Description

Megalograptus represents a genus of eurypterids, which are extinct aquatic arthropods. Megalograptus fossils have been uncovered within Late Ordovician Katian deposits in North America. The genus name, originating from the initial mistaken belief that Megalograptus was a kind of graptolite, translates to "great writing," reflecting the common naming pattern of graptolites with names ending in -graptus (meaning 'writing').

Megalograptus, a formidable megalograptid eurypterid, assumed the role of a substantial predator. The most notable species within this genus, M. ohioensis, achieved impressive body lengths of up to 78 centimeters (2 ft 7 in).

Megalograptus exhibited distinctive morphological characteristics that set it apart from other eurypterids. The head plate or carapace of Megalograptus displayed a somewhat square shape, devoid of a marginal rim, which distinguished it from certain other eurypterids. At the carapace's anterior end, there was a downward curve accompanied by six small spikes oriented downward, potentially an adaptation for digging into sediment. Positioned near the carapace's edge, Megalograptus had medium-sized, kidney-shaped compound eyes. Additionally, it possessed small ocelli, or simple eyes, positioned more centrally. Notably, certain appendages of Megalograptus contained an extra joint compared to the typical eurypterid structure.

One of the most remarkable features among the appendages was the third pair, regarded as the most prominent. In Megalograptus, these appendages were substantial structures adorned with pairs of prominent spines, akin only to similar structures in Mixopterus, another eurypterid. These Megalograptus appendages, measuring about 3.5 times the carapace's length, outstripped the size of Mixopterus counterparts. On the fourth joint of these appendages, one pair of spines terminated in rounded structures instead of sharp points, suggesting possible sensory function. These third appendages, seemingly designed for seizing and restraining prey, possessed notable flexibility, implying that their enlarged spines were employed to immobilize prey and deliver them to the chelicerae and coxal gnathobases, allowing the second pair of appendages to dismantle the immobilized prey.

The fourth appendages were short and adorned with spines. In contrast, the fifth pair, situated just before the swimming paddles (positioned on the sixth and final pair of appendages), lacked spines entirely. This unique limb shared similarities with a comparable appendage in the distantly related genus Eurypterus, where it has been interpreted as a balancing organ. The third joint of Megalograptus' swimming paddles had a forward bend, a rare trait within the eurypterids.

Colored fossils

In certain fossils, the original coloration has been preserved without any replacement occurring, indicating that the fossilization process did not alter the initial color scheme of these fossils.

According to scientists, Megalograptus 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.