Amphicoelias - Classification

Classification

Edward Drinker Cope described his finds in two 1878 issues of the American Naturalist, and assigned them to the new genus Amphicoelias. He placed it in a unique family, Amphicoeliidae, though this is now considered a nomen oblitum (forgotten name). The genus is usually assigned to the family Diplodocidae, though some modern analyses have found it at the base of the larger group Diplodocoidea or as a diplodocid incertae sedis (uncertain placement). The first named species in the genus, Amphicoelias altus (holotype specimen AMHD 5764), was discovered by Cope in 1877. But while it is only represented by a partial skeleton, there are enough diagnostic characteristics to provisionally define the genus. A. altus is known from better remains, but is smaller than A. fragillimus. Cope also named a third species in 1878: Amphicoelias latus.

Osborn and Mook, in 1921, provisionally synonymized the three species, sinking A. latus into A. altus, and suggesting also that A. fragillimus is just a very large individual of A. altus, a position McIntosh agreed with in 1998. Carpenter (2006) disagreed about the synonymy of A. altus and A. fragillimus, however, citing numerous differences in the construction of the vertebra also noted by Cope, and suggested these differences are enough to warrant a separate species or even a separate genus for A. fragillimus. However, he went on to caution that the validity of A. fragillimus as a separate species is nearly impossible to determine without the original specimen to study.

In 2007, John Foster suggested that the differences usually cited to differentiate Amphicoelias altus from the more well known Diplodocus are not significant and may be due to individual variation. Foster argued that Amphicoelias is probably the senior synonym of Diplodocus, and that if further research bears this out, the familiar name Diplodocus would need to be abandoned in favor of Amphicoelias, as was the case with Brontosaurus and its senior synonym Apatosaurus.