The new find also highlights the downside of the evolutionary shift from loosely-fitted teeth that fall out but grow back to having a single set of permanent chompers, a drawback shared by adult humans, the researchers said.
Labidosaurus hamatus -- a fat-headed, omnivorous reptile about 75 centimeters (30 inches) long -- adapted over millions of years to life on land rather than the watery marshes of its amphibious forebear.
It's stouter legs and armour-like skin were better adapted to running and warding off predators.
And its non-replaceable teeth, deeply anchored in its jaw, were better suited for eating fibrous plants and stems, alongside its more ancient diet of flying and crawling insects.
But having fixed-for-lifetime dentition made hamatus vulnerable to the same type of bacterial decay that plague humans and keep approximately two million dentists around the world employed.
"Our findings allow us to speculate that our own human system of having just two sets of teeth, baby and permanent -- although of obvious advantage because if its ability to chew and process many different food stuffs -- is more susceptible to infection," the authors concluded.
Researchers led by Robert Reisz, a professor at the University of Toronto at Mississauga, analysed an "exquisitely preserved" jaw found near Coffee Creek, Texas using CT-scan technology.
They found evidence of massive infection, likely resulting in the loss of several teeth and bone destruction in the jaw in the form of an abscess.
The study was published in the peer-reviewed journal The Nature of Science.