* Dinosaurs evolved much later: The first dinosaurs appeared around 245 million years ago, during the Triassic period. Rocks from 250 million years ago predate the existence of dinosaurs.
* The Permian-Triassic Extinction: The rock you're describing is from the Permian period. This period ended with the largest mass extinction event in Earth's history, wiping out a vast majority of life on the planet. Dinosaurs evolved *after* this event.
What you might find in a 250-million-year-old rock:
* Fossil evidence of Permian life: You could find fossils of creatures that lived during the Permian period, such as therapsids (mammal-like reptiles), ancient amphibians, and early reptiles.
* Fossils from the earliest Triassic: If the rock is very close to the Permian-Triassic boundary, you might find very early Triassic fossils. However, these wouldn't be dinosaur fossils.
