Complex sentence: Two fossil species, an earlier form, "L. robusta", and the more recent "L. palaeoleptonyx", are known, and may have fed primarily on shellfish, rather than on fish as the current species does.
Split into simpler sentences: Two fossil species, an earlier form, "L. robusta", and the more recent "L. palaeoleptonyx", are known. They probably fed primarily on shellfish, rather than on fish as the current species does.