"In the street" is the more common usage, but an argument can be made for both, depending on nuance.
The use of "in" suggests that the street is more than simply a surface. Rather it's part of a "right of way" that presumably extends above the pavement, at least as far as the tallest vehicles. It's similar to a parking space. You park "in" the parking space. (however, your tire might be "on" the line)
"On" suggests the street is simply the pavement. "Dead animals are on the asphalt".
It's pretty hard to imagine dead animals being anywhere other than "on" the pavement, but using the wider definition of street, the pavement is also technically "in" the street.
btw, I live on a dirt road....but let's not go there!