T. Bakkerargued in the 1990s that A.ajax and A.excelsus were in fact sufficiently distinct for the latter to merit a separate genus. In 2015, Emanuel Tschopp, Octávio Mateus, and Roger Benson released a paper on diplodocoid systematics, and proposed that genera could be diagnosed by thirteen differing characters, and species separated based on six. The minimum number for generic separation was chosen based on the fact that A.ajax and A.louisae differ in twelve characters, and Diplodocus carnegiei and D.hallorum differ in eleven characters. Thus, thirteen characters were chosen to validate the separation of genera. The six differing features for specific separation were chosen by counting the number of differing features in separate specimens generally agreed to represent one species, with only one differing character in D.carnegiei and A.louisae, but five differing features in B.excelsus. Therefore, Tschopp etal. argued that Apatosaurus excelsus, originally classified as Brontosaurus excelsus, had enough morphological differences from other species of Apatosaurus that it warranted being reclassified as a separate genus again. The conclusion was based on a comparison of 477 morphological characteristics across 81 different dinosaur individuals. Among the many notable differences are the widerand presumably strongerneck of Apatosaurus species compared to B.excelsus. Other species previously assigned to Apatosaurus, such as Elosaurus parvus and Eobrontosaurus yahnahpin were also reclassified as Brontosaurus. Some features proposed to separate Brontosaurus from Apatosaurus include: posterior dorsal vertebrae with the centrum longer than wide; the scapula rear to the acromial edge and the distal blade being excavated; the acromial edge of the distal scapular blade bearing a rounded expansion; and the ratio of the proximodistal length to transverse breadth of the astragalus 0.55 or greater. Sauropod expert Michael D'Emic pointed out that the criteria chosen were to an extent arbitrary and that they would require abandoning the name Brontosaurus again if newer analyses obtained different results. Mammal paleontologist Donald Prothero criticized the mass media reaction to this study as superficial and premature, concluding that he would keep "Brontosaurus" in quotes and not treat the name as a valid genus. Valid species Many species of Apatosaurus have been designated from scant material. Marsh named as many species as he could, which resulted in many being based upon fragmentary and indistinguishable remains. In 2005, Paul Upchurch and colleagues published a study that analyzed the species and specimen relationships of Apatosaurus. They found that A.louisae was the most basal species, followed by FMNHP25112, and