Apatosaurus "Apatosaurus" may have been more solitary than other Morrison Formation dinosaurs. Fossils of the genus have only been found in the upper levels of the formation. Those of "Apatosaurus ajax" are known exclusively from the upper Brushy Basin Member, about 152–151 mya. "A.louisae" fossils are rare, known only from one site in the upper Brushy Basin Member; they date to the late Kimmeridgian stage, about 151mya. Additional "Apatosaurus" remains are known from similarly aged or slightly younger rocks, but they have not been identified as any particular species, and thus may instead belong to "Brontosaurus". The Morrison Formation records a
Dinosaur egg and a system of classification based on the structure of eggshell was developed in China before gradually diffusing into the West. Dinosaur eggshell can be studied in thin section and viewed under a microscope. The interior of a dinosaur egg can be studied using CAT scans or by gradually dissolving away the shell with acid. Sometimes the egg preserves the remains of the developing embryo inside. The oldest known dinosaur eggs and embryos are from "Massospondylus", which lived during the Early Jurassic, about 190 million years ago. In 1859, the first scientifically documented dinosaur egg fossils were discovered in southern