Acutiramus is a genus of giant predatory eurypterid, an extinct group of aquatic arthropods. Fossils of Acutiramus have been discovered in deposits of Late Silurian to Early Devonian age. Eight species have been described, five from North America (including A. cummingsi, the type species) and two from the Czech Republic (with one of them potentially found in Australia as well). The generic name derives from Latin acuto ("acute" or "sharp") and Latin ramus ("branch"), referring to the acute angle of the final tooth of the claws relative to the rest of the claw.
Acutiramus is classified as part of the eurypterid family Pterygotidae, a family differentiated from other eurypterids by their flattened telsons (the most posterior segment of the body) and their modified chelicerae (frontal appendages), ending in well-developed chelae (claws). Many pterygotid eurypterids, such as Jaekelopterus and Pterygotus, grew to gigantic proportions. Acutiramus was no exception, with the largest species, A. bohemicus, surpassing in length. All species were not this large though, with some (such as A. floweri) only reaching lengths of and likely an apex predator.
Other giant pterygotid eurypterids, such as Jaekelopterus and Pterygotus, were very visually acute (possessed good and sharp eyesight) in a similar way to modern predatory arthropods and possessed robust and crushing chelae. Studies on the compound eyes and chelicerae of Acutiramus has revealed that it would have had a comparably low visual acuity and claws adapted for slicing and shearing, rather than crushing. This suggests that the ecological role of Acutiramus was distinct from that of other pterygotids, it potentially lived a lifestyle of ambush predation or scavenging on soft-bodied animals, feeding during the night or in otherwise low-light conditions.
Description
Acutiramus was one of the largest eurypterids, perhaps only smaller than the gigantic Jaekelopterus rhenaniae. At a length of , A. bohemicus is the largest known species of the genus, whilst the smallest were A. floweri and A. perryensis both at a length of . The body of Acutiramus was very slender, with members of the genus being almost five times as long as they were wide.Clarke, J. K., Ruedemann R. (1912) "The Eurypterida of New York"
Acutiramus can be distinguished from other pterygotids by the distal margin of the chelae (pincers), where the final tooth is at an acute angle relative to the rest of the claw. The large tooth in the center of the claw is distally inclined (points forwards). The prosoma (head) is subquadrate (nearly square-shaped), with compound eyes located at the edge of the front corners. The telson (the posteriormost segment of its body) has a low row of knobs running down its center.Størmer, L. 1955. Merostomata. Treatise on Invertebrate Paleontology, Part P Arthropoda 2, Chelicerata, P: 30–31.
Acutiramus is classified as part of the pterygotid family of eurypterids, a group of highly derived eurypterids of the Silurian to Devonian periods that differ from other groups by a number of features, perhaps most prominently in the chelicerae (the first pair of limbs) and the telson. The chelicerae of the Pterygotidae were enlarged and robust, clearly adapted to be used for active prey capture and more similar to the claws of some modern crustaceans, with well-developed teeth on the claws, than to the chelicerae of other eurypterid groups. Another feature distinguishing the group from other eurypterid groups were their flattened and expanded telsons, likely used as rudders when swimming. Their walking legs were small and slender, without spines, and they were likely not capable of walking on land.
A. bohemicus was once estimated to have reached lengths of based on the largest known chelicerae and coxae. This would have made it the largest known arthropod, but recent estimates have revised the upper size limits to . A. bohemicus is surpassed in size among the eurypterids only by Jaekelopterus rhenaniae, which measured in length. Likewise, A. cummingsi was also once thought to have been the largest eurypterid (before the discovery of larger individuals in Acutiramus and Jaekelopterus), with fragmentary fossils suggesting great lengths. The largest preserved tergite suggests that the species could grow up to 1.65 metres in length, while a gnathobase (a lower appendage used to macerate, or soften and cut, food) from Buffalo was estimated to have belonged to a specimen reaching in length. A. macropthalmus was also large in size, with fragmentary tergites from Litchfield, New York suggesting sizes in the range of .
History of research
Original description as Pterygotus
The earliest species of Acutiramus to be named was A. macropthalmus (as a species of Pterygotus, Pterygotus macrophthalmus) in 1859. The type specimen, the carapace of a young individual, was discovered in waterlime deposits of Upper Silurian age in Litchfield, New York. At virtually the same time, the type specimen of another species of Pterygotus, P. osborni, was described from the same locality, this specimen preserving the prosoma and four abdominal segments but not the carapace. Later more complete remains from the locality allowed the two species to be synonymized in 1912 by John Mason Clarke and Rudolf Ruedemann as the fossils of P. osborni proved identical to more complete remains referred to P. marcophthalmus.
A. bohemicus was likewise first named as a species of Pterygotus in 1872 based on an incomplete coxa (L23505) from the Přídolí Formation, Upper Silurian in age. Since the discovery of that specimen, other fossils referred to A. bohemicus have yielded other parts of the body, including chelicerae, more coxae, segments of the appendages, genital appendages and incomplete metastomas (a large plate that is part of the abdomen) and telsons. Furthermore, some Early Devonian Acutiramus fossils found in Australia have been tentatively assigned to A. bohemicus, although it is possible that they represent a new species.
The species that would eventually be designated as the type species of Acutiramus, A. cummingsi, was described as Pterygotus cummingsi in 1875, based on a fragmentary coxa (the base of the leg, with which it attaches to the body) of the fourth walking leg discovered during quarrying operations in cement rock near Buffalo, New York. Another species, P. buffaloensis would be named six years later based on an incomplete fourth walking leg with the coxa included. Clarke and Ruedemann synonymized these taxa in 1912, choosing P. buffaloensis as the name because they considered P. cummingsi to "not be properly defined".Tollerton, V. P. Jr 1997 "The Type Species of the Eurypterid Genus Acutiramus Ruedemann, 1935" Journal of Paleontology 71(5):950-951
The principal differences between A. cummingsi and other species of Acutiramus is in its telson. The telson of A. cummingsi is not as obovate (ovate with a narrow end at its base) or elongated as that of A. macrophthalmus (which possesses a telson that is six times as long as it is wide), the telson of A. cummingsi is most often just as long as it is wide. The shape of the metastoma of A. bohemicus has been compared to other species in the genus, especially to that of A. cummingsi, which preserves a metastoma that is almost identical in morphology. A. bohemicus is generally agreed to be the species closest in relation to A. cummingsi, though they are differentiated by characteristics in the dentition of the chelae, many of the teeth being larger in A. bohemicus. There may be additional differences, but the incomplete nature of the A. bohemicus material makes further comparisons impossible.
Identification as separate genus Clarke and Ruedemann noted in 1912 that P. macrophthalmus was easily distinguished from other species of Pterygotus. While it was obviously closely related to P. anglicus based on features of the telson, the two species could easily be differentiated by several characteristics, notably the fishhook-like shape of the teeth of the chelae (claws) and the direction they pointed in being different. These features of the claw were noted as being similar to those of P. osiliensis, a species with a highly distinct bilobed telson. Furthermore, Clarke and Ruedemann noted that P. macrophthalmus appeared to have a free ramus intermediate in shape between species of Pterygotus and A. cummingsi. With the exception of the primary tooth, which is long and curved, all teeth on the claws are small and lack serrations. The gnathobase is similar to that of A. cummingsi, and A. macrophthalmus would likely possess appendages and carapaces that are similar in form and size.
With species being named as part of Pterygotus becoming increasingly diverse, researchers began to name subgenera, such as Pterygotus (Erettopterus), named by John William Salter in 1859 for species of Pterygotus with a bilobed telson, such as P. osiliensis. The family Pterygotidae would be erected by Clarke & Ruedemann in 1912, and new subgenera were named for Pterygotus by Ruedemann in 1935. These subgenera included Pterygotus (Curviramus) and Pterygotus (Acutiramus) and were differentiated from other Pterygotus by the curvature of the denticles (teeth) of the chelicerae. The name "Acutiramus" derives from Latin acuto ("acute" or "sharp") and Latin ramus ("branch"), referring to the acute angle of the final tooth of the claws relative to the rest of the claw. The species originally included in the subgenus were P. (A.) bohemicus, P. (A.) buffaloensis and P. (A.) macrophthalmus. P. buffaloensis was designated as the type species of the subgenus Acutiramus. Adhering to the rule of nomenclatural priority, the earliest used name should have priority. This was pointed out by Erik N. Kjellesvig-Waering in a 1955 publication, stating that using P. buffaloensis over P. cummingsi was "subjective" and in 1961, Kjellesvig-Waering recognised P. cummingsi as the name of the type species. Acutiramus continued to be treated as a subgenus of Pterygotus until it was raised to the level of a separate genus by Leif Størmer in 1974.
Discovery of further species
A. floweri was described in 1955, based on a single fossilized claw (NYSM 10712), preserving the fixed and free rami and parts of the palm. The fossil was recovered from Oneida Creek, located to the southeast of Kenwood, New York, in deposits that suggest that it lived in a lagoonal environment in the Ludfordian stage of the Silurian. A. floweri occurred together with other eurypterids, notably Eurypterus pittsfordensis and, more rarely, Mixopterus.