Acutiramusis a genus of giant predatoryeurypterid,an extinct group of aquaticarthropods.Fossils ofAcutiramushave been discovered in deposits ofLate SiluriantoEarly Devonianage. Eight species have been described, five fromNorth America(includingA. cummingsi,the type species) and two from theCzech Republic(with one of them potentially found inAustraliaas well). The generic name derives fromLatinacuto( "acute" or "sharp" ) and Latinramus( "branch" ), referring to the acute angle of the final tooth of the claws relative to the rest of the claw.
| Acutiramus Temporal range:Late Silurian-Pragian,
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| Fossil ofA. cummingsi | |
Scientific classification
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Subphylum: | Chelicerata |
| Order: | †Eurypterida |
| Superfamily: | †Pterygotioidea |
| Family: | †Pterygotidae |
| Genus: | †Acutiramus Ruedemann,1935 |
| Type species | |
| †Acutiramus cummingsi Grote & Pitt, 1875
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| Species | |
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| Synonyms | |
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Synonyms ofA. bohemicus
Synonyms ofA. cummingsi
Synonyms ofA. macrophthalmus
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Acutiramusis classified as part of the eurypterid familyPterygotidae,a family differentiated from other eurypterids by their flattenedtelsons(the most posterior segment of the body) and their modifiedchelicerae(frontal appendages), ending in well-developedchelae(claws). Many pterygotid eurypterids, such asJaekelopterusandPterygotus,grew to gigantic proportions.Acutiramuswas no exception, with the largest species,A. bohemicus,surpassing 2 metres (6.6 feet) in length. All species were not this large though, with some (such asA. floweri) only reaching lengths of 20 centimetres (7.9 in).
Other giant pterygotid eurypterids, such asJaekelopterusandPterygotus,were veryvisually 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 ofAcutiramushas 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 ofAcutiramuswas distinct from that of other pterygotids, it potentially lived a lifestyle ofambush predationorscavengingon soft-bodied animals, feeding during the night or in otherwise low-light conditions.
Description
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Acutiramuswas one of the largest eurypterids, perhaps only smaller than the giganticJaekelopterus rhenaniae.At a length of 2.1 metres (6.9 feet),A. bohemicusis the largest known species of the genus,[1]whilst the smallest wereA. floweriandA. perryensisboth at a length of 20 cm (7.9 in).[2]The body ofAcutiramuswas very slender, with members of the genus being almost five times as long as they were wide.[3]
Acutiramuscan be distinguished from other pterygotids by the distal margin of thechelae(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). Theprosoma(head) is subquadrate (nearly square-shaped), withcompound eyeslocated at the edge of the front corners. Thetelson(the posteriormost segment of its body) has a low row of knobs running down its center.[4]
Acutiramusis classified as part of the pterygotid family of eurypterids, a group of highlyderivedeurypterids of theSiluriantoDevonianperiods that differ from other groups by a number of features, perhaps most prominently in thechelicerae(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 moderncrustaceans,with well-developed teeth on the claws, than to the chelicerae of other eurypterid groups.[5]Another feature distinguishing the group from other eurypterid groups were their flattened and expanded telsons, likely used as rudders when swimming.[6]Their walking legs were small and slender, without spines,[4]and they were likely not capable of walking on land.[1]
A. bohemicuswas 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.[7]This would have made it the largest known arthropod, but recent estimates have revised the upper size limits to 2.1 metres (6.9 feet).[1][2]A. bohemicusis surpassed in size among the eurypterids only byJaekelopterus rhenaniae,which measured 2.5 metres (8.2 feet) in length.[1]Likewise,A. cummingsiwas also once thought to have been the largest eurypterid (before the discovery of larger individuals inAcutiramusandJaekelopterus), 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 feet) in length.[3]A. macropthalmuswas also large in size, with fragmentary tergites fromLitchfield, New Yorksuggesting sizes in the range of 1.65 to 2 metres (5.4 to 6.6 ft).[3][2]
History of research
editOriginal description asPterygotus
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The earliest species ofAcutiramusto be named wasA. macropthalmus(as a species ofPterygotus,Pterygotus macrophthalmus) in 1859. The type specimen, thecarapaceof a young individual, was discovered in waterlime deposits ofUpper Silurianage in Litchfield, New York. At virtually the same time, the type specimen of another species ofPterygotus,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 byJohn Mason ClarkeandRudolf Ruedemannas the fossils ofP. osborniproved identical to more complete remains referred toP. marcophthalmus.[3]
A. bohemicuswas likewise first named as a species ofPterygotusin 1872 based on an incomplete coxa (L23505) from thePřídolí Formation,Upper Silurian in age. Since the discovery of that specimen, other fossils referred toA. bohemicushave yielded other parts of the body, including chelicerae, more coxae, segments of the appendages, genital appendages and incompletemetastomas(a large plate that is part of the abdomen) and telsons.[7]Furthermore, some Early DevonianAcutiramusfossils found inAustraliahave been tentatively assigned toA. bohemicus,although it is possible that they represent a new species.[8]
The species that would eventually be designated as the type species ofAcutiramus,A. cummingsi,was described asPterygotus cummingsiin 1875,[7]based on a fragmentarycoxa(the base of the leg, with which it attaches to the body) of the fourth walking leg discovered during quarrying operations in cement rock nearBuffalo, New York.[3]Another species,P. buffaloensiswould be named six years later based on an incomplete fourth walking leg with the coxa included. Clarke and Ruedemann synonymized these taxa in 1912, choosingP. buffaloensisas the name because they consideredP. cummingsito "not be properly defined".[9]
The principal differences betweenA. cummingsiand other species ofAcutiramusis in its telson. The telson ofA. cummingsiis not as obovate (ovate with a narrow end at its base) or elongated as that ofA. macrophthalmus(which possesses a telson that is six times as long as it is wide), the telson ofA. cummingsiis most often just as long as it is wide.[3]The shape of the metastoma ofA. bohemicushas been compared to other species in the genus, especially to that ofA. cummingsi,which preserves a metastoma that is almost identical in morphology.A. bohemicusis generally agreed to be the species closest in relation toA. cummingsi,though they are differentiated by characteristics in the dentition of the chelae, many of the teeth being larger inA. bohemicus.There may be additional differences, but the incomplete nature of theA. bohemicusmaterial makes further comparisons impossible.[7]
Identification as separate genus
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Clarke and Ruedemann noted in 1912 thatP. macrophthalmuswas easily distinguished from other species ofPterygotus.While it was obviously closely related toP. anglicusbased 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.[3]These features of the claw were noted as being similar to those ofP. osiliensis,a species with a highly distinct bilobed telson.[3]Furthermore, Clarke and Ruedemann noted thatP. macrophthalmusappeared to have a free ramus intermediate in shape between species ofPterygotusandA. cummingsi.With the exception of the primary tooth, which is long and curved, all teeth on the claws are small and lack serrations.[3]The gnathobase is similar to that ofA. cummingsi,andA. macrophthalmuswould likely possess appendages and carapaces that are similar in form and size.[3]
With species being named as part ofPterygotusbecoming increasingly diverse, researchers began to namesubgenera,such asPterygotus(Erettopterus), named byJohn William Salterin 1859 for species ofPterygotuswith a bilobed telson, such asP. osiliensis.[10]The familyPterygotidaewould be erected by Clarke & Ruedemann in 1912, and new subgenera were named forPterygotusby Ruedemann in 1935. These subgenera includedPterygotus(Curviramus) andPterygotus(Acutiramus) and were differentiated from otherPterygotusby the curvature of the denticles (teeth) of the chelicerae.[11]The name "Acutiramus"derives fromLatinacuto( "acute" or "sharp" ) and Latinramus( "branch" ), referring to the acute angle of the final tooth of the claws relative to the rest of the claw.[4]The species originally included in the subgenus wereP.(A.)bohemicus,P.(A.)buffaloensisandP.(A.)macrophthalmus.[12]P. buffaloensiswas designated as the type species of the subgenusAcutiramus.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 usingP. buffaloensisoverP. cummingsiwas "subjective" and in 1961, Kjellesvig-Waering recognisedP. cummingsias the name of the type species.[9]Acutiramuscontinued to be treated as a subgenus ofPterygotusuntil it was raised to the level of a separate genus by Leif Størmer in 1974.[11]
Discovery of further species
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A. floweriwas 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 fromOneida Creek,located to the southeast ofKenwood, New York,in deposits that suggest that it lived in alagoonalenvironment in theLudfordianstage of theSilurian.[13]A. flowerioccurred together with other eurypterids, notablyEurypterus pittsfordensisand, more rarely,Mixopterus.[11]
A. suwanneensiswas 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. suwanneensiscan be distinguished from otherAcutiramusby 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 ofA. suwanneensisis 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 (18.0 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.[14]Later size estimates place its length at 50 cm (20 in).[2]The fossil was recovered in a core from theGulf Oil CorporationinColumbia County, Floridaand was dated to have beenUpper Silurianin age. The fossil was found associated with fragmentary remains ofCeratiocaris,a genus ofphyllocaridcrustaceans.[14]
A. perneriwas named in honour of Czech paleontologistJaroslav Pernerin 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 inA. 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.[7]A. perneriwas regarded as a direct descendant ofA. bohemicusby Chlupáč (1994).[7]Using measurements of preserved chelicerae, the maximum body length ofA. perneriwas originally estimated to have been between 1.4 and 1.6 metres (4.6 and 5.2 ft),[7]later being revised to 1.1 metres (3.6 feet).[2]Just likeA. bohemicus,A. pernerioccurs inBohemia(in theCzech Republic), though in strata ofLochkovian(earliest Devonian) age.[7]
In addition, it has been demonstrated that the Enigma tic arthropodBunodella horridafrom the Silurian, known from one single fossil, actually represents the coxa of a swimming leg of an indeterminate species ofAcutiramus.The type species ofBunodella,NBMG 3000 (housed at theNew 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 toAcutiramusrather than other genera in Pterygotidae. However,B. horridahas not been formally synonymized withAcutiramusdue to the lack of more diagnostic material, and therefore remains as adubious name.[15]
Classification
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Acutiramusis classified within the familyPterygotidaein the superfamilyPterygotioidea.[16][1]Acutiramusis notably similar to the other two derived giant genera of the family,PterygotusandJaekelopterus.Based on similarities of the genital appendage it has been suggested that the three genera could be synonymous,[17]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.[17]Genital appendages can vary even within genera, for instance the genital appendage ofAcutiramuschanges from species to species, being spoon-shaped in earlier species and then becoming bilobed and eventually beginning to look similar to the appendage ofJaekelopterus.An inclusive phylogenetic analysis with multiple species ofAcutiramus,PterygotusandJaekelopterusis required to resolve whether or not the genera are synonyms of each other.[17]
The cladogram below is based on the nine best-known pterygotid species and two outgroup taxa (Slimonia acuminataandHughmilleria 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 perCope's rule( "phyletic gigantism" ).[1][18]
| Pterygotioidea |
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Paleobiology
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The cheliceral morphology andvisual acuityof 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 theircompound eyesand 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.[19]
In contrast to all other pterygotid genera, notably the very visually acutePterygotusandJaekelopterus,the eyes ofAcutiramuswere 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".[19]The IOA values ofAcutiramuschanged 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 inJaekelopterus.Pterygotids may thus have been almost equally visually acute early in their life cycle, becoming more differentiated during growth.[19]
The chelicerae of other pterygotids mainly served grasping functions and could also potentially be used for crushing and puncturing inPterygotusandJaekelopterus,with large and robust claws. In contrast, the chelicerae ofAcutiramuslikely 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.[19]
The differences from other pterygotids on the basis of visual acuity and the morphology of the claws indicates thatAcutiramusoccupied an ecological role distinct from other members of the group and was a significantly less active predator.[19]The weaker visual system and shearing claws ofAcutiramussuggest 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.[19]
Paleoecology
editIn Bohemia, pterygotid eurypterids occur in strata that were once marine environments, associated with common and diverse marine fossils.A. bohemicusoccurs together withtrilobitefossils such asPrionopeltisandSchariyaas well as in assemblages whereorthoconecephalopodsare common. In these regions,Acutiramusappears to have been a dominant predator, with some (though far fewer) occurrences of species ofPterygotus.[7]Fossils tentatively classified asA. bohemicushave also been discovered in thePragian-aged Wilson Creek Shale inVictoria,Australia.[8]In the SilurianCunningham Creek FormationofNew Brunswick,Canada,fossils ofAcutiramusoccur together with fossils of various jawless fish, such asCtenopleuron nerepisense,Thelodus macintoshiandCyathaspis acadica.[20]In thePozary Formationin the Czech Republic,Acutiramusoccurs together withconodontsof various genera, includingOzarkodina,Wurmiella,Oulodus,BelodellaandPseudooneotodus.[21]The environment of both these formations wasmarineduring the Silurian.[20][21]
The Silurian-Devonian boundary did not have any noticeable impact onAcutiramus,withA. bohemicusevolving intoA. perneriin 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 ofbenthiclife andanoxicconditions but generally apelagicliving environment with normal levels of salinity.[7]In some regions,Acutiramusrepresents the most commonly recovered eurypterid, such asA. bohemicusin Bohemia,[7]but in others, notably in southernOntario,Canada(A. macrophthalmusandA. cummingsi) andWilliamsville, New York(A. cummingsi) the genus represents a very rare component of the fauna.[22][23]
See also
editReferences
edit- ^abcdefBraddy, Simon J.; Poschmann, Markus; Tetlie, O. Erik (2007)."Giant claw reveals the largest ever arthropod".Biology Letters.4(1): 106–109.doi:10.1098/rsbl.2007.0491.PMC2412931.PMID18029297.
- ^abcdeLamsdell, James C.; Braddy, Simon J. (2009)."Cope's rule and Romer's theory: patterns of diversity and gigantism in eurypterids and Palaeozoic vertebrates".Biology Letters.6(2): 265–269.doi:10.1098/rsbl.2009.0700.ISSN1744-9561.PMC2865068.PMID19828493.Supplemental material.
- ^abcdefghijClarke, J. K., Ruedemann R. (1912) "The Eurypterida of New York"
- ^abcStørmer, L. 1955. Merostomata.Treatise on Invertebrate Paleontology, Part P Arthropoda 2, Chelicerata,P: 30–31.
- ^Tetlie, O. Erik (2007)."Distribution and dispersal history of Eurypterida (Chelicerata)"(PDF).Palaeogeography, Palaeoclimatology, Palaeoecology.252(3–4): 557–574.Bibcode:2007PPP...252..557T.doi:10.1016/j.palaeo.2007.05.011.Archived fromthe original(PDF)on 2011-07-18.
- ^Plotnick, Roy E.; Baumiller, Tomasz K. (1988-01-01). "The pterygotid telson as a biological rudder".Lethaia.21(1): 13–27.Bibcode:1988Letha..21...13P.doi:10.1111/j.1502-3931.1988.tb01746.x.
- ^abcdefghijkChlupáč, Ivo (1994)."Pterygotid eurypterids (Arthropoda, Chelicerata) in the Silurian and Devonian of Bohemia"(PDF).Journal of the Czech Geological Society.39/2-3: 147–162. Archived fromthe original(PDF)on 2018-02-27.Retrieved2018-02-27.
- ^abBicknell, Russell D. C.; Smith, Patrick M.; Poschmann, Markus (2020)."Re-evaluating evidence of Australian eurypterids".Gondwana Research.86:164–181.Bibcode:2020GondR..86..164B.doi:10.1016/j.gr.2020.06.002.S2CID225748023.
- ^abTollerton, V. P. Jr 1997 "The Type Species of the Eurypterid GenusAcutiramusRuedemann, 1935"Journal of Paleontology71(5):950-951
- ^Kjellesvig-Waering, Erik N. (1964). "A Synopsis of the Family Pterygotidae Clarke and Ruedemann, 1912 (Eurypterida)".Journal of Paleontology.38(2): 331–361.JSTOR1301554.
- ^abcCiurca, Samuel J.; Tetlie, O. Erik (2007)."Pterygotids (Chelicerata; Eurypterida) from the Silurian Vernon Formation of New York".Journal of Paleontology.81(4): 725–736.doi:10.1666/pleo0022-3360(2007)081[0725:PEFTSV]2.0.CO;2.ISSN0022-3360.S2CID140668235.
- ^Kjellesvig-Waering, Erik N. (1964). "A Synopsis of the Family Pterygotidae Clarke and Ruedemann, 1912 (Eurypterida)".Journal of Paleontology.38(2): 331–361.JSTOR1301554.
- ^Kjellesvig-Waering, Erik N.; Caster, Kenneth E. (1955). "The Pterygotidae of the Silurian Vernon Shales of New York".Journal of Paleontology.29(6): 1041–1047.JSTOR1300451.
- ^abKjellesvig-Waering, Erik N. (1955). "A New Phyllocarid and Eurypterid from the Silurian of Florida".Journal of Paleontology.29(2): 295–297.JSTOR1300471.
- ^Miller, Randall F.; Tetlie, O. Erik (2007). "The presumed SynziphosuranBunodella horridaMatthew, 1889 (Silurian; Cunningham Creek Formation, New Brunswick, Canada) is a eurypterid ".Journal of Paleontology.81(3): 588–590.Bibcode:2007JPal...81..588M.doi:10.1666/05127.1.S2CID85708511.
- ^Dunlop, J. A., Penney, D. & Jekel, D. 2018.A summary list of fossil spiders and their relatives.In World Spider Catalog. Natural History Museum Bern
- ^abcLamsdell, James C.; Legg, David A. (2010/11)."An isolated pterygotid ramus (Chelicerata: Eurypterida) from the Devonian Beartooth Butte Formation, Wyoming".Journal of Paleontology.84(6): 1206–1208.doi:10.1666/10-040.1.
- ^Gould, Gina C.; MacFadden, Bruce J. (2004-06-01). "Chapter 17: Gigantism, Dwarfism, and Cope's Rule:" Nothing in Evolution Makes Sense without a Phylogeny "".Bulletin of the American Museum of Natural History.285:219–237.doi:10.1206/0003-0090(2004)285<0219:C>2.0.CO;2.S2CID73556985.
- ^abcdefMcCoy, Victoria E.; Lamsdell, James C.; Poschmann, Markus; Anderson, Ross P.; Briggs, Derek E. G. (2015-08-01)."All the better to see you with: eyes and claws reveal the evolution of divergent ecological roles in giant pterygotid eurypterids".Biology Letters.11(8): 20150564.doi:10.1098/rsbl.2015.0564.ISSN1744-9561.PMC4571687.PMID26289442.
- ^abBurrow, Carole J. (2011-09-01). "A partial articulated acanthodian from the Silurian of New Brunswick, Canada".Canadian Journal of Earth Sciences.48(9): 1329–1341.Bibcode:2011CaJES..48.1329B.doi:10.1139/e11-023.ISSN0008-4077.
- ^abJ. Kříž. 1992.Silurian field excursions: Prague Basin (Barrandian), Bohemia.National Museum of Wales, Geological Series13:1-111
- ^Moelling, Leah I. (2013)."Eurypterid Paleoecology: Characterization and Analysis of a Late Silurian Shallow Marine Fossil Assemblage from Southern Ontario, Canada"(PDF).Senior Honors Thesis, Faculty of the University of Utah.Archived fromthe original(PDF)on 2018-02-28.Retrieved2018-02-27.
- ^Lau, Kimberly (2009). "Paleoecology and Paleobiogeography of the New York Appalachian Basin Eurypterids".Senior Honors Thesis, Department of Geology and Geophysics, Yale University.S2CID14229267.
External links
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Media related toAcutiramusat Wikimedia Commons- Acutiramus in Eurypterids.netArchived2008-07-23 at theWayback Machine
- Acutiramus at BioLib
- Pincer fossil of Pterygotus (Acutiramus)
- Acutiramus at Palaeos
