[1][2] Another specimen, the paratype MACN-CH 18689, consists of a cast of six articulated caudal vertebrae, the partial sacrum, and a fragmentary pubic peduncle from the right ilium.

[2] The autapomorphies that distinguish Epachthosaurus from other genera are:[2] middle and caudal dorsal vertebrae with unique articular processes extending ventrolaterally from the hyposphene; a strongly developed intraprezygapophyseal lamina, and processes projecting laterally from the dorsal portion of the spinodiapophyseal lamina; hyposphene-hypantrum articulations in caudals 1–14; and a pedal phalangeal formula of 2-2-3-2-0.

The genus shares the following apomorphies with various titanosaurians:[2] caudal vertebrae with ventrally expanded posterior centrodiapophyseal laminae; six sacral vertebrae; an ossified ligament or tendon above the sacral neural spines; procoelous proximal, middle, and distal caudal centra with well-developed distal articular condyles; semilunar sternal plates with cranioventral ridges; humeri with squared proximolateral margins and proximolateral processes; unossified carpals; greatly reduced manual phalanges; nearly horizontal, craniolaterally expanded iliac preacetabular processes; pubes proximodistally longer than ischia; and transversely expanded ischia.

[2] Below is a phylogenetical cladogram showing the position of Epachthosaurus within Titanosauria:[3] Phuwiangosaurus Andesaurus Argyrosaurus Paralititan Janenschia Aeolosaurus Santa Rosa indet.

[4] Malawisaurus Paludititan Lohuecotitan Epachthosaurus Alamosaurus Opisthocoelicaudia Neuquensaurus Rocasaurus Saltasaurus Lirainosaurus Atsinganosaurus Ampelosaurus Bonatitan Rapetosaurus Nemegtosaurus Gondwanatitan Aeolosaurus Rinconsaurus Muyelensaurus Bonitasaura Mendozasaurus Futalognkosaurus Epachthosaurus is known from the early Late Cretaceous of the Bajo Barreal Formation.