Brontosaurus excelsus has a history of being a very popular dinosaur, yet many recent studies find it a species of Apatosaurus, A. excelsus. It shows many features that are similar to A. ajax, A. louisae, and A. parvus. However, as was brought up in one SVPOW talk, Brontosaurus might be revived again, based on quite numerous and major differences. When comparing Scott Hartman’s Apatosaurus ajax, Apatosaurus louisae, Apatosaurus excelsus, and the original figures and description of Apatosaurus parvus, one might notice such differences. Namely, based on comparative anatomy, the species form a group of Apatosaurus ajax + (Apatosaurus excelsus + (Apatosaurus louisae + Apatosaurus parvus)). “Apatosaurus” mimimus probably belongs to a diplodocoid, and is very different from all the close Apatosaurus/Brontosaurus species. Below are features distinguishing the species from one another, which, as are fairly obvious, quite numerous.
Apatosaurus ajax: A gracile species, A. ajax is distinguished from all other species by lacking a posterior bulge on the proximal end of the scapula, and possessing a large anterior bulge on the proximal end of the scapula; a pubis whose shaft directly attaches to the ischium; lacking a developed posterior bulge on the proximal end of the humerus; possessing a pubis closest oriented to a horizontal grade; possessing the shortest and slenderest dorsal neural spines; possessing a low ilium; possessing a rectangular coracoid with convex edges; by having a pubic boot developed the least along the ventral edge, and the most along the dorsal edge; and a scapular blade that lacks a second anterior bulge seen on the other species.
Apatosaurus excelsus: An intermediate species, B. excelsus can be distinguished from all other species by possessing an preacetabular blade of the ilium that extends anteroventrally when flairing; possessing an upward notch in the pubic extension that attaches to the ischium when viewed laterally; having tall yet gracile dorsal neural spines; possessing a scapular shaft that curves posteriorly; possessing a coracoid that is ovoid; having an elongate humerus that extends posteriorly at its proximal end; and having a pubic boot that extends both anteroventrally and posterodorsally while not symetrical.
Apatosaurus louisae: An robust species, A. louisae can be distinguished from all other species by possessing a pubic boot almost symmetrical when divided; an ilium that inclines anterodorsally; a pubis with an almost rectangular extension attaching to the ischium; tall yet robust dorsal neural spines; a scapular shaft curving to become almost vertical; a short humerus thats proximal end has a posterior extension; and a broader, more robust tibia.
Apatosaurus parvus: An robust species, A. parvus can be distinguished from all other species by possessing a pubic boot with a larger posteriodorsal expansion that curves to become perpendicular with the shaft; A pubic shaft expanding to about one quarter greater than its original width; and a scapular shaft perpendicular to the posterior extension of the scapular blade, yet at an angle of approximately 130 degrees to the anterior expansion.
Even thought the above might be quite technical, it can all be summed up in about three words. Example. Of. Lumping. All the species have some similarities, but also have quite a few differences. Possibly the best way to solve this is by reassigning species to their original genera, or in the case of A. louisae, its closest species’ genus. Based on this, A. parvus and A. louisae would become Elosaurus, and A. excelsus would become Brontosaurus. However, I have heard somewhere that E. parvus is actually a juvenile, in which case, E. louisae might become its junior synonym. This I don’t think will be the case as even though they are quite similar in some features, they are also quite different in others.
I’m also putting up this illustration of an Apatosaurus. This is free to use anywhere as long as it retains the original license. This image has also been reviewed by the entire wikipedia review team, so it is as accurate as it can. Guess which species!
Note: The anatomical differences above are not the result of detailed examinations, just obvious differences in morphology and proportions.