shared with Brontosaurus parvus. 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. Description Brontosaurus was a large, long-necked, quadrupedal animal with a long, whip-like tail, and forelimbs that were slightly shorter than its hindlimbs. The largest species, B. excelsus, measured up to long from head to tail and weighed up to ; other species were smaller, measuring long and weighing . The skull of Brontosaurus has not been found, but was probably similar to the skull of the closely related Apatosaurus. Like those of other sauropods, the vertebrae of the neck were deeply bifurcated; that is, they carried paired spines, resulting in a wide and deep neck. The spine and tail consisted of 15 cervicals, ten dorsals, five sacrals, and about 82 caudals. The number of caudal vertebrae was noted to vary, even within a species. The cervical vertebrae were stouter than other diplodocids, though not as stout as in mature specimens of Apatosaurus. The dorsal ribs are not fused or tightly attached to their vertebrae, instead being loosely articulated. Ten dorsal ribs are on either side of the body. The large neck was filled with an extensive system of weight-saving air sacs. Brontosaurus, like its close relative Apatosaurus, had tall spines on its vertebrae, which made up more than half the height of the individual bones. The shape