Dinosaurs & Paleontology: The Cretaceous Period

Understanding Dinosaurs & Paleontology: The Cretaceous Period

Rock layers provide the basic timeline, but paleontologists need more than a layer order to date a fossil. Sedimentary rock usually forms from mud, sand, or shells that collect over time. Dinosaur bones are often buried in these sediments.

The rock below a fossil is normally older, while the rock above it is younger. This gives a relative age. For a numerical age, scientists often date volcanic ash beds near the fossil layer.

Minerals in ash can contain radioactive atoms that change into other atoms at a steady rate. A half life is the time needed for half of the original parent atoms to change. After one half life, one half remains.

After two half lives, one quarter remains. Careful lab measurements can place fossil sites on a much more precise timeline.

The Cretaceous world was not one uniform landscape. Continents moved apart, creating new coastlines and shallow inland seas. In North America, a broad sea once split much of the continent into western and eastern land areas.

Sea level changed as climate, tectonic activity, and the amount of water stored in ice changed. Fossils from marine rocks can include ammonites, sharks, fish, and tiny plankton. On land, river channels, floodplains, and coastal swamps preserved different kinds of animals.

The type of rock matters. Sandstone can record a fast flowing river.

Mudstone often forms in calmer water. Coal can point to thick plant growth in wet ground.

Plant changes shaped dinosaur food webs. Flowering plants spread widely during this time, though conifers, ferns, and cycads remained important. Different teeth reveal what an animal could process.

Long rows of grinding teeth in hadrosaurs helped them chew tough plant material. Sharp, serrated teeth in large predatory dinosaurs cut flesh rather than crushing bones. Scientists do not identify diet from teeth alone.

They compare tooth wear, jaw shape, fossilized stomach contents, trackways, and the plants found in the same rocks. A single fossil is a clue. A whole fossil site can show how organisms shared an environment.

The final extinction is studied through evidence from many places, not just dinosaur bones. A thin clay layer at the boundary contains unusually high amounts of iridium, an element rare in Earth crust but more common in some space rocks. Shocked quartz shows that minerals experienced extreme pressure.

The Chicxulub crater in Mexico provides the impact site. An incoming asteroid had enormous kinetic energy because kinetic energy equals one half times mass times speed squared. Dust, soot, and gases thrown into the atmosphere likely reduced sunlight, damaged plant growth, and disrupted food chains.

Students should remember that extinction was selective. Birds survived because they are the living dinosaur branch, while every other dinosaur lineage disappeared.