Footprints that are almost forever

kw: book reviews, monographs, nonfiction, science, paleontology, trace fossils

"Take nothing but pictures, leave nothing but footprints." This advice for visiting national parks and other sensitive areas is ironic in its assumption that footprints are ephemeral. They usually are, but not always. The big footprint in the middle of the picture below is more than 100 million years old.

My wife took this picture in 1994 when our son was six. We were on a family trip to Colorado and nearby states. While I attended a business conference the two of them did some sightseeing. Right at the center of the picture is a "Bronto Bulge", the footprint of a giant dinosaur. The reddish rocks are mudstones, and while the mud was soft the creatures' feet left deep impressions.

Our son has his hand on the layer the Brontosaur (or other sauropod) stepped in. Just above is a thin, gray layer of ash, and then more mudstone, which filled the footprints. There are several Bronto Bulges along this exposure. They are an example of trace fossils. They occur in a great many places.

Prehistoric Trackways National Monument was set up in 2009 on a bit over 8 square miles northwest of Las Cruces, New Mexico. Its purpose is to preserve the greatest assemblage of trace fossils in North America. The 48-page brochure Traces of a Permian Seacoast was published in 2011 by the New Mexico Museum of Natural History and Science to illustrate the riches of this unique area.

The brochure cover shows a trace fossil in the making; a Permian proto-lizard walking in mud, leaving tracks. On rare occasions (that occurred millions of times over millions of years), footprints and other traces of animal activity have been preserved, to be seen today.

I once thought of fossils as seashells or bones found embedded in rock. When I was a geology student I took a course in Invertebrate Paleontology, There I first learned that worms crawling on sand and mud or burrowing into it, or the scuttling of crabs and trilobites, leave marks on or under a surface that can become trace fossils.

There are many more invertebrates (every kind of animal without a backbone) than there are vertebrates, by a factor of millions. Thus, most trace fossils record the presence or passage of worms, crabs, clams, and so forth. But a small number (still many thousands) have the tracks of vertebrate animals such as lizards, frogs, and turtles.

This picture shows a common type of trace fossil: worm burrows. Any time you are at a place with soft sediment, and animals are crawling, walking, burrowing, or squirming around, take note of the trails they leave. If another layer of sediment is soon deposited on top, particularly if it is of a contrasting type, like sand over mud or vice versa, there is the potential that, long in the future, those tracks will be seen again as a trace fossil.

The brochure illustrates and describes a number of kinds of animal life recorded in their traces. This picture shows the "discovery slab". The footprints are not those of a dinosaur. The National Park is a Permian site. The Permian period preceded the "age of Dinosaurs" which began after 250 million years ago; the Permian period was from 299 to 251 million years ago. The Park's rocks are primarily aged about 280 million years.

Thus, the footprints seen at right are from a reptile that may be similar to the one on the brochure's cover, but more likely belong to a species of  "sailback lizard" such as Dimetrodon. The biggest tracks in this picture are from that animal, and their are tracks from a few smaller reptiles, plus worm or insect crawl marks. Since the area was discovered in 1987, many fossils of animals new to science, plus their footprints or other traces, have been found there.

Dimetrodon is such an iconic creature that models of them are frequently included in packs of toy dinosaur models. They are not dinosaurs. They were present between 292 and 270 million years ago. The first dinosaurs arose about 230 million years ago, 20 million years after the end of the Permian period.

The brochure whets my appetite for spending a few days there. I love the national parks, and I'd like to visit this one.