Though originally described as a giant spider, a multitude of features support the classification of Megarachne as a eurypterid. Among them, the raised lunules (the vaguely moon-shaped ornamentation, similar to scales) and the cuticular sculpture of the mucrones (a dividing ridge continuing uninterrupted throughout the carapace, the part of the exoskeleton which covers the head) are especially important since these features are characteristic of eurypterids.Megarachne possessed blade-like structures on its appendages (limbs) which would have allowed it to engage in a feeding method known as sweep-feeding, raking through the soft sediment of aquatic environments in swamps and rivers with its frontal appendage blades to capture and feed on small invertebrates.
Initially described as a gigantic spider, further analysis reveals that Megarachne is actually an eurypterid due to several notable characteristics. The lunules and mucrone cuticle are particularly significant, as they are hallmarks of eurypterids. Furthermore, the presence of blade-like appendages on its limbs allowed for sweep-feeding, a feeding method involving the use of frontal blades to capture small invertebrates from soft sediment.