This week's Friday Field Photo is from some Eocene sedimentary rocks (~56-34 million years old) I visited in the Spanish Pyrenees a couple of weeks ago.
You are looking straight up at an overhanging ledge and the basal bedding plane of a sandstone bed. It is up on a cliff face about 10 meters and out of reach, hence no scale -- the field of view is approximately 60-70 cm across.
If you haven't noticed already, check out the sinuous, double-ridged feature in the center-top of the photo. This is a trace fossil known as Scolicia. Trace fossils are neither fossilized remnants of hard body parts nor are they perfect imprints of soft body parts. Rather, they are traces of burrowing, grazing, and other behavior preserved in the sediment. Dinosaur tracks are trace fossils. If your footprints in a muddy tidal flat were buried, preserved, and lithified they would be trace fossils.
Although trace fossils have their own names to help categorization it's important to note that one trace can be made by multiple species and one species can make multiple traces. In the case of Scolicia, it is thought to be the trace of a type of echinoderm (e.g., sea urchins, sand dollars, etc.) plowing it's way through the sand.
Happy Friday!
Image: Scolicia trace fossil; from my Flickr collection
