Megalograptus is a genus of eurypterid, an extinct group of aquatic arthropods. Fossils of Megalograptus have been recovered in deposits of Katian (Late Ordovician) age in North America. 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. Fossils unassigned to any particular species have also been found in Canada. The generic name translates to "great writing" and originates from the mistaken original belief that Megalograptus was a type of graptolite, often given names ending with -graptus (meaning 'writing').
Megalograptus was a large predatory megalograptid eurypterid, with the largest and best known species, M. ohioensis, reaching body lengths of . Some species were substantially smaller, with the smallest, belonging to a hitherto undetermined species, only growing to about in length. Morphologically, Megalograptus was highly distinct. 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. Certain fossils of three different species, M. ohioensis, M. shideleri and M. williamsae, are so well-preserved that researchers have been able to infer the coloration they might have possessed in life. All three were deduced to have been brown and black in color, with M. ohioensis being darker than the others.
First described by Samuel Almond Miller in 1874, based on fragmentary fossil remains of the species M. welchi, Megalograptus being a graptolite was not formally questioned until 1908, when Rudolf Ruedemann recognized the fossils as eurypterid remains. 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. In 2015, the genus Pentecopterus was also assigned to the family. 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. Caster and Kjellesvig-Waering conducted further work on Megalograptus over the following years. In 1964, they named the species M. shideleri and M. williamsae and reclassified the fragmentary eurypterid Ctenopterus alveolatus as a species of Megalograptus.
Megalograptus lived in near-shore marine environments, where it used its large appendages, and possibly its telson and cercal blades, to capture prey. Possible coprolites (fossilized dung) are known from M. ohioensis, which contain fossil trilobite fragments as well as fragments of M. ohioensis itself. This suggests that Megalograptus might have been cannibalistic at times, like many modern chelicerates.
Description
Size
Megalograptus was a genus of large megalograptid eurypterids. The largest species was M. ohioensis, which ranged in length from . M. ohioensis was the second largest megalograptid and the second largest eurypterid of the Ordovician period, smaller only than the related Pentecopterus, which could grow to . Previous estimates have placed the size of some species of Megalograptus as substantially larger, with the type species M. welchi once believed to have reached lengths of in length. According to a 2009 study by James Lamsdell and Simon J. Braddy, such estimates are dubious as they are based on ornamentation in incomplete fossils. In the case of M. shideleri, once estimated to have reached lengths of , the size estimate was based only on two fragmentary tergites (upper portions of body segments), wherein the dimensions of the ornamental scales were unusually large, interpreted as suggesting a giant body size. The fact that scales can vary in size across the bodies of megalograptid eurypterids and that one of the relevant tergites of M. shideleri was not longer than suggests that this species did not reach lengths of more than . The length of the species M. alveolatus is uncertain, but it was much smaller than M. ohioensis, and M. williamsae grew to about . The smallest known species of Megalograptus was an as yet undescribed Canadian species which only grew to in length.