Traces in the sand

Brooke Jonson

What can you learn about an ancient world just by looking at the rocks?  Quite a lot as it happens. In 2011, I was patrolling a childhood fossiling location when I found the slab in figure 1. I stood in growing excitement on the cold beach peering back into the lives of the inhabitants of tropical Jurassic Yorkshire.

Dinosaur footprints, invertebrate traces and current ripples in sandstones of the Saltwick Formation, Aalenian stage, Middle Jurassic, c.175Ma. At this time, the North Yorkshire coast was part of a large coastal flood plain threaded by meandering rivers.

The red, green, and blue annotations in Figure 1 are the footprints of therapod dinosaurs. These are an example of what geologists call trace fossils. Most people will be familiar with body fossils, remains of animals, usually shell and bone, preserved in rock. Trace fossils represent preserved behaviour rather than body parts, moments in the lives of ancient animals frozen in time. These particular prints pose some interesting questions about the animals that have created. Are they related? Do they represent a small group or family? The human tendency is to interpret our observations as forming part of a nice neat narrative. I like to think I see the smaller prints prancing playfully about the weary plod of the largest set. Perhaps the children were pestering the adult; “Are we there yet!”, “I’m hungry!”, “the other one keeps teasing me!” In fact, these animals may have never met, but the tidal ripples (yellow lines) indicate that they were at least active in the same twelve-hour window. Maybe the smaller ones followed the larger at a safe distance, hoping to steal a morsel of food.

The pink marks have been left by various invertebrates. Some, like worms, burrowed up and down in the sand. But the longer traces to the right could be the trail of a snail patiently searching the surface of the sands for food. The circular traces could have been probe marks from the snouts of pterodactyls. Similar traces have been found on rocks along the coastline, along with marks from various other dinosaurs, crocodiles, turtles and even that great survivor, the Horseshoe crab.

The slab was too heavy to carry and the rising tide was nipping my heels so I could only steal a few shots before retreating. Besides, I liked the idea of leaving the prints for others to enjoy before time and the sea reclaimed them.

Trace fossils may lack the immediate impact of body fossils but they are no less important. They preserve how organisms interacted with each other and their environment, and as suchthey are invaluable for reconstructing ancient environments and bringing deep time back to life. The fossils in Figure 1 tell us that Yorkshire in the Middle Jurassic (170 million years ago) had a diverse fauna of large animals, even if none of their bones remain. This is what we can learn of ancient environments and their inhabitants from simply observing the rocks.

To me trace fossils and sedimentary structures are wonderful; there is something ghostly and ethereal about them. I feel as though, had I been a few moments earlier, I might have caught the animal in the act. As though if I listen carefully, I might catch the breath of ancient winds or hear the sigh of long gone tides in the flicker between seconds. I feel a connection across the abyss of time to lives long passed. I see these animals go about their lives and see the passing of daily time in the ripples of tides in a time when our species was just a quiet whisper in a distant future.

I’m not that different from these dinosaurs. We both patrolled a coastline on a never ending hunt. The therapods sought food, and I crave the secrets of lost worlds. Neither hunger is ever satiated for long. I remember walking away and looking at the traces on the beach including my own, and wondering if, in some distant unknowable future age, would someone look back at my footprints and feel the same connection across the gulf, to me?


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