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Osborn and Mook (1921) actually synonymize Amphicoelias latus with Camarasaurus supremus, but Carpenter (1998) considers it a synonym of Camarasaurus grandis because it was found lower in the Morrison Formation and the deeply concave articular faces on the caudal vertebrae are more consistent with C. grandis. Tschopp et al. (2015, 2017) recover Amphicoelias as either a basal diplodocid or an apatosaurine more derived than Brontosaurus excelsus. Therefore, you may be right to point out that Amphicoelias shares a circular femoral cross-section with Diplodocus, but the paper should take into account the differences between Amphicoelias and other diplodocids jotted out by Tschopp et al. (2015).
Carpenter, K., 1998, Vertebrate biostratigraphy of the Morrison Formation near Canon City, Colorado: In: The Upper Jurassic Morrison Formation: An Interdisciplinary Study. Edited by Carpenter K., Chure D. J., and Kirkland J. I., Modern Geology, v. 23, part 2, p. 407-426.
Tschopp, E.; Mateus, O. V.; Benson, R. B. J. (2015). "A specimen-level phylogenetic analysis and taxonomic revision of Diplodocidae (Dinosauria, Sauropoda)". PeerJ. 3: e857. doi:10.7717/peerj.857. PMC 4393826 . PMID 25870766.
Emanuel Tschopp; Octávio Mateus (2017). "[Osteology of Galeamopus pabsti sp. nov. (Sauropoda: Diplodocidae), with implications for neurocentral closure timing, and the cervico-dorsal transition in diplodocids].". PeerJ. 5: e3179. doi:10.7717/peerj.3179.
Carpenter (2018) has reclassified Amphicoelias fragillimus as a rebbachisaurid and erected Maraapunisaurus for it based on comparisons with known rebbachisaurids. Andrea Cau (http://theropoda.blogspot.com/2012/01/amphicoelias-fragillimus-e-un.html) previously floated a rebbachisaurid placement for Amphicoelias fragillimus, but Carpenter (2018) is the first author to formalize Cau's hypothesis. In other words, even if Maraapunisaurus isn’t as huge as estimated by Carpenter (2006), the new size estimates of 98-104 feet given by Carpenter (2018) still make it a monster sauropod on a par with the giant lognkosaurian titanosaurs, Xinjiangtitan, and the diplodocines Supersaurus and Diplodocus hallorum.
As a side note, the distal femur that you list as being part of AMNH 5777 was never assigned to M. fragillimus in the original description by Cope and is actually first mentioned in Cope's 1879 field notes; the distal femur itself is considered by Carpenter (2018) to possibly belong to either Camarasaurus and Amphicoelias rather than Maraapunisaurus. In short, just because the dorsal vertebrae of Maraapunisaurus were tall relative to other Morrison sauropods doesn't change the fact it would have had the same femoral length and circumference as other Morrison diplodocoids, because neural spine height doesn't translate into increased femoral length.
In my opinion, the revised size estimates for Maraapunisaurus by Carpenter (2018) make sense because a sauropod 190 feet long and weighting over 122,400 kilograms would not have had legs gracile enough to support such a high weight.
Carpenter, K. (2018). Maraapunisaurus fragillimus, n.g. (formerly Amphicoelias fragillimus), a basal Rebbachisaurid from the Morrison Formation (Upper Jurassic) of Colorado. Geology of the Intermountain West. 5 (9): 227–244. ISSN 2380-7601.