Among fossil footprints of dinosaurs, crocodiles and mammals that once visited a 110 million-year-old lakeshore in the Jinju Formation of South Korea’s Gyeongsang Basin, researchers have found a set of tracks belonging to the smallest raptor ever discovered.

The new research, published this week in the journal Scientific Reports, is authored by Martin Lockley, PhD, professor emeritus from the University of Colorado Denver’s College of Liberal Arts and Sciences, in collaboration with colleagues at the Chinju National University of Education and the Cultural Heritage Administration in South Korea, among others.

Are tracks from new Microraptor species?

The eighteen, half-inch-long didactyl, or two-toed, tracks came from a slab excavated from a Jinju City construction site. Researchers believe the tracks belong to a juvenile raptor or a new, sparrow-sized Microraptor species. The researchers are calling the tracks Dromaeosauriformipes rarus  to indicate its “rare” value. A replica of the slab, which contains the two trackways of D. rarus and a scattering of other tracks, was donated to the University of Colorado Museum of Natural History for analysis as part of the study.

The tracks attributed to the carnivorous dromaeosaurs— feathered, three-toed dinosaurs of the Cretaceous Period—are rare outside of formations found in Korea and China, where two-thirds of known raptor tracks exist. The rich formations in Asia are due to the layers of fine mud that once rimmed ancient lakes, able to preserve the details of the skin and lightweight footsteps of dinosaurs like the D. rarus trackmaker.

Abundance of well-preserved fossils

“If you look at similar deposits around the world, paleontologists may find only one or two kinds of tracks, like those of birds or pterosaurs,” says study co-author Lockley. “We’ve given the Jinju Formation a special label, Konservat-Lagerstätte, German for ‘exceptional conservation.’ That’s what we’re seeing in the case of this formation: fine details of frogs, mammals, lizards, and more, which are remarkably rare.”

Illustration of raptor footprint
Illustrations of Dromaeosauriformipes rarus ichnogen. et ichnosp nov. trackway described here at same scale as type Dromaeosauripus jinjuensis with reconstruction of trackmaker anatomy at scale corresponding to footprint sizes. Upper left color inset is photogrammetric image of Trackway 1, and right side photogrammetric color image shows the type trackway of Dromaeosauripus jinjuensis6. Original illustration, artwork and 3D models compiled by MGL, XL, and K-S Kim in Photoshop (version CS6 www.adobe.com/Photoshop) and Canvas X (version, 2017 Build 160, http://www.canvasgfx.com/).

Modern birds related to raptors

This discovery sheds light on important, unanswered questions, says Lockley. Small, carnivorous dinosaurs and birds are closely related, both in size and because their offspring hatch from eggs. Scientists still don’t know the size of the smallest dinosaurs hatched, but if the tracks belong to a juvenile, scientists can now determine that they may have been at least sparrow size.

If they are juveniles, there’s a chance they belong to one of the Microraptor species found in China, whose skeleton’s foot bones measure an inch long. But because their proportions are slightly different, researchers believe the new tracks may belong to a new species of raptor. Evidence of another tiny dinosaur’s footprints, the more commonly found Minisauripus, adds to the argument.

Most likely a new species, not baby dinosaurs, made tracks

“Rapidly growing dinosaurs don’t remain small or leave little footprints for very long,” says Lockley. “But of all of the footprints we’ve found of the Minisauripus, none grew larger than one inch; a preponderance of evidence of a small species and not babies. There’s a chance that we just found something smaller.”

Guest contributor, Office of Research Services Science Writer Rachel Sturtz

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