Opisthiamimus gregori is an extinct, small-bodied relative of the tuatara (Sphenodon punctatus), the only living member of the reptilian order Rhynchocephalia.
Opisthiamimus gregori lived in North America during the Late Jurassic epoch, about 150 million years ago.
The ancient reptile belongs to Rhynchocephalia, the group that includes the living tuatara.
It measured about 16 cm (6 inches) from nose to tail and likely survived on a diet of insects and other invertebrates.
“The tuatara looks a bit like a particularly stout iguana, but the tuatara and its newly discovered relative are in fact not lizards at all,” said Dr. Matthew Carrano, curator of Dinosauria in the Department of Paleobiology at the Smithsonian National Museum of Natural History.
“They are actually rhynchocephalians, an order that diverged from lizards at least 230 million years ago.”
In their Jurassic heyday, rhynchocephalians were found nearly worldwide, came in sizes large and small, and filled ecological roles ranging from aquatic fish hunters to bulky plant munchers.
But for reasons that still are not fully understood, they all but disappeared as lizards and snakes grew to be the more common and more diverse reptiles across the globe.
This evolutionary chasm between lizards and rhynchocephalians helps explain the tuatara’s odd features such as teeth fused to the jaw bone, a unique chewing motion that slides the lower jaw back and forth like a saw blade, a 100-year-plus lifespan and a tolerance for colder climates.
“These animals may have disappeared partly because of competition from lizards but perhaps also due to global shifts in climate and changing habitats,” Dr. Carrano said.
Several specimens of Opisthiamimus gregori were found at a site centered around an Allosaurus nest in the Morrison Formation, Wyoming, the United States.
“Whereas many fossil rhynchocephalians are based on isolated incomplete jaws, the type specimen of Opisthiamimus gregori includes most of the skull and postcranium and therefore represents one of the most complete specimens of Rhynchocephalia known from North America,” Dr. Carrano and colleagues said.
They used micro-computed tomography to examine the skeletal anatomy of Opisthiamimus gregori in detail and to develop a 3D reconstruction of the skull.
“Given Opisthiamimus gregori’s diminutive size, tooth shape and rigid skull, it likely ate insects,” said Dr. David DeMar Jr., also from the Department of Paleobiology at the Smithsonian National Museum of Natural History.
“Prey with harder shells such as beetles or water bugs might have also been on the menu.”
The results appear today online in the Journal of Systematic Palaeontology.
David G. DeMar Jr. et al. 2022. A nearly complete skeleton of a new eusphenodontian from the Upper Jurassic Morrison Formation, Wyoming, USA, provides insight into the evolution and diversity of Rhynchocephalia (Reptilia: Lepidosauria). Journal of Systematic Palaeontology 20 (1); doi: 10.1080/14772019.2022.2093139
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