A small reptile discovered in the United States is helping scientists better understand a group that once lived alongside dinosaurs but has almost completely disappeared. Named __Opisthiamimus gregori__, it stands out thanks to a rare, nearly complete fossil.
What makes this find so interesting is the group it belongs to. The rhynchocephalians, often called ‘living fossils’ because they are the last descendants of a group that dominated ecosystems over 240 million years ago, are now reduced to a single species: the tuatara in New Zealand. However, they were once far more widespread and diverse.”
These reptiles split from lizards and snakes around 230 million years ago. Back in the Jurassic, they occupied many ecological roles. Research published in the Journal of Systematic Palaeontology notes that some species fed on fish, others on plants, and some, like this newly described one, likely fed on insects.
Found Right Next To A Dinosaur Nest
The fossils were uncovered in the Morrison Formation in Wyoming, a well-known site for dinosaur remains. In this case, the bones were found near a nest of Allosaurus, a large predator from the Jurassic.
That detail is unexpected, but the real highlight is the condition of the fossil itself. Most rhynchocephalians are known from small fragments. This time, researchers found several specimens, including one that is almost complete, missing only parts of the tail and hind legs.
Data from the Smithsonian Institution highlight how rare such preservation is for this group. To study it properly, the team used micro-CT scans from three different facilities to build a detailed 3D reconstruction of the skeleton. The animal measured around 16 centimeters long, so it was quite small.
“Such a complete specimen has huge potential for making comparisons with fossils collected in the future and for identifying or reclassifying specimens already sitting in a museum drawer somewhere,” research associate David DeMar Jr. explained. “With the 3D models we have, at some point we could also do studies that use software to look at this critter’s jaw mechanics.”
A Bite Unlike Any Other Reptile
As explained in the study, looking at its skull and teeth, scientists think __Opisthiamimus gregori__ was an insect-eater. It probably fed on insects and maybe harder prey like beetles. The researchers point out that the shape of the teeth supports this interpretation.
“Rhynchocephalians are largely known from fragmentary fossils of their teeth and jaws,” noted Smithsonian National Museum of Natural History paleontologist Matthew Carrano.“Opisthiamimus was likely an insectivore with the ability to chew through fairly resistant foods like insect exoskeletons.”
Rhynchocephalians have a very unusual way of eating, their teeth are not separate structures but part of the jawbone itself. The tuatara still has this feature today, along with a jaw that moves back and forth in a sawing motion.
This fossil gave scientists a rare chance to study those features in detail. Some parts of the skull were crushed or shifted, but the scans helped reconstruct them accurately. That kind of detail is often missing when fossils are incomplete.
Still No Clear Answer For Their Decline
Even though rhynchocephalians were doing well during the Jurassic, they eventually declined while squamates, meaning lizards and snakes, became dominant. Why that happened is still not fully understood. They had traits that might have helped them survive, like long lifespans up to 100 years for the tuatara and an ability to live in cooler environments. But none of that clearly explains why most of them disappeared.
The study also explains that this new species could help researchers better classify other fossils that are harder to identify. It does not solve the bigger mystery, but it adds a useful piece to the puzzle.
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