Pliosaurid plesiosaur with five tooth pairs on the mandibular symphysis. Head about 20% of overall body length. Cervical vertebral count at least 13, with cervical centra shortened and with enlarged neural canal, but not heavily ossified except in the region of the neural spine. Cervical ribs much shortened and thickened; double headed. Pectoral centra triangular in outline. Dorsal vertebral centra heavily ossified, with indistinct subcentral foramina and fiat anterior and posterior faces. Anterior and posterior zygapophyses reduced, neural spines thickened, with midline anterior and posterior excavations. Dorsal ribs thickened and pachyostotic, with concave heads, round cross-section and sausage-like' profile. Gastralia heavily ossified, pachyostotic. Humeri short and lightly constructed. Teeth stout and conical, of circular cross-section, and with prominent striae around entire crown.

Greek, Pachus, 'thick', Latin, Costa, 'Rib', Greek, Sauros, 'Lizard': Thick rib lizard.