Anyone with an interest in paleontology is likely familiar with the concepts of fossilization and fossils themselves, also known as remains or petrified objects. Fossilization is a complex of physical and chemical processes that transform living organisms into fossils. It’s no secret that fossilization takes thousands, if not tens of thousands, of years. But let’s delve deeper into this topic to understand the processes involved, the conditions required, and the unique fossils we can discover.

Fossilization is a prolonged process of replacing the tissues of living organisms with minerals. After the remains of an animal or plant are buried, the voids within them are filled with groundwater, followed by mineralization. This process preserves small organisms particularly well, as they are quickly covered by various sediments—silt or sand. Thanks to mineralization, we can study detailed structures, such as those of tiny pterosaurs, down to the intricacies of wing membrane organization. Large organisms, however, cannot be covered with the necessary layer as quickly, leading to significant degradation before they fossilize. This is why we know the structure of feathers from small birds but know relatively little about large sauropods.

The preservation of fossils also depends on the time period in which the organism lived. The older the geological strata, the more they have been subjected to erosion (destruction). Cambrian deposits (540-485 million years ago) are much less abundant than Neogene deposits (23-2.5 million years ago), and as a result, we have far fewer preserved organisms from the Cambrian period.

Conditions can vary widely, but it’s crucial to isolate the organism from environmental influences and microorganisms. Ideal conditions for preserving organisms can be found in hot, dry deserts, icy wastelands, humid swamps, or rivers with thick layers of silt. Complete skeletons of large animals, millions of years old, have been discovered by paleontologists in permafrost and in layers of sandstone. One of the most impressive finds was the brain of an Iguanodon over 130 million years old, with well-preserved blood vessels and tissues. A fortunate combination of circumstances buried the brain in the acidic environment of a swamp before the tissues began to decompose and be consumed by bacteria. Another monumental discovery was the mammoth calves Lyuba and Khroma. Today, they have been extensively studied, with some segments of their DNA even restored. But the most astonishing discoveries awaited us in 2017.

Sometimes, the very bacteria that consume the remains of a deceased animal actually help us preserve it. For example, after the death of a fish, a whole layer of various organisms forms on the decomposing flesh, eager to feast. These organisms, along with the fish, are then covered by sediments. The black silhouettes of animals on stone that we find are nothing more than imprints left by these bacteria.

Jurassic Park introduced us to one of the most reliable sources for preserving ancient organisms—amber. Beautiful insects, tens of millions of years old, preserved in amber no longer surprise anyone. Unfortunately, no matter how much blood they drank, DNA doesn’t survive beyond six million years, so creating a dinosaur park remains a fantasy. However, the discoveries made in 2017 are truly mind-blowing. A whole Cretaceous-era bird chick and a feathered dinosaur tail, both dating back to the same period, were found. Thanks to amber, these finds have reached us in perfect preservation, allowing us to examine the minute details of feathers that are 100 million years old.

It is astonishing that after tens of millions of years, not only stones but also some organic substances or their traces can reach us. For instance, in the early 2000s, Mary Schweitzer published the results of her research on the remains of a Tyrannosaurus rex over 68 million years old. Her findings indicated that not only the structures of the dinosaur’s soft tissues but also collagen—a protein found in bones and cartilage—had been preserved. Just a few decades ago, such a discovery would have seemed impossible. Equally surprising were the discoveries of melanosomes (organelles that store melanin and other pigments) in dinosaurs. This has allowed us to approximate the coloration of some Mesozoic animals.

Despite the poor preservation of giant animals, entire groups of large animals sometimes reach us in good condition. How does this happen? Perhaps the animals chose the wrong path and fell into a trap, such as a tar pit. Or predators drove their prey into a swamp, where they all drowned together. Maybe a catastrophe occurred, wiping out an entire family during egg incubation. The area now known as the Hell Creek Formation was, 66 million years ago, rich in rivers and streams with clayey bottoms and swamps, which contributed to such rich and high-quality burials. For example, during high tides or heavy rains, water levels in some bodies of water would rise significantly, but as soon as the water receded, certain pools became isolated from rivers and lakes. Fish and turtles perished there, much to the delight of paleontologists.

Fossilization, or petrification, is a lengthy physico-chemical process that transforms the remains of deceased organisms and traces of their life activity into a complex mineral structure with imprints of various tissues, and sometimes even organic molecules. The more favorable the fossilization conditions of the period, and the closer to us on the temporal scale, the more fascinating details we can uncover about that time.