New fossilized specimens of monstersaurs — including a previously unknown genus and species, Bolg amondol — from the Kaiparowits Formation of Grand Staircase-Escalante National Monument in southern Utah reveal unexpected richness of these large-bodied lizards in Late Cretaceous North America.
Life reconstruction of Bolg amondol. Image credit: Cullen Townsend.
Bolg amondol lived in North America during the Campanian age of the Cretaceous period, around 72 million years ago.
The ancient animal represents an evolutionary lineage that sprouted within a group of large-bodied lizards called monstersaurs, the most familiar example being the Gila monsters, which still roam the deserts where the new species was recovered from.
“The Monstersauria are characterized by their large size and distinctive features like pitted, polygonal armor attached to their skulls and sharp, spire-like teeth,” said Dinosaur Institute paleontologist Hank Woolley and colleagues.
“They have a roughly 100 million-year history, but their fossil record is largely incomplete, making the discovery of Bolg amondol a big deal for understanding these charismatic lizards, and Bolg amondol would have been a bit of a monster to our eyes.”
“Almost 1 m (3 feet) tip to tail, maybe even bigger than that, depending on the length of the tail and torso.”
“So by modern lizard standards, a very large animal, similar in size to a Savannah monitor lizard; something that you wouldn’t want to mess around with.”
An associated skeleton of Bolg amondol — pieces of the skull, vertebrae, girdles, limbs, and the bony armor called osteoderms — was collected in 2005 from the Kaiparowits Formation of Utah.
“Bolg amondol is a great example of the importance of natural history museum collections,” said Dr. Randy Irmis, a paleontologist at the University of Utah.
Other fossils described by the team, including remarkably armored skull bones, demonstrate that the ancient, seasonally tropical forests of what is now southern Utah hosted at least three different types of large, predatory lizards.
“Even though these lizards were large, their skeletons are quite rare, with most of their fossil record based on single bones and teeth,” said Dr. Joe Sertich, a paleontologist at the Smithsonian Tropical Research Institute and Colorado State University.
“The exceptional record of big lizards from Grand Staircase-Escalante National Monument may prove to be a normal part of dinosaur-dominated ecosystems from North America, filling key roles as smaller predators hunting down eggs and small animals in the forests of Laramidia.”
“The identification of a new species of monstersaur highlights the likelihood that there were many more kinds of big lizards in the Late Cretaceous,” the researchers said.
“Additionally, this find shows that unexplored diversity is waiting to be dug up in the field and in paleontology collections.”
“Bolg amondol’s closest known relative hails from the other side of the planet in the Gobi Desert of Asia.”
“While dinosaurs have long been known to have traveled between the once connected continents during the Late Cretaceous epoch, the discovery of Bolg amondol reveals that smaller animals also made the trek, suggesting there were common patterns of biogeography across terrestrial vertebrates during this time.”
The findings appear in the journal Royal Society Open Science.
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C. Henrik Woolley et al. 2025. New monstersaur specimens from the Kaiparowits Formation of Utah reveal unexpected richness of large-bodied lizards in Late Cretaceous North America. R. Soc. Open Sci 12 (6): 250435; doi: 10.1098/rsos.250435
