The Morrison Formation is a fossil paradise. Within these multicolored sedimentary stone layers, the remains of allosaurus, stegosaurus, apatosaurus and more have been found in more than a century of fossil hunting. But these Jurassic rocks contain much more than the bones of “terrible lizards” that lived in a big way. Fossils of tiny creatures have been filling in what the Morrison Formation world was really like, including a new fossil reptile with ties to the modern tuatara.
named Opisthiamimus gregori, the little creature was an insect-hunting reptile that lived in Wyoming about 150 million years ago. However, despite the animal’s lizard-like appearance, it was not a lizard. Instead, it belonged to a different group of reptiles known as rhynchocephalians. Described by Smithsonian National Museum of Natural History paleontologist Matthew Carrano and colleagues This day in the Journal of Systematic Paleontology, opisthiamimus it was part of that group that flourished in the time of the dinosaurs but today is only represented by the New Zealand tuatara.
Finding tiny critters in the Morrison Formation is a challenging task, akin to finding a needle in a haystack that stretches from Montana to Oklahoma. Big dinosaurs, with their sturdy bones, are easier to find among the gray and maroon outcroppings, and that was the first step in a story that would eventually reveal opisthiamimus.
Decades ago, paleontologists had found a allosaurus nesting site at a place called Fox Mesa in Wyoming. To reach the layer of the dinosaur nests, the excavation required removing tons of overlying rock. “In 2010, while we were looking into that,” recalls Carrano, “Pete Kroehler noticed a couple of little specks of bone on the surface of a small block that had come out of the layer just around the eggshells.” Kroehler located a few other pieces that fit that curious piece of stone and marked them for later investigation. It was only when the museum’s preparators began removing the surrounding rock that they discovered that those little flecks of bone turned out to be something new.
The rocks revealed one of the most complete small reptiles ever found in the Morrison Formation. “The Rhynchocephalians are largely known from fragmentary fossils of their teeth and jaws,” says Carrano. But there was so much more to opisthiamimus. Preparation of the Fox Mesa fossils revealed most of the skull and body of this reptile, a fossil bonanza considering how little such small animals were preserved. The assembled bones were enough to know that opisthiamimus It was something very different from other reptiles that lived at the same time. By comparing the reptile’s teeth and jaws with those of other rhynchocephalians found in the same formation, Carrano says, experts determined that “opisthiamimus It was probably an insectivore with the ability to chew fairly tough foods like insect exoskeletons.”
The new reptile’s teeth and size align with the idea that opisthiamimus it was an insectivore, says Harvard University paleontologist Tiago Simões, who was not involved in the new study. that doesn’t mean that opisthiamimus However, it only ate hard-shelled insects. “It’s always complicated to infer dietary behavior in fossil animals,” says Simões, noting that opisthiamimus it would eat a variety of small prey even with its specializations in crushing shells.
the discovery of opisthiamimus it has broader implications than simply adding another species to the list of wondrous creatures that roamed North America during the Jurassic. Previously, scientists thought that most of the rhynchocephalians found in the Morrison Formation belonged to a genus called opistias. Many fragmentary finds were attributed to that reptile and experts thought that opistias it was a common and widely distributed species. But in the investigation opisthiamimus, Carrano and colleagues found that many of these older fossil assignments were incorrect. Many different relatives of tuatara existed side by side during the time the Morrison Formation was forming. Carrano calls this “a much more interesting picture” that raises questions about how various rhynchocephalian species managed to co-exist with each other during the Jurassic.
The new reptile is part of that image. “I think the name of the taxon says it all,” says Simões, while opisthiamimus medium “opistias imitate.” The two were so similar that paleontologists had apparently collected opisthiamimus fossils even before the Fox Mesa discoveries, they just didn’t realize the reptile was something new. Surely other similar cases of new species have been miscategorized or passed unnoticed by previous generations of experts. “I wonder how many other important specimens are in museum collections waiting to be described,” says Simões.
Although paleontologists have been examining the Morrison Formation for about a century and a half, many new finds remain to be made. That’s especially true of smaller animals that got lost in the great “bone parties” of the past. “I think there is a real focus among paleontologists now towards these undiscovered or even unannounced animals that were contemporaries of the larger and more famous dinosaurs,” Carrano says. Such creatures are difficult to locate, often looking like small fragments scattered among larger bones, but they complete a vision of ancient floodplains and forests that was previously incomplete. “These animals have often taken a backseat to the dinosaurs,” says Carrano, “which means there’s a lot left to discover, study and understand about them.”