Even after two centuries of research, paleontologists still know relatively little about certain expanses of time in the Age of Dinosaurs. The middle part of the Cretaceous, from about 83 million to 113 million years ago, is among them. Paleontologists have long wondered how the famous faunas of the end of the period—Cretaceous celebrities like Triceratops, Tyrannosaurus and Ankylosaurus—came to be, yet researchers have been thwarted by the scarcity of fossils in the preceding time periods. Now a new dinosaur from the 99-million-year-old rock of Utah is helping to fill in the fossiliferous gap.
Named Iani smithi after the Roman god Ianus, said in mythology to oversee transitions, and paleontologist Joshua Aaron Smith, the newly discovered dinosaur is known from a partial skeleton that includes much of the skull as well as portions of the spine and limbs. In life, the beaked, plant-munching creature was about 12 feet from the tip of its snout to the end of its tail. And as North Carolina Museum of Natural Sciences paleontologist Lindsay Zanno and colleagues described Wednesday in PLOS One, this dinosaur helps outline the changes that led up to the classic dinosaur communities of the Late Cretaceous.
At a glance, the dinosaur might seem somewhat plain. Iani lacks any horns, plates, spikes or other outstanding features that we often associate with dinosaurs. The details of the reptile’s skeleton, however, identify Iani as a rhabdodontomorph—a little-known group of herbivorous dinosaurs that were only recognized in 2016.
“I was really skeptical about that identification,” Zanno says, but that uncertainty led the researchers to be extra careful in their analysis. “Skepticism is what makes good science, so I spent a long time scrutinizing the anatomy and our analyses,” she adds. Iani shares some traits with other rhabdodontomorphs that can’t be denied, she says, particularly features of its teeth and skull. These dinosaurs, like the 8-foot long Zalmoxes from Romania and the 26-foot-long Muttaburrasaurus from Australia, make up a group of small- to medium-sized herbivores that spread around the world during the Cretaceous in the days before the more familiar duckbilled dinosaurs evolved.
In the broader picture of dinosaur evolution, rhabdodontomorphs were related to other bipedal herbivores like the iconic Iguanodon. They were somewhat like the antelope or deer of their time, filling the medium-sized herbivore niche in their communities. Even though such herbivores often gain less popular attention than the sharp-toothed carnivores that fed upon them, Iani’s discovery is an important part of a challenging ongoing effort to understand and describe the dinosaurs of what experts call the Mussentuchit Member of Utah’s Cedar Mountain Formation.
“The Cedar Mountain Formation captures over 40 million years of dinosaur evolution in western North America,” Zanno says, making it a great place to look for broader trends in the reptiles’ history. And while “fossil bone is not hard to stumble across in the gray-green Mussentuchit badlands,” Zanno says, a well-preserved skeleton is hard to find.
For decades, the dinosaurs found in these rocks have principally come to paleontologists as isolated, hard-to-identify teeth. The teeth allowed experts to guess at the different dinosaur groups present in western North America around 99 million years ago—such as raptors, titanosaurs and horned dinosaurs—but the bones that would reveal the full identity of each dinosaur were extremely hard to find.
Years of fieldwork by Zanno and colleagues, as well as other groups of paleontologists, have begun to answer some of those fossil question marks. In 2013, Zanno and colleagues named Siats meekerorum, a large Allosaurus relative found in the Mussentuchit Member, and in 2019 they followed up with the small tyrannosaur Moros intrepidus. Now Iani adds a new herbivore to the mix.
“Iani certainly helps to fill in gaps in the mid-Cretaceous fossil record,” says New York Institute of Technology paleontologist Karen Poole, who was not involved in the new research.
Rhabdodontomorphs were originally named from finds in Europe, then expanded to fossils known from Australia. Poole and colleagues have previously hypothesized that rhabdodontomorphs were present in Cretaceous North America, and the discovery from Utah adds new evidence that the dinosaurs thrived there.
Paleontologists are still searching for more fossils, but, from the available material, Zanno and colleagues have identified several major groups of herbivorous dinosaurs that lived side by side in the eastern Utah of 99 million years ago. Iani joins the hadrosaur relative Eolambia; an as-yet-unnamed species of small, sleek herbivore from a group called thescelosaurs; armored dinosaurs; and horned dinosaurs. Together they represent at least five groups of herbivorous dinosaurs that were present as the classic dinosaur communities of the Late Cretaceous were evolving.
“This discovery makes it clear that ornithopod dinosaurs were not on a single, steady march toward hadrosaurs,” Poole says, “but were diversifying into a wide array of animals all living alongside one another.” The cast of characters was broader and more variable than experts knew.
The middle of the Cretaceous was also a time of ecological upheaval. “North American dinosaurs of the mid-Cretaceous were hit by a double whammy,” Zanno says. New groups of organisms, including dinosaurs, were reaching North America from ancient Asia at the same time as the global climate was rapidly shifting. North America’s dinosaurs and ancient Asia’s dinosaurs were mixing together, some groups thriving and others disappearing. “I can’t help but think of it as Grand Central Station caught in time,” Zanno says, “with different dinosaur lineages coming and going, and a lot of chaos.”
In western North America, the hadrosaurs, horned dinosaurs, armored dinosaurs and even thescelosaurs persisted until the very end of the Cretaceous, 66 million years ago. But Iani and its relatives died out. Somehow these medium-sized herbivores didn’t make it even as many of the groups around them continued on for millions of years. When this cutoff happened, and why, is unknown. Other spots in Utah, such as the Straight Cliffs in the southern part of the state, may contain the relevant fossils to answer those questions.
Popular depictions of dinosaur eras often compress whole time periods down to relatively narrow slices. The whole of the Cretaceous lasted about 79 million years, from 66 million to 145 million years ago. That’s a broader span of time than the entire post-Cretaceous history of the world, and plenty of time for different groups of dinosaurs to thrive and fall back. The straightforward march of Early Cretaceous dinosaurs to their Late Cretaceous descendants we love to see in museum halls didn’t occur in such a neat and straightforward fashion. The story was much more complicated, involving entire groups of dinosaurs that experts are only just beginning to know, and Iani is really just the tip of the Cretaceous iceberg. “There’s a lot of herbivores in any ecosystem,” Poole notes, “and we haven’t come close to finding them all.”