| Hughmilleria | |
|---|---|
 | |
| Fossils ofH. socialis. | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Subphylum: | Chelicerata |
| Order: | †Eurypterida |
| Superfamily: | †Pterygotioidea |
| Family: | †Hughmilleriidae |
| Genus: | †Hughmilleria Sarle, 1903 |
| Type species | |
| †Hughmilleria socialis Sarle, 1903 | |
| Species | |
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Hughmilleria is a genus ofeurypterid, an extinct group of aquaticarthropods. Fossils ofHughmilleria have been discovered in deposits of theSilurian age inChina and theUnited States. Classified as part of the basal familyHughmilleriidae, the genus contains three species,H. shawangunk from the easternUnited States,H. socialis fromPittsford, New York, andH. wangi fromHunan,China. The genus is named in honor of the ScottishgeologistHugh Miller.
H. socialis is the type species ofHughmilleriidae, aeurypterid family classified in the superfamilyPterygotioidea that is differentiated by their streamlined bodies, the enlargement of their medium-sizedchelicerae and the presence of pairedspines on the walkingappendages. With the biggest specimen measuring 20 centimetres (8 inches) in length,Hughmilleria is considered a eurypterid of small size.
Hughmilleria is the most basal (primitive) known member of thePterygotioidea.[1] It was a small-sized eurypterid, with the largest specimen measuring 20 cm (8 in), being surpassed by other members of its superfamily, such asSlimonia acuminata, which measured 100 cm (39 in) in length, andPterygotus grandidentatus, which could reach 1.75 meters (5 ft 8 in).[2] Thetelson (the most posterior segment of the body), which was lanceolate and styliform, is distinctly aEurypterus-like feature. The marginalcompound eyes, the relatively largechelae and the cordate (heart-shaped)metastoma (a large plate that is part of the abdomen) show a great resemblance toPterygotus.[3] Thecarapace was parabolic or subquadrate with oval marginal eyes, the chelicerae were able to extend beyond the carapace margin and the appendages II–V were spiniferous.[1] The genus is in various ways similar to the morederived eurypterids of its superfamily, the Pterygotioidea, however, it lacked the expanded and flattened telson that thepterygotids andSlimonia had. This suggests thatHughmilleria did not need to use the telson as a rudder to swim.[4]
Hughmilleria is distinguished from other members ofPterygotioidea by its streamlined body, its subquadrate prosoma (head), its medium-sized chelicerae, its small overall size and the various characteristics it shares withEurypterus.[5][3]
The genusHughmilleria was erected by the Americangeologist Clifton J. Sarle in 1903 to contain the speciesH. socialis, which was recovered for the first time in the Pittsford Shale Member of theVernon Formation,New York.[3] This species is the most abundant species of pterygotioid in the area with around 450 specimens found.[6] The generic name derives fromHugh Miller, a Scottish geologist and writer who found fossils of eurypterids of the Silurian, among themHughmilleria.[7] A variety ofH. socialis was also described,Hughmilleria socialis var.robusta, but currently it is considered a synonym of the species mentioned.[8]
Four years later, a second species was discovered among the fauna of the Shawangunk grit atOtisville. It was described asHughmilleria shawangunk and was smaller thanH. socialis.[3] The temporal range ofH. shawangunk has been placed between theLlandovery andLudlow epoch.[1] The largest specimens measured 12 cm (5 in) in length,[2] and differ from the type species by the carapace, which was broader, and the compound eyes which were larger and more prominent than the eyes ofH. socialis, among other aspects.[3]
The familyPterygotidae was erected in 1912 byJohn Mason Clarke andRudolf Ruedemann to constitute a group for the generaPterygotus,Slimonia,Hastimima andHughmilleria. However, Erik N. Kjellesvig-Waering emended the family in 1951, when the generaHastimima,Hughmilleria,Grossopterus andSlimonia were referred to their own family, theHughmilleriidae, which leftPterygotus as the only genus within the Pterygotidae. In 1961, Erik N. Kjellesvig-Waering splitHughmilleria into twosubgenera,Hughmilleria (Hughmilleria) andHughmilleria (Nanahughmilleria),[9] althoughNanahughmilleria would be later raised to the genus level.
In 2007, a new species ofHughmilleria from Hunan, China, was described asH. wangi based on an almost complete specimen (CNU-E-HLT2006001). Thespecific name is in honor of Junqing Wang, who found the fossil of the species in 1992. This species was recovered from theXiaoxiyu Formation of Hunan, in deposits that suggest that it lived in theTelychian age of the Silurian, which makes it the oldest eurypterid discovered in China.H. wangi differs from the North American species by the presence of epimera (lateral "extensions" of the segment) on the entirepostabdomen and a slightly wider carapace than in the other species.[1] Measuring 6 cm (2 in) in length,H. wangi is the smallest known species ofHughmilleria and of the Pterygotioidea superfamily.[2]
In addition, if the problematicH. lanceolata, which has been suggested by some researchers to belong to this genus, really did so, it would extend the range ofHughmilleria to the Silurian of Scotland.[8] It was originally described as a new species of the new genusHimantopterus (apreoccupied name, nowErettopterus) by the English geologist and paleontologistJohn William Salter. The body of this species was elongate and attenuated behind. Its telson was lanceolated and its swimming legs were narrow.[10] This species has also been related toNanahughmilleria, but recent studies suggest thatH. lanceolata was probably closer toEurypteroidea. However, the lack of eyes on all specimens ofH. lanceolata hinders the resolution of its phylogenetic position.[11]
Hughmilleria is classified within the familyHughmilleriidae in the superfamilyPterygotioidea.[8] HistoricallyHughmilleria was first considered a member of thePterygotidae until it was assigned to its own family alongside other genera that would later be reclassified.[12]
Hughmilleria andHerefordopterus shared a subtriangular carapace outline with the pterygotids and the wide telson and genital appendages of all three taxa were similar, although the genital appendages ofHughmilleria andHerefordopterus were more similar to those ofSlimonia by the division of these into three segments, in contrast to the undivided morphology in the pterygotids. Still,Hughmilleria andHerefordopterus differed from pterygotids andslimonids by the presence of paired spines on the walking appendages, that along with the characteristics thatSlimonia andCiurcopterus share, suggest that the hughmilleriids were more distant from the pterygotids thanSlimonia was.[13] Within Hughmilleriidae, both genera possessed a marginal rim much broader anteriorly than posteriorly and appendages spiniferous ofHughmilleria-type, butHughmilleria had 18-20 gnathobasic (of the gnathobase, a lower appendage used in the alimentation) teeth on appendage VI, unlikeHerefordopterus and the pterygotids, who had 12-13. Therefore,Hughmilleria represents the most basal form of Pterygotioidea.[14]
According to Clifton J. Sarle,Hughmilleria was very similar toEurypterus, and could be confused with a species of this genus if it was not for the presence of the marginal position of the eyes and the relatively largechelae. However, by its cordatemetastoma, the intramarginal to marginal position of thecompound eyes, the slightly longer preoral appendages, less developed swimming legs and theopercular appendage,Hughmilleria was more likePterygotus.[3]
The cladogram presented below, derived from a 2007 study by researcher O. Erik Tetlie, showcases the interrelationships between the pterygotioid eurypterids.[15]
| Pterygotioidea |
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Fossils ofHughmilleria have been found inSilurian deposits from theLlandovery andLudlowepochs.[15] Together with its close relatives,Hughmilleria dominated the communities inbrackish andfresh water, whilePterygotus andEurypterus dominated marine environments.[16]
The Silurian deposits of the Pittsford Shale Member in which fossils ofH. socialis have been found shelter various faunas of eurypterids, includingMixopterus multispinosus,Erettoperus osiliensis,Eurypterus pittsfordensis andCarcinosoma spiniferus, among others. Fossils from other organisms were also found, such as thecrustaceanCeratiocaris and theostracodLeperditia.[17] Geological features of the formation, such as the friable andcalcareousmudstone, theargillaceousdolomite and thelithology and associatedbiota suggests that the environment was marginal marine, very shallow and probably brackish.[17]
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