The genus contains five species: M. alveolatus, M. ohioensis, M. shideleri, M. welchi and M. williamsae, all based on fossil material found in the United States.

Megalograptus was a large predatory megalograptid eurypterid, with the largest and best known species, M. ohioensis, reaching body lengths of 78 centimeters (2 ft 7 in).

The two most distinctive features of Megalograptus were its massive and spined forward-facing appendages, far larger than similar structures in other eurypterids, and its telson (the last division of the body).

The sharp spike-shaped telson of Megalograptus was not venomous, but it was specialized in that it was surrounded by unique cercal blades, capable of grasping.

Megalograptus was noted as being similar to Echinognathus by August Foerste in 1912 and the two genera have been considered closely related since then, and have been grouped together in the Megalograptidae since 1955.

Kenneth E. Caster and Erik N. Kjellesvig-Waering revised Megalograptus in 1955, owing to the discovery of more complete fossil material of the new species M. ohioensis.

Megalograptus lived in near-shore marine environments, where it used its large appendages, and possibly its telson and cercal blades, to capture prey.

The carapace (head plate) of Megalograptus was vaguely quadratic in shape and flattened, lacking a marginal rim, which was present in some other eurypterids.

The third joint of the swimming paddles of Megalograptus bent the appendages forwards, a rare feature in the eurypterids, otherwise mostly known from the distantly related genus Dolichopterus.

The metastoma (a large plate located on the underside of the body) of Megalograptus was roughly egg-shaped, unusually wide and broadly subtriangular (almost triangular) in shape, differentiating it from the same structure in all other eurypterids, where it was usually cordate (heart-shaped).

[3] The most unusual feature of Megalograptus was the structure formed by the telson (the last division of the body) and the immediately preceding segment (the pretelson).

Megalograptus had, alongside the sharp and stout telson spike, two paired and rounded blade-formed lobes, the so-called cercal blades.

[3] In the 1964 description of M. ohioensis, M. shideleri and M. williamsae, Kenneth E. Caster and Erik N. Kjellesvig-Waering made inferences of the life coloration of the species based on a collection of well-preserved specimens.

Miller mistakenly believed the fossil material, consisting of a postabdominal (segments 8–12) tergite and two fragments of an appendage, was the integument of a graptolite (a member of Graptolithina, an extinct group of colonial pterobranchs),[3] and gave it the name Megalograptus, meaning "great writing" (deriving from the Greek megale, "great", and graptos, "writing", commonly used for graptolite fossils).

[3] That same year, Ruedemann's suspicions were confirmed in discussions with August Foerste and Edward Oscar Ulrich, who also agreed that the fossils were of a eurypterid.

[3][8] Foerste was invited to contribute with his understanding of Megalograptus to Ruedemann's and John Mason Clarke's 1912 monograph The Eurypterida of New York.

[8] Megalograptus was considerably revised in 1955 by Caster and Kjellesvig-Waering in Leif Størmer's 1955 Treatise on Invertebrate Paleontology, from which the modern understanding of the genus originates.

[3] In 1964, Caster and Kjellesvig-Waering named two new species of Megalograptus[3] based on additional fossil material from the Katian[2] of Ohio: M. shideleri and M. williamsae.

M. shideleri was named based on fragmentary fossil specimens originally found by William H. Shideler in the Saluda Formation near Oxford, Ohio, and in Indiana.

Caster and Kjellesvig-Waering noted in 1964 that there were very fragmentary eurypterid fossils[3] known from the Katian-age[2] deposits of the Whitewater Formation near Oxford, Ohio, referring these specimens to Megalograptus.

[3] In 2002, fossils belonging to a small variety of Megalograptus were first reported from Katian-age deposits of the Nicolet River Formation in Quebec, Canada.

[2][12][13] Megalograptus fossils found in Katian-age deposits in the US state of Georgia and in the Shawangunk Ridge of New York may also represent two distinct new species.

[14] Shortly after being recognized as a eurypterid in the early 20th century, Megalograptus was noted by Foerste in 1912 as being similar, and likely closely related, to the genus Echinognathus.

[17] In 2004, O. Erik Tetlie determined Megalograptus, and by extension the Megalograptidae, to be taxonomically problematic, perceiving the genus to share several potential synapomorphies (derived, "advanced", traits unique to a clade) with both the Eurypteroidea and the Mixopteroidea (now considered a synonym of Carcinosomatoidea), as well as having a large number of apomorphies (characteristics different from what existed in the ancestor of an organism).

Depending on how the analysis was conducted, Megalograptus changed position in the phylogenetic tree, only sometimes being recovered as basal within the Mixopteroidea (if taxa where less than one third of the body was preserved were removed).

[13] No phylogenetic analysis ever recovered Megalograptus in these positions and the genus was often excluded from analyses due to its perceived strange mix of features.

[1] The description of Pentecopterus, the only other genus in the Megalograptidae, in 2015 by Lamsdell and colleagues saw the megalograptids again considered to be close relatives of the mixopterids and carcinosomatids.

The phylogenetic analysis accompanying the description of the new genus resolved the Megalograptidae as basal within the relatively derived Carcinosomatoidea superfamily, which also includes the Carcinosomatidae and Mixopteridae.

[4] The cercal blades of Megalograptus are believed to have been a considerable aid when swimming, acting like a biological rudder,[1][3] but they were also able to articulate and move like a scissor.

M. ohioensis occurred alongside a typical Late Ordovician fauna, including trilobites (Isotelus and Flexicalymene), bryozoans, gastropods, pelecypods, brachiopods, ostracods and scolecodonts.