How do we know which dinosaurs walked on 2 or 4 legs?

From the discovery of the first fossil dinosaurs at the beginning of the 19^and century, questions arose about their posture: on two legs, or on four legs? Facing a Diplodocus or a T. rex, no hesitation. But for many others, the message of the bones is not so clear. So how do paleontologists determine what the posture of these creatures of the past might have been?

From a vision of heavy creatures, to that of agile animals

Let’s talk about the first known dinosaur! It is Megalosaurus, alias “large lizard”, which lived in the Middle Jurassic (approx. 168-166 million years ago) in present-day England and whose scientific description dates from 1824.

At that time, William Buckland, his descriptor, had only a jaw with several teeth, as well as the vertebrae and long bones of the limbs. The first representations show him as a giant lizard, about fifteen meters long, and therefore as a quadruped with the legs arranged on the sides. Following later discoveries, and in particular the work of Richard Owen, dinosaurs are no longer represented with legs apart from the body laterally, but with more vertical limbs. It is in this upright quadruped posture that Megalosaurus is represented at Crystal Palace Park of London, where life-size reconstructions of dinosaurs were exhibited in 1853. Today, paleontologists know that this species, which measured 6 to 7 meters in length, was actually bipedal.

Other reconstruction errors have thus been corrected over time and the advance of knowledge in paleontology. This is the case for example for theIguanodon, represented as a massive quadruped, then as a biped whose tail trailed on the ground and finally, today, as a quadruped capable of being optionally bipedal (as in the Disney cartoon Dinosaur).

These changes in posture also accompany a change in the perception of dinosaurs that occurred in the 1970s, especially when it was recognized that birds are indeed dinosaurs. We went from an image of rather amorphous heavy creatures to that of relatively agile dynamic animals.

First bipeds! Then several returns to the quadruped

Dinosaurs are an interesting group to study postural transitions. They show at least four cases of transition from bipedalism to quadrupedalism: in sauropodomorphs (group of Diplodocus) and at least three times in ornithischians (other herbivores such as Triceratops, Stegosaurus and Ankylosaurus).

Indeed, the first dinosaurs were bipedal. One hypothesis suggests that the dinosaurs owe their evolutionary success, and therefore their great and rapid diversification (radiation) from the beginning of the Triassic, to the advantages linked to their bipedal posture. They would thus have been faster and more agile than their contemporaries, the pseudosuchians, a group currently represented by the crocodilians. This hypothesis is called the locomotor superiority hypothesis.

But when one becomes massive, bipedalism is no longer necessarily the most advantageous posture, hence the return to quadrupedalism.

What can we learn from dinosaur bones?

There are a number of skeletal features of the limbs that help distinguish dinosaur posture. This is particularly the case for length ratios and the presence of certain bony structures, such as muscle attachments. These attachments are indeed generally relatively more developed in quadrupeds on the humerus (bone of the arm), the ulna (or ulna, bone of the forearm) and less developed on the femur (bone of the thigh), where the tail muscles attach. Quadrupedal dinosaurs also have proportionately longer forelimbs and shorter metatarsals (foot bones) than bipedal dinosaurs. The muscle mass of bipeds is in fact concentrated more towards the top of the hind limbs, moving the animal’s center of gravity away from the ground, which notably allows rapid movements and better running abilities.

The feet are positioned vertically below the pelvis in quadrupedal dinosaurs while they are closer to the midline of the body (which would pass vertically through the middle of the body) in bipeds, as in humans in which the knee is closer of this line as the top of the thigh. Indeed, moving the feet away from the midline in a biped would cause it to lose a lot of stability, due to the distribution of the weight on only two limbs and its center of mass located further back from the body and further from the ground.

The bipedal-quadrupedal passage in dinosaurs is generally linked to a large increase in size, so that characteristics linked to massive weight are often associated with those linked to quadrupedism and it is not always easy to distinguish them. For example, the giant forms show an even more marked elongation of the forelimbs.

The determination of posture usually relies on the identification of several of these different anatomical features. It requires having a good representation of the overall proportions of the skeleton and therefore fairly complete remains. However, researchers have tried to find features that predict whether an organism was bipedal or quadrupedal from a single bone. This is what an international team did on the femur. Their study not only makes it possible to highlight the morphological characteristics linked to an increase in size (such as the ends of the bone more robust in the most massive) and linked to a change in posture (such as a more curved bone in the bipeds) , but also to differentiate them. Like what, with a single bone, we can sometimes learn a lot!

And from representations of the whole animal?

Researchers have modeled the position of the center of mass (CoM) in various dinosaurs. An Anglo-Canadian team studied this in several ornithischians in whom a too anterior position of the CoM, naturally making bipedal locomotion impossible, made it possible to identify a quadrupedal mode of locomotion. This study also suggested that the development of growths at the level of the skull, but also of the back and the tail (such as the club of the ankylosaurs) could have had an important role in the evolution of the position of the CoM, pulling it forward for the frills and horns for example, and so posture, at least within ceratopsians (like Triceratops).

An international team has also analyzed whether criteria could make it possible to study changes in posture during growth. These researchers thus suggested, on the basis of the circumferences of the humerus (bones of the arm) and femur (thigh), a change of posture from quadruped to biped in the primitive sauropod. Mussaurus and in the primitive ceratopsian Psittacosaurus. Mussaurus and Psittacosaurus occupy positions in the evolutionary tree of dinosaurs close to where evolutionary transitions in posture (between adults of different species and not between juveniles and adults of the same species) seem to have occurred (bipedal-quadrupedal passage in the sauropodomorphs and ceratopsians). This discovery thus suggests that these juvenile postural characteristics would then have been retained in adults during evolution.

For further

Without a skeleton, what to do?

In addition to morphological characteristics, footprints can also be a very good indicator of posture. The study of footprints is called ichnology. Unfortunately, it remains very difficult to precisely associate footprints with the dinosaur that left them. They nevertheless make it possible to characterize the locomotion of groups of dinosaurs identified at a broader taxonomic rank (such as genus or family).

In sauropods, many handprints in small to medium-sized forms show a lateral orientation, while they are more forward-oriented in large forms. This has led researchers to suggest a reduction in hand mobility related to increasing body size or caused by continued ossification of joints as individuals increase in age. These researchers also highlighted, by comparing the hand and foot prints, that a more anterior orientation of the hands was also used at higher speeds, allowing the forelimb to be involved in propulsion as well. ‘animal. Indeed, propulsion is always essentially provided by the hind limbs in quadrupeds. Moreover, their study showed an uncoupling in forelimb and hindlimb posture and orientation variations, likely reflecting marked anatomical and functional differences. So even without a skeleton, we manage to obtain information on the posture.

Thus, many studies are underway to try to elucidate the posture of dinosaurs, whether to specify the locomotion of forms for which there is no longer any ambiguity between bipedalism and quadrupedalism, or to understand the mode of locomotion of forms whose posture remains much more enigmatic. In particular, there is still work with all these essentially quadrupedal forms, but able to move in a bipedal way and those essentially bipedal, but able to move in a quadrupedal way. Because they are not rare! The combined studies of various research teams using different approaches gradually allow us to complete our knowledge in this field.

Alexandra Houssaye, Paleobiology/Functional Morphology Researcher, National Museum of Natural History (MNHN) and Romain Pintore, PhD student in Paleobiology and Functional Morphology, National Museum of Natural History (MNHN)

This article is republished from The Conversation under a Creative Commons license. Read the original article.