Front Range Fossil Footprints

Martin Lockley 1,2, Neffra Matthews 3, Brent Breihaupt 3, Beth Simmons 2,4, Sue Hirschfeld 5, 1 University of Colorado, 2 Friends of Dinosaur Ridge, 3 U.S. Bureau of Land Management, 4 formerly Metro State University, 5 City of Boulder Open Space Mountain Parks.

As paleontologists working at Dinosaur Ridge, Fossil Trace, and other well-known locations along the Front Range and elsewhere in Colorado, we are often asked if we still make new discoveries. On the understanding that ‘new’ means previously unknown to science, the answer is a definite YES! Although fossils (bones, tracks, and plants) from Dinosaur Ridge and Fossil Trace have been known since the late 19th and early 20th centuries, further important finds have been made and reported in the scientific literature through the late 20th century and up to the present day. Important 21st-century footprint reports include:

• ~66-67 million-year-old mammal tracks, named Schadipes, from Triceratops Trail, in 2003
• ~97-98 million-year-old courtship traces, named Ostendichnus from Dinosaur Ridge, in 2016
• ~110 million-year-old raptor tracks, named Dromaeosauripus from Dinosaur Ridge, in 2016
• ~66-67 million-year-old dinosaur and other tracks from near Marshall, Boulder County, in 2018

It takes time for new discoveries to find their way into the scientific literature, and then into local guidebooks and educational media, interpretative signs, and brochures. For example, while mammal footprints and courtship and raptor traces made it into the 5th edition of the Dinosaur Ridge guidebook in 2017, the discovery of abundant dinosaur tracks from Boulder County (item 4 above) was only made in 2018. In this short article, we continue to update readers on the latest research finds and developments, with emphasis on new discoveries from near Marshall in Boulder County, which we designated as the Cherryvale Site.

Fruitful Findings from Cherryvale.

Surprisingly, although the Marshall area has been known since the late 1880s for coal mining operations in the Late Cretaceous (Maastrichtian: 65-70 million-year-old) Laramie Formation, no tracks had been reported. Thanks to the economic importance of coal, especially in the pioneer days of the late 19th century, the site has been visited by many generations of geologists interested in the structure, stratigraphy, and other features, but no one suspected that the area might yield fossil footprints. We heard via the trackers grapevine that a colleague, Jerry Harris, had found a three-toed track in the vicinity of Marshall when he was working at the Denver Museum of Nature & Science in the 1990s, but no illustration or details of the specimen are known. Initially, we were unaware that this clue was significant, as we had gone to the site to look at other geological features. In short, the results were rather spectacular. We discovered at least two relatively abundant dinosaur track types, as well as other less common dinosaur and turtle tracks. In total, we documented more than 30 tracks representing at least six distinct trackmaker groups.

The abundant and easily identified tracks include Saurexallopus, a slender-toed bird-like track up to about 30-35 cm (12-14 in), wide with widely splayed toe impressions up to 100o-115o) (Figure 1A,B on next page), and usually a fourth, reversed toe trace. Saurexallopus, interpreted as the track of a bird-like oviraptosaurid dinosaur, and also known from Triceratops Trail, where the tracks are poorly preserved, occurs from at least seven sites in North America, always in the latest Cretaceous (Maastrichtian): i.e., at three sites in Colorado, two in Wyoming, and one each in Utah and British Columbia. At first sight, Saurexallopus looks rather like Magnoavipes the familiar three-toed, bird-like (probably ornithomimid) track from Dinosaur Ridge. However, besides registering four toe traces in most cases, Saurexallopus is larger and about 25 million years younger than Magnoavipes. The second common track type from the Cherryvale site is Hadrosauropodus, attributed to the large duck-billed dinosaurs or hadrosaurs (Figure 1C,D), which had large fleshy feet that left wide toe impressions. The largest of these tracks are 60 cm (2 feet) long and wide. These are also common in the latest Cretaceous, including at Triceratops Trail, where there is a hadrosaur pit with some footprints up to 90 cm (3 ft) long. Other dinosaur tracks from the Cherryvale Site include other 3-toed (theropod) and 4-toed ceratopsian (horned dinosaur) footprints and turtle tracks. Traces of invertebrates and other enigmatic features are still being studied. So stay tuned.

The steady march of Science at Dinosaur Ridge and beyond.

As discussed in a companion article this issue, Dinosaur Ridge and Triceratops Trail (Fossil Trace), like Red Rocks and other Colorado Front Range locations, have become important, globally-ranked geo-destinations in the world of paleontology, geoscience, and environmental education. As background to the new 21st century discoveries mentioned here we draw attention to the 30-year his- tory of Dinosaur Ridge (1989-2019) and the 65 or more scientific publications which have appeared on the subject of fossil footprints alone in this period. This is to say nothing of the articles by academic and applied geologists on the rocks, fossils, and economic geology of the Front Range and Denver Basin, or the large number of popular dinosaur books and film documentaries that mention Dinosaur Ridge, sometimes in detail. This is why the Dinosaur Ridge guidebook, also covering Triceratops Trail and the Golden area, has run to five editions, each one with new, updated information.

The 2018 Cherryvale discovery site is the most recent, but the above-listed identification of the nest scrape display traces made by large theropods (Figure 2 A-C) and the first raptor tracks from Colorado (Figure 2D-F) are also significant. The scrapes show carnivorous dinosaurs, like their modern bird descendants, used nest scrape display as part of their sexual display rituals. The largest scrape from Dinosaur Ridge is ~two meters (~six feet) long and a meter (3.3 ft) wide, and was featured in the Secrets of the Underground documentary. Paleontologists have suggested for years that carnivorous dinosaur courtship likely resembled that of modern birds. However, this educated guess was pure speculation until the scrapes from Colorado provided the physical evidence. This potent combination of dinosaurs and sexual display activity received much international press attention in 2016.