For three amazing days in early September, we went dinosaur hunting along the Jurassic Coast in northern Spain. We were seeking the tracks of dinosaurs that inhabited this coastal region around 155–145 million years ago, and we found a lot!
An example of what we found. Left photo: Jay’s foot in the ~150 million-year-old footprint of a dinosaur. Right photo: Jay’s foot next to a footprint he made in the mud during our trip. It was quite astonishing to see footprints in the ancient sedimentary rocks that looked like they’d just been made yesterday!
Some background information
Please see my last post for location of the Jurassic coast in northern Spain, the location of Spain on Earth during the Jurassic Period, and a stratigraphic column of the geologic units we viewed along this part of the coast: https://landscapes-revealed.net/the-jurassic-dinosaur-coast-of-asturias-part-1/. Part 1 focused on the sediments deposited in oceanic environments during the Early Jurassic Period. This post focuses on the sediments deposited in terrestrial and coastal environments during the Late Jurassic Period—this is the part with dinosaur footprints.
A fabulous place to visit for more background information is the Museo Jurásico de Asturias (MUJA), whose location is shown on the map below. The museum, built in the shape of a three-toed dinosaur, is filled with superb specimens and explanations of the region. On the right side of the photo is Playa de La Griega (Griega Beach), an easy place to visit (more about this location below). This photo is from the field guide reference listed at the bottom of this post.Left photo: A map showing locations of the museum (MUJA), Griega Beach, and Llastres (also spelled Lastres) where we stayed. Right photo: A sign at Griega Beach for a restaurant named La Cueva de Dino (The Dinosaur Cave). Dinosaurs are a big tourist draw in this region. We enjoyed a two-night stay in Lastres, a scenic fishing town that has been listed as one of the “most beautiful villages in Spain” (see location on map above). Hotel Eutimio is an excellent family-owned lodge and restaurant. In a village known for fish and seafood, there are many delicious restaurants. Griega Beach is on the other side of the bay in this photo.Here’s what the Asturian coast looked like during the Late Jurassic Period (diagram from the MUJA). In the Early Jurassic, the coast was flat and the preserved sedimentary rocks were deposited in the ocean (see previous post). At the beginning of the Late Jurassic, tectonic activity created mountains and caused the sea to retreat. This change is marked by the Vega Formation of river sediments (see previous post).
This post focuses on the Tereñes and Lastres Formations where most of the dinosaur footprints are found. Interpretation of diagram (from left to right): Mountainous relief in the southeast of the region; Alluvial plain with meandering rivers (Vega Fm); Fluvially-dominated delta (Lastres Fm); Interior sea (Tereñes Fm); Barrier reef. Fluvial=rivers. Alluvial=sediment carried by water, mostly in rivers.
Tereñes Formation
We observed the Tereñes Formation at Ribadesella, Mirador Tereñes, Griega Beach, and Tazones (see map in previous post).
These tracks at Griega Beach are the biggest footprints known in Spain and among the largest in the world. They were created by herbivorous sauropods, a group that includes the largest animals that ever lived on land. This type of trace fossil is a mold—the mud “molds” to the shape of the dinosaur’s foot and is preserved when sand fills in the depressions. In these exposures along the beach, the sand has been eroded away, exposing the footprints in the mud.This is a mural in the MUJA illustrating sauropods in a coastal setting with trees whose foliage they would have eaten.Left photo: This fossil footprint is a mold of a three-toed dinosaur. It was probably a theropod, a group of bipedal carnivores that eventually included Tyrannosaurus rex. Right photo: This is a different type of mold of a sauropod footprint. We are seeing the underside of the footprint, so in this case, the mud was eroded away and we see the sand that filled in the footprint’s depression.Left photo: This is the Tazones Port (see location below under Lastres Formation), where we saw the three-toed footprint above. Right photo: In the layers at Tazones Port, we also saw a trackway of three-toed dinosaurs. A trackway shows the animal’s path of movement.Here are some features in the Tereñes Formation that provide clues about the depositional environment. Left photo: Mud cracks on the surface of a sedimentary layer shows the area was flooded by river or ocean water to create mud that then dried up. Center photo: The while areas are reduction spots where tree trunks produced local oxygen-reducing environment. The reddish areas were oxygenated in the surrounding soil layer. Right photo: Ripple marks on the surface of a sedimentary layer indicate flowing water.This photo of Griega Beach shows how the ripples in the photo above probably formed. The tidal range was about 3 meters (9.8 feet) that day. When the tidal level was high, the waves were on this part of the beach and the moving water created ripples that were then exposed when the water receded during low tide (the time of this photo). In the background is the town of Lastres where we stayed. As shown in the mural of sauropods above, the dinosaurs were living in a coastal setting where the water level would have risen and fallen with the tides.
Lastres Formation
The best place for footprint fossils that we visited was Tazones, located west of Lastres (see map in previous post).
Left image: A map of Tazones. We visited Tazones port, where we saw three-toed dinosaurs (photos in Tereñes section above), and the beach northwest of Faro (lighthouse) de Tazones, where we saw the largest number of footprints in the Lastres Formation. Right photo: The beach site near the lighthouse. Here is an unlikely layer balanced on an inserted boulder overlying a mudstone layer with dinosaur footprints (side view).
An amazing “trample zone” with numerous sauropod footprints at Tazones. It’s not too hard to imagine those giant dinosaurs walking around in the mud. After that, sand would have been swept into the area, filling the depressions and preserving the footprints. This exposure is at the coastline where waves are attacking the rocks. The overlying sand layers was removed and, inevitably, the mud layer will eventually be eroded away too. The layers were all tilted during the Alpine-Pyrenean Orogeny about 40 million years ago.Left photo: A manus (forefoot) and pes (back foot) pair of sauropod footprints at Tazones. Right photo: A three-toed footprint, probably of a theropod. The light color is the sandy filling of the footprint.These photos are side views of the sauropod footprints, showing how the depressions in the mud were filled with sand. Left photo: Notice how the mud is compressed by the foot of the dinosaur. Right photo: This is what the sand mold above (in Tereñes section) would look like in a side view.These photos show features in the sedimentary rocks that provide clues about the environment of deposition. Upper left photo: A piece of fossil wood carried to the coast by rivers. Upper center photo: A trace fossil called ophiomorpha, made by burrowing organisms in a sandy inter-tidal environment. The organisms used their own fecal pellets to line the walls of the burrow and keep the tube from collapsing. The mud of the pellets is less resistant than the surrounding sand, which is why the circular exterior has eroded more than the sand. Upper right photo: The angular black pieces are charcoal from a forest fire carried to the coast by rivers. Bottom photo: Layers of cross-bedded sand (layers at an angle to the flat layers) formed by bars migrating in a river. Jay is making circles to illustrate the holes that are an erosive feature called tafoni commonly found in thick sandstone layers.
Cultural note about Asturias
Left photo: A church and the Asturian flag in Tazones. Right photo: Cider (sidra) is incredibly popular in Asturias. It must be a little fizzy. This is achieved by either (1) holding the bottle high above the glass and pouring (practice is needed for this!) or (2) using a simple machine that inputs some bubbles into the cider. One must pour the slightly fizzy cider into a glass and then drink it immediately. We saw this machine at work on an adjacent table while dining in Lastres.
In my next post, we’ll go to the Picos de Europa for a 10-day trek.
Reference
García-Ramos et al., Austurian Jurassic Coast Field Trip Guide, 2008: International conference in Oviedo, Spain.
