🦕 How did giant sauropod dinosaurs become quadrupedal?

Sauropods, those giant long-necked dinosaurs, are the largest known animals to have ever existed, even though they descended from significantly smaller bipedal ancestors. Many of them reached several tens of meters in length (over 65 feet) and masses exceeding several tens of tons (over 20 short tons).

Yet, these gigantic quadrupeds belong to the sauropodomorphs, whose first representatives were small bipeds no larger than a turkey. This evolution toward extreme gigantism therefore suggests a response of their morphology to considerable physical constraints.


Since the work of Galileo, it has been established that bones should grow disproportionately to body mass. When an object doubles in size, for example, its volume is multiplied by eight. Applied to land animals, this relationship implies that bones should thicken much faster than the rest of the body to support this weight gain.

The specialization of limbs for gigantism can also correspond to more complex structural modifications to better manage the stresses linked to increased mass. For example, in sauropods and/or elephants, the bones are particularly straight, unlike those of lighter animals, whose bones are more curved.

Using bone scanning and 3D modeling tools, a team of researchers from the Institute of Systematics, Evolution, Biodiversity (ISYEB - CNRS/MNHN/Sorbonne Université/EPHE) and the Adaptive Mechanisms and Evolution laboratory (MECADEV - CNRS/MNHN) reveals that the appearance of morphological specializations varies depending on the type of bone considered. Thus, these specializations appear very abruptly in the bones of the forearm, while they are established more gradually in the bones of the hind limbs.

Since the first sauropodomorphs were originally small bipeds, this difference very likely reflects the transition toward quadrupedal locomotion, imposing a new body support constraint on the forelimbs, whereas the hind limbs already fulfilled this function.


In this study, the scientists focused particularly on the humerus, the bone connecting the shoulder to the elbow. They thus discovered that the specializations of sauropods do not appear uniformly within the bone itself. While the morphology of its lower half evolves abruptly, that of the upper half changes more progressively.

The shape variations observed in the lower part of the humerus follow a pattern similar to that observed for the forearm bones (the radius and ulna), these elements together constituting the elbow joint. This result agrees with some studies conducted in mammals, particularly rhinoceroses, where a regionalization around joints has also been identified. This might allow for finding, in the future, a generalized trend among different groups of land animals.

This result indicates that the evolution of limbs is not necessarily organized bone by bone, as traditionally assumed, but according to functional modules centered on joints. The elbow joint thus appears as a coherent unit of evolutionary transformation, involving several bones, or parts of bones, subjected to the same mechanical constraints.

A comparable regionalization around the elbow has also been observed in several groups of mammals, such as rhinoceroses or mustelids (weasels). The presence of similar patterns in such distantly related groups suggests that these are not isolated cases.

These results will help identify, in the future, generalized trends of limb evolution in different groups of land animals.