Round geographical coordinates for classes with intervals of 0.5° latitude/longitude

Question

I have the following data.frame with the occurrence coordinates of a species:

 df <- data.frame(
    especie = c("sp1", "sp2", "sp3"),
    longitude = c(-48.97376, -36.08333, -41.66931),
    latitude = c(-21.138258, -9.933333, -14.197054))
       
df

 especie longitude   latitude
     sp1 -48.97376 -21.138258
     sp2 -36.08333  -9.933333
     sp3 -41.66931 -14.197054

I need to round these coordinates so that they are distributed in classes of 0.5 degrees latitude/longitude. For example:

 longitude latitude
    -71.75    12.25
    -71.25    12.25
    -72.25    11.75
    -71.75    11.75
    -73.75    11.25
    -73.25    11.25