Imagine that your a scientist digging for fossils on a low-lying grassland.
Many of the fossil you dig up seem to belong to creatures sadapted to live in the mountains.
Million of years ago what do you think the landscape looked like in the region where you are diging.
Many times when scientists dig for fossils in an area, the fossils seem to be adapted to a different climate, environment, habitat, etc. This is due to the fact that although presently as in your question, the area may be a grassland, millions of years ago, this may have been a mountainous area. The theory of plate tectonics demonstrates that the continents of the world move along "plates" continually, over the mantle below. Also, the seafloor is moving as well. Due to this movement, sometimes events occur where plates collide, which can result in mountain building and uplift. Also, earthquakes can be a result. Plates can move apart changing the locations of continents. In the past, an area might have been tropical near the equator, and now, it is located in a cooler area. It is possible that the fossils in your question were located on a mountain which might have eroded over time forming a low lying area. There are many reasons why this landscape might have changed over time.