached its closing act 66 million years ago.

For example, paleontologists used to wonder why there seemed to be far more dinosaur species roaming western North America 75 million years ago than 66 million years ago, just prior to the K/Pg extinction that decimated them. Some experts reasoned that the creatures were already in decline. But when researchers looked at how prehistoric habitats shifted during that 9-million-year window, they found that environments better at preserving fossils diminished over time. A warm, shallow sea that divided North America drained off the continent, taking with it the wet, marshy lowlands that immortalized dinosaurs so extensively. So there were probably more dinosaur species running around the continent 66 million years ago than we’ll ever know about, a gap created by changes to the land itself.

So where would these missing fauna have dwelled? There’s every reason to think that dinosaurs clambered around ancient mountain ranges—but mountains are hotspots of erosion, not deposition, so the accumulations of sediment needed to preserve dinosaur bodies weren’t present there. The erosion problem also applies to some deserts, like the one in modern-day Arches National Park. Even though it’s perfectly natural to think of dinosaurs wandering between the expanses of bright sandstone, these landscapes were too dry and disintegrated for dinosaurs to be buried and fossilized in them.

It’s also entirely possible that some “rare” species in well-documented fossil beds were transported there by the elements after death. Think of the heavily armored ankylosaurs that were swept out to burials at sea, or the long-necked sauropods that dwelled in the hills but are known by the bones washed into cave systems, where they were buried. Fossil beds often represent where organisms became preserved, not necessarily where they lived.

Because the Earth is not an onion, much of the fossil record remains uneven and unexposed. While somewhat dated now, one 2006 estimate proposed that more than 70 percent of discoverable dinosaur species are still hidden beyond detection.

Much of what we’ve learned about the dinosaur story comes from the later parts of the Triassic, Jurassic, and Cretaceous. We have way less information on the middle of each Mesozoic period, times when new dinosaur dynasties were forming and the ecosystems they thrived in were evolving with them. When experts uncover these animals, they enrich our knowledge of these mysterious times in dinosaur history. In 2019, for example, paleontologists described the sharp-toothed Asfaltovenator from the Middle Jurassic in Argentina. The species offers some context for the rise of the world’s first truly giant carnivores, like Allosaurus. It’s in these middle chapters that we stand to learn the most—to learn about the dinosaurs that are most likely to change and challenge what we think we know about a world millions of years ago.

We hope you enjoyed Riley Black’s column, Dinosaur Mysteries. Check back on PopSci+ in June for the next article.