Baby sauropod tracks from the Swiss Jura

Tiny footprints and evidence of a juvenile herd

At Bois de Sylleux near Porrentruy, a surface was uncovered that preserved very small sauropod footprints in association with other much larger tracks of sauropods and theropod dinosaurs.

These very small footprints have average lengths ~12 cm in length (hind feet). Their shape and walking pattern show that they were probably made by very young members of the same kind of sauropod that produced the larger tracks.

We have a record that showed that at least eight juveniles walked in the same direction, close together and at a similar speed. Their parallel trackways provide rare evidence that young sauropods travelled as a group.

Map and orthophoto of the Bois de Sylleux dinosaur tracksite showing numerous sauropod trackways
The Bois de Sylleux tracksite and its mapped sauropod trackways. Can you spot the group of tiny juvenile trackways? What about the large tridactyl theropod trackway?
Age
About 152.7 million years old, from the Late Jurassic.
Location
Bois de Sylleux, near Courtedoux in the Swiss Jura.
Footprint size
The tiny hind footprints average 11.6 centimetres long.
The group
At least eight juvenile sauropods moved together.

What do the tiny tracks look like?

Sauropods walked on all four feet, leaving different impressions from their hind feet, called pes tracks, and their front feet, called manus tracks.

  • The tiny pes tracks are oval to slightly rounded, with an average length of 12 cm and width of 8 cm.
  • Some preserve four toe impressions, including small claw traces.
  • The manus tracks are wider than long and are oval to kidney-shaped.
  • The larger sauropod pes tracks on the surface average 42 cm long.
Photographs, three-dimensional models and scientific comparisons of tiny and larger sauropod footprints
Follow the trackway of one tiny sauorpod ("S19"). Manus = hand (fore foot), pes = foot (hind foot). are they baby sauropods?

How do we know they were juveniles?

A small footprint does not automatically belong to a baby dinosaur. It could have been made by an adult of a naturally small or dwarf species. The Bois de Sylleux tracks therefore had to be compared in detail.

Foot shape
The tiny and larger tracks share important anatomical features, including comparable manus shape and toe orientation.
Walking pattern
Stride-length comparisons indicate similar styles of locomotion in the tiny and larger trackmakers.
Growth
Differences in footprint shape are consistent with changes that occurred as the sauropods grew.
Shared surface
Tiny, intermediate and larger tracks occur together on the same palaeosurface.

Taken together, the evidence supports an ontogenetic relationship: the tiny tracks were made by young individuals related to the larger trackmakers, rather than by an unrelated small-bodied sauropod.

Walking together

The juvenile trackways run roughly parallel for 12 to 18 m and the animals likely travelled in the same direction with several walking side by side.

This coordinated movement is evidence of gregarious behaviour which is typically noted for sauropod trackways. The trackmakers may have belonged to the same brood but footprints alone cannot tell us whether they were siblings.

What the tracks show: at least eight young sauropods moving closely together in the same direction.

How small were the trackmakers?

Estimated hip height
Approximately 41 cm - 47 cm
Possible age
Their size is compatible with very young animals

Sauropods grew enormously between hatching and adulthood. Rapid growth may have helped young animals move more quickly beyond the body sizes at which they were most vulnerable to predators.

Were adults caring for them?

Larger sauropod tracks occur on the same surface, but most head in the opposite direction. Five larger trackways move in the same general direction as the juveniles, although they lie about 6 to 10 m away. The evidence is intriguing, but it does not demonstrate parental care.

Why are these tracks important?

Social behaviour
They provide rare direct evidence that very young sauropods moved together.
Movement
They show that small juveniles could travel across an exposed tidal landscape.
Jura ecosystem
They add to evidence that dinosaurs of several sizes repeatedly used the Late Jurassic Jura carbonate platform.

Fun fact!

The tiny sauropods left hind footprints only a little longer than an adult human hand, yet members of their broader group included the largest land animals ever known.

Although the original tracksite was removed during construction of the A16 motorway, slabs bearing the juvenile trackways, along with casts and other specimens, are preserved in the JURASSICA Museum collections.

Scientific basis: research by Matteo Belvedere, Lara Sciscio, Daniel Marty, Kent Stevens, Scott Ernst, Géraldine Paratte and Christian A. Meyer on the Bois de Sylleux sauropod tracksite.