It is puzzling to think that a fossil of Scream Tail's size was found in Mesoprotozoic rocks, but in retrospect, Jigglypuff's biology is just wildly different enough to make it a viable candidate for the oldest branch of pokemon's evolutionary tree. Yet even more remarkable than its age is its well-preserved throat, for its vocalization, were this pokemon brought forward in time to the modern era, must have exceeded even Exploud's in volume.
We do not know what Scream Tail shouted to communicate, yet surely it was not necessary, with so few rivals extant, to scream at a volume which shatters glass. Speculation has revolved around two distinct if not irreconcilable notions; that the ear, so early in its evolution, could only detect loud noises such as earthquakes, or that the acoustics of this world a billion years ago were radically worse than in the present time.
There is a crater at the center of Paldea which exists outside of time, shielded from the present day by high mountain walls. And yet, at night in the towns near those mountains, people are kept awake at night by the sounds of screams and loud music. The victims have compared the song to a Jigglypuff's, were it to trade in lullabies for metal or hard rock. None have made it over the mountains and returned to sleep quietly in bed, but an unfortunate few claim to have been thrown out entirely by the force of this ancient pokemon's long, catapult-like tail, their screams drowned out by those of their assailant.
Perhaps, with proper equipment, the screams of a billion years ago could be recorded, and human understanding of the Precambrian dramatically improved – or perhaps Area Zero is permeable enough that Scream Tail's screams have been influenced by the modern Jigglypuff style.