Dinosaurs: How Fossil Evidence Explains Their Evolution and Extinction

Three separate lines of scientific evidence explain the dinosaur story: precisely dated rock layers that show when dinosaurs first appeared, fossils that preserve their bones and behavior, and isotope signatures in rock from the end of the Cretaceous Period that identify what killed them off. Researchers have refined all three in the past decade using methods ranging from uranium-lead dating to isotope geochemistry.

Origins: Rewriting the Triassic Timeline

Massachusetts Institute of Technology (MIT) researchers dated volcanic zircon crystals from the Chinle Formation, a rock sequence spanning Arizona, New Mexico, Utah and Colorado, and found dinosaurs present in North America roughly 223 million years ago. The method relies on measuring the ratio of uranium to lead isotopes in zircon crystals, which begin accumulating lead at a known rate once the mineral forms, giving scientists a precise age for the surrounding rock.

That finding complicated an earlier view that dinosaurs arose only in South America around 230 million years ago before spreading elsewhere. Geologist Jahan Ramezani, who worked on the MIT study, said the two continents show very different evolutionary patterns during this period.

In South America, by contrast, the researchers found very little overlap between the earlier and later evolutionary stages of dinosaurs. The team also identified a 16-million-year gap in the fossil record just below the oldest known dinosaur-bearing rocks, meaning dinosaurs may have originated even earlier than current fossils show, if evidence from that period was never preserved or has not yet been found.

How Fossils Preserve Evidence of Dinosaurs

The Natural History Museum in London explains that most dinosaur fossils form through a process called petrification. An animal that dies near a lake or river can be buried quickly in mud, sand or silt; the resulting lack of oxygen slows decomposition long enough for groundwater to deposit minerals inside the bone, gradually turning it to stone. The museum notes this process can take millions of years and typically requires rapid burial, which is one reason finds are rare.

• Body fossils: preserved bones, teeth and claws, often with a sponge- or honeycomb-like internal texture left by mineral crystals.
• Trace fossils: footprints, skin or feather impressions, and coprolites (fossilized droppings) that record behavior rather than body parts.
• Mould and cast fossils: cavities left when groundwater dissolves buried bone, later filled by sediment or minerals to form a cast of the original shape.

The Asteroid That Ended the Age of Dinosaurs

The Cretaceous Period ended abruptly 66 million years ago when an asteroid struck what is now the Yucatán Peninsula in Mexico, leaving the Chicxulub crater. A 2024 study led by Mario Fischer-Gödde of the University of Cologne, reported by the Natural History Museum and by Sci.News, measured ruthenium isotopes in Cretaceous-Paleogene boundary rock from sites including Spain, Italy and Denmark. The isotope signatures matched carbonaceous chondrites rather than the siliceous asteroids typical of the inner Solar System, indicating the impactor originated beyond the orbit of Jupiter.

Researchers compared the boundary samples with material from five other, smaller impact structures dated between 36 million and 470 million years old, as well as with ancient impact-spherule layers and two carbonaceous meteorites, to confirm the match was specific to the Chicxulub event. According to Sci.News, University of Cologne professor Carsten Münker, a co-author of the study, said the finding makes the Chicxulub strike "a very rare and unique event in geological time" among known asteroid impacts on Earth.

National Geographic reports that the asteroid struck at roughly 45,000 miles per hour, throwing large volumes of dust, debris and sulfur into the atmosphere and triggering severe climate disruption worldwide. The same article states that about 76 percent of all species on Earth went extinct, including every non-avian dinosaur, while birds — classified by scientists as avian dinosaurs — survived. Sci.News cites a close but slightly different figure, attributing to the University of Cologne researchers a finding that the impact and its aftermath killed off 75 percent of life on the planet.

Was Volcanism Also a Factor?

National Geographic notes that large volcanic eruptions at the Deccan Traps in present-day India may have compounded the environmental crisis around the same time as the impact. The outlet also states that the Chicxulub event remains the only one of Earth's major mass extinctions definitively linked to an asteroid strike, as the other four were primarily associated with volcanic activity and long-term climate shifts rather than a single impact.