Part of a juvenile dinosaur unearthed in Utah may upend what experts know about the evolutionary history of ceratopsids — the group of frilled, horn-faced dinosaurs that include Triceratops and Styracosaurus.
The nearly 100 million-year-old fossil may be evidence of the oldest ceratopsid ever found, pre-dating the previous record-holder — the little Zuniceratops, from New Mexico — by nearly 10 million years.
While among the most recognizable dinosaurs in the fossil record, ceratopsids are also one of the most diverse — as shown by the discoveries of many new horn-faced dinosaur species in recent years, from New Mexico to Canada. [See a recently discovered Triceratops cousin: “‘Hellboy’ Dinosaur Discovered in Alberta Reveals Evolutionary Surprises“]
But how all of these species fit together remains a puzzle.
And the newly found specimen from Utah might prove to be a key piece, said Dr. Kenneth Carpenter.
“It provides strong supportive evidence for evolutionarily more advanced ceratopsians in North America 98 million years ago, give or take a million or two,” said Carpenter, a paleontologist at Utah State University Eastern Prehistoric Museum.
More basal, or ancestral, forms of horned dinosaurs have been found throughout the American West, like the small but spry Aquilops, which was about the size of a house cat. [Adorable dinosaur alert! Learn more about Aquilops: “Oldest Horned Dinosaur in North America Discovered in Montana“]
But the Utah specimen suggests that the much bigger and more developed ceratopsids came on the scene far earlier than had been suspected — perhaps only 10 million years after the tiny Aquilops thrived.
Reporting their find in the journal Cretaceous Research, Carpenter and his colleague, Dr. Richard Cifelli from the Oklahoma Museum of Natural History, write, “If correctly identified, the [Utah specimen] … shows that the diversity of early ceratopsians was greater than previously realized.”
Unlike more recent ceratopsid finds, this specimen was originally excavated in 1991, in the Cedar Mountain formation of east-central Utah.
But until recently, it was stashed away in storage at the Oklahoma Museum of Natural History.
The specimen consists solely of an ilium, or pelvic bone, found in a stratum of Earth dated to around 100 million years ago, at the very beginning of the Cenomanian age.
Some 26 centimeters, or 10 inches, long, the bone was originally thought to have belonged to Eolambia, a Iguanodon-like plant-eater whose remains are abundant in the area where it was found.
But Carpenter and Cifelli soon noticed some important anatomical clues.
For one thing, they found that the bone came from a juvenile dinosaur, as shown by the fibers and striations that run through its outer layers.
“The fibrous bone texture indicates the Cedar Mountain ceratopsoid was not fully grown,” Carpenter said.
“The big question is how much larger was the adult?”
What’s more, its shape and orientation differ significantly from the pelvis of Eolambia.
To find out what animal the bone truly came from, Carpenter compared it to the ilia of several ceratopsids, including an older, ancestral protoceratops from Idaho; a flashy Agujaceratops from Texas; a juvenile dinosaur from Montana known as Brachyceratops, and finally some specimens from China.
“The ilium shares features with the only good juvenile ilium of an advanced ceratopsian, Agujaceratops,” he said.
Noted for its long brow horns and heart-shaped, spiky frill, Agujaceratops is well-documented, thanks to bone beds in Texas that contained many of the animals of various ages, from babies to adults.
This has allowed for a detailed analysis of how the dinosaurs changed through their life cycle, including their pelvic bones.
“This juvenile Agujaceratops is different from the adult in that the front part, called the preacetabular process, curves towards the side and down. In the adult, this part is more horizontal.
“The Cedar Mountain ilium shows the same feature as the juvenile Agujaceratops.”
The ancient ilium unearthed in Utah is “definitely not” that of Agujaceratops itself, he added. or another.
“Agujaceratops juveniles simply retain the primitive ceratospid pattern in the ilium,” he said.
“We see this in other dinosaurs, such as the more elongate face in juvenile tyrannosaurs reflecting the elongate face of more primitive theropods – such as Coelophysis.” [See new insights into tyrannosaurs: “Tyrannosaur Tracks Discovered in Wyoming Reveal Dinosaur’s Speed“]
But what ceratopsid the bone actually belongs to is, and will probably remain, unknown.
Nonetheless, Carpenter and Cifelli say, their discovery shows that there’s more to the story of ceratopsian evolution than experts currently realize.
Writing in Cretaceous Research, they note: “The presence of a ceratopsid or ceratopsoid in the earliest Late Cretaceous supports the hypothesis that the group had a long history in North America.”
Carpenter, K., & Cifelli, R. (2016). A possible juvenile ceratopsoid ilium from the Cenomanian of central Utah, U.S.A. Cretaceous Research, 60, 167-175 DOI: 10.1016/j.cretres.2015.11.016