| Bassipterus | |
|---|---|
 | |
| Reconstruction of the swimming leg | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Subphylum: | Chelicerata |
| Order: | †Eurypterida |
| Superfamily: | †Adelophthalmoidea |
| Family: | †Adelophthalmidae |
| Genus: | †Bassipterus Kjellesvig-Waering & Leutze, 1966 |
| Type species | |
| †Bassipterus virginicus Kjellesvig-Waering & Leutze, 1966 | |
Bassipterus ("wing fromBass") is agenus ofeurypterid, an extinct group of aquaticarthropods.Bassipterus is classified as part of thefamilyAdelophthalmidae, the onlyclade within thederived ("advanced")Adelophthalmoideasuperfamily of eurypterids. Fossils of the single andtype species,B. virgnicus, have been discovered in deposits of the LateSilurian age inWest Virginia andMaryland,United States. The genus is named after Bass, where most of the fossils have been recovered.
Bassipterus was a well-knownbasal ("primitive") genus that was distinguished from the more derived adelophthalmids by the specialization of its genital operculum (a plate-like segment which contains the genital aperture) and its long and narroweyes, beingPittsfordipterus's closest relative.
Bassipterus is almost completely known, with the first to fifth pair of limbs and type B genital appendage being the only unknown body parts. The body had a lanceolate (lance-shaped) and streamlined shape. Theprosoma ("head") was long, rounded and tray-shaped. Thecarapace (dorsal plate of the head) was surrounded by a narrow marginal rim, witheyes placed frontally, placed intramarginally (within the margin) and unusually long. PE 6139, a 15.2 millimeter (0.6 inch) long carapace, had eyes 4.5 mm (2 in) long and 2.2 mm (0.1 in) wide. Theocelli are preserved, and were placed in the center of the carapace, immediately behind the eyes. Themetastoma (a large plate part of theabdomen) was long, truncated (shortened as by cutting it) at the base and cordate (heart-shaped) in theanterior (frontal) part. One specimen preserves a metastoma of 8.3 mm (0.3 in) in length and 5 mm (0.2 in) in width.[1]
Of theappendages (limbs), the swimming legs (sixth pair of appendages) are the only known ones. These were ofHughmilleria-type (with enlarged seventh and eighth leg segments and the ninth very small[2]), but in thisspecies, it was serrated along the anterior edge of the sixth to seventh joints. The terminal joint was a sharped spur. Themesosoma (comprisingbody segments 1 to 6) was tapered, with the firsttergite (dorsal part of the segment) considerably narrower than the rest. Themetasoma (comprising segments 7 to 12) followed the tapering of the mesosoma and ended in a long dirk-liketelson (the posteriormost division of the body). The telson was smooth in the ventral part, but carinated (with a keel) dorsally towards the end, which was a very sharped point. The specimen PE 6208 had a 32.5 mm (1.3 in) long telson. Theornamentation inBassipterus is well known and developed. The carapace was covered by triangular scales. The tergites and abdominal plates had mucrones (sharped points) typical ofAdelophthalmidae, but these were considerably larger and more prominent than in related genera. In some places, the mucrones touched each other and formed a rhombic ornamentation, while in others, they were so elongated that they resembled the linear striations present in the morederived ("advanced")genusAdelophthalmus.[1]
Themorphology of the genital operculum (a plate-like segment which contains the genital aperture) allows an easy differentiation betweenBassipterus and the other adelophthalmids. The type A genital appendage (assumed to represent females) was long, reaching the second pair of abdominal plates, and was divided into three joints. The first had a short, triangular and hastate (with protruding lobes) portion followed by a tubular shaft that ended in two lateral triangular projections at the point of union with the following joint. The second joint was shorter and composed of three different areas. On both sides of the central shaft, two long narrowsclerites (hardened parts), possibly plates, lengthened themselves at their ends. The third joint was very short and finished in two long points reminiscent ofEurypterus, a more basal genus. The appendage in its entirety was covered by triangular mucrones which pointed outwards.[1] The genital operculum differentiatesBassipterus from other eurypterid genera and suggests a close relationship withPittsfordipterus.[3]
Bassipterus virginicus was described by the paleontologists Erik Norman Kjellesvig-Waering and Kenneth Edward Caster based on theholotype (PE 6201, an almost complete specimen), sevenparatypes and dozens of fragments. Most of the fossils were found in theWills Creek Formation ofBass,West Virginia, as well as an isolated carapace of the sameformation inCumberland,Maryland (previously referred toHughmilleria).[1] The nameBassipterus is translated as "wing from Bass", with the first word of the name referring to the West Virginianunincorporated community of Bass and the last word composed of theGreek word πτερόν (pteron, wing).[3] Although Kjellesvig-Waering and Caster initially classifiedBassipterus in thefamilyHughmilleriidae,[1] Victor P. Tollerton Jr. moved the genus along with others to the new family Adelophthalmidae.[2]
In 2004, the paleontologist Odd Erik Tetlie concluded thatBassipterus was asynonym ofParahughmilleria bellistriata and the more basalStoermeropterus nodosus.[4] Although subsequent authors have followed this study,[5] the most acceptedphylogenetic classification placesBassipterus together withPittsfordipterus, forming abasal clade.[3]
Bassipterus is classified as part of the family Adelophthalmidae, the onlyclade ("group") within thesuperfamilyAdelophthalmoidea.[6] It was originally described as a hughmilleriid, but since then it has been considered closer toAdelophthalmus than toHughmilleria.[2]
Bassipterus andPittsfordipterus form a basal ("primitive") group that differentiate them from the other adelophthalmids. This clade is backed by a pair ofsynapomorphies (shared characteristics different from that of theirlatest common ancestor); relatively long and narrow eyes and a complex termination of the genital appendage. Therefore,Pittsfordipterus is thesister group (closest relative) ofBassipterus.[3]Bassipterus was also similar toParahughmilleria, a derived adelophthalmid whose metastoma, telson and body were slightly differentiated.[7] This has led some authors to synonymizeBassipterus withParahughmilleria,[4] though this is not currently supported.[3]
Thecladogram below presents the inferred phylogenetic positions of most of the genera included in the three most derived superfamilies of theDiploperculata infraorder of eurypterids (Adelophthalmoidea,Pterygotioidea and thewaeringopteroids), as inferred by Odd Erik Tetlie and Markus Poschmann in 2008, based on the results of a 2008 analysis specifically pertaining to the Adelophthalmoidea and a preceding 2004 analysis.[3]
Bassipterus fossils have been recovered fromSilurian deposits of the LateLudlow (Ludfordian) and EarlyPridoliepochs of the Wills Creek Formation of West Virginia and Maryland,United States. In the Marylander part of the formation, fossils of other eurypterids have been found, such asEurypterus remipes orWaeringopterus cumberlandicus together with an indeterminate species of theostracodLeperditia.[8] On the other hand, in the West Virginian section, remains ofW. cumberlandicus,Erettopterus exophthalmus,Parahughmilleria bellistriata andStoermeropterus nodosus have been associated, as well as unclassifiedbrachiopods andbivalves.[9]Bassipterus lived in alagoonal or restricted shallowsubtidal (the area wheresunlight reaches the bottom of theocean) environment. Thelithology (description of the physical characteristics of the rocks) of the zone was composed ofargillaceous (composed of clay-like materials)limestone andcalcareous (containingcalcium)shale.[8][9]
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| Geography | ||
| Notable genera | ||
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