Alberta wouldn't technically be part of the Osai region but I liked this idea too much. Oh well. There's a pun to be made here about Shell gas I'm sure. Also something something Turtle Mountain (1903 Frank slide, not that I'm blaming tragic loss of life on a pokemon).

Chertle, the asphalt pokemon
Chertle is a small rock-type pokemon from the Osai region. Though it was once restricted to a small area of north-western Osai, chertle and its evolutions have become a staple of civil engineering projects so they are very common in and around urban environments. Chertle's small, fluid body is very vulnerable so the first thing it does upon hatching is to look for some suitable rocks to adhere to itself. When threatened it tucks its body into the spaces between the rocks, protecting it from harm. Its major predators are steel-type pokemon, which were uncommon in the Osai region before settlers came from Kalos and Galar. It has a peculiar smell like warm tar, and is very sluggish in cold weather. Chertle evolves into tortoil at level 20.

Tortoil, the asphalt pokemon
Tortoil is a medium-sized rock-and-poison type pokemon from the Osai region. It spends most of its life below ground in a semi-fluid state, looking for tasty rocks to eat and sturdy rocks for its shell. It tills up the rockbed as it moves, and its feeding activity is the source of superior-quality aggregate. With a ready supply of pre-crushed rock, a single tortoil and armadozer can lay down a fully paved stretch of road in a fifth of the time it would take through conventional means. Teams of these pokemon working together keep the roads of Osai in remarkable shape in spite of the environment's many challenges. The inclination of tortoil to wear a fossil skull on its head seems to stem from this partnership - it seems to admire armadozer's headgear. Tortoil evolves into Tartoise at level 45.

Tartoise, the asphalt pokemon
Tartoise is a large rock-and-poison type pokemon from the Osai region. Its heavy carapace of rocks makes it difficult to move, so it relies on its ability to make tendrils of sticky tar to grab things just out of its reach. Tartoise excavates huge pits to look for food, and tends to do so heedlessly - it doesn't care if it is buried in a rockslide, it just oozes itself up to the surface and carries on eating. Sometimes it finds rocks with fossils in them - it seems to like these and finds places on its body to display them. If Tartoise can be coaxed into a fossil bed, it can be tasked with excavating fossils with great care - the magnificent Shale Bed collection of fossils was uncovered by a particularly diligent tartoise. Getting fossils back from tartoise is another matter...