Naturally occurring tritium is extremely rare on Earth, where trace amounts are formed by the interaction of the atmosphere with cosmic rays.
This is an unfalsifiable hypothesis, and is unscientific.
The most common isotopes in natural uranium are uranium-238 (which has 146 neutrons and accounts for over 99%) and uranium-235 (which has 143 neutrons).
Uranium has the highest atomic weight of the primordially occurring elements.
Please note that any compilation of Stone Age art is bound to be both selective and subject to revision as new archeological discoveries are made.
Furthermore, despite the widening range of archeological dating techniques - such as radiometric carbon dating, Uranium/Thorium and thermo-luminescence dating - not every work of art can be dated with great accuracy if the geological environment lacks important measurable elements.