Peculiar scrape-like dinosaur tracks discovered in Colorado are likely the traces of courtship displays performed by male dinos, paleontologists say — the first physical evidence of mating rituals among extinct dinosaurs.
Found at four sites in southwestern and central Colorado, the scrapes are up to two meters [6 feet] in diameter — as long and deep as bathtubs — in beds of sandstone dating back 100 million years.
While the species that made them is unknown, the three-toed prints around them indicate that the scrapes were made by two-legged carnivores known as theropods.
These newly identified fossil traces bear a striking resemblance to the marks left by modern birds, researchers say. The particularly resemble the scrapes made by males as they strut and scratch to attract the attention of females.
“These are the first sites with evidence of dinosaur mating display rituals ever discovered, and the first physical evidence of courtship behavior,” said Dr. Martin Lockley, a paleontologist at the University of Colorado-Denver, in a press statement.
“These huge scrape displays fill in a missing gap in our understanding of dinosaur behavior.”
Lockley and his colleagues found more than 50 of the distinctive markings around the state, he reported.
Some were discovered on two parcels of public land near Gunnison Gorge in western Colorado, and others just like them turned up along the famously fossil-rich Dinosaur Ridge near the Denver suburb of Lakewood.
For instance, they are too deep and dynamic to be so-called locomotion tracks, made by dinosaurs as they walked or ran. [See a new dinosaur-track find: “Thousands of Dinosaur Tracks Discovered Along Alaska’s Yukon River“]
And while they somewhat resemble nests in their dimensions, they lack the protective rims that typically surround theropod nests, as well as shells or any other signs of brooding. [See dinosaur nests recently discovered in Montana: “Remarkable’ Dinosaur Nest Discovered in Montana Suggests Some Dinos Re-Used Breeding Sites“]
However, in his description of the tracks in the journal Scientific Reports, Lockley notes that the marks do have “compelling parallels” with scrapes found near the breeding sites of a variety of modern birds, from ostriches to puffins.
“Ornithological literature provides many reports of scrape ceremonies and nest scrape display activity,” he writes.
Some male birds are known to perform a sort of mock nest-digging performance, Lockley explains, a display intended to flaunt home-making skills for females in the market for mates.
Although no fossil nests have been found around the newly found marks in Colorado, he adds, “[t]he scrapes most probably occur near nesting colonies, [which are] as yet unknown or no longer preserved in the immediate study areas.”
He concludes that the scrape marks are signs of “stereotypical avian [or bird-like] behavior.”
The ramifications of the discovery are far-reaching, Lockley notes, not only because of the intriguing insights it provides into the behavior of individual dinosaurs, but also because it offers yet more evidence of the close evolutionary relationship between extinct dinosaurs and modern birds.
“The scrape evidence has significant implications,” Lockley said in his statement.
“This is physical evidence of prehistoric foreplay that is very similar to birds today.
“Modern birds using scrape ceremony courtship usually do so near their final nesting sites.
“So the fossil scrape evidence offers a tantalizing clue that dinosaurs in ‘heat’ may have gathered here millions of years ago to breed and then nest nearby.”
As Lockley writes in his paper, “It is intriguing to imagine the vocalizations of large theropods during courtship scrape ceremonies and copulation.”
Lockley, M., McCrea, R., Buckley, L., Deock Lim, J., Matthews, N., Breithaupt, B., Houck, K., Gierliński, G., Surmik, D., Soo Kim, K., Xing, L., Yong Kong, D., Cart, K., Martin, J., & Hadden, G. (2016). Theropod courtship: large scale physical evidence of display arenas and avian-like scrape ceremony behaviour by Cretaceous dinosaurs Scientific Reports, 6 DOI: 10.1038/srep18952