Humans appear 30 seconds before midnight at end of day. Most fossils are embedded in sedimentary rock, weathered particles that provide strata from lower older layers to upper newer layers. Paleontologists study the fossil record based on boundaries between strata, where one mix of fossils gives way to another. Transitional links are intermediate between major groups. Seymouria is amphibian ancestral to reptiles.
Therapsids are reptiles ancestral to mammals.
Geological Time Scale a. Geological history of earth is divided into eras, periods, and epochs. Absolute dating method uses radioactive isotopes. Isotopes each have particular half-life or time it takes for half of isotope to decay and become nonradioactive. Following extinctions, remaining groups expand to fill habitats vacated by extinct species. Extinction of dinosaurs at end of Cretaceous.