The genus is distinguished from other eurypterids by the large number of elongated and curved spines, blade-like in life, on its limbs.

Initially assumed to represent a species of Eurypterus, these distinguishing features were quickly noticed and deemed important enough to designate Echinognathus as its own genus.

With some additional fossil assigned to Echinognathus in the early 20th century, the genus was noted to be similar to Megalograptus, another eurypterid with spiny limbs.

The fossils of Echinognathus are known from what was once marine environments, and it lived alongside a fauna including graptolites, brachiopods, cephalopods and trilobites.

[2] The body segments of Echinognathus were ornamented with prominent oblong scales, similar in shape to "raindrops running down a windowpane".

[5] The fossils were recovered in deposits of Katian (Late Ordovician) age[3] north of Utica, New York by William N. Cleveland, a friend of Walcott.

[9] Fragments of a spine, or possibly the telson (the final body segment, often in the shape of a spike), were referred to Echinognathus by Ruedemann in 1916.

[2] In The Eurypterida of New York, August Foerste compared the Echinognathus fossils to those of Megalograptus, also fragmentarily known at the time, and concluded that the two were likely closely related, if not congeneric.

[11] Though the taxonomic position of this family has been contested historically, phylogenetic analyses support the Megalograptidae as grouped with the Carcinosomatidae and Mixopteridae in the superfamily Carcinosomatoidea.

[4] Based on the related Megalograptus, the second hypothesis, that the spines of Echinognathus were used for active prey capture and to move food to the mouth, is more likely.