Whitney can say that with a degree of certainty because one of the first things she and her team of researchers had to do was to define exactly what a tusk is. Previously it had been a somewhat ambiguous term.
They determined that for a tooth to be a tusk it must extend out from the mouth, be made solely of dentine, and grow continuously so it would regenerate if damaged or broken. Regular mammal teeth are also made of dentine but are coated in hard enamel, which would preclude continuous growth.
And therein lies a trade-off and perhaps a clue about the environmental pressures and relative evolutionary advantages for animals with each. Enamel-covered teeth are more durable but irreplaceable; tusk teeth could be used for fighting or digging because if damaged they could repair themselves and keep growing.
Whitney and her colleagues then set out to see how tusks evolved. They analyzed paper-thin slices of fossilized teeth from 19 dicynodont specimens, representing 10 different species, and used micro-CT scans to examine how the tusks attached to the skull to see whether there was any evidence of continuous growth.
They found that some of the dicynodont tusks they examined didn’t make the cut. Many of the early dicynodont tusks were coated in enamel instead of dentine, making those “tusks” just very large teeth.