Crawling through tight underground passages in
southern France, paleontologist Jean-David Moreau and his colleagues have to descend
500 meters below the surface to reach the only known footprints of long-necked dinosaurs
called sauropods ever found in a
natural cave.
The team discovered the prints, left by behemoths
related to Brachiosaurus, in Castelbouc Cave
in December 2015 (SN: 2/21/18). But
getting to the site might make even the most hardened field scientists balk. Wriggling
through such dark, damp and cramped spaces every time they visit is challenging
for elbows and knees, and even trickier when carrying delicate equipment such
as cameras, lights and laser scanners.
It’s both physically exhausting and “not comfortable
for someone claustrophobic,” with the researchers spending up to 12 hours
underground each time, says Moreau, of the Université
Bourgogne Franche-Comté in Dijon. It can be dangerous too, as some parts of the
cave are periodically flooded, so accessing the deep chambers must be limited
to periods of drought, he says.
Moreau has studied fossilized dinosaur footprints and
plants for more than a decade in southern France’s Causses Basin, one of the
richest areas for aboveground dinosaur tracks in Europe. When spelunkers
chanced upon some underground prints in 2013, Moreau and his colleagues realized
there could be lots of dinosaur prints within the region’s many deep, limestone
caves. Footprints left in soft mud or sand hundred million years ago could have
been turned to rock and forced underground over many eons.
And deep caves, being less exposed to wind and rain, “can
occasionally offer larger and better-preserved surfaces [imprinted by dinosaur
steps] than outdoor outcrops,” Moreau says.
Moreau’s team is the only one to have discovered dinosaur footprints in
natural caverns, though prints also have been found around the world in human-made
railway tunnels and mines. “The discovery of dinosaur tracks inside a natural
karstic cave is extremely rare,” he says.
The first subsurface dinosaur prints that the team found
were 20 kilometers away from Castelbouc at a site called Malaval Cave, reached
via an hour-long clamber through an underground river with several 10-meter
drops. “One of the main difficulties in the Malaval Cave is to walk taking care
to not touch or break any of the delicate and unique [mineral formations],” Moreau
says.
Those three-toed prints, each up to 30 centimeters long and detailed
in 2018 in the International Journal of Speleology, were left by carnivorous
dinosaurs walking upright on their hind legs through marshland about 200
million years ago.
In contrast, the five-toed herbivore tracks in Castelbouc Cave are each
up to 1.25 meters long and were left by three enormous herbivorous sauropods
that walked the shoreline of a sea about 168 million years ago. What’s more, these prints
are on the cave’s ceiling 10 meters above the floor, the team reports in a
study published online March 25 in Journal of Vertebrate Paleontology.
In fact, “the tracks we see on the roof are not ‘footprints,’ they are
‘counterprints,’” Moreau explains. “The dinosaurs walked on a surface of clay,
which is nowadays totally eroded to form the cave. Here, we only see the
overlying layer [of sediment that filled in the footprints],” leaving reverse
prints bulging out of the ceiling. It’s similar to what you’d see if you filled
a footprint in mud with plaster and then washed all of the mud away to leave
the cast.
The tracks are important as they hail from a time in the early to mid-Jurassic
Period from 200 million to 168 million years ago when sauropods
were diversifying and spreading across the world, but relatively few
fossil bones have been found (SN: 12/1/15).
These prints confirm that sauropods then inhabited coastal or wetland environments
in what is now southern France.
Moreau is now leading researchers in exploring “another deep and long cave, which has yielded hundreds of dinosaur footprints,” he says. The team has yet to publish those results, which he says may prove to be the most exciting of all.
