Bonnan, M.F. and P. Senter. 2007. Were the basal sauropodomorph dinosaurs Plateosaurus and Massospondylus habitual quadrupeds?; pp. 139-155 in Barrett, P. M. and D.J. Batten (eds.), Evolution and palaeobiology of early sauropodomorph dinosaurs. Special Papers in Palaeontology, 77.

Abstract: The basal sauropodomorph dinosaurs Plateosaurus

and Massospondylus are often portrayed as habitual quadrupeds

that were facultatively bipedal. Surprisingly, the

functional morphology of their forelimbs has rarely been

considered when reconstructing their locomotor habits. If

Plateosaurus and Massospondylus were efficient, habitual

quadrupeds we predict that the manus would have been pronated

such that it produced a caudally directed force in parallel

with the pes. We articulated and manipulated the

forelimbs of Plateosaurus, Massospondylus and several extant

outgroup taxa (Varanus, Alligator, Anser and Struthio) using

a standardized protocol. Moreover, we compared our results

with previously published estimates of forelimb movement in

saurischian outgroup taxa from Theropoda and Sauropoda

and with the basal sauropodomorph ⁄ sauropod Melanorosaurus.

Our results indicate that the range of motion in the

forelimbs of Plateosaurus and Massospondylus did not allow

efficient, habitual quadrupedal locomotion. The range of

humeral flexion and abduction is limited and the articular

surfaces of the radius and ulna orient the palmar surfaces of

the manus medially in semi-supination. Active or passive

pronation of the manus was not possible and the manus

could not function in a dynamically similar way to the pes

for efficient quadrupedal locomotion. Our results also rule

out specialized forms of quadrupedal locomotion, such as

the knuckle-walking gait of some mammals. We suggest that

most known ‘prosauropod’ trackways were probably not

made by animals such as Plateosaurus or Massospondylus, but

the ichnotaxon Otozoum may have been created by animals

similar to these taxa. Furthermore, we show that trunk and

limb ratios do not yield consistent results and should not be

used solely to determine posture. Although these two taxa

probably assumed a quadrupedal posture as hatchlings, we

show that the morphological orientations of the forelimb

elements remained consistent across ontogeny, precluding

efficient, quadrupedal locomotion at any age. As with theropods,

forelimb use in basal sauropodomorphs is difficult to

reconstruct and interpret. We speculate that the forelimb

could have aided in acquiring vegetation or defence in

Plateosaurus and Massospondylus only if these animals reared

or assumed a tripodal posture.

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