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In the Footsteps of the Largest Dinosaurs

In early October of last year, CNRS announced the discovery, near the city of Lyon, of the largest dinosaur footprints ever found. How significant is this discovery and what can these footprints tell us about the animals that left them?

When Marie-Hélène Marcaud and Patrice Landry, two nature enthusiasts, came across oval tracks 1.5 m long in Plagne in April 2009, they immediately reached out to Pierre Hantzpergue and Jean-Michel Mazin from the PEPS laboratory.1
“These tracks were left by some of the largest animals to ever walk the planet,” says Mazin, who is studying the large soil depressions, together with geologist Hantzpergue. Beyond their unusual size, Mazin also notes the presence of a fold of limestone sediment towards the front of the prints. “The fold was formed when the animal's paw sank into mud,” he explains.
He also recognizes evidence of quadruped locomotion, with alternating prints made by the front and back paws. “In this case, the animal footprint partly overlaps its handprint. The handprint thus only appears as a thin crescent-shaped print at the front of the footprint.”
As for Hantzpergue, he is dating the footprints by biostratigraphy. “This involves studying the different strata which have built up under the footprints to analyze the fossils within them, notably ammonites,” he explains. Ammonites went through a very rapid morphological evolution, which makes it possible to date, in relative terms, the strata where they are found to within 150,000 years. This is a precise enough estimate, given that the dinosaur tracks are more than 150 million years old.
Beyond simply breaking a size record (the largest footprints until now measured 1.2 m), “what interests us is the wealth of information these footprints will reveal about the animals who left them,” says Mazin. “We will be able to understand how they walked.” And it must have been something of a challenge, considering the animal's weight. According to known sauropod skeletons, the footprints suggest these dinosaurs were more than 25 meters in length and weighed more than 40 tons. The weight at which a living being would collapse under its own weight is only 10 or 20 tons off...
To reconstruct a few minutes of the life of a 40-ton giant from these footprints—some 20 have already been uncovered—the researchers want to work with biophysicists who will provide biomechanical equations for quadruped locomotion, models that have already been used on smaller dinosaurs from the same family. Using the distance between footprints, the researchers hope to calculate the speed at which the animal was moving—probably no more than 4 km/h—as well as determine its acceleration and deceleration. The angle between footprints left by the right and left feet is expected to reveal the maneuverability of the animal, which was probably very limited. “To turn, they probably needed to stop first,” adds Mazin. “They were certainly incapable of running, and were pretty much trudging along, keeping three paws on the ground at any one time. In fact, even to lift a single paw must have required a huge effort.”
What were these dinosaurs doing in an area that was covered by sea for millions of years during the Jurassic period? “We know that at the end of this period, the region was frequently immersed,” says Hantzpergue. These dinosaurs, which were 300 km further north, probably took advantage of a drop in sea level to head south. Vegetation-covered islets, interspersed throughout this vast muddy plain, probably provided food for the expedition. “To know more, we need to reconstruct the lives of these sauropods, mainly by using fossils of other animals we hope to find,” says the geologist.
So, how did the footprints survive until now? Made in carbonated mud and preserved from the paths of other animals, they probably dried fairly quickly in the sun. “They were then covered by several hundreds of meters of sediment, and later exposed through the erosion that created the Jura's topography,” says Hantzpergue. More recently, a path used to carry wood led to the removal of the top layer of soil, virtually exposing the footprints.
Further exploration has suggested the presence of other tracks, hundreds of meters away, under 10 to 50 cm of earth and plant cover. Uncovering them will mean digging up a meadow of several hectares. The whole area needs to be mapped, through laser survey of the tracks and aerial photography. This research is expected to take three to four years for a team of 30 people whom the researchers hope to recruit. They will also need to secure funding, possibly from the regional authorities of the area, in order to pursue their work and uncover more sauropod footprints.

Charline Zeitoun

Notes :

1. Paléoenvironnements et paléobiosphère (CNRS/Université Claude Bernard Lyon-I).

Contacts :

PEPS, Lyon.
Jean-Michel Mazin,
Pierre Hantzpergue,


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