Robert T. Bakker
Almost all 20th-century paleontologists agreed with Riggs that all "Apatosaurus" and "Brontosaurus" species should be classified in a single genus. According to the rules of the ICZN (which governs the scientific names of animals), the name "Apatosaurus", having been published first, had priority as the official name; "Brontosaurus" was considered a junior synonym and was therefore discarded from formal use. Despite this, at least one paleontologist—Robert T. Bakker—argued in the 1990s that "A. ajax" and "A. excelsus" are in fact sufficiently distinct that the latter continues to merit a separate genus. In 2015, an extensive study of diplodocid relationships by Emanuel Tschopp, Octavio Mateus, and Roger Benson concluded that "Brontosaurus" was indeed a valid genus of sauropod distinct from "Apatosaurus". The scientists developed a statistical method to more objectively assess differences between fossil genera and species, and concluded that "Brontosaurus" could be "resurrected" as a valid name. They assigned two former "Apatosaurus" species, "A. parvus" and "A. yahnahpin", to "Brontosaurus", as well as the type species "B. excelsus". Paleontologist Michael D'Emic made a critique. Paleontologist Donald Prothero criticized the mass media reaction to this study as superficial and premature, concluding: