Cervical vertebra 11 of Apatosaurus louisae CM 3018 (A; modified from Gilmore, 1936) and Diplodocus carnegii CM 84 (B; modified from Hatcher, 1901) in left (A) and right (B) lateral view. Note the posteriorly extending posterior centrodiapophyseal lamina in Apatosaurus (A; C135-1), the anteriorly restricted pneumatic foramen typical for most apatosaurs (A; C172-1), the pre-epipophysis (A; C181-1), the subdivided prezygapophyseal centrodiapophyseal fossa, characterizing A. louisae (A; C184-1), the posteriorly expanded interpostzygapophyseal lamina of Diplodocus (B; C190-1), the posteriorly restricted prezygapophysis of A. louisae (A; C194-1), compared to the state in Diplodocus, where it reaches the anterior edge of the condyle (B; C194-0), the vertical accessory spinal lamina marking Diplodocus (B; C203-1), the different positions of the cervical ribs (ventrally projecting, A, C216-1; or level with centrum, B, C216-0), and the absence (A; C219-1) or presence (B; C219-0) of the anterior process of the cervical rib. Vertebrae scaled to same posterior cotyle height. Abb: apf, anterior pneumatic fossa; CR, cervical ribs; podl, postzygodiapophyseal lamina; poz, postzygapophysis; ppf, posterior pneumatic fossa; prdl, prezygodiapophyseal lamina; pvfo, posteroventral fossa; spol, spinopostzygapophyseal lamina; sprl, spinoprezygapophyseal lamina.