Scientific American: The Physics of “Bloat and Float” – How Dinosaur Carcᴀsses Drift Out to Sea and Become Seabed Fossils

When a large dinosaur died near a river or coastal plain during the Mesozoic, its body often did not stay on land. Instead, it embarked on a remarkable final journey governed by the well-understood process known as “bloat and float.”

As decomposition begins, bacteria inside the carcᴀss produce large volumes of gas — primarily methane, carbon dioxide, and hydrogen sulfide. In big-bodied animals, these gases accumulate in the body cavity, dramatically inflating the torso like a natural balloon. The increased buoyancy can keep a multi-tonne dinosaur afloat for days, weeks, or even months, depending on water temperature, salinity, and the integrity of the body wall.

Once afloat, ocean currents, tides, and storms could carry the carcᴀss tens to hundreds of kilometres offshore. Modern analogues confirm the physics: elephant and hippopotamus carcᴀsses in African rivers routinely drift 100–500 km out to sea before sinking. The same forces operated during the Age of Dinosaurs, when higher sea levels and extensive river systems made such journeys even more common.

Eventually the body wall ruptures or the gases slowly diffuse, causing the carcᴀss to lose buoyancy and sink to the seafloor. There it lands in fine marine mud alongside fish, ammonites, and sharks. Rapid burial by sediment protects the bones from scavengers and currents, allowing fossilisation over millions of years. Tectonic subsidence then carries the remains deeper — sometimes kilometres beneath the modern seabed, as seen with the record-deep Plateosaurus bone from 2,256 metres under the North Sea.

This mechanism explains virtually every verified dinosaur bone found in marine sedimentary rocks worldwide. The bones are almost always isolated and disarticulated — exactly what is expected after long-distance floating and eventual sinking. No evidence supports the idea that non-avian dinosaurs lived or hunted in the ocean.

From riverbank to abyssal plain, the “bloat and float” process reveals how dynamic Mesozoic coastlines once were — and how ordinary biology and physics can move terrestrial giants into the most unexpected fossil graveyards.