How Do Fossils Form?: Crash Course Geology #18
CrashCourse
What Fossils Actually Are 0:00
You might picture fossils as bones or shells, but they cover much more ground than that. Fossils are preserved remains or traces of organisms that are at least 10,000 years old, and they can include skin, feathers, flowers, footprints, tunnels, nests, even whole baby mammoths or worm sperm. The episode opens with a notosaur, a 110 million year old creature whose body drifted down a river into the sea, bloated, burst, and sank belly up into seafloor mud, which set up the burial that let it fossilize at all. Even though every living thing dies, very few actually turn into fossils, since the process needs very specific conditions.
Two Main Paths To Fossilizing 1:31
After burial, one common path is permineralization, where minerals from groundwater seep into buried bone, wood, shells, or soft tissue, replace them, and leave a denser stone version behind, sometimes even opalizing into something glossy. A quick field trick paleontologists use is licking a suspected fossil, since porous bone sticks to your tongue while ordinary rock does not. The other main path is compression and carbonization, which usually happens to lighter organisms like leaves, fish, and insects, first flattening them under sediment and then chemically turning their decomposing remains into a thin carbon silhouette. Not every fossil preserves a body at all. Trace fossils capture behavior, like the 11,500 year old footprints in New Mexico left by children playing in the tracks of a giant ground sloth.
Rare Whole Body Preservation 4:30
In rarer cases, whole bodies survive with skin intact, such as insects sealed in tree resin that hardens into amber, bodies that dry into mummies in desert caves, or remains preserved in tar pits, bogs, or ice. The two biggest factors that raise the odds of fossilization are rapid burial soon after death and having tough body parts like shells, bones, or teeth, since soft tissue decomposes too easily. Even after fossilizing, a specimen has to survive erosion and weathering long enough to be found, which is why the fossil record stays incomplete, and why the notosaur's survival for millions of years until oil sands miners in Alberta accidentally drilled into it in 2011 was so exceptional.
What Fossils Teach Us 6:31
Paleontologist Jennifer Clack's 1987 discovery in Greenland of a fossil she nicknamed Boris, showing gill and lung structures alongside paddle shaped legs, revealed that early tetrapods evolved legs and lungs before moving onto land. Rich fossil sites called lagerstätten, like Canada's Burgess Shale from over half a billion years ago, hold huge numbers of well preserved fossils, including ancestors of living animals and strange dead ends like hallucigenia, whose head took scientists three decades to identify. Fossils also reveal Earth's own history, showing that sea creature fossils atop mountains prove ancient seafloors were pushed upward, and matching fern and reptile fossils on separate continents helped confirm that continents move.
AI-generated summary. It can be wrong or incomplete - check anything that matters against the original.
