those birds unknown, that left only footprints

Big toes! Huge bird!!

Along the rim of Bull Canyon, on the north slope of the La Sal Mountains in southeast Utah, my field assistant and I followed footprints in sand. Three-toed two-footed creatures had passed this way when the sand was wet, before it turned to rock. They were common then, traveling in packs.

Similar 3-pronged impressions occur 2000 miles to the east, in the Connecticut River Valley in Massachusetts. Some are famous—among the earliest to be studied and published. They were found in 1835 by Dexter Marsh, who was laying a flagstone sidewalk. He showed the slabs to the owner of the property who gave them to a physician who then gave them to state geologist Edward Hitchcock.

"They consist of two slabs, about forty inches square, originally united face to face; but on separation, presenting four most distinct depressions on one of them, with four correspondent projections on the other; precisely resembling the impressions of the feet of a large bird in mud." (Hitchcock 1836, italics mine)

Sandstone slabs, each 36.5" x 34"; depressions (molds) on left, projections (casts) on right (source, click on "Fossil Slabs Found by Dexter Marsh").

Hitchcock was understandably excited. Very few bird fossils had been found anywhere, and geologists had decided that because most birds were lightweight creatures of the air, they were unlikely to be submerged and preserved on the bottom of lakes, oceans and such. "Even when they chance to perish in the water, they float so long upon the surface, as to be most certainly discovered, and devoured by rapacious animals."

As it turned out, such marks were fairly common in the area. Hitchcock studied slabs from five quarries, concluding that the impressions must have been made by birds:

1. These impressions are evidently the tracks of a biped animal. For I have not been able to find an instance, where more than a single row of impressions exists.

2. They could not have been made by any other known biped, except birds. On this point, I am happy to have the opinion of more than one distinguished zoologist.

3. They correspond very well with the tracks of birds.

Some of Hitchcock's drawings of modern-day bird tracks, from his 1836 publication (source).

However, many of the tracks Hitchcock studied were too large to have been made by the birds we know—to 18 inches long and 13 inches wide, and separated by 6-foot strides. Therefore these tracks must have been made by large birds now extinct. Eminent geologists of the day agreed with Hitchcock (2). But by the end of the century they had been "proven" wrong. These tracks were not avian, they were reptilian (Dean 1969).

Science marches on of course, and we now know that in a sense, Hitchcock and his colleagues were correct. The creatures that left footprints in the Connecticut River Valley and southeast Utah were indeed birds. But they also were dinosaurs, specifically theropods (study the images below before discussing this at cocktail parties).

Dinosaur classification (3). While all birds are dinosaurs, not all dinosaurs are birds; similarly, birds are a subset of theropods (from Zureks).

Evolution of birds from a dinosaur ancestor; Manti-La Sal National Forest, Bull Canyon Tracksite.

You might be wondering why dinosaurs traveled the rim of Bull Canyon. Well ... actually they didn't. There was no Bull Canyon 157 million years ago. Instead this was a broad coastal plain, where large bipeds could cruise along at 2.5–3.5 mph (Hunt-Foster 2016).

The Bull Canyon Tracksite includes at least 50 well-preserved large theropod tracks, to 18 x 14 inches in size. But aside from footprints little is known about these creatures, for no bones have been found. So rather than naming a species, paleontologists named their tracks: Megalosauripus. These are ichnofossils—"a fossil record of biological activity by lifeforms but not the preserved remains of the organism itself." They're also called trace fossils, the term I learned.

 Theropods passed this way, in a pack perhaps.

They were big! (40-pound dog for scale).

Megalosauripus is a theropod track, not the theropod itself.

The wet sand where theropods once walked is now sandstone, part of the Moab Member of the Curtis Formation, dating from 157 million years ago (Late Jurassic). The setting was dynamic—changing sea level, oscillating shoreline, occasional sand dunes—making classification and dating of rock units difficult (Mathis 2021). But no matter. Whatever geologists decide to call the rock, its theropod tracks go on and on and on. They occur across Arches National Park, east to the Bull Canyon area and the Colorado–Utah state line, and perhaps as far south as Blanding. This is the Moab Megatracksite, also known as the Dinosaur Freeway. A conservative estimate of its size is 700 square miles; as of 2016, c. 3000 tracks had been reported from 30 sites (Hunt-Foster 2016).

Dinosaur Stomping Grounds, aka Jurassic Dancefloor, with at least 2000 theropod tracks (Sierra Club).

Our visit to the Bull Canyon Tracksite last fall was but a brief introduction. Many more opportunities to commune with large extinct birds await. Fortunately many of the sites are on public land, and there's a handy guide available. We shall return!

Dreaming of giant birds after a day in the field.

Notes

(1) The title of this post comes from a poem by Henry Wadsworth Longfellow—To the Driving Cloud. He referred to fossil bird tracks in several poems (Dean 1969).

(2) Edward Hitchcock was a cleric and amateur geologist. In his time, so-called amateurs made major contributions, his study of fossil birds being a good example. Charles Lyell confirmed Hitchcock's findings, and included them in lectures and later editions of Principles of Geology. Louis Agassiz and others also spread the word (more in Dean 1969).

(3) Are you wondering, as I did, why birds are NOT included in the seemingly eponymous Ornithischia (lower right in diagram)? That group includes dinosaurs with hips that superficially resemble those of birds. Maybe the name predates the realization that birds evolved from a theropod.

Sources (in addition to links in post)

Dean, DR. 1969. Hitchcock's Dinosaur Tracks. American Quarterly 21:639–644. https://www.jstor.org/stable/2711940

Hitchcock, E. 1836. Ornithichnology—description of the footmarks of birds, (Ornithichnites) on New Red Sandstone in Massachusetts. American Journal of Science and Arts, XXIX:307-40. Internet Archive.

Hunt-Foster, RK, et al. 2016. Tracking Dinosaurs in BLM Canyon Country, Utah. Utah Geological Association, Geology of the Intermountain West, Vol. 3. PDF

Mathis, A. 2021. Moab, Goblin Valley, and the Curtis Formation. Moab Happenings Archive.