Hi Veronicalane,
My eyesight is poor and I can't see the photos clearly enough to tell what the dark spots are on your
Common Staghorn Fern (Platycerium bifurcatum) Are they appearing on the reverse sides of the fronds or the top? My first thought was maybe you were seeing spores which appear as brown on the reverse side but if it's black (as it appears in the photos) the issue may be due to cold damage; possibly a fungal infection if the plant has experienced wet and cold conditions.
Hopefully others will be able to see your photo more clearly and offer advice.