"buttressed vertebrae" Based on partial material. The different species of Allosaurus varied in weight. Allosaurus fragilis, A. atrox, and A. ferox weighed about 1.1 to 1.9 tons (1 tonne to 1.7 tonnes); A. amplexus was much heavier and weighed about 2.7 to 5.5 tons (3 tonnes to 5 tonnes). E. amplexus is included with Allosaurus fragilis (or A. amplexus), although Bakker believes it is a separate and larger genus. Robert Bakker also referred a tail vertebrae (measuring 148 mm in length) from the upper Morrison Formation near Masonville, Colorado to Epanterias amplexus.