How can dinosaur tracks be preserved as fossils?
No problem, but first of all a distinction: tracks (be they dinosaurs or any other animal) are not true fossils, but are called trace fossils. A “fossil” is a piece of organic material, usually bone, that has been lithified (turned to stone). A trace fossil is a cast, left behind by something that was once there. Almost anything can be preserved as a trace fossil.
Tracks, yes…
…but examples of trace fossils include all sorts of things, from preserved worm burrows (these will mostly be my own photos, btw… I’ve seen a lot of great trace fossils in my teaching days)
to sticks that have decayed, leaving their surface textures beautifully preserved
Wasp larvae
Nearly anything can be preserved if it was in the right environment. Especially what you were asking about: footprints!
(some people have a hard time seeing them so I’ve circled them)
To get preserved footprints, you need the following:
- Footprints. Your animal needs to be heavy enough to leave significant prints behind and it needs to be walking across something soft. No one leaves prints on a slab of granite. This usually means walking through a swamp or along the banks of a river or something of that nature.
- If the print has been left in something wet, as is usually the case, it would help a lot for it to dry out. That isn’t required, but most prints in mud will be washed away unless the mud dries first. Once the mud dries, it can be pretty resistant.
- You need a flood to bury it with sediment. You know what happens to footprints that aren’t buried? They disappear with erosion. It’s why national parks are so cool with you leaving footprints behind. They don’t tend to last:
Once the footprint has been filled in with a different material, almost always the result of a flood (but not technically always), then that print can be preserved for as long as it remains underground. In some cases, it may become slightly warped over millions of years, but prints are generally very very well preserved by rocks once they’re buried. This is because the prints are protected from the erosive forces that would break them down so easily: wind and flowing water. Do you know how collectors like to encase their most precious things in plastic? Well, it turns out that encasing them in rock and burying them thousands of feet underground is even better (except for the fact that it’s hard to look at and enjoy whenever you want after that).
To get your footprints preserved, you need a lot of things happening just right. You need something heavy walking across something soft and squishy. You need that surface to harden and then you need a flood to come along and bury it. Sometimes the prints can be preserved by falling volcanic ash instead of a flood. Additionally, if it’s to be preserved for a very long time, it needs to be in an environment where deposition is occurring and steadily burying it deeper and deeper, like at the foot of a mountain or in a river delta or something like that. Footprints aren’t preserved for long at the top of mountains because they are quickly re-exposed to the elements and weathered away. There is an example below of fossilized footprints emplaced at the top of a mountain, but they’re very recent and they won’t last long.
Footprints can be found around the globe and are super exciting for those of us who appreciate them. The circled theropod tracks above are from just south of Cheyenne Mountain, near Colorado Springs. The astute may have noticed that the tracks are actually coming out of the rock toward the viewer, which is totally wrong; this is because the rock unit is overturned and you’re actually looking at the underside of the tracks. There’s a much better example along the Skyline Drive outside Canon City, Colorado.
My beautiful wife for scale. The trackway is extensive above her head and from here you’re looking up at the bottom of the rocks and the footprints. By the way, the drive is gorgeous. Not that it’s relevant, but this is what the road there looks like. You should visit if you’re ever nearby.
There are footprints buried all around the world, but they only make very brief appearances at the surface. For a print to be exposed for us to recognize it, the rock layer it’s in has to have all of the layers on top of it eroded away to reach the surface. Then, the material filling the footprint has to be eroded away, leaving the more resistant (typically muddy) material the print was pressed into behind. With all of this erosion involved, you might guess that these footprints aren’t very long-lived features once they’re in a place where we can actually see them. You’re right. From the moment they’re exposed, their days are numbered. Look at where the Skyline Drive tracks above are located. They are not long for this Earth. Hell, just look at the opposite side of the theropod tracks I posted above:
They are not going to be there for very long! I measured dinosaur tracks near Arches National Park as part of a field class in summer of 2000. I returned in 2016. It was not my imagination; they were not as sharply defined as they were 16 years earlier. In another 50 or 60 years, I doubt they’ll be recognizable at all.
Measuring dinosaur tracks is actually a regular part of many field geology classes in the western US. It helps to do it after a rainstorm when they are filled with water, making them much more obvious.
Above is the most amazing dinosaur track I’ve ever seen. The river has eroded the rocks away perfectly to reveal a cross section (side view) of a sauropod (huge dino) track! You can see just how it squished in; so cool!!
And it’s not just ancient dinosaur tracks that have been preserved. Along with countless other animals, human tracks have been too. There are the Laetoli prints from Tanzania that are 3.6 million years old, left by our ancient hominid ancestors
At Laetoli, the “person” who left these tracks was walking through volcanic ash during an eruption. There is another far more recent example of this that you can visit if you drop by the Big Island of Hawaii. The 1790 Footprints (not a clever or attention-grabbing name, by the way… just saying, National Park Service) on Kilauea preserve the tracks of a retreating band of warriors after a battle; they were made during the ashfall of an eruption in which most of the men died.
So, to wrap up, footprints frequently are preserved. Step one (literally): walk through something squishy to leave tracks behind. Step two: have them harden. Step three: bury them with another material. Step four: wait tens/hundreds/thousands/millions of years until just the moment erosion exposes them again and then enjoy them/learn from them while they last. Sedimentary rocks are fantastic at preserving structures that were trapped/imprinted within them.
