Acutiramus

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. Seven 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 2 metres (6.6 ft) in length. All species were not this large though, with some (such as "A. floweri") only reaching lengths of 20 centimetres (8 in).
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 2.1 metres (6.89 feet), "A. bohemicus" is the largest known species of the genus, whilst the smallest were "A. floweri" and "A. perryensis" both at a length of 20 cm (8 in). The body of "Acutiramus" was very slender, with members of the genus being almost five times as long as they were wide.
"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.
"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 2.3 to 2.5 metres (7.5 to 8.2 ft) 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 2.1 metres (6.9 ft). "A. bohemicus" is surpassed in size among the eurypterids only by "Jaekelopterus rhenaniae", which measured 2.5 metres (8.2 ft) 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 2 metres (6.6 ft) in length. "A. macropthalmus" was also large in size, with fragmentary tergites from Litchfield, New York suggesting sizes in the range of 1.65 to 2 metres (5.4 - 6.6 ft).
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".
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".
"A. suwanneensis" was named from a single specimen in 1955. The specimen includes the part and counterpart of an almost complete fixed ramus and hand of a chelicera. Together, the ramus and hand of the chelicera measure 31 mm in length. The chelicera is similar to that of other species in the genus and to other members of the Pterygotidae with the hand being rectangular and measuring about 7 mm in length. A highly fragmentary species, "A. suwanneensis" can be distinguished from other "Acutiramus" by the numerous teeth in its chelicerae that, while high in number, are not as many as in other species of the genus. Additionally, the central and main tooth (measuring 5.7 mm, or 0.2 inches, in length) is pointed forward but lacks the serrations commonly present in other species. Overall, the morphology of the chelicerae of "A. suwanneensis" is thin and slender. Estimates of the total length of the species revealed that it would have been relatively small, measuring at most 45.7 cm (18 in) in length. As such, it has been speculated that it might represent a juvenile or subadult, the lack of serrations on the teeth could be due to its possibly juvenile nature. Later size estimates place its length at 50 cm (19.7 in). The fossil was recovered in a core from the Gulf Oil Corporation in Columbia County, Florida and was dated to have been Upper Silurian in age. The fossil was found associated with fragmentary remains of "Ceratiocaris", a genus of phyllocarid crustaceans.
"A. perneri" was named in honour of Czech paleontologist Jaroslav Perner in 1994 and is known from fossilized remains consisting of several chelicerae, operculum with a genital appendage, coxae and several fragmentary body segments. The species was originally included in "A. bohemicus", which is very similar and is known from the same time and region. The pattern of denticulation on the chelicerae is virtually identical, but the chelicerae themselves are slightly more narrow, with more angled tips and the teeth are less prominent and shorter. "A. perneri" was regarded as a direct descendant of "A. bohemicus" by Chlupáč (1994). Using measurements of preserved chelicerae, the maximum body length of "A. perneri" was originally estimated to have been between 1.4 and 1.6 metres (4.6 to 5.3 ft), later being revised to 1.1 metres (3.6 ft). Just like "A. bohemicus", "A. perneri" occurs in Bohemia (in the Czech Republic), though in strata of Lochkovian (earliest Devonian) age.
In addition, it has been demonstrated that the enigmatic arthropod "Bunodella horrida" from the Silurian, known from one single fossil, actually represents the coxa of a swimming leg of an indeterminate species of "Acutiramus". The type species of "Bunodella", NBMG 3000 (housed at the New Brunswick Museum), is incomplete and poorly preserved, but shows the characteristic ornamentation of the pterygotids composed of semilunate scales. The first to eight denticles of originally twelve or thirteen are also preserved. The enlarged anterior denticle and the curvature present in the anterior margin of the coxal neck suggest an assignment to "Acutiramus" rather than other genera in Pterygotidae. However, "B. horrida" has not been formally synonymized with "Acutiramus" due to the lack of more diagnostic material, and therefore remains as a dubious name.
Classification.
"Acutiramus" is classified within the family Pterygotidae in the superfamily Pterygotioidea. "Acutiramus" is notably similar to the other two derived giant genera of the family, "Pterygotus" and "Jaekelopterus". Based on similarities of the genital appendage it has been suggested that the three genera could be synonymous, though differences have been noted in chelicerae, chelicerae have been questioned as the basis of generic distinctions in eurypterids since their morphology is dependent on lifestyle and vary throughout ontogeny. Genital appendages can vary even within genera, for instance the genital appendage of "Acutiramus" changes from species to species, being spoon-shaped in earlier species and then becoming bilobed and eventually beginning to look similar to the appendage of "Jaekelopterus". An inclusive phylogenetic analysis with multiple species of "Acutiramus", "Pterygotus" and "Jaekelopterus" is required to resolve whether or not the genera are synonyms of each other.
The cladogram below is based on the nine best-known pterygotid species and two outgroup taxa ("Slimonia acuminata" and "Hughmilleria socialis"). The cladogram also contains the maximum sizes reached by the species in question, which have been suggested to possibly have been an evolutionary trait of the group per Cope's Rule ("phyletic gigantism").
Paleoecology.
The cheliceral morphology and visual acuity of the pterygotid eurypterids separates them into distinct ecological groups. The primary method for determining visual acuity in arthropods is by determining the number of lenses in their compound eyes and the interommatidial angle (shortened as IOA and referring to the angle between the optical axes of the adjacent lenses). The IOA is especially important as it can be used to distinguish different ecological roles in arthropods, being low in modern active arthropod predators.
In contrast to all other pterygotid genera, notably the very visually acute "Pterygotus" and "Jaekelopterus", the eyes of "Acutiramus" were low in visual acuity (with few lenses in the compound eyes and high IOA values), inconsistent with the traditionally assumed pterygotid lifestyle of "active and high-level visual predators". The IOA values of "Acutiramus" changed during ontogeny but in a way opposite to other pterygotids. Vision becomes less acute in larger specimens, whilst vision tends to get more acute in adults in other genera, such as in "Jaekelopterus". Pterygotids may thus have been almost equally visually acute early in their life cycle, becoming more differentiated during growth.
The chelicerae of other pterygotids mainly served grasping functions and could also potentially be used for crushing and puncturing in "Pterygotus" and "Jaekelopterus", with large and robust claws. In contrast, the chelicerae of "Acutiramus" likely served a slicing or shearing function. The chelicerae themselves were large, but with differentiated denticles out of which one was serrated, long and strongly inclined.
The differences from other pterygotids on the basis of visual acuity and the morphology of the claws indicates that "Acutiramus" occupied an ecological role distinct from other members of the group and was a significantly less active predator. The weaker visual system and shearing claws of "Acutiramus" suggest that it might have been an ambush predator, or possible a scavenger, that fed on soft-bodied animals, feeding during the night or in otherwise low-light conditions.
In Bohemia, pterygotid eurypterids occur in strata that were once marine environments, associated with common and diverse marine fossils. "A. bohemicus" occurs together with trilobite fossils such as "Prionopeltis" and "Schariya" as well as in assemblages where orthocone cephalopods are common. In these regions, "Acutiramus" appears to have been a dominant predator, with some (though far fewer) occurrences of species of "Pterygotus". Fossils tentatively classified as "A. bohemicus" have also been discovered in the Pragian-aged Wilson Creek Shale in Victoria, Australia. In the Silurian Cunningham Creek Formation of New Brunswick, Canada, fossils of "Acutiramus" occur together with fossils of various jawless fish, such as "Ctenopleuron nerepisense", "Thelodus macintoshi" and "Cyathaspis acadica". In the Pozary Formation in the Czech Republic, "Acutiramus" occurs together with conodonts of various genera, including "Ozarkodina", "Wurmiella", "Oulodus", "Belodella" and "Pseudooneotodus". The environment of both these formations was marine during the Silurian.
The Silurian-Devonian boundary did not have any noticeable impact on "Acutiramus", with "A. bohemicus" evolving into "A. perneri" in Bohemia and still constituting a major part of the typical marine fauna present in its environment. The environment itself was offshore, but relatively shallow water, with some influence of benthic life and anoxic conditions but generally a pelagic living environment with normal levels of salinity. In some regions, "Acutiramus" represents the most commonly recovered eurypterid, such as "A. bohemicus" in Bohemia, but in others, notably in southern Ontario, Canada ("A. macrophthalmus" and "A. cummingsi") and Williamsville, New York ("A. cummingsi") the genus represents a very rare component of the fauna.