In addition to the lightweight giant eurypterids, some deep-bodied forms in the family Hibbertopteridae were also very large. A carapace from the Carboniferous of Scotland referred to the species "Hibbertoperus scouleri" measures wide. As "Hibbertopterus" was very wide compared to its length, the animal in question could possibly have measured just short of in length. More robust than the pterygotids, this giant "Hibbertopterus" would possibly have rivalled the largest pterygotids in weight, if not surpassed them, and as such be among the heaviest arthropods.