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 megalograptids. 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.
Morphology Megalograptus was morphologically unique and easily distinguishable from other eurypterids. The carapace (head plate) of Megalograptus was vaguely quadratic in shape and flattened, lacking a marginal rim, which was present in some other eurypterids. At the front of the carapace there was a downturn and six small downward-facing spikes, possibly an adaptation for digging in the mud. The compound eyes of Megalograptus were medium-sized and reniform (kidney-shaped), located close to the edge of the carapace. The ocelli (simple eyes located more centrally) were small. Some of the appendages of Megalograptus unusually had one more joint than was common in eurypterids.
Among the appendages, the third pair (counting the simple chelicerae, pincers or mouth parts, as the first pair) are the most notable. In Megalograptus these were massive structures, covered in pairs of great spines, only comparable to the same structures in Mixopterus, another eurypterid. The appendages of Megalograptus, about 3.5 times the length of the carapace, were significantly larger than those of Mixopterus. On the fourth joint of the appendages, one pair of spines end in bulbous structures, rather than sharp points, and were perhaps sensory. This third pair, presumably used for seizing and constraining prey, had notable flexibility, indicating that its hypertrophied spines would have been used to hold prey and bring them to chelicerae and the coxal gnathobases, so that the second pair then could rip the immobilized prey.
The fourth pair of appendages were short and spiny, but the fifth pair, immediately preceding the swimming paddles (placed on the sixth and final pair of appendages), were completely spineless. This unusual limb is similar to the same appendage in the distantly related genus Eurypterus, where it has been interpreted as a balancing organ. 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 wide swimming paddles of Megalograptus were formed from the sixth joint of the appendage. The seventh joint, which in many genera formed a major part of the paddle, was reduced to a relatively small structure. The eighth joint, not preserved in any known Megalograptus fossil material, is indicated as existing by attachment points in the seventh joint, which also indicate that it was significantly smaller than in other eurypterids.
The mesosoma of Megalograptus (the first six segments after the head) was distinctly similar to the same segments in modern scorpions and different from the mesosomas of other eurypterids. The body contracted after the last segment of the mesosoma, rather than after the first segment of the metasoma (the last six segments), which was otherwise typical for eurypterids. In most eurypterids, the mesosoma was widest at the fourth or fifth segment, but in Megalograptus it was widest at the third. The first segment of the mesosoma was considerably shorter than the succeeding segments, which were otherwise approximately of the same length. The last few segments of the body were slightly longer than the preceding segments. 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).
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. These were attached beneath the telson, directly to the pretelson. The blades were capable of articulation, in effect forming a large grasping organ. In other eurypterids, the telson tends to be an undivided structure in the shape of a paddle or spike, meaning that the cercal blades distinguish Megalograptus from nearly all other eurypterids. Cercal blades are only known from one other eurypterid, Holmipterus, and are lacking in the basal ("primitive") megalograptid Pentecopterus. Taken together with the telson, the telson–pretelson assemblage of Megalograptus forms a flattened structure, superficially similar to the flattened telsons of many genera in the superfamily Pterygotioidea.
Megalograptus was ornamented with small scales of irregular size across its body. On the carapace, they were flat and disc-like and scattered without any obvious pattern. On the fifth pair of appendages, the scales were more elongated. On the main body, the scales were rounded, raised and nearly elliptical in shape. Many of the scales on the carapace, the fifth pair of appendages, the mesosoma and metasoma and some on the appendages had holes in their center, suggesting that they once supported bristles (stiff hairs). In life, Megalograptus may have had an almost hirsute (hairy) appearance.
Coloration
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. In some fossils of M. shideleri, the fossils retained their original coloration, with no replacement having taken place (meaning that mineralization during fossilization did not distort the original color scheme of the fossils). M. shideleri was brown, with scales varying in color from dark brown to black and the integument (the scales) was a lighter brown color. A similar, but darker, brown and black color scheme has been inferred for M. ohioensis, and its fossils being better preserved allows for more detailed examination. M. williamsae also had a similar color scheme, with its tergites indicating black scales against light brown integument. The colors of Megalograptus inferred by Caster and Kjellesvig-Waering are similar to those inferred by Kjellesvig-Waering of Carcinosoma newlini, another eurypterid also inferred to have been brown and black, in 1958.
According to Caster and Kjellesvig-Waering, M. ohioensis was mostly dark brown, with some black elements. Though no obvious integument pattern has been determined, most of the body, including the head, had dark brown integument, contrasted by black scales. The coxae (base segments of the appendages) were dark brown, with black scales and black gnathobases ("tooth plates" surrounding the mouth). As such, the appendages began as dark brown in color, but quickly darkened towards their ends. Most of the appendages of larger specimens, including the spiny and large forelimbs, were almost entirely black in color and with black spines, although in smaller specimens, the appendages were typically lighter in color. Connective tissue in the appendages was pale brown in color. The appendage immediately preceding the swimming paddles was not entirely black, instead just darkening to a very dark brown. The metastoma was dark brown, with black scales, postules (small elevations) and mucrones (tiny spines). The telson of M. ohioensis, as well as much of the preceding segment, was entirely black in color.
History of research
Type material